subsurface exploration program and geotechnical evaluation …€¦ · 22/10/2014 · additional...

Subsurface Exploration Program and Geotechnical Evaluation

Proposed Preschool 8361 South Park Lane

Littleton, Colorado

Prepared for:

Mr. Tom Slavens

45-508 Puoni Place

Kaneohe, Hawaii 96744

Job Number: 14-3083 October 22, 2014

TABLE OF CONTENTS

Page

Purpose and Scope of Study ...................................................................................... 1

Proposed Construction ................................................................................................ 1

Site Conditions ............................................................................................................ 2

Geologic Setting ........................................................................................................... 2

Subsurface Exploration ............................................................................................... 3

Laboratory Testing ...................................................................................................... 3

Subsurface Conditions ................................................................................................ 3

Seismic Classification .................................................................................................. 5

Geotechnical Considerations for Design ...................................................................... 5

Shallow Foundations ..................................................................................................... 7

Slab-on-Grade Floor .................................................................................................... 9

Lateral Loads ............................................................................................................... 13

Water Soluble Sulfates ................................................................................................. 14

Soil Corrosivity ............................................................................................................. 15

Project Earthwork ....................................................................................................... 16

Excavation Considerations .......................................................................................... 20

Utility Lateral Installation .............................................................................................. 21

Surface Drainage ......................................................................................................... 23

Subsurface Drainage ................................................................................................... 27

Pavement Sections ....................................................................................................... 29

Exterior Flatwork .......................................................................................................... 34

Closure ........................................................................................................................ 37

Locations of Test Holes ..................................................................................... Figure 1

Logs of Test Holes ........................................................................................... Figure 2

Legend and Notes ............................................................................................. Figure 3

Gradation Plot .................................................................................................... Figure 4

Typical Underdrain Detail ................................................................................... Figure 5

Summary of Laboratory Test Results ........................................................ Tables 1 & 2

Pavement Section Calculations ................................................................... Appendix A


Littleton, Colorado

Job No. 14-3083 GROUND Engineering Consultants, Inc. Page 1

PURPOSE AND SCOPE OF STUDY

This report presents the results of a subsurface exploration program performed by

GROUND Engineering Consultants, Inc. (GROUND) to provide a geotechnical

evaluation in support of design and construction of a preschool facility to be constructed

at 8361 South Park Lane in Littleton, Colorado. Our study was conducted in general

accordance with GROUND’s Proposal No. 1408-1450 Revised, dated August 11, 2014.

Field and office studies provided information obtained at the test hole locations regarding

surface and subsurface conditions, including the existing site vicinity improvements.

Material samples retrieved during the subsurface exploration were tested in our

laboratory to assess the engineering characteristics of the site earth materials, and

assist in our geotechnical analysis. Results of the field, office, and laboratory studies are

presented below.

Additional information was obtained from a preliminary geotechnical evaluation of the

site prepared by GROUND, dated September 17, 2014.1

This report has been prepared to summarize the data obtained and to present our

findings and conclusions based on the proposed construction and the subsurface

conditions encountered. Design parameters and a discussion of engineering

considerations related to construction of the proposed improvements are included

herein.

PROPOSED CONSTRUCTION

We understand that the proposed facility will consist largely of a single-story,

approximately 11,000-square foot school building. We anticipate that structural loads

will be low, typical of this type of structure. No below-grade level is planned. A paved

parking area will be installed to north of the building. Underground utility laterals will be

installed to serve the new facility.

1 GROUND Engineering Consultants, Inc., 2014, Preliminary Geotechnical Evaluation, Proposed Preschool, 8361 South Park Lane, Littleton, Colorado, Job No. 14-3069, prepared for Mr. Tom Slavens, dated September 17.


Littleton, Colorado


The building will be surrounded by landscaping including sidewalks and other

hardscaping. Limited cuts are planned for the northern portion of the site. Fill increasing

in thickness southward across the site, reaching up to about 7 feet in nominal thickness

are planned to construct the building pad. As a result, the building finished floor

elevation will be 3 to 6 feet above existing grades.

If the proposed construction differs significantly from that described above, GROUND

should be notified to re-evaluate the conclusions and parameters contained herein.

SITE CONDITIONS

At the time of our subsurface exploration program, the site consisted of a largely un-

improved lot approximately 1½ acres in area. The ground surface supported grasses

and weeds, although apparently some grading had been performed previously.

Scattered small trees were observed along the northern margin of the property and

mature trees were noted near the southern margin. The ground surface sloped gently to

the south over the majority of the site. Total relief across the property was

approximately 5 feet. Near the southern boundary, closer to the Highline Canal, the

southward slope steepened markedly.

The site was bordered by South Park Lane to the north, the High Line Canal, the High

Line Canal Trail and County Line Road to the south. Commercial properties were noted

to the east, north and west. Ground that was unimproved or in use for agriculture lay to

the south. A moderately sized reservoir lay about 1,000 feet to the west.

GEOLOGIC SETTING

Published geologic maps, e.g., Trimble and Machette (1979)2 depict the site as underlain

by Quaternary Colluvium (‘slope-wash’ deposits), underlain in turn by bedrock of the

upper Cretaceous / lower Tertiary Denver Formation. Colluvial soils typically consist of

sands, silts and clays in varying proportions typically reflecting the compostion of the

underlying, source materials. The Denver Formation in the project area consists of

interbedded claystones and sandstones with local siltstones.

2 Trimble D. and Machette, M., 1979, Geologic Map of Greater Denver Area, Front Range Urban Corridor,

Colorado: U.S. Geological Survey, Geologic Quadrangle Map I-856-H.


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SUBSURFACE EXPLORATION

Subsurface exploration for the project was conducted in October, 2014. A total of four

(4) test holes were drilled with a conventional, truck-mounted, drilling rig to evaluate the

subsurface conditions as well as to retrieve samples for laboratory testing and analysis.

Two test holes advanced to depths of about 50 and 51 feet in the approximate footprint

of the proposed building. The other test holes were drilled to depths of about 5 feet in

the area proposed for parking. A GROUND engineer directed the subsurface

exploration, logged the test holes in the field, and prepared the samples for transport to

our laboratory.

Samples of the subsurface materials were retrieved with a 2-inch I.D. ‘California’ liner

sampler. The sampler was driven into the substrata with blows from a 140-pound

hammer falling 30 inches, a procedure similar to that described by ASTM Method

D1586. Penetration resistance values, when properly evaluated, indicate the relative

density or consistency of soils. Depths at which the samples were obtained and

associated penetration resistance values are shown on the test hole logs.

The approximate locations of the test holes, including the test hole drilled for GROUND’s

preliminary evaluation, are shown on Figure 1. Logs of the test holes are presented on

Figure 2. Explanatory notes and a legend are provided on Figure 3.

LABORATORY TESTING

Samples retrieved from our test holes were examined and visually classified in the

laboratory by the project engineer. Laboratory testing of soil samples included standard

property tests, such as natural moisture contents, dry unit weights, grain size analyses,

and Atterberg limits. Swell-consolidation, water-soluble sulfate content and a suite of

corrosivity tests were completed on selected samples, as well. Resilient modulus testing

was performed on a bulk sample of the shallow soils. Laboratory tests were performed

in general accordance with applicable ASTM and AASHTO protocols. Results of the

laboratory testing program are summarized in Tables 1 and 2.

SUBSURFACE CONDITIONS

Beneath a variably thick layer of poorly developed topsoil, the test holes penetrated

about 5 feet of clayey fill soils at test holes B-1 and P-2. At the other test hole locations


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and beneath the fill soils, native sands and clays were encountered that extended to

depths of about 13 to 21 feet. The sands and clays were underlain by silty to clayey

sands that extended to the depths explored. Bedrock was not encountered to the

depths explored (about 51 feet).

We interpret the shallow sands and clays to be upper Pleistocene Loess (a wind-

deposited material) possibly interbedded with alluvial (stream-laid) soils. We interpret

the underlying silty and clayey sands to be Pleistocene alluvial soils, likely Slocum

Alluvium.

Loess commonly exhibits a potential for hydro-consolidation or “collapse.” This refers to

volume loss in the soil due to re-structuring of the constituent grains of the soil to a more

compact arrangement upon wetting.

Fill soils were recognized in two of the test holes and likely are present elsewhere at the

site. Delineation of the complete lateral and vertical extents of any fills at the site, or

their compositions, was beyond our present scope of services. If fill soil volumes and

compositions at the site are of significance, they should be evaluated by the contractor

using test pits.

Coarse gravel and larger clasts are not well represented in small diameter liner samples

collected from 4-inch diameter test holes. Therefore, such materials may be present

even where not called out in the material descriptions herein.

Fill consisted of sandy clays and clayey sands with local gravel. The sand fractions

were fine to medium. They were dry to moist, moderately to highly plastic, stiff / medium

dense to hard / dense, and brown in color.

Sands and Clays ranged from clayey sands to sandy clays with local silts. The sand

fractions were fine to medium to fine to coarse. They were moist to wet, moderately to

highly plastic, stiff / medium dense, and brown in color.

Silty and Clayey Sands included silty sands, clayey sands and scattered lenses of

clay. The sand fractions were fine to medium to fine to coarse. Cleaner sands were

encountered in the deeper portion of the section. Gravels were encountered locally.


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Groundwater was encountered in Test Holes B-2 and B-3 at depths of about 38 to 39

feet below existing grades at the time of drilling. Subsequent re-measurement 8 days

later indicated water a depth of about 33½ feet at B-2.

Groundwater levels can be expected to fluctuate, however, in response to annual and

longer-term cycles of precipitation, irrigation, surface drainage, land use, and the

development of transient, perched water conditions.

Swell-consolidation Testing indicated consolidation potentials between 1 and 2

percent in the sands and clays under surcharges approximately equal to foundation or

overburden pressures.

SEISMIC CLASSIFICATION

Based on extrapolation of available data to depth and our experience in the project area,

we consider the site likely to meet the criteria for a Seismic Site Classification of D

according to the 2012 IBC classification (Table 1613.5.2). Exploration and/or shear

wave velocity testing to a depth of 100 feet or more (necessary to evaluate the Seismic

Site Classification quantitatively, or possibly to obtain a higher classification) was not

part of our present scope of services. GROUND can provide a proposal in that regard

upon request. We consider the likelihood of achieving a Site Classification of C at the

subject location to be low.

The USGS’s Earthquake Ground Motion Tool v.5.0.9a indicated an SDS value of 0.237g

and an SD1 value of 0.092g for the site’s latitude/longitude coordinates and Seismic Site

Class of D.

GEOTECHNICAL CONSIDERATIONS FOR DESIGN

Geotechnical Risk The soils underlying the site exhibited potentials for post-

construction settlement that can cause damaging, post-construction, structural

movements. This condition, if not mitigated, could affect nearly all improvements at the

site, and represent the primary geotechnical constraint on design at the facility. Specific

geotechnical measures in this regard are provided in subsequent sections of this report.

Additional discussion and information regarding the geotechnical risks that they address

are provided below.


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The conclusions and parameters provided in this report were based on the data

presented herein, our experience in the general project area with similar structures, and

our engineering judgment with regard to the applicability of the data and methods of

forecasting future performance. A variety of engineering parameters were considered as

indicators of potential future soil movements. Our conclusions were based on our

judgment of “likely movement potentials,” (i.e., the amount of movement likely to be

realized if site drainage is generally effective, estimated to a reasonable degree of

engineering certainty) as well as our assumptions about the owner’s willingness to

accept geotechnical risk. “Maximum possible” movement estimates necessarily will be

larger than those presented herein. They also have a significantly lower likelihood of

being realized in our opinion, and generally require more expensive measures to

address. We encourage the client, upon receipt of this report, however, to discuss these

risks and the geotechnical alternatives with us. In addition to the risks and remedial

approaches presented in this report, the client also must understand the risk-cost trade-

offs addressed by the civil engineer and structural engineer in order to direct his design

team to the portion of the Higher Cost / Lower Risk – Lower Cost / Higher Risk spectrum

in which this project should be designed. If the client does not understand these risks, it

is critical that additional information or clarification be requested so that the owner’s

expectations reasonably can be met.

Likely Post-Construction Movements Based on the data obtained for this study and

our experience on other projects in similar geotechnical settings, we estimate likely,

post-construction, vertical movements on the order of 2 to 3 inches where improvements

are supported directly on the existing earth materials. Lateral movements will result, as

well. Significant structure distress could result from movements of these magnitudes.

General Building Foundation Types Supporting the building on a deep foundation

systems (e.g., drilled piers) and providing it with a structural floor also supported on deep

foundations can reduce estimated post-construction movements to about ½ inch. A

deep foundation system and a structural floor is the most positive approach to limit the

risk of post-construction settlement at this site. We have assumed, however, that the

use of deep foundations may not be practical for this facility. Geotechnical parameters

for deep foundation design can be provided upon request, however.

Conventional, shallow, foundations can be used for the proposed buildings, together with

slab-on-grade concrete floors, if supported on a section of properly compacted fill that

Paul Kastner

Highlight

Paul Kastner

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Paul Kastner

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Littleton, Colorado


extends down to elevation 5534 (based on the approximate elevations on Figure 2).

(Remedial earthwork to that elevation likely will re-work all the existing, undocumented

fill soils beneath the building, as well as the upper portion of the collapsible soils.) If

footings and floor slabs bear on such a fill section, and effective drainage is established

and maintained, we estimate likely post-construction foundation movements to be on the

order of 1 inch, with similar differential movements over spans of about 40 feet.

However, because earthwork to the above indicated depths will leave a portion of the

collapsible deposits in place, a risk for greater, post-construction settlement will remain.

Therefore, it is critical to establish and maintain effective surface and subsurface

drainage at the site.

The limited risk of greater than estimated, post-construction movement can be reduced

further by construction of a deeper fill section beneath the building. Geotechnical

parameters in that regard can be provided upon request. Excavation and replacement

to depths of at least 20 feet below existing grades would be necessary to re-work all of

the potentially collapsible soil.

More detailed parameters for design and construction of shallow, spread footing

foundations and slab-on-grade floors are provided in the Shallow Foundations and Slab-

on-Grade Floor sections of this report.

SHALLOW FOUNDATIONS

Geotechnical Parameters for Shallow Foundation Design

1) Footings should bear on a section of properly compacted fill soils below the

footings and to an elevation of 5534 feet based on the approximate elevations

shown on Figure 2. This is in addition to scarification and re-compaction of the

underlying surface, as outlined in the Project Earthwork section of this report.

If project earthworking exposes existing, undocumented fill soils that extend to

depths of more than 6 feet below foundation bearing elevation, then those soils

should be excavated to their full depth and replaced with properly compacted fill.

Where grades are raised in areas where existing fills are present, any existing fill

soils still should be excavated fully and replaced with properly compacted fill.


Littleton, Colorado


The fill section should extend should extend at full thickness across the building

footprint and at least 6 feet laterally beyond the building perimeter.

Considerations for fill placement and compaction are provided in the Project

Earthwork section of this report.

The fill section beneath the building should be laterally consistent and of uniform

depth to reduce differential, post-construction foundation movements. A

differential fill section will tend to increase differential movements.

The contractor should provide survey data of the excavation beneath each

building indicating the depth and lateral extents of the remedial excavation.

2) Footings bearing on firm native soils or properly compacted fill may be designed

for an allowable soil bearing pressure of 1,500 psf for footings up to 6 feet in

width (least lateral dimension). This value may be increased by ⅓ for transient

loads such as wind or seismic loading. For larger footings, a lower allowable

bearing pressure may be appropriate.

The estimated 1 inch of likely, post-construction settlement associated with this

fill section and allowable bearing pressure is based on an assumption of effective

site drainage. If foundation soils are subjected to an increase/fluctuation in

moisture content, the effective bearing capacity will be reduced and greater post-

construction movements than those estimated above may result.

In order to reduce differential settlements between footings or along continuous

footings, footing loads should be as uniform as possible. Differentially loaded

footings will settle differentially.

3) Spread footings should have a minimum lateral dimension of 16 or more inches

for linear strip footings and 24 or more inches for isolated pad footings. Actual

footing dimensions, however, should be determined by the structural engineer.

4) Footings should bear at an elevation 3 or more feet below the lowest adjacent

exterior finish grades to have adequate soil cover for frost protection.

5) Continuous foundation walls should be reinforced top and bottom to span an

unsupported length of at least 10 feet.


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6) Geotechnical parameters for lateral resistance to foundation loads are provided

in the Lateral Loads section of this report.

7) Connections to the building of all types must be flexible and/or adjustable to

accommodate the anticipated, post-construction movements.

Shallow Foundation Construction Considerations

8) Care should be taken when excavating the foundations to avoid disturbing the

supporting materials. Hand excavation or careful backhoe soil removal may be

required in excavating the last few inches.

9) Footing excavation bottoms may expose loose, organic or otherwise deleterious

materials, including debris. Firm materials may become disturbed by the

excavation process. All such unsuitable materials should be excavated and

replaced with properly compacted fill.

10) Foundation soils may be disturbed or deform excessively under the wheel loads

of heavy construction vehicles as the excavations approach footing bearing

levels. Construction equipment should be as light as possible to limit

development of this condition. Track-mounted vehicles generally should be used

because they exert lower contact pressures. The movement of vehicles over

proposed foundation areas should be restricted.

11) All footing areas should be compacted with a vibratory plate compactor prior to

placement of concrete.

12) Compacted fill placed against the sides of the footings should be compacted in

accordance with the criteria in the Project Earthwork section of this report.

SLAB-ON-GRADE FLOOR

Geotechnical Parameters for Design of a Slab-on-Grade Floor

1) A slab-on-grade floor should bear on a section of properly compacted fill soils

below the footings and to an elevation of 5534 feet based on the approximate

elevations shown on Figure 2. This is in addition to scarification and re-


Littleton, Colorado


compaction of the underlying surface, as outlined in the Project Earthwork

section of this report.

If project earthworking exposes existing, undocumented fill soils that extend to

depths of more than 8 feet below slab bearing elevation, then those soils should

be excavated to their full depth and replaced with properly compacted fill. Where

grades are raised in areas where existing fills are present, the existing fill soils

still should be excavated fully and replaced with properly compacted fill.

The remedial fill section should extend at full depth 6 or more feet laterally

beyond the slab perimeter.

The thickness of the re-worked section should be taken from the bottom of the

slab + gravel layer system. (If the gravel layer is not installed, the re-worked

section should be correspondingly thickened.)

Criteria and compaction standards for fill placement and compaction are provided

in the Project Earthwork section of this report.

2) Floor slabs should be adequately reinforced. Floor slab design, including slab

thickness, concrete strength, jointing, and slab reinforcement should be

developed by a structural engineer.

3) A vertical modulus of subgrade reaction (Kv) of 78 tcf (90 pci) may be used for

design of a concrete, slab-on-grade floor bearing on firm, properly compacted,

clayey fill. This value is for a 1-foot x 1-foot plate; it should be adjusted for slab

dimension.

4) Floor slabs should be separated from all bearing walls and columns with slip

joints, which allow unrestrained vertical movement.

Slip joints should be observed periodically, particularly during the first several

years after construction. Slab movement can cause previously free-slipping

joints to bind. Measures should be taken to assure that slab isolation is

maintained in order to reduce the likelihood of damage to walls and other interior

improvements.

5) Concrete slabs-on-grade should be provided with properly designed control

joints.


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ACI, AASHTO and other industry groups provide guidelines for proper design

and construction concrete slabs-on-grade and associated jointing. The design

and construction of such joints should account for cracking as a result of

shrinkage, curling, tension, loading, and curing, as well as proposed slab use.

Joint layout based on the slab design may require more frequent, additional, or

deeper joints, and should reflect the configuration and proposed use of the slab.

Particular attention in slab joint layout should be paid to areas where slabs

consist of interior corners or curves (e.g., at column blockouts or reentrant

corners) or where slabs have high length to width ratios, significant slopes,

thickness transitions, high traffic loads, or other unique features. The improper

placement or construction of control joints will increase the potential for slab

cracking.

6) Interior partitions resting on floor slabs should be provided with slip joints so that

if the slabs move, the movement cannot be transmitted to the upper structure.

This detail is also important for wallboards and doorframes. Slip joints which will

allow 1½ inches or more of differential vertical movement should be considered.

Accommodation for differential movement also should be made where partitions

meet bearing walls.

7) Post-construction soil movements may not displace slab-on-grade floors and

utility lines in the soils beneath them to the same extent. Design of floor

penetrations, connections and fixtures should accommodate at least 2 inches of

differential movement.

8) Moisture can be introduced into a slab subgrade during construction and

additional moisture will be released from the slab concrete as it cures. A properly

compacted layer of free-draining gravel, 4 or more inches in thickness, should be

placed beneath the slabs. This layer will help distribute floor slab loadings, ease

construction, reduce capillary moisture rise, and aid in drainage.

The free-draining gravel should contain less than 5 percent material passing the

No. 200 Sieve, more than 50 percent retained on the No. 4 Sieve, and a

maximum particle size of 2 inches.

The capillary break and the drainage space provided by the gravel layer also

may reduce the potential for excessive water vapor fluxes from the slab after

construction as mix water is released from the concrete.

Paul Kastner

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Paul Kastner

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Littleton, Colorado


We understand, however, that professional experience and opinion differ with

regard to inclusion of a free-draining gravel layer beneath slab-on-grade floors. If

these issues are understood by the owner and appropriate measures are

implemented to address potential concerns including slab curling and moisture

fluxes, then the gravel layer may be deleted.

9) A vapor barrier beneath a building floor slab can be beneficial with regard to

reducing exterior moisture moving into the building, through the slab, but can

retard downward drainage of construction moisture. Uneven moisture release

can result in slab curling. Elevated vapor fluxes can be detrimental to the

adhesion and performance of many floor coverings and may exceed various

flooring manufacturers’ usage criteria.

Per the 2006 ACI Location Guideline, a vapor barrier is required under concrete

floors when that floor is to receive moisture-sensitive floor covering and/or

adhesives, or the room above that floor has humidity control.

Therefore, in light of the several, potentially conflicting effects of the use vapor-

barriers, the owner and the architect and/or contractor should weigh the

performance of the slab and appropriate flooring products in light of the intended

building use, etc., during the floor system design process and the selection of

flooring materials. Use of a plastic vapor-barrier membrane may be appropriate

for some building areas and not for others.

In the event a vapor barrier is utilized, it generally should consist of a minimum

15 mil thickness, extruded polyolefin plastic (no recycled content or woven

materials), maintain a permeance less than 0.01 perms per ASTM E-96 or ASTM

F-1249, and comply with ASTM E-1745 (Class “A”). Vapor barriers should be

installed in accordance with ASTM E-1643.

Polyethylene (“poly”) sheeting (even if 15 mils in thickness which polyethylene

sheeting commonly is not) does not meet the ASTM E-1745 criteria and

generally should not be used as vapor barrier material. It can be easily torn

and/or punctured, does not possess necessary tensile strength, gets brittle, tends

to decompose over time, and has a relatively high permeance.


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Construction Considerations for a Slab-on-Grade Floor

10) Loose, soft or otherwise unsuitable materials exposed on the prepared surface

on which the floor slab will be cast should be excavated and replaced with

properly compacted fill.

11) The fill section beneath a slab should be of uniform thickness. Where existing, fill

soils are encountered and re-worked (See the Project Earthwork section of this

report.) a fill section thicker than that indicated above may result. The increased

section thickness should be constructed beneath the entire slab.

12) Concrete floor slabs should be constructed and cured in accordance with

applicable industry standards and slab design specifications.

13) All plumbing lines should be carefully tested before operation. Where plumbing

lines enter through the floor, a positive bond break should be provided.

LATERAL LOADS

Shallow Foundations Resisting Lateral Loads Footings and similar elements

designed for frictional resistance to lateral loads may be designed using a friction

coefficient between the foundation element and fill soils of 0.31.

Passive soil pressure may be estimated at this site using an equivalent fluid pressure of

305 pcf for drained conditions to a maximum of 3,050 psf. The upper 1 foot of

embedment should be neglected for passive resistance. Where passive soil pressure is

used to resist lateral loads, it should be understood that significant lateral strains will be

required to mobilize the full value indicated above, likely 1 inch or more. A reduced

passive pressure can be used for reduced anticipated strains, however.

Note that the values indicated above (and preliminarily, below) were based on a moist

unit weight (γ') of 125 pcf and an angle of internal friction () of 25 degrees for the

shallow, on-site soils and are un-factored. Appropriate factors of safety should be

included in design calculations.

Preliminary At-Rest and Active Lateral Earth Pressures for Retaining Walls We are

not aware that retaining walls are planned at this time as part of the proposed facility

improvements. Preliminary ‘active’ and ‘at-rest’ equivalent fluid pressures are provided


Littleton, Colorado


below for initial wall cost estimates. They assume that on-site soils will be used as wall

backfill, conditions will be well-drained, and that the backfill surface will be horizontal. In

the event that retaining walls are added once project design begins, a geotechnical

engineer should be retained to develop parameters for retaining wall parameter design.

Global stability analysis may be needed, as well. The owner should realize that

additional subsurface exploration may be necessary.

Preliminary Equivalent

Condition Fluid Pressure

Active: 51 pcf

At-Rest: 73pcf

Actual wall design should incorporate any upward sloping backfills, live loads such as

construction equipment, material stockpiles, etc., and other surcharge pressures. The

build-up of hydrostatic pressures behind a wall also will increase lateral earth pressures

on the walls.

WATER-SOLUBLE SULFATES

The concentration of water-soluble sulfates measured in a selected sample of site soils

was approximately 0.01 percent by weight. (See Table 2.) Such a concentration of

soluble sulfates represents a negligible environment for sulfate attack on concrete

exposed to these materials. Degrees of attack are based on the scale of 'negligible,'

'moderate,' 'severe' and 'very severe' as described in the “Design and Control of

Concrete Mixtures,” published by the Portland Cement Association (PCA). The Colorado

Department of Transportation (CDOT) utilizes a corresponding scale with four classes of

severity of sulfate exposure (Class 0 to Class 3) as described in the table below.


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REQUIREMENTS TO PROTECT AGAINST DAMAGE TO CONCRETE BY SULFATE ATTACK FROM EXTERNAL SOURCES OF SULFATE

Severity of Sulfate

Exposure

Water-Soluble Sulfate (SO4

=) In Dry Soil

(%)

Sulfate (SO4=)

In Water

(ppm)

Water / Cementitious Ratio

(maximum)

Cementitious Material

Requirements

Class 0 0.00 to 0.10 0 to 150 0.45 Class 0

Class 1 0.11 to 0.20 151 to 1,500 0.45 Class 1

Class 2 0.21 to 2.00 1,501 to 10,000 0.45 Class 2

Class 3 2.01 or greater 10,001 or greater 0.40 Class 3

Based this datum, no special, sulfate-resistant cement need be used in project concrete.

SOIL CORROSIVITY

Data were obtained to support an initial assessment of the potential for corrosion of

ferrous metals in contact with earth materials at the site, based on the conditions at the

time of GROUND’s evaluation. The test results are summarized in Table 2.

Soil Resistivity In order to assess the “worst case” for mitigation planning, samples of

materials retrieved from the test holes were tested for resistivity in the laboratory, after

being saturated with water, rather than in the field. Resistivity also varies inversely with

temperature. Therefore, the laboratory measurements were made at a controlled

temperature. Measurement of electrical resistivity indicated a value of approximately

4,911 ohm-centimeters in a sample of site soils.

pH Where pH is less than 4.0, soil serves as an electrolyte; the pH range of about 6.5 to

7.5 indicates soil conditions that are optimum for sulfate reduction. In the pH range

above 8.5, soils are generally high in dissolved salts, yielding a low soil resistivity.3

Testing indicated a pH value of about 7.9.

Reduction-Oxidation testing indicated a red-ox potential of -84 millivolts. Such a low

potential typically creates a more corrosive environment.

Sulfide Reactivity testing indicated a ‘negative’ results in the local soils. The absence

of sulfides in the soils suggests a less corrosive environment.

3 American Water Works Association ANSI/AWWA C105/A21.5-05 Standard.


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Corrosivity Assessment The American Water Works Association (AWWA) has

developed a point system scale used to predict corrosivity. The scale is intended for

protection of ductile iron pipe but is valuable for project steel selection. When the scale

equals 10 points or higher, protective measures for ductile iron pipe are indicated. The

AWWA scale is presented below. The soil characteristics refer to the conditions at and

above pipe installation depth.

Table A.1 Soil-test Evaluation Soil Characteristic / Value Points

Soil Resistivity

<1,500 ohm-cm ..........................................................................................… 10 1,500 to 1,800 ohm-cm ................................................................……......…. 8 1,800 to 2,100 ohm-cm .............................................................................…. 5 2,100 to 2,500 ohm-cm ...............................................................................… 2 2,500 to 3,000 ohm-cm .................................................................................. 1 >3,000 ohm-cm ................................................................................… 0

pH

0 to 2.0 ............................................................................................................ 5 2.0 to 4.0 ......................................................................................................... 3 4.0 to 6.5 ......................................................................................................... 0 6.5 to 7.5 ......................................................................................................... 0 * 7.5 to 8.5 ......................................................................................................... 0 >8.5 .......................................................................................................... 3

Redox Potential

< 0 (negative values) ....................................................................................... 5 0 to +50 mV ................................................................................................…. 4 +50 to +100 mV ............................................................................................… 3½ > +100 mV ............................................................................................... 0

Sulfide Reactivity

Positive ........................................................................................................…. 3½ Trace .............................................................................................................… 2 Negative .......................................................................................................…. 0

Moisture

Poor drainage, continuously wet ..................................................................…. 2 Fair drainage, generally moist ....................................................................… 1 Good drainage, generally dry ........................................................................ 0

* If sulfides are present and low or negative redox-potential results (< 50 mV) are

obtained, add three (3) points for this range.

We anticipate that drainage at the site after construction will be effective. Nevertheless,

based on the values obtained for the soil parameters, the overburden soils and bedrock

appear to comprise a moderately corrosive environment for ferrous metals (5 points).


Littleton, Colorado


If additional information or evaluations are needed regarding soil corrosivity, then the

American Water Works Association or a corrosion engineer should be contacted. It

should be noted, however, that changes to the site conditions during construction, such

as the import of other soils, or the intended or unintended introduction of off-site water,

might alter corrosion potentials significantly.

PROJECT EARTHWORK

The earthwork criteria below are based on our interpretation of the geotechnical

conditions encountered in the test holes. Where these criteria differ from applicable

municipal specifications, e.g., for backfill compaction for a publicly maintained utility line,

the latter should be considered to take precedence.

Prior to earthwork construction, existing construction debris, vegetation and other

deleterious materials should be removed and disposed of off-site. Relic underground

utilities should be abandoned in accordance with applicable regulations, removed as

necessary, and properly capped.

Use of Existing Fill Soils Fill soils were recognized in some of the test holes and likely

are present elsewhere on-site. In general, we anticipate that most fill soils will be

suitable for re-use as compacted fill. Because they were not sampled and tested,

however, it is possible that some existing fill materials when excavated may not be

suitable for re-use as compacted fill, including trash, organic material, coarse cobbles

and boulders, and construction debris. Excavated fill materials should be evaluated and

tested, as appropriate, with regard to re-use.

All existing fill soils exposed under the proposed building footprint, pavements, flatwork

or other improvements should be excavated and replace with properly compacted fill.

Cobbles and fragments of rock, as well as inert construction debris, e.g., concrete or

asphalt, up to 6 inches in maximum dimension may be included in project fills, in

general. Such materials should be evaluated on a case-by-case basis where identified

during earthwork.

Use of Native Site Soils The existing site soils – free of organic materials, coarse

cobbles, boulders, or other deleterious materials – appeared suitable, in general, for re-

use as compacted fill.


Littleton, Colorado


Imported Fill Materials Materials imported to the site as fill should be free of organic

material, and other deleterious materials. Imported material should exhibit 40 percent

or less passing the No. 200 Sieve and a plasticity index of 15 or less. Materials

proposed for import should be approved prior to transport to the site.

Fill Platform Preparation Prior to filling, the top 12 inches of in-place materials on

which fill soils will be placed (except for utility trench bottoms where bedding will be

placed) should be scarified, moisture conditioned and properly compacted in accordance

with the criteria below to provide a uniform base for fill placement.

General Considerations for Fill Placement Fill soils should be thoroughly mixed to

achieve a uniform moisture content, placed in uniform lifts not exceeding 8 inches in

loose thickness, and properly compacted. No fill materials should be placed, worked,

rolled while they are frozen, thawing, or during poor/inclement weather conditions.

Where soils supporting foundations or on which foundation will be placed are exposed to

freezing temperatures or repeated freeze – thaw cycling during construction – commonly

due to water ponding in foundation excavations – bearing capacity typically is reduced

and/or settlements increased due to the loss of density in the supporting soils. After

periods of freezing conditions, the contractor should re-work areas affected by the

formation of ice to re-establish adequate bearing support.

Care should be taken with regard to achieving and maintaining proper moisture contents

during placement and compaction. Materials that are not properly moisture conditioned

may exhibit significant pumping, rutting, and deflection at moisture contents near

optimum and above. The contractor should be prepared to handle soils of this type,

including the use of chemical stabilization, if necessary.

Compaction areas should be kept separate, and no lift should be covered by another

until relative compaction and moisture content within the specified ranges are obtained.

Compaction Standards Fill soils should be compacted to 100 percent or more of the

maximum dry density at moisture contents within 2 percent of the optimum moisture

content as determined by ASTM D698, the ‘standard Proctor.’


Littleton, Colorado


Use of Squeegee Relatively uniformly graded fine gravel or coarse sand, i.e.,

“squeegee,” or similar materials commonly are proposed for backfilling foundation

excavations, utility trenches (excluding approved pipe bedding), and other areas where

employing compaction equipment is difficult. In general, this procedure should not be

followed.

Even when properly densified, uniformly graded granular materials are comparatively

permeable and allow water to reach and collect in the lower portions of the excavations

backfilled with those materials. This leads to wetting of the underlying soils and resultant

potential loss of bearing support as well as increased local heave or settlement.

Wherever possible, excavations should be backfilled with approved, on-site soils placed

as properly compacted fill. Where this is not feasible, use of “Controlled Low Strength

Material” (CLSM), i.e., a lean, sand-cement slurry (“flowable fill”) or a similar material for

backfilling should be considered.

Where “squeegee” or similar materials are proposed for use by the Contractor, the

design team should be notified by means of a Request for Information (RFI), so that the

proposed use can be considered on a case-by-case basis. Where “squeegee” meets

the project requirements for pipe bedding material, however, it is acceptable for that use.

Settlements Settlements will occur in filled ground, typically on the order of 1 to 2

percent of the fill depth. For a 6-foot fill, for example, that corresponds to a total

settlement of about 1 inch. If fill placement is performed properly and is tightly

controlled, in GROUND’s experience the majority (on the order of 60 to 80 percent) of

that settlement typically will take place during earthwork construction, provided the

contractor achieves the compaction levels indicated herein. The remaining potential

settlements likely will take several months or longer to be realized, and may be

exacerbated if these fills are subjected to changes in moisture content.

Cut and Filled Slopes Permanent, graded slopes supported by local soils up to 10 feet

in height should be constructed no steeper than 3:1 (horizontal : vertical). Minor raveling

or surficial sloughing should be anticipated on slopes cut at this angle until vegetation is

well re-established. Surface drainage should be designed to direct water away from

slope faces into designed drainage pathways or structures.


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EXCAVATION CONSIDERATIONS

Excavation Difficulty Test holes for the subsurface exploration were advanced to the

depths indicated on the test hole logs by means of conventional, truck-mounted,

geotechnical drilling equipment. We anticipate no unusual excavation difficulties in

these materials, in general, for the proposed construction with conventional, heavy duty,

excavating equipment.

Coarse gravel and cobbles, as well as construction debris may be encountered,

however. The contractor should be prepared to excavate, handle, process and export

these materials.

Temporary Excavations and Personnel Safety Excavations in which personnel will

be working must comply with all applicable OSHA Standards and Regulations,

particularly CFR 29 Part 1926, OSHA Standards-Excavations, adopted March 5, 1990.

The contractor’s “responsible person” should evaluate the soil exposed in the

excavations as part of the contractor’s safety procedures. GROUND has provided the

information in this report solely as a service to the client and is not assuming

responsibility for construction site safety or the contractor’s activities.

The contractor should take care when making excavations not to compromise the

bearing or lateral support for any adjacent, existing improvements. The contractor also

should take sufficient care to locate the existing, buried utility lines and other in-ground

improvements which appeared to be relatively numerous.

Temporary, un-shored excavation slopes up to 15 feet in height, in general, should be

cut no steeper than 1½:1 (horizontal : vertical) in the on-site soils in the absence of

seepage. Some surface sloughing may occur on the slope faces at these angles.

Should site constraints prohibit the use of the above-indicated slope angle, temporary

shoring should be used. GROUND is available to provide shoring design upon request.

Groundwater and Surface Water Groundwater was encountered at depths of as

shallow as about 33 feet below existing grades. Therefore, groundwater is not

anticipated to be a significant factor in project excavations except for limited volumes of

perched groundwater.


Littleton, Colorado


Should seepage or flowing groundwater be encountered in project excavations, the

slopes should be flattened as necessary to maintain stability or a geotechnical engineer

should be retained to evaluate the conditions. The risk of slope instability will be

significantly increased in areas of seepage along excavation slopes.

The contractor should take pro-active measures to control surface waters during

construction and maintain good surface drainage conditions to direct waters away from

excavations and into appropriate drainage structures. A properly designed drainage

swale should be provided at the tops of the excavation slopes. In no case should water

be allowed to pond near project excavations.

Temporary slopes should also be protected against erosion. Erosion along the slopes

will result in sloughing and could lead to a slope failure.

UTILITY LATERAL INSTALLATION

The measures and criteria below are based on GROUND’s evaluation of the local,

geotechnical conditions. Where the parameters herein differ from applicable municipal

requirements, the latter should be considered to govern.

Pipe Support The bearing capacity of the site soils appeared adequate, in general, for

support of typical utility lines. The pipes + contents are less dense than the soils which

will be displaced for installation. Therefore, GROUND anticipates no significant pipe

settlements in these materials where properly bedded.

Areas allowed to pond water will require excavation and replacement with properly

compacted fill or bedding. The contractor should take particular care to ensure

adequate support near pipe joints which are less tolerant of extensional strains.

Where thrust blocks are needed, the parameters provided in the Lateral Loads section of

this report may be used for design.

Trench Backfilling Some settlement of compacted soil trench backfill materials should

be anticipated, even where all the backfill is placed and compacted correctly. Typical

settlements are on the order of 1 to 2 percent of fill thickness. However, the need to

compact to the lowest portion of the backfill must be balanced against the need to

protect the pipe from damage from the compaction process. Some thickness of backfill

may need to be placed at compaction levels lower than specified (or smaller compaction


Littleton, Colorado


equipment used together with thinner lifts) to avoid damaging the pipe. Protecting the

pipe in this manner can result in somewhat greater surface settlements. Therefore,

although other alternatives may be available, the following options are presented for this

project:

Controlled Low Strength Material Because of the above considerations, the entire depth

of the trench should be backfilled (both bedding and common backfill zones) with

“controlled low strength material” (CLSM), i.e., a lean, sand-cement slurry, “flowable fill,”

or a similar material (e.g., air-entrained “flash fill”) along all trench alignment reaches

with low tolerances for surface settlements.

CLSM used as pipe bedding and trench backfill should exhibit a 28-day unconfined

compressive strength between 50 to 150 psi so that re-excavation is not unusually

difficult. The contractor should establish design mix and a quality control program to

facilitate keeping the CLSM strengths within this range.

Placement of the CLSM in several lifts or other measures likely will be necessary to

avoid ‘floating’ the pipe. Measures also should be taken to maintain pipe alignment

during CLSM placement.

Compacted Soil Backfilling As an alternative that will result in surface settlements, for

use in reaches that are tolerant of such settlements, the utility trenches may be

backfilled with compacted soil using the on-site soils or similar materials. Where

compacted soil backfilling is employed as backfill, the risk of trench backfill settlements

entailed must be anticipated and accepted by the client.

We anticipate that the on-site soils excavated from trenches will be suitable, in general,

for use as common trench backfill within the above-described limitations. Backfill soils

should be free of vegetation, organic debris and other deleterious materials. Fragments

of rock, cobbles, and inert construction debris (e.g., concrete or asphalt) coarser than 6

inches in maximum dimension should not be incorporated into trench backfills. As

discussed in the Project Earthwork section of this report, the excavated soils commonly

will exhibit moisture contents above the optimum which will require drying prior to

placement as compacted fill.

If it is necessary to import material for use as backfill, the imported soils should be free

of vegetation, organic debris, and other deleterious materials and meet the criteria for

imported soils provided in the Project Earthwork section of this report.


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Soils placed for compaction as trench backfill should be conditioned to a relatively

uniform moisture content, placed and compacted in accordance with the criteria in the

Project Earthwork section of this report.

Pipe Bedding Pipe bedding materials, placement and compaction should meet the

specifications of the pipe manufacturer and applicable municipal standards. Bedding

should be brought up uniformly on both sides of the pipe to reduce differential loadings.

As discussed above, we CLSM or similar material should be used in lieu of granular

bedding and compacted soil backfill where the tolerance for surface settlement is low.

(Placement of CLSM as bedding to at least 12 inches above the pipe can protect the

pipe and assist construction of a well-compacted conventional backfill, although possibly

at an increased cost relative to the use of conventional bedding.)

If a granular bedding material is specified, then with regard to potential migration of fines

into the pipe bedding, design and installation should follow ASTM D2321, Appendix

X1.8. The on-site sols largely classify as Class III and Class IV soils with regard to that

standard. If the granular bedding does not meet filter criteria for the enclosing soils, then

non-woven filter fabric (e.g., TenCate Mirafi® 140N, or the equivalent) should be placed

around the bedding to reduce migration of fines into the bedding which can result in

severe, local surface settlements. Where this protection is not provided, settlements can

develop/continue several months or years after completion of the project.

In addition, clay or concrete cut-off walls should be installed at the margins of the pad to

interrupt the granular bedding section to reduce the rates and volumes of water

transmitted along utility alignments which can contribute to migration of fines. We

anticipate that cut-off walls also will be beneficial with regard to limiting migration of

methane along the pipe bedding.

If granular bedding is specified, some site soils may be suitable for that use with

significant processing. Materials proposed for use as pipe bedding should be tested for

suitability prior to use. Imported materials should be approved prior to transport to the

site.

SURFACE DRAINAGE

The site soils are relatively stable with regard to moisture content – volume relationships

at their existing moisture contents. Other than the anticipated, post-placement


Littleton, Colorado


settlement of fills, post-construction soil movements will result primarily from the

introduction of water into the soils underlying the proposed structure, hardscaping and

pavements. Wetting of the soils may result from infiltrating surface waters (precipitation,

irrigation, etc.), water flowing along constructed pathways such as bedding in utility pipe

trenches, or a rise in the local water table.

The following drainage measures should be followed both for during construction and as

part of project design. The facility should be observed periodically to evaluate the

surface drainage and identify areas where drainage is ineffective. Routine maintenance

of site drainage should undertaken throughout the design life of the proposed facility. If

these measures are not implemented and maintained effectively, the movement

estimates provided in this report could be exceeded.

1) Wetting or drying of the foundation excavations and underslab areas should be

avoided during and after construction as well as throughout the improvements’

design life. Permitting increases/variations in moisture to the adjacent or

supporting soils may result in a decrease in bearing capacity and an increase in

volume change of the underlying soils, and increased total and/or differential

movements.

2) Positive surface drainage measures should be provided and maintained to

reduce water infiltration into foundation soils.

The ground surface surrounding the exterior of each building should be sloped to

drain away from the foundation in all directions. A minimum slope of 12 inches

in the first 10 feet should be incorporated in the areas not covered with

pavement or concrete slabs, or a minimum 3 percent in the first 10 feet in the

areas covered with pavement or concrete slabs. Reducing the slopes to comply

with ADA requirements may be necessary by other design professionals but may

entail an increased potential for moisture infiltration and subsequent volume

change of the underlying soils and resultant distress.

In no case should water be allowed to pond near or adjacent to foundation

elements, utility trench alignments, etc.


Littleton, Colorado


It should be noted that the maintenance necessary to maintain effective surface

drainage may include removal and replacement of improvements, local

earthwork to restore drainage gradients, etc.

3) Drainage also should be established to direct water away from sidewalks and

other hardscaping as well as utility trench alignments which are not tolerant of

moisture-volume changes in the underlying soils or flow of infiltrating water.

The ground surface near foundation elements should be able to convey water

away readily. Cobbles or other materials that tend to act as baffles and restrict

surface flow should not be used to cover the ground surface near the

foundations.

Where the ground surface does not convey water away readily, additional post-

construction movements and distress should be anticipated.

4) In GROUND’s experience, it is common during construction that in areas of

partially completed paving or hardscaping, bare soil behind curbs and gutters,

and utility trenches, water is allowed to pond after rain or snow-melt events.

Wetting of the subgrade can result in loss of subgrade support and increased

settlements / increase heave. By the time final grading has been completed,

significant volumes of water can already have entered the subgrade, leading to

subsequent distress and failures. The contractor should maintain effective site

drainage throughout construction so that water is directed into appropriate

drainage structures.

In no case should water be permitted to pond adjacent to or on sidewalks,

hardscaping, or other improvements as well as utility trench alignments, which

are likely to be adversely affected by moisture-volume changes in the underlying

soils or flow of infiltrating water.

5) Roof downspouts and drains should discharge well beyond the perimeter of the

structure foundations (minimum 10 feet) and backfill zones and be provided with

positive conveyance off-site for collected waters.

If roof downspouts and drains are not used, then surface drainage design should

anticipate concentrated volumes of water adjacent to the buildings.


Littleton, Colorado


6) Irrigation water – both that applied to landscaped areas and over-spray –

commonly is a significant cause of distress to improvements. Where (near-)

saturated soil conditions are sustained, distress to nearby improvements should

be anticipated.

To reduce to potential for such distress, vegetation requiring watering should be

located 10 or more feet from building perimeters, flatwork, or other

improvements. Irrigation sprinkler heads should be deployed so that applied

water is not introduced near or into foundation/subgrade soils. Landscape

irrigation should be limited to the minimum quantities necessary to sustain

healthy plant growth.

Use of drip irrigation systems can be beneficial for reducing over-spray beyond

planters. Drip irrigation also can be beneficial for reducing the amounts of water

introduced to building foundation soils, but only if the total volumes of applied

water are controlled with regard to limiting that introduction. Controlling rates of

moisture increase beneath the foundations, floors and other improvements

should take higher priority than minimizing landscape plant losses.

Where plantings are desired within 10 feet of a building, the plants should be

placed in water-tight planters, constructed either in-ground or above-grade, to

reduce moisture infiltration in the surrounding subgrade soils. Planters should be

provided with positive drainage and landscape underdrains.

As an alternative involving only a limited increase in risk, the use of water-tight

planters may be replaced by local shallow underdrains beneath the planter beds.

7) Plastic membranes should not be used to cover the ground surface near the

building without careful consideration of other components of project drainage.

Plastic membranes can be beneficial to directing surface waters away from the

building and toward drainage structures. However, they effectively preclude

evaporation and transpiration of shallow soil moisture. Therefore, soil moisture

tends to increase beneath a continuous membrane. Where plastic membranes

are used, additional shallow, subsurface drains should be installed.

Perforated “weed barrier” membranes that allow ready evaporation from the

underlying soils may be used.


Littleton, Colorado


SUBSURFACE DRAINAGE

As a component of project civil design, properly functioning, subsurface drain systems

(underdrains) can be beneficial for collecting and discharging saturated subsurface

waters. Underdrains will not collect water infiltrating under unsaturated (vadose)

conditions, or moving via capillarity, however. In addition, if not properly constructed and

maintained, underdrains can transfer water into foundation soils, rather than remove it.

This will tend to induce heave or settlement of the subsurface soils, and may result in

distress. Underdrains can, however, provide an added level of protection against

relatively severe post-construction movements by draining saturated conditions near a

structure should they arise, and limiting the volume of wetted soil.

If, as we anticipate, some of the potentially collapsible soils are left in-place beneath the

structure, then the building should be protected by an underdrain system. In addition, if

below-grade or partially below-grade level(s) are added to a building, then damp-

proofing should be applied to the exteriors of below-grade elements. The provision of

Tencate MiraFi® G-Series backing (or comparable wall drain provisions) on the exteriors

of (some) below-grade elements may be appropriate, depending on the intended use. If

a (partially) below-grade level is limited in extent, it may be efficient to install a separate,

local underdrain system, in that area.

Geotechnical Parameters for Underdrain Design Where an underdrain system is

included in project drainage design, design should incorporate the parameters below.

The actual underdrain layout, outlets, and locations should be developed by a civil

engineer. A typical, cross-section detail of an underdrain for this project is provided on

Figure 5.

An underdrain system should be tested by the contractor after installation and after

placement and compaction of the overlying backfill to verify that the system functions

properly.

1) An underdrain system for a building should consist of perforated, rigid, PVC

collection pipe at least 4 inches in diameter, non-perforated, rigid, PVC

discharge pipe at least 4 inches in diameter, free-draining gravel, and filter

fabric, as well as a waterproof membrane.


Littleton, Colorado


2) The free-draining gravel should contain less than 5 percent passing the No. 200

Sieve and more than 50 percent retained on the No. 4 Sieve, and have a

maximum particle size of 2 inches. Each collection pipe should be surrounded

on the sides and top (only) with 6 or more inches of free-draining gravel.

3) The gravel surrounding the collection pipe(s) should be wrapped with filter fabric

(MiraFi 140N® or the equivalent) to reduce the migration of fines into the drain

system.

4) The waterproof membrane should underlie the gravel and pipe, and be attached

to the foundation stem wall as shown in the detail.

5) The underdrain system should be designed to discharge at least 5 gallons per

minute of collected water.

6) The high point(s) for the collection pipe flow lines should be below the grade

beam or shallow foundation bearing elevation as shown on the detail. Multiple

high points can be beneficial to reducing the depths to which the system would

be installed.

The collection and discharge pipe for the underdrain system should be laid on a

slope sufficient for effective drainage, but a minimum of 1 percent.

Pipe gradients also should be designed to accommodate at least 1 inch of

differential movement after installation along a 40-foot run.

Gradients flatter than 1 percent may be used but will convey water less efficiently

and entail an increased risk of local post-construction movements. Steeper

gradients, e.g., 1½ percent, will make the system more tolerant of local,

differential settlements, possibly resulting in less frequent repairs.

7) Underdrain ‘clean-outs’ should be provided at intervals of no more than 100 feet

to facilitate maintenance of the underdrains. Clean-outs also should be provided

at collection and discharge pipe elbows of 60 degrees or more.


Littleton, Colorado


8) The underdrain discharge pipes should be connected to one or more sumps from

which water can be removed by pumping, or to outlet(s) for gravity discharge.

We suggest that collected waters be discharged directly into the storm sewer

system, if possible.

PAVEMENT SECTIONS

A pavement section is a layered system designed to distribute concentrated traffic loads

to the subgrade. Performance of the pavement structure is directly related to the

physical properties of the subgrade soils and traffic loadings.

Standard practice in pavement design describes a typical flexible pavement section as a

“20-year” design pavement. However, most pavements will not remain in satisfactory

condition without routine maintenance and rehabilitation procedures performed

throughout the life of the pavement.

Subgrade Materials Based on the results of our field and laboratory studies, the

subgrade materials at the subject site consisted largely of silty sands, commonly with

gravel. These materials were classified largely as A-2-6 and A-7-5 soils in accordance

with the AASHTO classification system, with Group Index values of 0 to 3.

Resilient Modulus (MR) testing (AASHTO T-307) was performed on a representative

composite sample of the subgrade materials encountered at the site. For the sandy

materials typical of the proposed grades, a resilient modulus of 4,294 psi was obtained

at 2 percent above the optimum moisture content for use in development of the

pavement sections.

It is important to note that significant decreases in soil support as quantified by the

resilient modulus have been observed as the moisture content increases above the

optimum. Therefore, pavements that are not properly drained may experience a loss of

the soil support and subsequent reduction in pavement life.

Anticipated Traffic No project specific traffic loadings were available at the time of

preparation of this report. Therefore, for the small parking lot an EDLA of 4 was

assumed for the pavements. The EDLA value of 4 was converted to equivalent 18-kip

single-axle load (ESAL) values of 29,200, for a 20-year design life. An EDLA of 50,


Littleton, Colorado


corresponding to an ESAL value of 365,000, was assumed for trash collection zones

and other routes subject to heavier traffic including large trucks.

If design traffic loadings differ significantly from these assumed values, GROUND should

be notified to re-evaluate the pavement sections below.

Pavement Sections The soil resilient modulus and the anticipated ESAL values were

used to determine the required structural number for the project pavements. The

required structural number was then used to develop minimum, pavement sections for

the project. Pavement sections were based on the DARWin™ computer program that

solves the 1993 AASHTO pavement design equation. Pavement parameters and

calculations are summarized in Appendix A. A Reliability Level of 80 percent was

utilized develop the pavement sections. A structural coefficients of 0.40 and 0.12 were

used for hot bituminous asphalt and aggregate base course, respectively. The resultant

minimum pavement sections that should be placed at the site are tabulated below.

MINIMUM PAVEMENT SECTIONS

Location: Parking Lot Heavy Vehicles & High

Turning Stress

Full Depth Asphalt: 5½ in. AC -

Composite Section: 4 in. AC / 5 in. Ag. Base -

Concrete Section: (6 in. PCC / 6 in. Ag. Base)

6½ in. PCC / 6 in. Ag. Base

Heavy traffic areas/routes serving the facility such as the trash collection areas should

be provided with rigid pavements consisting of 6½ or more inches of portland cement

concrete underlain by 6 or more inches of properly compacted CDOT Class 6

Aggregate Base Course. (A mathematically equivalent composite flexible section for

these areas would be 5 inches of asphalt over 10 inches of aggregate base, but this

section will not perform as well as the concrete section where heavy vehicles are

parked, turn repeatedly, etc.)

Pavement Materials Asphalt pavement should consist of a bituminous plant mix

composed of a mixture of aggregate and bituminous material. Asphalt mixture(s) should


Littleton, Colorado


meet the requirements of a job-mix formula established by a qualified engineer as well

as applicable City of Littleton design requirements.

Aggregate base material should meet the criteria of CDOT Class 6 aggregate base

course. Base course should be placed in and compacted in accordance with the criteria

in the Project Earthwork section of this report.

Where rigid (concrete) pavements are placed, the concrete should consist of a plant mix

composed of a mixture of aggregate, portland cement and appropriate admixtures

meeting the requirements of a job-mix formula established by a qualified engineer as

well as applicable design requirements of the City of Littleton. Concrete should have a

minimum modulus of rupture of third point loading of 650 psi. Normally, concrete with a

28-day compressive strength of 4,500 psi should develop this modulus of rupture value.

The concrete should be air-entrained with approximately 6 percent air and should have a

minimum cement content of 6 sacks per cubic yard. Maximum allowable slump should

be 4 inches.

These concrete mix design criteria should be coordinated with other project

requirements including any criteria for sulfate resistance presented in the Water-Soluble

Sulfates section of this report. To reduce surficial spalling resulting from freeze-thaw

cycling, we suggest that pavement concrete meet the requirements of CDOT Class P

concrete. In addition, the use of de-icing salts on concrete pavements during the first

winter after construction will increase the likelihood of the development of scaling.

Placement of flatwork concrete during cold weather so that it is exposed to freeze-thaw

cycling before it is fully cured also increases its vulnerability to scaling. Concrete placing

during cold weather conditions should be blanketed or tented to allow full curing.

Depending on the weather conditions, this may result in 3 to 4 weeks of curing, or more.

Concrete pavements should contain sawed or formed joints. CDOT and various industry

groups provide guidelines for proper design and concrete construction and associated

jointing. In areas of repeated turning stresses, the concrete pavement joints should be

fully tied and doweled. Example layouts for joints, as well as ties and dowels, that may

be applicable can be found in CDOT’s M standards, found at the CDOT website:

http://www.dot.state.co.us/DesignSupport/. PCA, ACI and ACPA publications also

provide useful guidance in these regards.


Littleton, Colorado


Subgrade Preparation Although subgrade preparation to a depth of 12 inches is typical

in the project area, pavement performance commonly can be improved by a greater

depth of moisture-density conditioning of the soils. Also, the contractor should be

prepared to prepare the subgrade as outlined herein even where elevated subgrade

moisture contents are encountered beneath the existing pavements.

Remedial Earthwork Based on CDOT guildlines, shortly before paving, the pavement

subgrade should be excavated and/or scarified to a depth of 12 inches or more,

moisture-conditioned and properly re-compacted. However, as noted in the Project

Earthwork section of this report, all existing, undocumented fill soils encountered in the

pavement subgrade should be excavated and replaced with properly compacted fill.

Because soils vulnerable to “collapse” underlay the site to significant depths, the extent

of earthwork indicated above will entail a greater than typical potential for pavement

distress at this project site. Greater than typical pavement maintenance is likely.

Greater depths of remedial earthwork beneath the pavement, e.g., 4 feet or to the full

depth of the shallow soils vulnerable to “collapse,” will improve flatwork performance.

We do not anticipate that excavation and replacement of the existing soils to depths of

21 feet or more will be feasible in all areas of the project. However, more detailed

geotechnical parameters can be provided in that regard, upon request.

Subgrade preparation should extend the full width of the pavement from back-of-curb to

back-of-curb. The subgrade for sidewalks and other project hardscaping also should be

prepared in the same manner.

Criteria and standards for fill placement and compaction are provided in the Project

Earthwork section of this report. The contractor should be prepared either to dry the

subgrade materials or moisten them, as needed, prior to compaction.

Where adequate drainage cannot be achieved or maintained, excavation and

replacement should be undertaken to a greater depth, in addition to the edge drains

discussed below.

Proof Rolling Immediately prior to paving, the subgrade should be proof rolled with a

heavily loaded, pneumatic tired vehicle. Areas that show excessive deflection during

proof rolling should be excavated and replaced and/or stabilized. Areas allowed to pond

prior to paving will require significant re-working prior to proof-rolling. Establishment of a


Littleton, Colorado


firm paving platform (as indicated by proof rolling) is an additional requirement beyond

proper fill placement and compaction. It is possible for soils to be compacted within the

limits indicated in the Project Earthwork section of this report and fail proof rolling,

particularly in the upper range of specified moisture contents.

Additional Considerations The collection and diversion of surface drainage away from

paved areas is extremely important to satisfactory performance of the pavements. The

subsurface and surface drainage systems should be carefully designed to ensure

removal of the water from paved areas and subgrade soils. Allowing surface waters to

pond on pavements will cause premature pavement deterioration. Where topography,

site constraints or other factors limit or preclude adequate surface drainage, pavements

should be provided with edge drains to reduce loss of subgrade support. The long-term

performance of the pavement also can be improved greatly by proper backfilling and

compaction behind curbs, gutters, and sidewalks so that ponding is not permitted and

water infiltration is reduced.

Landscape irrigation in planters adjacent to pavements and in “island” planters within

paved areas should be carefully controlled or differential heave and/or rutting of the

nearby pavements will result. Drip irrigation systems should be used for such planters to

reduce over-spray and water infiltration beyond the planters. Enclosing the soil in the

planters with plastic liners and providing them with positive drainage also will reduce

differential moisture increases in the surrounding subgrade soils.

In our experience, infiltration from planters adjacent to pavements is a principal source of

moisture increase beneath those pavements. This wetting of the subgrade soils from

infiltrating irrigation commonly leads to loss of subgrade support for the pavement with

resultant accelerating distress, loss of pavement life and increased maintenance costs.

This is particularly the case in the later stages of project construction after landscaping

has been emplaced but heavy construction traffic has not ended. Heavy vehicle traffic

over wetted subgrade commonly results in rutting and pushing of flexible pavements,

and cracking of rigid pavements. Where the subgrade soils are expansive, wetting also

typically results in increased pavement heave. In relatively flat areas where design

drainage gradients necessarily are small, subgrade settlement or heave can obstruct

proper drainage and yield increased infiltration, exaggerated distress, etc. (These

considerations apply to project flatwork, as well.)


Littleton, Colorado


Also, GROUND’s experience indicates that longitudinal cracking is common in asphalt-

pavements generally parallel to the interface between the asphalt and concrete

structures such as curbs, gutters or drain pans. This of this type is likely to occur even

where the subgrade has been prepared properly and the asphalt has been compacted

properly.

The anticipated traffic loadings do not include excess loading conditions imposed by

heavy construction vehicles. Consequently, heavily loaded concrete, lumber, and

building material trucks can have a detrimental effect on the pavement.

Most pavements will not remain in satisfactory condition without regular maintenance

and rehabilitation procedures performed throughout the life of the pavement.

Maintenance and rehabilitation measures preserve, rather than improve, the structural

capacity of the pavement structure. Therefore, an effective program of regular

maintenance should be developed and implemented to seal cracks, repair distressed

areas, and perform thin overlays throughout the lives of the pavements. The greatest

benefit of pavement overlaying will be achieved by overlaying sound pavements that

exhibit little or no distress.

Crack sealing should be performed at least annually and a fog seal/chip seal program

should be performed on the pavements every 3 to 4 years. After approximately 8 to 10

years after construction, patching, additional crack sealing, and asphalt overlay may be

required. Prior to overlays, it is important that all cracks be sealed with a flexible,

rubberized crack sealant in order to reduce the potential for propagation of the crack

through the overlay. If actual traffic loadings exceed the values used for development of

the pavement sections, however, pavement maintenance measures will be needed on

an accelerated schedule.

EXTERIOR FLATWORK

We anticipate that the exteriors of proposed buildings and other portions of the site will

be provided with concrete flatwork. Like other site improvements, flatwork will

experience post-construction settlements as soil moisture contents increase after

construction and distress likely will result. The following measures will help to reduce

damages to these improvements.


Littleton, Colorado


The soils beneath project sidewalks, paved entryways and patios, masonry planters and

short, decorative walls, and other flatwork should be excavated and/or scarified to a

depth of 12 inches or more, moisture-conditioned and properly re-compacted.

However, as noted in the Project Earthwork section of this report, all existing,

undocumented fill soils encountered in the flatwork subgrade should be excavated and

replaced with properly compacted fill. Because soils vulnerable to “collapse” underlay the

site to significant depths, the extent of earthwork indicated above will entail a greater

than typical potential for hardscaping distress at this project site. Greater than typical

hardscape maintenance should be anticipated.

Greater depths of remedial earthwork beneath hardscaping, e.g., 4 feet or to the full

depth of the shallow soils vulnerable to “collapse,” will improve flatwork performance.

We do not anticipate that excavation and replacement of the existing soils to depths of

20 feet or more will be feasible in all areas of the project. However, more detailed

geotechnical parameters can be provided in that regard, upon request.

1) Prior to placement of flatwork, a proof roll should be performed to identify areas

that exhibit instability and deflection. The deleterious soils in these areas should

be removed and replaced with properly compacted fill. The contractor should

take care to achieve and maintain compaction behind curbs to reduce differential

sidewalk settlements. As in the case of pavements, passing a proof roll is an

additional requirement to placing and compacting the subgrade fill soils within the

specified ranges of moisture content and relative compaction in the Project

Earthwork section of this report. Subgrade stabilization may be cost-effective in

this regard.

2) Flatwork should be provided with control joints extending to an effective depth

and spaced no more than 10 feet apart, both ways. Narrow flatwork, such as

sidewalks, likely will require more closely spaced joints.

3) In no case should exterior flatwork extend to under any portion of the building

where there is less than 2 inches of vertical clearance between the flatwork and

any element of the building. Exterior flatwork in contact with brick, rock facades,

or any other element of the building can cause damage to the structure if the

flatwork experiences movements.

Construction and Drainage Between Buildings and Pavements Proper design,

drainage, construction and maintenance of the areas between individual buildings and


Littleton, Colorado


parking/driveway areas are critical to the satisfactory performance of the project.

Sidewalks, entranceway slabs and roofs, fountains, raised planters and other highly

visible improvements commonly are installed within these zones, and distress in or near

these improvements is common. Commonly, proper soil preparation in these areas

receives little attention during overlot construction because they fall between the building

and pavement areas which typically are built with heavy equipment. Subsequent

landscaping and hardscape installation often is performed by multiple sub-contractors

with light or hand equipment, and necessary over-excavation and soil processing is not

performed. Consequently, subgrade soil conditions commonly deviate significantly from

specified ranges. Therefore, the contractor should take particular care with regard to

proper subgrade preparation in the immediate building exteriors.

Frost and Ice Considerations Nearly all soils other than relatively coarse, clean,

granular materials are susceptible to loss of density if allowed to become saturated and

exposed to freezing temperatures and repeated freeze – thaw cycling. The formation of

ice in the underlying soils can result in heaving of pavements, flatwork and other

hardscaping (“ice jacking”) in sustained cold weather of 2 inches or more. This heaving

can develop relatively rapidly. A portion of this movement typically is recovered when

the soils thaw, but due to loss of soil density some degree of displacement typically will

remain. This can result even where the subgrade soils were prepared properly.

Where hardscape movements are a design concern, e.g., at doorways, replacement of

the subgrade soils with 3 or more feet of clean, coarse sand or gravel with a drain should

be considered, or the element supported on foundations similar to the building and

spanning over a void. Detailed parameters and discussion in this regard can be

provided upon request. It should be noted that where such open graded granular soils

are placed, water can infiltrate and accumulate in the subsurface relatively easily, which

can lead to increased settlement or heave from factors unrelated to ice formation. The

relative risks from these soil conditions should be taken into consideration where ice

jacking is a concern. GROUND will be available to discuss this upon request.

Concrete Scaling Surface scaling of sidewalks and other exterior concrete can result

from poor workmanship during construction, such as ‘over-finishing’ the surface. It also

can result from exposure to relatively severe weather conditions with repeated freeze-

thaw cycles. In GROUND’s experience, if reducing the potential for freeze-thaw scaling

is a design consideration, the following measures are beneficial: a) maintaining a

maximum water/cement ratio of 0.45 by weight for exterior concrete, b) including Type F

fly ash in the mix for exterior concrete as 20 percent of the cementitious material, and c)


Littleton, Colorado


use of exterior concrete that exhibits a minimum compressive strength of 4,500 psi.

Inclusion of ‘fibermesh’ in the concrete mix also may be beneficial for reducing surficial

scaling. (These concrete mix design criteria should be coordinated with other project

requirements including the criteria for sulfate resistance presented in the Water-Soluble

Sulfates section of this report.) In addition, the use of de-icing salts on exterior concrete

flatwork during the first winter after construction will increase the likelihood of the

development of scaling. Placement of flatwork concrete during cold weather so that it is

exposed to freeze-thaw cycling before it is fully cured also increases its vulnerability to

scaling. Concrete placing during cold weather conditions should be blanketed or tented

to allow full curing. Depending on the weather conditions, this may result in 3 to 4 weeks

of curing, and possibly more.

CLOSURE

Geotechnical Review The author of this report should be retained to review project

plans and specifications to evaluate whether they comply with the intent of the measures

discussed in this report. The review should be requested in writing.

The geotechnical conclusions and parameters presented in this report are contingent

upon observation and testing of project earthworks by representatives of GROUND. If

another geotechnical consultant is selected to provide materials testing, then that

consultant must assume all responsibility for the geotechnical aspects of the project by

concurring in writing with the parameters in this report, or by providing alternative

parameters.

Materials Testing The client should consider retaining a geotechnical engineer to

perform materials testing during construction. The performance of such testing or lack

thereof, however, in no way alleviates the burden of the contractor or subcontractor from

constructing in a manner that conforms to applicable project documents and industry

standards. The contractor or pertinent subcontractor is ultimately responsible for

managing the quality of his work; furthermore, testing by the geotechnical engineer does

not preclude the contractor from obtaining or providing whatever services that he deems

necessary to complete the project in accordance with applicable documents.

Limitations This report has been prepared for the client as it pertains to design and

construction of the proposed building and related improvements as described herein. It

may not contain sufficient information for other parties or other purposes.


Littleton, Colorado


In addition, GROUND has assumed that project construction will commence by Summer,

2015. Any changes in project plans or schedule should be brought to the attention of a

geotechnical engineer, in order that the geotechnical conclusions in this report may be

re-evaluated and, as necessary, modified.

The geotechnical conclusions and criteria in this report relied upon subsurface

exploration at a limited number of exploration points, as shown in Figure 1, as well as

the means and methods described herein. Subsurface conditions were interpolated

between and extrapolated beyond these locations. It is not possible to guarantee the

subsurface conditions are as indicated in this report. Actual conditions exposed during

construction may differ from those encountered during site exploration.

If during construction, surface, soil, bedrock, or groundwater conditions appear to be at

variance with those described herein, a geotechnical engineer should be retained at

once, so that re-evaluation of the conclusions for this site may be made in a timely

manner. In addition, a contractor who relies upon this report for development of his

scope of work or cost estimates may find the geotechnical information in this report to be

inadequate for his purposes or find the geotechnical conditions described herein to be at

variance with his experience in the greater project area. The contractor is responsible

for obtaining the additional geotechnical information that is necessary to develop his

workscope and cost estimates with sufficient precision. This includes current depths to

groundwater, etc.

ALL DEVELOPMENT CONTAINS INHERENT RISKS. It is important that ALL aspects

of this report, as well as the estimated performance (and limitations with any such

estimations) of proposed improvements are understood by the client. Utilizing these

criteria and measures herein for planning, design, and/or construction constitutes

understanding and acceptance of the conclusions with regard to risk and other

information provided herein, associated improvement performance, as well as the

limitations inherent within such estimates.

If any information referred to herein is not well understood, then the client or anyone

using this report, should contact the author or a GROUND principal immediately. We will

be available to meet to discuss the risks and remedial approaches presented in this

report, as well as other potential approaches, upon request.


Littleton, Colorado


This report was prepared in accordance with generally accepted soil and foundation

engineering practice in the Douglas County, Colorado, area at the date of preparation.

Current applicable codes may contain criteria regarding performance of structures

and/or site improvements which may differ from those provided herein. Our office should

be contacted regarding any apparent disparity. GROUND makes no warranties, either

expressed or implied, as to the professional data, opinions or conclusions contained

herein.

This document, together with the concepts and conclusions presented herein, as an

instrument of service, is intended only for the specific purpose and client for which it was

prepared. Reuse of or improper reliance on this document without written authorization

and adaption by GROUND Engineering Consultants, Inc., shall be without liability to

GROUND Engineering Consultants, Inc.

GROUND appreciates the opportunity to complete this portion of the project and

welcomes the opportunity to provide the client with a proposal for construction

observation and materials testing prior to construction commencement.

Sincerely,

GROUND Engineering Consultants, Inc. Brian H. Reck, P.G., C.E.G., P.E. Reviewed by James B. Kowalsky, P.E.

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APPENDIX A

Pavement Section Calculations

1993 AASHTO Pavement Design

DARWin Pavement Design and Analysis System

A Proprietary AASHTOWareComputer Software Product

Network Administrator

Flexible Structural Design Module

Job No. 14-3083Parking Lot

Asphalt Section

Flexible Structural Design

18-kip ESALs Over Initial Performance Period 29,200 Initial Serviceability 4.5 Terminal Serviceability 2 Reliability Level 80 %Overall Standard Deviation 0.44 Roadbed Soil Resilient Modulus 4,294 psiStage Construction 1

Calculated Design Structural Number 2.17 in

Specified Layer Design

Layer

Material Description

StructCoef.(Ai)

DrainCoef.(Mi)

Thickness(Di)(in)

Width

(ft)

Calculated

SN (in)1 Asphalt 0.4 1 5.5 - 2.20

Total - - - 5.50 - 2.20





Flexible Structural Design Module

Job No. 14-3083Parking Lot

Composite Section

Flexible Structural Design

18-kip ESALs Over Initial Performance Period 29,200 Initial Serviceability 4.5 Terminal Serviceability 2 Reliability Level 80 %Overall Standard Deviation 0.44 Roadbed Soil Resilient Modulus 4,294 psiStage Construction 1

Calculated Design Structural Number 2.17 in

Specified Layer Design

Layer

Material Description

StructCoef.(Ai)

DrainCoef.(Mi)

Thickness(Di)(in)

Width

(ft)

Calculated

SN (in)1 Asphalt 0.4 1 4 - 1.602 Aggregate Base 0.12 1 5 - 0.60

Total - - - 9.00 - 2.20





Rigid Structural Design Module

Job No. 14-3083Concrete Pavements

Rigid Structural Design

Pavement Type JPCP 18-kip ESALs Over Initial Performance Period 365,000 Initial Serviceability 4.5 Terminal Serviceability 2 28-day Mean PCC Modulus of Rupture 650 psi28-day Mean Elastic Modulus of Slab 3,400,000 psiMean Effective k-value 16 psi/inReliability Level 80 %Overall Standard Deviation 0.34 Load Transfer Coefficient, J 3.6 Overall Drainage Coefficient, Cd 1

Calculated Design Thickness 6.36 in

subsurface exploration program and geotechnical evaluation …€¦ · 22/10/2014 · additional...

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