dougherty mills structural foundation report

STRUCTURE FOUNDATION REPORT

Dougherty Mills Bridge over Slippery Rock CreekSlippery Rock Twp, 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

April 28, 2014

INDEXPage

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

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

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

2. GEOLOGIC INFORMATION………………………………………………………………3

3. SITE INVESTIGATION……………………………………………………………………..5

3.1 LABORATORY TEST RESULTS………………………………………………..5

3.2 CHEMICAL TESTING……………………………………………………………..6

3.3 SUBSURFACE CONDITIONS…………………………………………………...7

4. RECOMMENDATIONS…………………………………………………………………….8

4.1 FOUNDATIONS………………………..………………………………………….8

4.2 EXCAVATIONS…………………………………………………………………... 8

4.3 SLOPE STABILITY………………………………………………………………..9

4.4 CORROSION……………………………………………………………………...10

5. LIMITATIONS & DESIGN CONSIDERATIONS………………………………………...12

APPENDIX:

1. INTRODUCTIONThis report contains the subsurface exploration, geotechnical analysis, and

structural foundation 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.

After the on-site investigation, and all the laboratory tests, it is determined that a

deep foundation will not be required to meet structural stability of this bridge. For both

abutments that reference boring holes DOU-903 and DOU-906, a sandstone bedrock

layer is located at 16.6 feet and 14.7 feet. The presence of this strong stable layer

provides the necessary support for the foundation without embedding it deep in the soil.

1.1 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.

1.2 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.

2. 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 wireline

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

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

locations around the bridge. DOU-905a was augured to 22.5ft before samples were

taken.

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 contain

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

side of the bridge borehole DOU-905 encountered debris 20.5ft under the ground

surface. Among the debris found was concrete fragments and wood. It also

encountered concrete and wood after another foot. The boring was soon abandoned

due to the fragments and solid concrete and wood that could have been the footing to

the prior bridge. This resulted in the boring of 905a. A layer of organic material was

found in DOU-003, but should not interfere with the structure. No other major

inconsistencies turned up from the borings.

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 the 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

3. SITE INVESTIGATION

3.1 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 triaxial 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 triaxial shear test are listed in Table 3 on the next page.

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

3.2 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.


3.3 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.

4. RECOMMENDATIONS

4.1 FOUNDATIONSSackett Engineers have developed two different foundation plans that will provide

structural stability for the applied loadings on the bridge. Both shallow and deep

foundations were considered for design and it was concluded that a shallow foundation

will meet the necessary requirements. Please refer to Appendix D for design

calculations.

Option 1 was the design of drilled shafts below the abutments for the bridge. The

design included the following:

Two piles are proposed for Abutment 1. Both piles will start at an elevation of

1148.5 ft. The pile will have a diameter of 3.5 ft to an elevation of 1141ft (7.5ft) and a

socket diameter of 3ft to an elevation of 1134.5 ft (6.5ft). The piles should be placed

evenly 11 ft apart at the center of the bridge abutment..

Two piles are also proposed for Abutment 2. Both piles will start at an elevation

of 1149 ft, meaning the entire pile length will be within the bedrock. Each pile will have

a length of 6.5 ft and a diameter of 3.5 ft. The piles should be placed evenly 11 ft apart

at the center of the bridge abutment.

Option 2 included a strip footing foundation that laid on top of bedrock. Although

the depths of the bedrock are different from Abutment 1 to Abutment 2, all other

constraints and variables are constant. Abutment 1 will sit on a layer of bedrock 16.6

feet below the surface whereas Abutment 2 will lay on the same bedrock layer at 14.7

feet below the surface. In addition after various earthwork calculations, Sackett

Engineering has approved a strip foundation 45’x10’x2’ which can resist sliding, tilting

and bearing capacity.

4.2 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”.

4.3 SLOPE STABILITYThe Slope-Stability Analysis was conducted at the cross section of station

181+00 at the culvert. The analysis was conducted using Geo-Slope student version

from 2012. The culvert was analyzed to determine a failure plane on both the right and

left side of the embankment. For the purposes of the analysis the additional surcharge

loading of 360 psf was included by adding an additional 3 feet of the topsoil layer.

For the analysis of a slope failure along the right side of the culvert, R class rock

fill was placed along the 20 feet of the embankment to widen the roadway. All material

properties for this analysis are from the provided design of the embankment and shown

in Table 6. The result for the preliminary analysis for the factor of safety for a slope

failure of the right side is 1.165. The failure plane can be seen in Figure 3 in Appendix

E below.Table 6 - Soil properties for Slope Stability along Right Side

Material Type γ (psf) C (psf) Φ (deg)Silty Sand Fill 110 35 26

Clay 125 630 0

R class rock fill 130 0 44

For the analysis of a slope failure along the left side of the culvert, the existing

gabion wall was not considered during the analysis and replaced with the silty sand fill

of the topsoil layer. Two analyses were done for a failure plane along this side, one

being the embankment without backfill and the other being the embankment once the

backfill was placed. All the material properties for this analysis are from the provided

design of the embankment and shown in Table 7. The result for the preliminary

analysis for the factor of safety for a slope failure of the left side of the culvert without

backfill is 0.780. This failure plane can be seen in Figure 1 In Appendix E below.

Because this is an unacceptable factor of safety, backfill will be needed to increase the

stability. The analysis for the factor of safety for a slope failure of the left side of the

culvert with backfill is 1.491. This failure plane can be seen in Figure 2 In Appendix E

below.

Table 7- Soil properties for Slope Stability along Left Side

Material Type γ (psf) C (psf) Φ (deg)Silty Sand Fill 110 35 26

Clay 120 630 0

Residual Sandy Gravel 130 0 35

The analysis was conducted using the provided culvert design and the

engineering judgment of Sackett Engineering. The provided figures show the location of

the factor of safety and failure plane for each analysis.

4.4 CORROSIONSlippery Rock Creek has had previous issues with pollutants from Acid Mine

Drainage. This resulted in elevated acid loads in watershed streams and sub

watersheds. Further testing should be done to see what other effects this has on the

properties of the soil and water.

Two representative soil samples were used for the possible corrosively of any

subsurface structures (DOU-903, DOU-907). DOU-903 was slightly acidic at 6.1 and

had a chloride concentration of 406 ppm and a sulfate concentration of 200 ppm. This

sample had a resistivity of 1083 ohm*cm, which is considered to have poor corrosion

resistance.

Sample DOU-907 has a pH of 3.2, a chloride concentration of 466 ppm, and a

sulfate concentration of 300 ppm. The specific resistance is 1575 ohm*cm, which is

considered to have poor corrosion resistance.

Special corrosion resistance measure should be taken in order to protect the

concrete and reinforcing steel below the surface. Both samples possessed high levels

of chloride and sulfate concentrations. When sulfur is present with chlorides, the

corrosion of steel is accelerated compared to normal conditions. The pH of both

samples fall in a range that is not suitable for normal concrete. According to the PCA

(Portland Cement Association) concrete deterioration increases as the pH of the acid

decreases from 6.5., and that no solution will hold up for long if exposed to a solution

with a pH of 3 or lower. Both samples had pH’s lower than 6.1.

Specific resistivity is a measure of how corrosive a soil is. Both of the samples

possessed specific resistivity well below the requirement to be characterized as poor.

One item that leads to a lower resistivity is the salt and moisture content of the soil.

After on site investigation and further lab tests, it is apparent that corrosion

prevention methods must be used in order to ensure lasting stability of the structure.

The main components of this bridge that are susceptible to corrosion are the strip

footing foundation, concrete deck slabs, and the steel beams.

The footing composed of reinforced concrete is susceptible to both the weather

and the earth that is exposed above, beneath and laterally of the foundation. In

accordance with section 5.12 Durability in the PennDot DM-4, a minimum of 4 inches of

cover to cast in place concrete permanently exposed to the earth and a minimum of 3

inches for precast concrete piles. In addition, the concrete mix should limit the w:c ratio

to .45 with no additives that contain chlorides. It is the designer’s recommendation to

use concrete class type V that will provide a stronger resistance. ACI Committee 222

has set a standard limit for chloride ions in a concrete mix prior to being set and

exposed. In the case for reinforced concrete exposed to chloride, the ion concentration

is set to 0.10% as per percent weight of cement.

Cathodic protections systems are vastly used to coat various members of a

bridge. For example, cathodic protection coating has been successful in preventing

corrosion of reinforced bars in concrete. This is done by applying a coating to the rebars

in the concrete which will be exposed to weathering as opposed to the steel members

themselves. Although this coating can provide resistance to weather and water

exposure, this system and maintenance is very expensive, and a full analysis must be

done in order to ensure its cost effectiveness.

The painting of steel bridge members has been an ongoing solution to further

extend a bridge’s structural integrity. The corrosion of steel members has been

unpreventable due to the gaps between concrete decks. Although joint systems are in

place to limit deck dripping, splash zones as they are called, are still very prevalent and

will corrode the members beneath. Most paint products are of an inorganic zinc rich

paint, which will corrode instead of the steel members themselves. This protection

provides a second skin, which if applied properly, can withstand weathering and other

conditions up to 20 years without replacement.

Considering the inclement weather this bridge will face in Butler County PA, a

dual deck protection system is warranted. Our recommendation is for a filled

galvanized metal grid deck system, and the use of 1½ inches of microsilica modified

concrete overlay as per PennDOT Standard Drawings BD-604M. This system has been

frequently used in populated areas with poor weather conditions, which serves as a

baseline for continued use. In addition, Federal Aid projects have been continuously

implementing this new deck system in bridges nationwide.

5. LIMITATIONS & DESIGN CONSIDERATIONS

All 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 relatively 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 information, for the design of 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 cohesionless 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 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 is 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.

The previously stated design considerations were implemented in the design

calculations for the spread footing and drilled shaft. Please refer to Appendix D for the

full calculations.

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.

This structural foundation report and all its contents have been prepared by

Sackett Engineering. All of the content in the report express Sackett Engineerings

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 ESLOPE STABILITY ANALYSIS

Figure 1 Left Side with Existing Gabion Wall Backfill

Figure 2 Left Side with New Slope and Backfill

Figure 3 Right Side with Rock Fill

dougherty mills structural foundation report

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