occoquan multipurpose pedestrian trail bridge … of...occoquan multipurpose pedestrian trail bridge...

Occoquan Multipurpose Pedestrian Trail Bridge Project Fairfax County, Virginia

B&N Project No. 26032 September 15, 2017

ii

TABLE OF CONTENTS Section Page No.

EXECUTIVE SUMMARY ....................................................................................................................... iii

I. INTRODUCTION.......................................................................................................................... 1

II. SITE DESCRIPTION AND PROPOSED CONSTRUCTION .................................................. 1

III. GEOLOGIC SETTING................................................................................................................. 2

IV. SCOPE OF THE FIELD EXPLORATION ................................................................................ 4

V. SUBSURFACE CONDITIONS .................................................................................................... 5

A. Stratification ............................................................................................................................. 5 B. Groundwater Conditions .......................................................................................................... 6

VI. GEOTECHNICAL RECOMMENDATIONS ............................................................................. 6

A. Foundation ............................................................................................................................... 6 Helical Piers ..................................................................................................................................... 6 Shallow Foundation ......................................................................................................................... 7 B. Abutment Walls ........................................................................................................................ 8 Abutments Backfill .......................................................................................................................... 9 C. Site Classification for Seismic Design ..................................................................................... 9

VII. CONSTRUCTION RECOMMENDATIONS ............................................................................. 9

A. Site Preparation ........................................................................................................................ 9 B. Foundation Construction ........................................................................................................ 10 Foundation Construction – Shallow Foundation ........................................................................... 11 C. Compacted Fill ....................................................................................................................... 12 D. Construction Water Control ................................................................................................... 13

VIII. ADDITIONAL SERVICES RECOMMENDED ....................................................................... 14

IX. LIMITATIONS ............................................................................................................................ 14

APPENDICES

APPENDIX A - FIGURES & TABLES

Figure 1: Site Location Map

Figure 2: Topographic Map

Figure 3: Boring Location Plan

Figure 4: Soil Survey Map

APPENDIX B - FIELD OPERATIONS

Key to Symbols and Classifications

Test Boring Logs (2)

Hand Auger Records (4)



iii

EXECUTIVE SUMMARY

This report contains the results of our subsurface exploration and geotechnical recommendations for the

proposed construction of a multipurpose pedestrian trail bridge over Mills Branch Creek, within Occoquan

Regional Park located in Fairfax County, Virginia. We understand that the project will consist of construction

of a new prefabricated steel and timber deck pedestrian trail bridge over the Mills Branch Creek. The bridge

will have a total span of 60 feet and a clear width of 10 feet. The bridge is suited for use of pedestrians and

bicyclists and provides access to other park areas. This executive summary is presented for convenience only.

While the executive summary is an integral part of the report, it should not be used in lieu of reading the entire

report, including the appendices.

Two (2) soil test borings (Q-series) were drilled in the area of the proposed pedestrian bridge. Due to presence

of a construction fence and steepness of the existing slope in the vicinity of boring Q-1, this boring was offset

and redrilled approximately 20 feet south and additional four (4) hand auger borings were drilled near

proposed south abutment to explore subsurface conditions. Borings Q-1 and Q-2 were drilled using a CME

45C track-mounted drill rig to depths of 7.9 and 18.5 feet below the ground surface respectively and soil test

borings HA-1 to HA-4 were attempted using small diameter hand augers to depths ranging from 1.0 to 3.7

feet. The borings generally encountered sandy SILT (ML), fine to medium silty SAND (SM), rock fragments

and weathered rock.

Based on this investigation, the site appears suitable for the proposed development. Subsurface conditions

indicate that a properly designed and installed helical pier system with a minimum of 1 3/4 -inch solid square

shaft should be suitable to support the proposed pedestrian bridge.

Based on the groundwater level readings, we anticipate that groundwater may be encountered during

excavation for the abutments, and temporary dewatering may be required. The contractor should be prepared

to handle both surface water runoff and groundwater during construction.

We recommend that B&N be engaged to review the final design plans and specifications to determine whether

any changes in the project affect the validity of our recommendations, and whether our recommendations

have been properly implemented. We will provide a Construction Material Testing & Inspection (T&I)

services based on the Fairfax County Special Inspections requirements. These services include providing

observations during construction to check that the soil conditions in general accordance with our design

assumptions, and to check that work is being performed in general accordance with the plans and

specifications.

Details of the exploration and design recommendations are provided in the text and appendices.



Page 1

I. INTRODUCTION

Presented in this report are the results of our subsurface exploration and geotechnical engineering analysis for

the proposed multipurpose pedestrian trail bridge construction over Mills Branch Creek at Occoquan Regional

Park in Fairfax County, Virginia. Our geotechnical engineering study included reviewing publicly available

geologic references, conducting a site reconnaissance, locating and drilling two (2) soil test borings and four

(4) hand auger borings, and classifying the soil samples obtained. The data obtained have been evaluated,

and engineering analyses were conducted to develop our recommendations for this report. Our geotechnical

report includes the following:

Our evaluation of subsurface conditions based on available data

Subgrade preparation recommendations

Compacted fill and backfill recommendations

Bridge foundation recommendations

Abutment wall recommendations

Site seismic classification

Surface Water and Groundwater considerations

Recommended construction phase testing and inspections

The assessment of site environmental conditions, including the detection of contaminants/pollutants in the

soil, rock, surface water, groundwater, or delineation of jurisdictional wetlands of the site was beyond the

scope of this geotechnical exploration. In addition, services with respect to slope stability analysis, cost or

quality analysis, plans, specifications, and construction services are not included in the scope of this report.

II. SITE DESCRIPTION AND PROPOSED CONSTRUCTION

The site is located at Occoquan Regional Park at 9751 Ox Road in Lorton, Fairfax County, Virginia. The

project will consist of construction of a new pedestrian trail bridge which will span over existing Mills

Branch Creek at Occoquan Regional Park in Fairfax County, Virginia. The entire project will include the

design and construction of a 60-foot long and 10-foot wide prefabricated steel and timber deck bridge with

curb and railing where needed. The bridge will provide a safe route for pedestrians and bicyclists to access

other areas around the park. The site is bound to the north, east and west by trees and tall grass and to the

south by existing park access road. In general, the topography of the site is gently rolling to moderately

sloped with grades varying from approximate elevation +32 feet east of the site to the elevation +5 feet in

the vicinity of the stream. Based on the grading plan and site topography, we anticipate that some minor

cut and fill will be expected to reach proposed grades. Please note that the recommendations provided in



Page 2

this report, pertains to the pedestrian trail bridge only. The site is located and laid out as shown in Figures

1, 2, and 3 in Appendix A.

III. GEOLOGIC SETTING

The project lies within a boundary of the Piedmont Physiographic Province and the Coastal Plain

Physiographic Province. The Piedmont is a rolling upland surface underlain by complexly folded and faulted

crystalline rocks. These metamorphic rocks date to the Early Cambrian and/or Late Proterozoic periods. The

rock units are generally fine to course grained, lustrous, greenish-gray to gray, reddish-weathering, quartz rich

schist, and lesser mica schist, phyllite, and gneiss. Natural soils derived from the weathering of the underlying

bedrock are predominantly silty and sandy and may contain weathered rock and quartz fragments.

A transitional zone between the soil and the bedrock, referred to as weathered rock, is generally present, with

the degree of weathering decreasing with depth. Weathered rock is a term used to describe material that

consistently exhibits standard penetration resistances of 60 or more blows per foot (bpf) of penetration, is

residual, and generally exhibits a rocklike or saprolitic structure. The transition from weathered rock to

bedrock is not always sharply defined, and the surface of the bedrock may vary over short distances. The

general soil profile consists of clays in the near surface underlain by silts, fine sands, decomposed rock, and

intact rock. It should be noted that, in some cases, intact rock more resistant to weathering, such as quartzite,

can be encountered in the decomposed rock zone, well above the intact rock zone.

The Coastal Plain Physiographic Province is in an area characterized by soils ranging from the Cretaceous

Age to Pleistocene Age. In general, the Pleistocene Age Terrace Deposits overlie the older Potomac Group

soils of Cretaceous age. The terrace deposits consist primarily of sands and gravels with some interbedded

clays and silts. The thickness of the Coastal Plain deposits over the Piedmont rocks increases to the east.

Beneath the sand and gravel Terrace Deposits, occur soils of the Potomac Group, consisting of interbedded

layers of sands, silts and clays. The most significant geologic characteristic of the Potomac Group soils in

this area is the landslide potential of the highly plastic, heavily overconsolidated clays of this formation. These

soils, referred to locally as “marine clay”, are known to be prone to landslide activity in areas of steep slopes,

shallow clays, and high ground-water conditions. Marine clays also have high shrink-swell potential. The

overall drainage is to the southeast. Many of the soils in the Coastal Plain have moderately slow to slow

permeability. Drainage restrictions create shallow seasonal high water tables in large areas.



Page 3

Based on the “Digital Representation of the 1993 Geologic Map of Virginia", 2003, CD ROM (ISO-9660)

that is available from https://www.dmme.virginia.gov/commerce/” which can be viewed with Google Earth,

the site is primarily underlain by the Potomac Formation, the Quantico Formation, and the Chopawamsic

Formation. The Potomac Formation of the Cretaceous age typically consist of gray, pink, and green poorly

graded fine to coarse quartzo-feldspathic sand that is interbedded with gray to green sandy clay and silt. The

Quantico Formation of the Ordovician age typically consists of slate and porphyroblastic schist with gray to

black, graphitic, pyritic phyllite and slate interbedded with felsic metatuff, metagraywacke, and micaceous

quartzite. The Chopawamsic Formation typically consists of laterally discontinuous lenses and tongues of

metamorphosed felsic, intermediate, and mafic volcanic flows and volcanoclastic rocks, with interlayered

quartzite, quartzose graywacke, schist, and phyllite.

The National Cooperative Soil Survey of Fairfax County, Virginia, from the website

http://websoilsurvey.nrcs.usda.gov/, indicated that the subject site is underlain primarily by the followings as

shown on Table 1:

TABLE 1 - Summary of Site Soil Survey and Soil Classification

Soil Series Name and Slope Soil Map

Symbol Problem Soil Class

Codorus and Hatboro Soils (0-2% Slopes) 30A III

Rhodhiss-Rock outcrop complex, (25 to 45% Slopes) 88E I

Sassafras-Marumsco Complex (7-15% Slope) 91C III

Urban Land – Grist Mill 98 IVB

According to the “Description & Interpretive Guide to Soils in Fairfax County”, the Rhodhiss –Rock Outcrop

Complex are classified as Class I soils which generally consist of undisturbed natural soils that typically have

few characteristics that would adversely affect building foundations; the Codorus and Hatboro Soils and

Sassafras-Marumsco Complex are classified as Class III soils due to high shrink/swell potential, landslide

susceptibility, high compressibility, low bearing strength, and shallow water tables; and the Urban Land –

Grist Mill are classified as IVB soils due to potential disturbance that may have resulted from previous site

grading or construction. In addition, Class IVB soils are soils that were originally Class I or Class II soils.

Furthermore, the guide also indicates that small portion of the Codorus Hatboro Soils, the Rhodhiss –Rock

Outcrop Complex may be mapped on top of asbestos-containing bedrock. However, a review of the Fairfax

County Soils Map of the area did not indicate the area as orange soils and no evidence of fibrous material was



Page 4

observed during visual classification of the jar samples. A copy of the soil survey and aerial image from the

National Cooperative Soil Survey for the site is shown in Figure 4 in Appendix A.

IV. SCOPE OF THE FIELD EXPLORATION

Conclusions and recommendations contained in this report are based on the results of two (2) soil test borings

drilled across the site, and evaluation of soil samples obtained from these borings. Due to steepness of slope

at the location of proposed south abutment and existence of silt fences, boring Q-1 was offset and redrilled 20

feet south and additional hand auger borings (HA series) were performed in the vicinity of proposed south

abutment.

The test borings were utilized to provide both visual identification and engineering properties of soil

underlying the site. Drilling was accomplished with a CME 45C rubber track-mounted drill rig utilizing 3-

1/4 inch- inside-diameter hollow-stem augers. Representative soil samples were obtained by means of the

split-barrel sampling procedure according to ASTM D-1586-84 methods. In the split-barrel sampling

procedure, a 2-inch-outside-diameter split-barrel sampler is driven into the soil a distance of 18 or 24 inches

by means of a 140-pound hammer falling freely a distance of 30 inches. After an initial seating interval of 6

inches, the number of blows required to drive the sampler through a 12-inch interval is termed the Standard

penetration test (SPT) resistance or N-value. The SPT value can be used to provide an indication of the in-

place relative density of cohesionless soils or the consistency of cohesive soils. Soil samples obtained by

conducting this test were used in the field to visually classify the soil types.

The subsurface conditions encountered at the boring locations are shown on the test boring logs in Appendix

B. These test boring logs represent our interpretation of the subsurface conditions based on visual examination

of field samples by a geotechnical engineer. The lines designating the interfaces between various strata on

the test boring logs represent the approximate interface locations. However, the actual transitions between

strata may be gradual. Water levels shown on the test boring logs represent the conditions only at the time of

our exploration, unless otherwise stated. The approximate locations of the borings are shown on the Boring

Location Plans (Figure 3) in Appendix A.



Page 5

V. SUBSURFACE CONDITIONS

A. Stratification

The generalized subsurface stratification was extrapolated from the results of the two (2) soil test borings.

The reader is referred to the Test Boring Logs for the detailed subsurface conditions encountered for each of

the borings drilled. The various soil strata encountered on the site were in general compliance with the

geological formation and history of the region.

The presented stratification of the different layers is based on our visual observations, and geologic origin. In

general, the soils encountered in the area of the proposed development can be designated and summarized in

the stratum presented below.

STRATUM A:

Stratum A consisted of residual soils that were sampled as very soft to hard gray, brown

and dark brown SILT (ML) with varying amounts of sand; loose to medium dense

yellowish brown, reddish brown and dark brown fine to medium silty SAND (SM) with

varying amounts of rock fragments. Stratum A was encountered below a 2 inches and 6

inches thick topsoil layer in soil test boring Q-1 and Q-2 and measured in thickness by 3.8

feet and 18.0 feet in these soil test borings, respectively. SPT resistances in Stratum A

generally ranged between WOH and 46 blows per foot (bpf).

STRATUM B:

Stratum B generally consisted of WEATHERED ROCK, sampled as gray and gray brown

sandy silt. Stratum B was encountered beneath Stratum A in soil test boring Q-1 and was

not encountered in soil test boring Q-2. Stratum B was measured to be 3.9 feet in thickness

and extended through the boring termination depth in soil test boring Q-1. WEATHERED

ROCK is defined for engineering purposes as residual material that retains the relict

structure of the parent bedrock and exhibits SPT resistances consistently in excess of 60

bpf. Refusal is defined as 50 blows for 1 inch or less penetration. It should be noted that

weathering that reduces intact rock to weathered rock does not necessarily occur uniformly

in any direction. As a result, intact rock can be encountered and should be anticipated in

the weathered rock zone. As a general rule, drilling equipment similar to that used for this

exploration can advance augers further into material than can be excavated with reasonable

effort by track-mounted backhoe.

It should be noted that the stratigraphy inferred from the boring logs is approximate. Actual changes between

material types (strata) may occur abruptly, more gradually, or at slightly different elevations than those

depicted. Soil areas and groundwater conditions between borings may vary from conditions observed at each

boring location. Furthermore, the borings depict the conditions only during the time of their excavation. Some

conditions, particularly groundwater levels, will fluctuate seasonally.



Page 6

B. Groundwater Conditions

Water levels were measured in our soil test borings at the time of drilling, and as indicated after a stabilization

period as noted in Appendix B on the soil test boring records. Groundwater was encountered at the time of

drilling at depth of 9.1 feet in soil test borings Q-2. Groundwater was not encountered at the time of drilling

in the soil test borings Q-1. After a stabilizing period (casing pull-out), no groundwater was encountered at

any of soil test borings. Please note that no 24 hr. water level readings were obtained at any of soil test borings

and borings were backfilled upon completion.

Cave-in depths were also measured in each of the soil test borings at depths ranging from 3.6 to 6.2 feet bgs.

Please note that cave-in depths where groundwater was not encountered may be indicative of groundwater

movement. Fluctuations in groundwater levels may occur seasonally and vary depending on factors such as

precipitation, surface run-off, evaporation, construction activity, and other site-specific factors. Since such

variations are anticipated, design drawings and specifications should accommodate such possibilities, and

construction planning should be based on the assumption that variations can occur. Local perched

groundwater conditions could develop at locations where clays and silts are present.

VI. GEOTECHNICAL RECOMMENDATIONS

The recommendations provided in this report are based on the previously discussed project information, our

observations at the site, interpretation of the field data obtained during the subsurface explorations performed

on August 25, 2017, our experience with similar subsurface conditions, and engineering analysis. If the

proposed construction scheme should vary from that previously described, then B&N should be contacted and

provided with the opportunity to review these recommendations. The proposed construction of the pedestrian

bridge appears feasible subject to the recommendations contained herein.

A. Foundation

Helical Piers

Based on the information that was provided to us, we understand that the prefabricated pedestrian trail bridge

will be designed to bear on a deep foundation system consisting of a concrete abutment supported by a row

of helical piers which are also known as screw piles. Typical helical pier consist of a lead section with one or

more helices welded to a steel shaft in order to provide the needed bearing capacity. Load is transferred from



Page 7

the shaft to the soil through these bearing plates. The helices are spaced at distances far enough apart that

they function independently as individual bearing elements; consequently, the capacity of a particular helix

on a helical anchor/pile shaft is not influenced by the helix above or below it. The piers are screwed into the

ground with a hydraulic torque motor, and extensions without helixes are added during driving until the

required torque is achieved. Based on the soils encountered at the site we believe that a properly designed

and installed helical pier system will be suitable to support the proposed pedestrian trail bridge. Each helical

pier should be designed with the uppermost helix to bear at least five feet in the medium dense silty sand of

stratum A. However, we understand that minimum embedment depth may not be reached at areas close to

south abutment due to shallow weathered rock elevations. Therefore, alternate foundation systems, if

necessary, are recommended in following section to support the pedestrian bridge. The piers may be designed

using the following typical soil parameters for the medium dense sands as shown in Table 2;

TABLE 2- Recommended Soil Characteristics for Medium Dense Sands

Internal angle of friction (o) Unit weight (lbs./ft3) Saturated unit weight (lbs./ft3)

32 115 125

For this project, we recommend that the helical piers be provided with a minimum of 1 3/4 -inch solid square

to be used for this application. We anticipate that helical piers designed with these parameters can achieve

allowable compression capacities in the range of 8 to 40 kips per individual pier, depending on the helix

configuration, embedment depth, and pier shaft type selected. If resistance to lateral forces is required, we

recommend that the piers be battered. Helical foundations should have a minimum side clearance of 3 times

the largest helix diameter.

Helical piers are usually installed by specialty contractors with a structural designer that determine the

numbers, sizes and spacing of the helices required to support the ultimate load and provide guarantees relative

to the capacities of the piers. We recommend that the installation of helical piers be monitored full-time by a

geotechnical engineer.

Shallow Foundation

Based in results of our subsurface investigation, we understand that minimum embedment depth may not

be achieved for helical piers drilled at the location of proposed south abutment, therefore, if necessary, the

proposed abutment may be supported on a shallow foundation system consisting of spread and strip



Page 8

footings. Based on the existing site and subsurface conditions, we recommend that footings be designed to

bear either on Residual soils of Stratum A or Weathered Rock of Stratum B with an allowable bearing

pressure of 3.0 kips and 4.0 kips per square foot (ksf) respectively. The allowable bearing pressures

recommended are based on correlations made previously between standard penetration test resistances, the

performance of foundations supported by soils similar to those at this site for this project. We recommend

that the subgrade soils be evaluated by a geotechnical engineer to determine whether they are capable of

supporting the design bearing pressure.

The allowable bearing pressures recommended are based on correlations made previously between standard

penetration test resistances, the performance of foundations supported by soils similar to those at this site, and

preliminary settlement analyses performed for this project. Maximum total settlements are anticipated to be

on the order of 1 inch, with maximum differential settlements on the order of 1/2 inch, if constructed in

accordance with standard engineering practice.

Some undercutting and replacement of loose, soft, organic, or unsuitable soils may be necessary for footings

bearing on or below existing grades. If unsuitable soils are encountered during construction at the foundation

subgrade elevation, the footing should either be lowered to suitable low-plasticity natural soil, or removed

entirely and replaced with compacted low-plasticity soil meeting the compacted fill requirements of this report

or lean concrete.

B. Abutment Walls

Based on the information that was provided to us, we understand that two concrete abutments will be

constructed as part of supporting system for the pedestrian trail bridge at both north and south sides of the

creek. The proposed abutment wall must also be designed to withstand lateral soil pressure. Where wall

corners are fixed and rotation is not desired or permitted, they should be designed to withstand an at-rest earth

pressure condition. A coefficient of at-rest earth pressure (Ko = horizontal stress/vertical stress) of 0.5 is

recommended for walls designed to resist at-rest earth pressures that assume no lateral movement. With an

assumed total unit weight of backfill of approximately 120 pounds per cubic foot (pcf), the walls should

support an equivalent hydrostatic pressure of approximately 60 psf per foot of wall height.

The abutment wall may also be designed using active earth pressures if rotation of the walls will be permitted.

With an active earth pressure coefficient of 0.33 and an assumed backfill soil unit weight of 120 pcf, an



Page 9

equivalent fluid pressure of 40 psf per foot of wall height should be used in the design. Abutment wall corners

should be designed using the at-rest earth pressure described in the previous paragraph.

In order to allow for future excavations for utilities, we recommend that the passive pressure at the toe of

abutment wall be neglected in design. Resistance to sliding should be provided by friction resistance at the

base of the abutment wall. A coefficient of friction against sliding of 0.35 should be used in the design of the

abutment wall bearing on compacted structural FILL or the residual soils of Stratum A. For walls footings

bearing on weathered rock of Stratum B coefficient of friction of 0.58 is recommended.

Abutments Backfill

The abutments should be backfilled with select backfill consisting of VDOT 21A/21B, or select material

Type I with a minimum CBR value of 30 and should be compacted in accordance with Sections 303 and

305 of the VDOT Road and Bridge Specifications, Latest Edition. The zone of select backfill should be

constructed in accordance with Section 17.3 of the Volume V - Part 2 – Design Aids of the VDOT Structure

and Bridge Manuals.

C. Site Classification for Seismic Design

Site classification for seismic design was determined in accordance with Section 1613 of the International

Building Code 2009. Based on the subsurface conditions encountered, the site should be classified as Site

Class D.

VII. CONSTRUCTION RECOMMENDATIONS

A. Site Preparation

Prior to foundation construction and earth fill placement, all topsoil, vegetation, debris, and surface soils

containing organic material should be removed from the construction area and either wasted from the site or

used as topsoil in areas to be landscaped. We estimate up to 6 inches of stripping may be required for site

clearing. Any voids created due to stump removal should be carefully backfilled with compacted fill in areas

where planned foundations are at or near existing ground surface. Stripping operations should extend at least

10 feet outside structures where fill will be placed. During the stripping and rough grading, positive surface

drainage should be maintained to prevent the accumulation of water. If the exposed subgrade becomes



Page 10

excessively wet or frozen, or if conditions are encountered different from those described previously in this

report, the geotechnical engineer should be notified.

We recommend that the proposed subgrade in fill areas be proofrolled to detect unsuitable soil conditions. If

proofrolling is not feasible, we recommend using hand augers and dynamic cone penetration (DCP) tests

during construction to verify the suitability of the subgrades. Areas that do not proofroll successfully or

provide adequate bearing should be undercut to firmer soils, and the undercut excavation backfilled using

compacted, engineered fill.

The exposed subgrades should be observed and documented by the geotechnical engineer. If unsuitable

conditions are encountered at the subgrade level, recommendations for dealing with the conditions should be

provided to the owner's representative by the geotechnical engineer. Any soft or loose soils encountered

during the site preparation, as determined by the geotechnical engineer, should be either improved in place or

undercut and replaced with compacted fill as detailed in this report. If organic, highly plastic, or excessively

wet soils are encountered during site preparation, they should be excavated and replaced with compacted fill.

Proper sedimentation and erosion controls should be implemented according to the approved grading plans

prior to commencement of site work. Grading should be planned to limit ponding of water and soil

disturbance. On site soils are considered suitable for cut and fill slopes of up to 3H:1V or flatter. Additional

analysis may be required if steeper cut and fill slopes are necessary.

B. Foundation Construction

We recommend that the helical pier foundation system be installed by a trained and certified installer using

site-specific approved construction documents and the manufacturer’s written installation documents. The

piers should be installed using a rotary hydraulic machine that is capable of exerting a torsional moment that

exceeds the maximum specified installation torque by at least 10 percent. In addition, an axial force should

be applied on top of the shaft such that the pier penetrates into the underlying soils at a rate of approximately

3 inches per revolution. During the installation of the pier, the contractor should have a torsional resistance

machine capable of measuring the torque to an accuracy of plus or minus 10 percent of the minimum specified

effective torsional resistance termination criterion.

Close observation and monitoring by a certified inspector working under close supervision with the



Page 11

geotechnical engineer, who is familiar with the subsurface conditions at the site and has considerable

experience with helical pier installation procedures, is considered necessary during construction in order to

confirm that the piers are installed satisfactorily to meet the design intent and criteria. The inspector is

responsible for observing all work performed during helical pier installation. During helical pier

installation, the inspector should:

Verify that the torque indicator that is being used has been calibrated within the past 12 months.

Verify that rotational speed between 5 and 25 revolutions per minute is maintained.

Verify that pier is moving between 2 ½ to 3 ½ inches per revolution, especially when torque readings

are taken.

Verify that pile, drive tool, and drive motor are in alignment.

Ensure that pile maximum torque rating is not exceeded.

Verify that extension section with helices are added in the proper sequence.

Record torque readings in increments no greater than 5 feet.

Verify that tip embedment and torsional resistance readings meet or exceed the termination criteria

before terminating installation.

Foundation Construction – Shallow Foundation

Exposure to the environment may weaken the soils at the footing bearing level if the foundation excavations

remain open for too long a time. Therefore, foundation concrete should be placed the same day that

excavations are dug. If the bearing soils are softened by surface water intrusion or exposure, the softened soils

must be removed from the foundation excavation bottom immediately prior to placement of concrete. Care

should be exercised during winter months to prevent freezing of subgrade soils and backfill prior to or after

placement of concrete. Foundation concrete should not be placed on frozen or saturated subgrades. If soft,

highly plastic, or expansive soils classified as CH and/or MH are encountered during construction, the footing

should be either lowered to suitable low-plasticity natural soil, or undercut entirely and replaced with

compacted low-plasticity soil meeting the requirements of the Compacted Fill Section of this report or lean

concrete.

The geotechnical engineer should observe the foundation excavations immediately prior to concrete

placement. The foundation bearing areas should be level or suitably benched and be free of loose soil, ponded

water, and debris prior to the observation. The geotechnical engineer should compare the soils exposed with



Page 12

those encountered in the soil test borings, and then document the results. Any significant differences should

be brought to the attention of the owner's representative along with appropriate recommendations.

C. Compacted Fill

Fill materials are expected for bringing areas up to grade or in any undercut areas. Before filling operations

begin, representative samples of each proposed fill material, including onsite material, should be collected

and the samples should be tested and approved by the geotechnical engineer to determine the maximum dry

density (ASTM D698), optimum moisture content (ASTM D698), natural moisture content (ASTM D 2216),

gradation (ASTM D422), and plasticity (ASTM D4318) of the soil. These tests are needed for quality control

during compaction and to determine if the fill material is acceptable. A review of test boring logs indicates

that the in-situ moisture content of the on-site soils was determined to be mostly wet of the optimum moisture

content by more than 2 percent. As such, we do not believe that the onsite soils will be suitable for use as

compacted fill, unless the on-site soils are dried so that the moisture content is within 2 percent of the optimum

moisture content as obtained from the standard Proctor test.

We recommend that areas on which fill are to be placed be scarified prior to fill placement. We recommend

that compacted fill be constructed by spreading acceptable soil in loose layers not more than 8 inches thick.

The soils used within the pedestrian bridge structure or for replacement in any undercut areas should be

compacted in thin lifts up to 8 inches to at least 95 percent of the standard Proctor maximum dry density

(ASTM D 698). The upper 12 inches of fill should be compacted to 98 percent of standard Proctor. The grass

areas should be compacted to 90 percent of standard Proctor.

The moisture content of the fill soils should be maintained within two percentage points of the optimum

moisture content determined from the standard Proctor density tests. Suitable compacted fill for the structure

should be free of deleterious matter and fibrous organic material, classified CL or better per ASTM D2487,

have a liquid limit of less than 45, and a plasticity index of less than 20. The minimum unit weight of the

proposed fill material should be 105 pcf. In addition, structural fill should not contain particles greater than 4

inches in diameter.

Placement and compaction of all structural fills should be monitored and tested by a certified soils technician

under the supervision of the geotechnical engineer. Significant deviations, either from specifications or good

practice, should be brought to the attention of the owner's representative, along with appropriate



Page 13

recommendations. The fill placed should be tested according to ASTM Standards D1556 (sand cone method)

or D6938 (nuclear method) to evaluate the density and moisture content. A minimum of one test per 2000

square feet of material placed should be performed on each lift. However, no fewer than two tests per day

should be performed on each lift. Recommended compaction effort should include the use of vibratory

smooth drum rollers for granular soils, sheeps foot rollers for fine-grained soils. Any areas that do not meet

the compaction specifications should be recompacted to achieve compliance.

The fill should be graded to provide positive surface drainage, and be sealed by a smooth drum roller or tire-

rolled to prevent infiltration or surface runoff into the approved fill layers. If fill placement is interrupted by

inclement weather, the surface of the fill should be retested, and reworked if necessary, prior to placement of

additional material.

It should be noted that the appropriate provisions of the Fairfax County Building Code and current

Occupational Safety & Health Administration (OSHA) regulations should be followed. Excavation deeper

than 4 feet into soil should be sloped as required by OSHA, or properly braced and shored.

D. Construction Water Control

Based on the groundwater levels noted in the soil test borings, extensive site dewatering should not be

required. However, groundwater may be encountered during excavation for the abutments, and temporary

dewatering may be required. The contractor should promptly remove any surface water or groundwater from

the construction site. This has been done effectively on similar construction projects by means of gravity

drainage and sump pumping. If any water is encountered that cannot be easily handled in such a manner, the

geotechnical engineer should be notified.

The groundwater level can vary from the level measured in our borings due to variations in rainfall,

construction activity, surface runoff, and other site-specific factors. Since such variations are anticipated, we

recommend that design drawings and specifications accommodate such possibilities and that construction

planning be based on the assumption that such variations can occur.



Page 14

VIII. ADDITIONAL SERVICES RECOMMENDED

Additional soil and foundation engineering, testing, and consulting services recommended for this phase of

the project are summarized below:

Site Preparation and Proofrolling: A geotechnical engineer or soils technician should be present onsite

after clearing and stripping operations to determine the degree of undercutting necessary or whether further

proofrolling is necessary to prepare the subgrade.

Fill Placement and Compaction: A geotechnical engineer or soils technician should observe any required

filling operations and should conduct sufficient in-place density tests to evaluate whether the specified degree

of fill compaction has been achieved. The geotechnical engineer must observe and approve imported or

borrow materials to be used, and must determine whether the existing moisture contents are suitable.

Concrete: All concrete placements should be monitored and tested by certified field personnel. All structural

concrete and reinforcing steel should be inspected by a certified concrete technician for compliance with

approved plan specifications.

IX. LIMITATIONS

This report has been prepared to aid in the evaluation of this site for the proposed construction. It is considered

that adequate recommendations have been provided to serve as a basis for design and preparation of plans and

specifications. Additional recommendations can be provided as needed.

The analyses and recommendations of this report are based on the information made available to us at the

time of the actual writing of the report and on site conditions, surface and subsurface, that existed at the time

the exploratory test borings were drilled. Further assumption has been made that the exploratory test borings,

in relation to the depth, are representative of conditions across the site. If subsurface conditions are

encountered that differ significantly from those reported herein, Burgess & Niple should be notified

immediately so that the analysis and recommendations can be reviewed and/or revised as necessary.

We have prepared this report in accordance with generally accepted geotechnical engineering practices, and

make no other warranties, either expressed or implied, as to the professional services performed under this

agreement.



Page 15

Our foundation recommendations are subject to confirmation or revision upon review of the final grades,

plans, and specifications covering all details of the proposed construction.

APPENDIX A

FIGURES & TABLES

FIGURE 1

PROJECT NAME: Occoquan Regional Park Multipurpose

Pedestrian Trail Bridge

Fairfax County, Virginia

PROJECT NO.: 26032

DATE: Sep. 11, 2017

SITE LOCATION MAP

14520 Avion Parkway, Suite 100

Chantilly, Virginia, 20151

Source: This image was extracted from the Bing.com website.

Scale: NTS

APPROXIMATE SITE

LOCATION

PROJECT NAME: Occoquan Regional Park Multipurpose



PROJECT NO.: 26032

DATE: Sep. 11, 2017

TOPOGRAPHIC MAP FIGURE 2

14520 Avion Parkway, Suite 100,

Chantilly, Virginia, 20151

Source: This image was extracted from the “Occoquan Quadrangle, Virginia, 7.5 Minute

Series” topographic map from the U.S. Geological Survey, dated 2016.

Scale: NTS

APPROXIMATE SITE LOCATION

Burgess & Niple, Inc.

14520 Avion Parkway,#100 Chantilly, VA, 20151

PROJECT NAME: Occoquan Regional Park

Multipurpose Pedestrian Bridge

9751 Ox Road, Lorton, VA, 22079


PROJECT NO.: 26032

DATE: Sep. 11, 2017

FIGURE 3

BORING LOCATION PLAN

SB-1

Soil Map—Fairfax County, Virginia(Figure 4 - Soil Survey Map - Occoquan Pedestrian Trail Bridge Project)

Natural ResourcesConservation Service

Web Soil SurveyNational Cooperative Soil Survey

9/6/2017Page 1 of 3

4283

730

4283

750

4283

770

4283

790

4283

810

4283

830

4283

850

4283

730

4283

750

4283

770

4283

790

4283

810

4283

830

304070 304090 304110 304130 304150 304170 304190 304210 304230 304250

304070 304090 304110 304130 304150 304170 304190 304210 304230 304250

38° 40' 53'' N77

° 1

5' 8

'' W38° 40' 53'' N

77° 1

5' 1

'' W

38° 40' 49'' N

77° 1

5' 8

'' W

38° 40' 49'' N

77° 1

5' 1

'' W

N

Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 18N WGS840 40 80 160 240

Feet0 10 20 40 60

MetersMap Scale: 1:860 if printed on A landscape (11" x 8.5") sheet.

Soil Map may not be valid at this scale.

MAP LEGEND MAP INFORMATION

Area of Interest (AOI)Area of Interest (AOI)

SoilsSoil Map Unit Polygons

Soil Map Unit Lines

Soil Map Unit Points

Special Point FeaturesBlowout

Borrow Pit

Clay Spot

Closed Depression

Gravel Pit

Gravelly Spot

Landfill

Lava Flow

Marsh or swamp

Mine or Quarry

Miscellaneous Water

Perennial Water

Rock Outcrop

Saline Spot

Sandy Spot

Severely Eroded Spot

Sinkhole

Slide or Slip

Sodic Spot

Spoil Area

Stony Spot

Very Stony Spot

Wet Spot

Other

Special Line Features

Water FeaturesStreams and Canals

TransportationRails

Interstate Highways

US Routes

Major Roads

Local Roads

BackgroundAerial Photography

The soil surveys that comprise your AOI were mapped at1:12,000.

Warning: Soil Map may not be valid at this scale.

Enlargement of maps beyond the scale of mapping can causemisunderstanding of the detail of mapping and accuracy of soilline placement. The maps do not show the small areas ofcontrasting soils that could have been shown at a more detailedscale.

Please rely on the bar scale on each map sheet for mapmeasurements.

Source of Map: Natural Resources Conservation ServiceWeb Soil Survey URL:Coordinate System: Web Mercator (EPSG:3857)

Maps from the Web Soil Survey are based on the Web Mercatorprojection, which preserves direction and shape but distortsdistance and area. A projection that preserves area, such as theAlbers equal-area conic projection, should be used if moreaccurate calculations of distance or area are required.

This product is generated from the USDA-NRCS certified data asof the version date(s) listed below.

Soil Survey Area: Fairfax County, VirginiaSurvey Area Data: Version 13, Sep 27, 2016

Soil map units are labeled (as space allows) for map scales1:50,000 or larger.

Date(s) aerial images were photographed: May 3, 2015—Feb22, 2017

The orthophoto or other base map on which the soil lines werecompiled and digitized probably differs from the backgroundimagery displayed on these maps. As a result, some minorshifting of map unit boundaries may be evident.

Soil Map—Fairfax County, Virginia(Figure 4 - Soil Survey Map - Occoquan Pedestrian Trail Bridge Project)



9/6/2017Page 2 of 3

Map Unit Legend

Fairfax County, Virginia (VA059)

Map Unit Symbol Map Unit Name Acres in AOI Percent of AOI

30A Codorus and Hatboro soils, 0to 2 percent slopes,occasionally flooded

0.2 15.3%

88E Rhodhiss-Rock outcropcomplex, 25 to 45 percentslopes

0.1 5.8%

91C Sassafras-Marumsco complex,7 to 15 percent slopes

1.1 73.5%

98 Urban land-Grist Mill 0.1 5.3%

Totals for Area of Interest 1.6 100.0%

Soil Map—Fairfax County, Virginia Figure 4 - Soil Survey Map -Occoquan Pedestrian Trail Bridge

Project



9/6/2017Page 3 of 3

APPENDIX B

FIELD OPERATIONS

Soil Type

Soil

Component

Descriptive

Term

Boulder Major

(Uppercase SILT

Cobble letters) CLAY

SAND

Gravel GRAVEL

- Coarse

- Fine Secondary

(Adjective) Clayey or Silty

Sand Sandy or Gravelly

- Coarse

- Medium (with) Clay or Silt

- Fine Sand or Gravel

soils. When a soil sample consists of two or more types, the percentages of the types are

estimated by weight and indicated by descriptive terminology.

Particle Size Percentage

VISUAL CLASSIFICATION PROCEDURE

KEY TO BORING LOG SOIL CLASSIFICATIONS

Soils Identification

Identification of soil type is made on the basis of an estimate of particle size for predominantly

coarse-grained soils and on the basis of cohesiveness (plasticity) for predominantly fine-grained

12 in.

3 -12 in.

3/4 - 3 in.

#4 - 3/4 in.

over 50%

over 12%

over 30%

5 - 12%

15 - 30%

#10 - #4

#40 - #10

#200 - #40

Silt / Clay (trace)

Notes: 1)

2)

APPENDIX

B N-VALUE CHART.

PENETRATION RESISTANCE (SPR) OR

SOILS CLASSIFICATION AND STANDARD

presence only

Particle size is determined by U.S. Standard Sieve sizes.

Atterberg Limit determinations are often used to classify

fine-grained soils (silts and clays).

The Standard Penetration Resistance values (N-values) are used to describe the relative density of

<#200

coarse-grained soils or the consistency of fine-grained soils.

Relative Density or Consistency

RELATIVE DENSITY

Term N-Value N-Value

CONSISTENCY

Term

Very Dense

0 - 4

5 - 9

10 - 30

31 - 50

Over 50

Very Loose

Loose

Medium Dense

Dense

Very Soft

Soft

Firm

0 - 1

2 - 4

5 - 8

Very Stiff

Hard

Very Hard

9 - 16

17 - 30

31 - 50

Over 50

Stiff

0

4

8

12

16

20

24

18.0 17.8

14.0

10.1

T

ML

W

TOPSOIL: 2"

Firm to hard gray and brown sandy SILT,moist

WEATHERED ROCK sampled as grayand gray brown sandy silt, moist

1

2

3

4

5

3-2-3-6

6-18-28-38

21-38-33-50/5

19-50/5

50/0

5

46

71

79

100+

16

24

12

11

0

Auger could not be advancedpast 7.9'

Boring terminated @ 7.9'

PROJECT NAME:Occoquan Regional Park Multipurpose


PROJECT NUMBER:

26032

BORING NUMBER:

Q-1CLIENT NAME:

Northern Virginia Regional Park Authority

CLIENT PROJECT NUMBER: SHEET 1 OF

DRILLER:

Jonathan Monroe

LOCATION:

As Staked

ELEVATION (FEET):

18.0' (Est.)

WATER LEVELS DATE TIME DEPTH CAVED DATE START: 8/25/17

ENCOUNTERED: 8/25/17 - Dry DATE FINISH: 8/25/17

BEFORE CASING PULLED: 8/25/17 - Dry Dry METHOD: 3 1/4" ID HSA

AFTER CASING PULLED: 8/25/17 - Dry 3.6' EQUIPMENT USED: CME 45C

LONG TERM: Boring Backfilled Upon Completion REVIEWER: Ali Sheikhbahaei

Notes: Boring Q-1 was offset 20' to the south from the proposed abutment location

EL

EV

AT

ION

(FE

ET

)

LE

GE

ND

US

CS

CLASSIFICATION

DE

PT

H(F

EE

T)

SA

MP

LE

#

BL

OW

SP

ER

6IN

CH

ES

N V

AL

UE

RE

CO

VE

RY

(IN

CH

ES

)

MO

IST

UR

EC

ON

TE

NT

REMARKS

BURGESS & NIPLE, INCTEST BORING LOG

1

0

4

8

12

16

20

24

12.5 12.0

4.5

-6.0

T

SM

ML

TOPSOIL: 6"

Loose to medium dense yellowish brown,reddish brown and dark brown fine to

medium silty SAND, trace to some rockfragments, moist

Very soft to firm dark brown sandy SILT,moist

1

2

3

4

5

6

2-6-6-7

8-3-2-5

4-2-4-7

11-7-5-5

WOH-WOH-WOH-WOH

3-4-3

12

5

6

12

0

7

18

21

18

16

15

10

Boring terminated @ 18.5'

PROJECT NAME:Occoquan Regional Park Multipurpose


PROJECT NUMBER:

26032

BORING NUMBER:

Q-2CLIENT NAME:

Northern Virginia Regional Park Authority

CLIENT PROJECT NUMBER: SHEET 1 OF

DRILLER:

Jonathan Monroe

LOCATION:

As Staked

ELEVATION (FEET):

12.5' (Est.)

WATER LEVELS DATE TIME DEPTH CAVED DATE START: 8/25/17

ENCOUNTERED: 8/25/17 - 9.1' DATE FINISH: 8/25/17

BEFORE CASING PULLED: 8/25/17 - Dry Dry METHOD: 3 1/4" ID HSA

AFTER CASING PULLED: 8/25/17 - Dry 6.2' EQUIPMENT USED: CME 45C

LONG TERM: Boring Backfilled Upon Completn REVIEWER: Ali Sheikhbahaei

Notes:

EL

EV

AT

ION

(FE

ET

)

LE

GE

ND

US

CS

CLASSIFICATION

DE

PT

H(F

EE

T)

SA

MP

LE

#

BL

OW

SP

ER

6IN

CH

ES

N V

AL

UE

RE

CO

VE

RY

(IN

CH

ES

)

MO

IST

UR

EC

ON

TE

NT

REMARKS

BURGESS & NIPLE, INCTEST BORING LOG

1

1. Exploratory soil test borings were drilled on August 25, 2017 using 3-1/4"inside-diameter hollow stem augers.

2. Groundwater measurements are recorded on the logs when encountered, atcompletion, and after a stabilization period as applicable.

3. These logs are subject to the limitations of visual, lab classifications,and the interpretations of the geotechnical engineer.

4. Boring locations were established and field located by B&N personnel andelevations were provided.

Notes:

Symbol Description

Strata symbols

Topsoil

Silt

Weathered Rock

Silty Sand

Misc. Symbols

Water level duringdrilling

KEY TO SYMBOLS

RECORD OF HAND AUGER LOG

B&N Project Number 26032

Hand Auger No.: HA-1 Elevation: 11.0’

Depth (ft.) Description

From To

0 2.0’ Rock fragments, Boulders

NOTE: Hand-Auger refusal at 2.0 ft. below ground surface.





From To

0 3.7 Gray, Gray brown sandy SILT with rock fragments






From To

0 1.0’ Gray, Gray brown sandy SILT with rock fragments






From To

0 2.0’ Gray, Gray brown sandy SILT with rock fragments


occoquan multipurpose pedestrian trail bridge … of...occoquan multipurpose pedestrian trail bridge...

Documents