geotechnical report

GEOTECHNICAL REPORT

Dougherty Mills Bridge over Slippery Rock CreekSlippery Rock Township, Butler County, Pennsylvania

Prepared for:

Shelley StoffelsAssociate Professor of Civil Engineering

208 Sackett BuildingUniversity Park, PA 16802

Prepared by:

Sackett Engineering Inc.439 West Beaver

State College, PA 16801

March 24, 2014

INDEXPage

INTRODUCTION…………………………………………………………………………..……1

SITE AND PROJECT DESCRIPTION…………………………………………………..…....1

GENERAL…………………………………………………………………………………...…. 2

GEOLOGIC INFORMATION…………………………………………………………….…….2

LABORATORY TEST RESULTS………………………………………………………..…….4

CHEMICAL TESTING…………………………………………………………………..………5

SUBSURFACE CONDITIONS………………………………………………………….……. 6

EXCAVATIONS………………………………………………………………….……………...6

LIMITATIONS & DESIGN CONSIDERATIONS………………..……………………...…….6

APPENDIX

GEOTECHNICAL REPORTDougherty Mills Bridge over Slippery Rock CreekSlippery Rock Twp, Butler County, PennsylvaniaMarch 24, 2014

INTRODUCTIONThis report contains the subsurface exploration and geotechnical analysis for the

proposed construction for Dougherty Mills Bridge over Slippery Rock Creek, located at

Slippery Rock Township in Butler County Pennsylvania. PennDOT is replacing the

existing structure.

Sackett Engineering is tasked, by PennDOT, with analyzing the geotechnical

data provided by R.Peel/Kimball and to provide recommendations for the foundation of

the proposed bridge. Sackett Engineering services also entail future consulting on the

project, however services do not cover environmental aspects of design.

SITE AND PROJECT DESCRIPTIONThis project involves the replacement of the Dougherty Mills Bridge located

approximately 2 miles south of Slippery Rock University. The proposed replacement of

the existing concrete arch bridge is located on PA 173 (Centreville Pike) over the

Slippery Rock Creek in Slippery Rock Township, Butler County, PA. The location of the

bridge is indicated in figure 1 in appendix A.

The existing bridge is located in Rock Falls Park and is considered a historic

location. It was built in 1929 as a two-lane 140-foot roadway bridge with a pedestrian

walkway and sees an average daily traffic of approximately 6370 vehicles. To the north

of the bridge and surrounding the park is Stoughton Beach Rd. This road intersects PA

173 approximately 30 feet from the north end of the bridge. Both ends of the bridge are

heavily forested.

Topographic information was taken from USGS topographic mapping from the

Slippery Rock US Topo map dated 2013. At both ends of the bridge, the road slopes

inward where they level out at the location of the bridge. The slope on the road north of

the bridge is more gradual while the south road is steeper. The Slippery Rock US Topo

map can be seen in figure 2 in appendix A.

The bridge is known to be a historic site; some features exist from previous

structures both above and below ground. There is a historic cut-stone arch near the

south section of the bridge. The stone-cut arch was from a previous bridge that crossed

at a similar location. This structure has historical importance, and plans should be made

to preserve this structure.

GENERALThe recommendations provided are based on the data available from the boring

logs in Appendix B. The boring logs only depict the subsurface profiles of the locations

drilled. It is important to note variations in subsurface profiles between boreholes are

probable and should be expected. Being that variations in subsurface conditions can

potentially undermine the integrity of the structure and/or the foundation, it is imperative

to consult the Geotechnical Engineer if any variations are encountered. If the designs

for the structure are submitted to revisions, the Geotechnical Engineer should be

informed, as changes to foundation recommendations may be required as well.

Generally accepted geotechnical principles and practices were used to evaluate

the data provided by R.Peel/Kimball. Assumptions and conclusions made by others

based upon the data herein are not the responsibility of Sackett Engineering.

A proposed plan for boring logs was developed using FHWA regulations. This

plan calls for 4 borings to be made at both the north and south ends of the bridge, two

being positioned at the beginning of the approach and two at the end of the abutment

for each side. On the north end the estimated depth to bedrock is 25 feet. Because

bedrock is expected near the surface, borings should reach a maximum depth of 32 feet

if bedrock is not encountered or 10 feet into bedrock once encountered. On the south

end the estimated depth to bedrock is approximately 20 feet. Because bedrock is

expected near the surface, borings should reach a maximum depth of 32 feet if bedrock

is not encountered or 10 feet into bedrock once encountered. An additional boring

should be taken on each end to determine if there are any inconsistencies in soil types.

If any differences are encountered, additional borings are recommended.

From the observed geological data, there are no obstructions affecting the site that

need further consideration.

GEOLOGIC INFORMATIONIn May of 2009 an on site field investigation was performed in which 14 boring

holes were drilled at varying locations as shown in figure 1 in Appendix B. These

drillings were performed by R.Peel/Kimball using hollow stem augers/NQ2 wire line

coring. The results of these test borings are numbered DOU-001- DOU-005 and DOU-

901- DOU-908 and DOU-905a. Of these borings, three were taken at various locations

around the bridge. During the boring, the hole was abandoned because of a crooked

hole. The debris encountered should not pose any major threat to future foundations.

Most pertinent information is at location DOU-005, in which a small patch of coal

was found. However, this layer seems to be localized and shallow which does not cause

a threat to the integrity of this bridge. In addition, the geologic strata at the site contains

no sinkholes, and no visible sources that may be a hazard in the future. On the north

side of the bridge, borehole DOU-905, debris was found 20.5ft under the ground

surface. Among the debris found was concrete fragments and wood. The boring was

soon abandoned due to the debris. These fragments could be the footing to a prior

bridge.

Additional bedrock information was gathered using a geologic map of the Mercer

quadrangle published by the Pennsylvania Bureau of Topographic and Geologic Survey

in 1962, Figure 3 Appendix A. According to the map, this region has underlying

Hempfield shale which is described as a sandy to silty shale and fine-grained

sandstone. After comparing the geologic map to our own site findings, it was found to

be concurrent with each other.

The underlying bedrock at our site is mostly fine-grained Sandstone with

moderately to slightly weathered rock. Over the extent of the bridge, the rock quality

designation of the bedrock ranges from 34%-85%. Most have values in the upper range

which is essential for resisting seismic activity and rock flex. Referencing Engineering

Rock Mass Classification by Z. T. Bieniawski. in Table 1 below.

Table 1 - Rock Mass Classification

LABORATORY TEST RESULTSThe samples obtained from drilling were brought back to the laboratory for

testing. All soil samples were categorized and tested using AASHTO classifications.

Sieve analyses were determined for five samples of data. Atterberg Limit tests were

conducted in order to determine the plasticity index, liquid limit, and plastic limit. The

uniformity coefficient (Cu) and coefficient of curvature (Cc) were determined from the

particle size distribution data. General soil properties such as water content, dry density,

saturation, void ratio, and specific gravity were determined for the soil samples. Results

of the soil testing are summarized below. The full lab test results are in Appendix C.

Table 2 - Soil Testing Summary

Source Sample No.

Depth (ft) USCS AASHTO LL (%) PI(%) PL(%) Wn (%)

DOU-001

ST 6-8 CH A-7-6 60 37 23 30.8

DOU-002

S10-S13 16.5-22.5 CL A-7-6 42 19 23 31.1

DOU-003

S3-S6 6-12 SM A-2-4 NV NP NP 33.5

DOU-003

S13-15 21-25.5 GW-GM A-1-a NV NP NP 11.2

DOU-905

S8-S11 13.5-19.5 CL A-6 30 12 18 20.4

A consolidated undrained tri-axial shear test was also used to determine soil

properties. Three test samples were taken from one of the borings to determine total

and effective stress, as well as the pore pressures. The initial results from the in situ soil

are listed in the table but both initial and at test results are listed later in Appendix C.

Some of the results for the tri-axial shear test are listed in the following table.

Table 3 - Triaxial Shear Test Results

Sample No. 1 2 3

Dry Density (pcf) 91.8 91.8 91.7

Void Ratio .9039 .9036 .9055

Eff. Cell Pressure (tsf) 0.72 1.44 2.88

Failure Stress (tsf) 0.58 .070 0.62

Total Pore Pressure (tsf) 5.47 6.22 7.49

Strain (%) 13.5 11.4 15.9

σ1 Failure (tsf) 0.87 0.95 1.05

σ3 Failure (tsf) 0.29 0.26 0.43

Chemical TestingChemical Testing was performed at multiple boring locations to determine the

corrosive properties of the soil. The results of the tested soil are represented in the

following table. The peak resistivity for samples DOU-903 and DOU-907 were

calculated to be 1,083(ohm*cm) and 1,575(ohm*cm) respectively. These values

occurred at 20% moisture content, and are considered to be poor against corrosion

resistance.Table 4 Chemical Soil Analysis

Sample Number

Boring Number

Depth (ft) pH Determination

Chloride Determination

(ppm)

Sulfate Determination

(ppm)

Min. Resistivity (ohm*cm)

S9-S11 DOU-903 15.0-19.0 6.1 406 200 1,083

S4-S7 DOU-907 6.0-12.0 3.2 466 300 1,575

A summary of the general guidelines given by USCS for the soil samples is shown in

Table 5. The soils used for design are clays, silty sand, and gravel with well graded silt.

The rocks encountered in the bores are siltstone, sandstone

Table 5 Required Geotechnical Engineering Analysis

Sample Number

Boring Number

USCS Soil Type Slope Stability Analysis

Settlement Analysis

Bearing Capacity Analysis

Settlement Analysis

Lateral Earth Pressure

Stability Analysis

ST DOU-001 CH Clay Required Required Required, Deep foundation generally required unless soil has been preloaded

Required, Consolidation test data needed to estimate setlement amount and time

Not recommended for use directly behind or in retaining walls

All walls shouldbe designed toprovide minimumF.S. = 2 againstoverturning &F.S. = 1.5 againstsliding along base.External slopestabilityconsiderationssame aspreviously givenfor cut slopes &embankments

S10-S13S8-S11

DOU-002DOU-905

CL Clay Required Required These soils are notrecommended foruse directly behindor in retaining orreinforced soilwalls.

All walls shouldbe designed toprovide minimumF.S. = 2 againstoverturning &F.S. = 1.5 againstsliding along base.External slopestabilityconsiderationssame aspreviously givenfor cut slopes &embankments.

S3-S6 DOU-003 SM Sand, Silty

Generally notrequired if cut orfill slope is 1.5Hto 1V or flatter,and underdrainsare used to drawdown the watertable in a cutslope. Erosion of slopes may be a problem.

Generally not required

Required forspread footings,pile or drilledshaftfoundations.Spread footingsgenerallyadequate exceptpossibly for SCsoils

Generally not needed

Empirical correlations with SPT values usually used to estimate settlement

generallysuitable if have lessthan 15% fines.Lateral earthpressure analysisrequired using soilangle of internalfriction.


S13-S15

DOU-003 GW-GM Gravel, well graded, silty

Generally notrequired if cut orfill slope is 1.5Hto 1V or flatter,and underdrainsare used to drawdown the watertable in a cutslope.

Generally notrequired

Required forspread footings,pile or drilledshaftfoundations.

Generally notneededEmpiricalcorrelations withSPT valuesusually used toestimatesettlement

GW, soils generallysuitable for backfillbehind or inretaining orreinforced soilwalls.GM, soils generallysuitable if have lessthan 15% fines.Lateral earthpressure analysisrequired using soilangle of internalfriction.


SUBSURFACE CONDITIONSSubsurface profiles (Figure 1 and 2 in Appendix B) were created from the boring log

data for six stations parallel to the roadway, and four perpendicular to the roadway.

These figures are an interpretation of the boring logs, and indicate a shallow depth of

bedrock between 10-20 feet.

EXCAVATIONSThe excavations are to be performed by the contractor. The contractor is

required to follow the current standards set down by the United States Department of

Labor, Occupational Safety and Health Administrations in order to ensure the safety of

the project site for employees. The contractor is responsible to follow, The Solid Waste

Management Act (35 P.S. 6018.101 et seq.), and the Department of Environmental

Protection Municipal Waste Regulations (25 Pa. Code Chapters 271, 273, 279, 281,

283, and 285), for proper disposal procedures. The Pennsylvania Municipal Waste

Regulations, 25 Pa. Code Section 271.101(b)(5), state that the Department will prepare

a manual for the management of waste from land clearing, grubbing, and excavation,

including trees, brush, stumps and vegetative material which identifies best

management practices and may approve additional best management practices on a

case-by-case basis”.

LIMITATIONS & DESIGN CONSIDERATIONSAll engineering recommendations provided in this report are based on the

information obtained from the subsurface exploration and laboratory testing. Due to the

shallow depth of bedrock, only shallow foundations will be considered for this project.

From the laboratory tests of the foundation yielded high RQD values indicating stable

soil under the proposed locations for the abutments as seen in Appendix C. The design

for both spread footings and drilled shafts will be analyzed for the foundation design.

Spread footings are most commonly used when the depth of bedrock is found at

shallow depths. Drilled shafts increase the lateral strength for bridge foundations and

help to decrease the vibration that is created in the foundation.

For the design of spread footings at the bridge abutments, the following design

considerations are recommended:

Backfill Layer

● Clay Layer with a unit weight of 130 lb/ft^3

● Friction angle taken as 33 degrees

● Abutment 1: 16.6 foot thick layer

● Abutment 2: 14.7 foot thick layer

● Both Foundations lay on top of the Sandstone bedrock

Foundation

● Base Width = 10 Feet

● Base Length = 45 Feet

● Concrete thickness = 2 Feet

● Use Concrete with a unit weight of 150 lb/ft^3

All information provided was checked against bearing capacity, sliding and tipping

resistance.

Forces

● Lateral Water pressure from the water table present on both sides of abutment

which counteract each other's forces

● Compressive Strength “Co” taken as 1,500 ksf

For the design for drilled shafts at Abutment 1, the following are design considerations

that should be used in the analysis:

The subsurface for design for the Abutment 1 should be based off of Boring Log

DOU-903. This site should be used as the representative location for calculations

because it has the shallowest depth to bedrock. The soil found during the site

exploration was a layer of sand gravelly with trace clay, and sandy clayey. The Unified

Soil Classification System classifies these as SP and SC respectively. The average unit

weights according to USCS are 124.15 lb/ft3 and 117.78 lb/ft3 respectively. The starting

elevation for design should be based off the elevation of 1148.5 ft. The water table is

found at 4.4ft under the starting elevation of 1148.5 ft. The soil profile is two sand

layers, so the design cohesion less soils is to be followed.

N60 values should be based off of the N values obtained in the boring log. An

average value for each layer shall be taken, and this should be corrected using

appropriate constants.

The bedrock encountered for this location is sandstone. The reduction factor to

account for rock joints, αe, should be according to DM-4 (10.4.6.5-2). The RQD for this

rock sample is 64%. A reference stress of 2.12 k/ft2 is should be used in calculations for

the rock socketed drilled shafts. An average compressive strength of 648 k/ft 2 should be

used for concrete in the design. The uniaxial compressive strength of the sandstone

should be 1500 k/ft2. This is a conservative value based on the range of 1400-3600 k/ft2

according to DM-4 Table 10.6.3.2.2-2. Based on the rock qualities, an m value of 1.231

and an s value of .00293 shall be used, according to AASHTO Table 10.4.6.4-4.

Resistance factors for the tip resistance in rock and side resistance should be .55

and .50 respectively. The side resistance in sand shall be .65. The resistance factors

used are from DM-4 Table 10.5.5.2.4-1.

For the design for drilled shafts at Abutment 2, the following are design

considerations that should be used in the analysis:

The subsurface for design for the Abutment 2 should be based off of Boring

Log DOU-906. This site should be used as the representative location for calculations

because it represents an average depth to bedrock. The soil found during the site

exploration was a layer of sandy clay with some gravel, and silty clay. The Unified Soil

Classification System classifies these as CL and CH respectively. The average unit

weights according to USCS are 121 lb/ft3 and 127lb/ft3 respectively. The starting

elevation for design should be based off the elevation of 1149 ft. The water table is

found at 10ft under the starting elevation of 1149 ft. The soil profile is two clay layers, so

the design for cohesive soils is to be followed.

N60 values should be based off of the N values obtained in the boring log. An

average value for each layer shall be taken, and this should be corrected using

appropriate constants.

The bedrock encountered for this location is a small layer of siltstone, with

underlying sandstone. The reduction factors to account for rock joints, αe, should be

according to DM-4 (10.4.6.5-2). The RQD values for the siltstone and sandstone are 0%

and 62%. A reference stress of 2.12 k/ft2 is should be used in calculations for the rock

socketed drilled shafts. An average compressive strength of 648 k/ft2 should be used for

concrete in the design. The uniaxial compressive strength of the sandstone should be

1500 k/ft2. This is a conservative value based on the range of 1400-3600 k/ft2 according

to DM-4 Table 10.6.3.2.2-2. Based on the rock qualities, an m value of 1.231 and an s

value of .00293 shall be used, according to AASHTO Table 10.4.6.4-4. The resistance

factors used for the side resistance in the siltstone are 0.5. The resistance factors for

the tip and side in the sandstone should be .55 and .50 respectively. The resistance

factors used are from DM-4 Table 10.5.5.2.4-1.

Due to the shallow depth of bedrock underlying the project site, any type of deep

foundation design will not be considered for this project. Deep foundations are only

required when shallow soils are not strong enough to maintain the load of the structure.

Since our bedrock is rather shallow, there is no need for the load to be transferred to

deeper soils. In addition, such method is used for hard driving conditions such as

cobbles and boulders, which is not present in this site.

Sackett Engineering has prepared this geotechnical report and all its contents.

All of the content in the report express Sackett Engineering’s interpretations of the

subsurface conditions, tests, and results of the conducted analyses. The opportunity to

review the plans and specifications as they pertain to the recommendations contained in

this report would be appreciated in order to submit further comments or feedback based

on this review.

APPENDIX A

Figure 1:

APPENDIX B

APPENDIX C

geotechnical report

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