GM18-18_Final_Bridge_Foundation_Recommendations_August_2018.pdf

M E M O R A N D U M Western Federal Lands Highway Division

610 E. Fifth Street Vancouver, WA 98661

DATE: August 31, 2018

TO: Benn Oltmann Structures Functional Manager

FROM: Diann Morehouse Geotechnical Engineer

SUBJECT: Geotechnical Memorandum GM18-18 Final Bridge Foundation Recommendations North of Moose Creek North

MT DOT 569(3)

INTRODUCTION

This memorandum finalizes the foundation recommendations for the bridge over California Creek based on final bridge loads. Preliminary recommendations were provided in WFLHD Geotechnical Memorandum, GM 31-17, dated December 15, 2017. This Memo summarizes final bridge loads and foundation type and size. All information and recommendations presented in GM 31-17 are valid with the exception of the 1) final bridge length, 2) number of piles per abutment, 3) minimum pile length and tip elevation, and 4) slightly revised recommended soil parameters for lateral analysis.

BRIDGE LENGTH AND LOADS

Final bridge length was increased from 80 feet to 83 feet to accommodate the two rows of closed end pipe piles in each abutment cap. Final, updated axial loads, as provided by the Central Federal Lands, CFLHD, Structural Group on 5/23/2018, are summarized below. Load factors are the same as presented in GM 31-17. Downdrag loads are not expected on the piles.

The final bridge design has two rows of six piles for a total of twelve pipe piles at each abutment; resulting in a maximum factored load per pile for the Strength I Limit State is 90 kips. Maximum unfactored load per pile for the Service I Limit State is 60 kips.

FOUNDATION RECOMMENDATIONS

As recommended in GM31-17, piles should be closed-ended, 18-inch steel pipe piles with 0.75-inch wall thickness driven using a conical driving point. The piles should be filled with concrete after driving. The piles should consist of ASTM A 252 Grade 3 steel having a minimum yield strength of 45 ksi.

The final bridge design calls for 12 piles at each abutment. The factored axial compressive load is 90 kips per pile.

Revised recommended soil parameters for lateral analysis are shown in Table 1.

These values are based on the representative values shown in Design and Construction of Driven Pile Foundations, Hannigan, et al, (2016), Geotechnical Engineering Circular No. 12, Publication No. FHWA-NHI-16-009, U.S. Department of Transportation, Federal Highway Administration, Tables 5-9, 7-21, and 7-22.

Due to the slight modification to the soil parameters, revised analyses results showing foundation profiles and soil conditions, and vertical analyses results for 18-inch diameter pipe piles at each abutment for the Service I and Strength I Limit States appear in Attachment A.

The graphs show the capacity for a ten-pile group. The axial capacity analyses are intended to be used to estimate pile group capacity and determine pile lengths for contract documents.

Each pile should be driven to a required nominal axial compressive resistance of 145 kips.

This is based on a factored load of 90 kips per pile, a resistance factor of 0.65, and an estimated 6 kips of side resistance in the zone of scour during pile driving. Final pile tip depth is estimated at 26 feet below the bottom of pile cap with the tip elevation for Abutment 1 at

6226.9 feet and Abutment 2 at 6227.7 feet. Dynamic load tests with signal matching are required on the first pile driven at each abutment to verify that the contractors driving system installs the piles to the required nominal axial compressive load without damaging the piles.

The dynamic load test results will be used to establish the driving criteria for the rest of the piles in each abutment. Actual pile tip elevation may vary based on site conditions and pile driving results. Artesian groundwater conditions were encountered near an elevation of 6220 feet. Due to these artesian groundwater conditions, do not drive piles below an elevation of 6224 feet prior to discussing with WFLHD Geotechnical Engineer.

Table 1: Recommended Soil Parameters for Lateral Analysis.

Location Elevation

ft. Material Description γ lb/ft3 γ' lb/ft3 ϕ' degree Cohesion lb/ft2

Modulus of

Subgrade Reaction, k lb/in3

Soil Strain, E50

Abut. 1 West

6254 - 6253 Mod. Stiff Clay w/o Free Water

110 - - 100 30 0.020

6253 - 6248 Sand - 57 30 - 20 - 6248 - 6237 Stiff Clay w/o Free Water - 90 - 1300 1000 0.010 6237 - 6227 Stiff Clay w/o Free Water - 90 - 1300 750 0.007 6227 - 6196 Stiff Clay w/o Free Water - 100 - 1000 1000 0.005 6196 - 6155 Stiff Clay w/o Free Water - 100 - 1500 1000 0.005

Abut. 2 East

6255 - 6253 Mod. Stiff Clay w/o Free Water

110 - - 100 30 0.020

6253 - 6251 Sand - 57 30 - 20 - 6251 - 6240 Stiff Clay w/o Free Water - 90 - 1300 1000 0.01 6240 - 6220 Stiff Clay w/o Free Water - 90 - 1300 750 0.007 6220 - 6207 Stiff Clay w/o Free Water - 100 - 1000 1000 0.005 6207 - 6185 Stiff Clay w/o Free Water - 100 - 1000 1000 0.005 6185 - 6156 Stiff Clay w/o Free Water - 100 - 1500 1000 0.005

NOTES:

• Estimated groundwater elevation is 6254 feet.

• γ = moist unit weight

• γ’ = effective unit weight

• ϕ' = effective internal angle of friction

• k = modulus of subgrade reaction (rate of increase of modulus E with depth)

CONSTRUCTION CONSIDERATIONS

Drilling Piling The upper soil profile across the site is generally consistent with sand with various fractions of clay, silt, gravel, and some cobbles to an elevation between about 6248 and 6243 feet where highly to completely weathered sandstone is interpreted to begin. A gravel layer was encountered starting near elevation 6252 and 6254 and is about 2 to 4 feet thick.

Groundwater was encountered near the elevation of California Creek.

Considering the proximity of the groundwater to the ground surface dewatering will be likely be needed for excavations extending below an elevation of about 6254 feet. This elevation may vary depending on the construction schedule and the weather conditions. The cohesionless soils encountered in the borings will cave-in if left unsupported when excavations extend below the groundwater table. The overburden soil will not stand up in an unsupported drill hole. It is therefore recommended that the piling be drilled using a drill rig that is able to install the pile to tip elevation by continuous advancement of the pile with an interior drill string such that cuttings removal is maintained within the pile.

Although boulders were not encountered in the borings both cobbles and boulders may be encountered at the drilled pile locations.

Excavations and Dewatering The contractor should be prepared to encounter water in the exaction to install the pile caps and wingwalls and plan accordingly. Excavations will need to be dewatered so that backfill can be placed and compacted in the dry.

It is anticipated that the soils encountered in the excavations will be submerged soil that fall into the Occupational Safety and Health Administration classification of Type C soil. The maximum allowable slope for excavations less than 20 feet deep in Type C soil is 1V:1.5H unless a support or shield system is used (U.S. Department of Labor). The cohesionless soils encountered in the borings are likely to cave when excavations extend below the groundwater table. Dewatering with multiple sumps and pumps and/or temporary shoring will likely be required for excavation.

All other geotechnical information and recommendations presented in WFLHD Geotechnical Memorandum GM 31-17 are valid

ATTACHMENT A

Foundation Profiles and Soil Conditions Vertical Analyses

Axial Geotechnical Pile Resistance Graphs

FOUNDATON PROFILE AND SOIL CONDITIONS

Boring terminated at elevation 6155 feet

California Creek Bridge West Abutment

FHWA

WFLHD

VERTICIAL ANALYSIS

California Creek Bridge West Abutment Strength I Limit State with Scour

FACTORED AXIAL RESISTANCE vs. FOUNDATION DEPTH

Single Pile

6x2 Pile Group

California Creek Bridge West Abutment Strength I Limit State with scour

FOUNDATION PROFILE AND SOIL CONDITONS

Boring terminated at elevation 6156 feet

California Creek Bridge East Abutment

VERTICAL ANALSYIS WITH SCOUR

Strength I Limit State

FACTORED AXIAL RESISTANCE vs. FOUNDATION DEPTH

Single Pile

6 x 2 Pile Group

Strength I Limit State