Final_Geotechnical_Report.pdf

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FREMONT PASS TRAIL

SUMMIT COUNTY, CO

CO FLAP SUM91(1)

Final Geotechnical Report Report No. CO-FLAP-SUM91(1)-19-01

Prepared by Federal Highway Administration

Central Federal Lands Highway Division

Geotechnical Services Branch March 2019 ii

SIGNATURE SHEET

Report Prepared by: __________________________________________

Dustin F. Robbins, P.E. Geotechnical Engineer

Report Reviewed by: __________________________________________

Devin Dixon, P.E., Geotechnical Engineer

Approved for Distribution by: __________________________________

Marilyn Dodson, P.E., Lead Geotechnical Engineer

Date

Distribution

Electronic:

N:\CO\SUM91(1)Geotech\7_FinalDocs K:\TechServices\Geotech\1. Geotech Project Reports Project Management Project Development/Design Construction Acquisitions Bridge

Hardcopies:

CFLHD, Tech Services Library (1) Geotechnical Files (2) Project Management (1) Project Development/Design (1) Construction (1) Bridge (1) i

TABLE OF CONTENTS

SECTION ONE - INTRODUCTION

1.1 BACKGROUND AND LOCATION

1.2 SCOPE AND PURPOSE

SECTION TWO - FIELD EXPLORATION AND LABORATORY TESTING

2.1 FIELD EXPLORATION

2.2 LABORATORY TESTING

SECTION THREE - GEOLOGY, SEISMICITY, AND SITE CONDITIONS

3.1 GEOLOGY

3.2 SEISMIC DEISGN PARAMETERS

3.3 SUMMARY OF SITE CONDITIONS

SECTION FOUR - ANALYSIS AND RECOMMENDATIONS

4.1 FOUNDATIONS - OVERPASS BRIDGE

4.1.1 Bridge Foundation Selection

4.1.2 Drilled Shaft Axial Resistance

4.1.3 Group Effects on Axial Resistance

4.1.4 Lateral Loads on Deep Foundations

4.1.5 Shallow Foundations Factored Bearing Resistance - Strength Limit State

4.1.6 Settlement Evaluation - Service Limit State

4.2 FOUNDATIONS - BOARDWALK

4.2.1 Bridge Foundation Selection

4.2.2 Helical Pile Axial Resistance

4.2.3 Helical Pile Lateral Resistance

4.3 ABUTMENT DESIGN

4.4 EMBANKMENT CONSTRUCTION

4.5 ROCKERY WALLS

4.6 CONSTRUCTION CONSIDERATIONS

4.7 SPECIFICATIONS

4.8 LIMITATIONS

SECTION FIVE - REFERENCES

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TABLES

TABLE 2.1 Summary of Field Exploration Program TABLE 2.2 Summaries of Laboratory Index Tests Results TABLE 3.1 Summaries of Seismic Parameters for Earthquake with 1000-Year Return Period for Site Class C TABLE 3.2 Depth to Practical Auger Refusal TABLE 4.1 Proposed Bridge Foundation Locations TABLE 4.2 LPILE Parameters TABLE 4.3 Factored Bearing Resistance for Shallow Foundations TABLE 4.4 Lpile Input Parameters - Boardwalk TABLE 4.5 Lateral Earth Pressures for Bridge Abutments TABLE 4.6 Presumptive Material Parameters for Design of Rockery Walls TABLE 4.7 Factors of Safety for Static and Seismic Design TABLE 4.8 Recommended Geometry for Rockery Abutments

FIGURES

FIGURE 1 Location Map FIGURE 2 Geology Map FIGURE 3 Boring Location Map, Main Alignment FIGURE 4 Boring Location Map, Proposed Pedestrian Bridge FIGURE 5 Test Pit Location Map, Wetland Mitigation Site

APPENDICES

APPENDIX A Boring Logs, Test Pit Logs APPENDIX B Laboratory Data APPENDIX C Photographs APPENDIX D Geophysical Exploration

Fremont Pass Trail, CO FLAP SUM91(1)

SECTION ONE - Introduction

1.1 BACKGROUND AND LOCATION

This report presents the geotechnical investigation findings and provides geotechnical recommendations for the Fremont Pass Trail Project, Summit County, CO. The project begins approximately 0.8 miles south of Interstate-70 and extends parallel to the east of Highway 91 for approximately 3.3 miles before crossing over the highway at the southern end of the project. The entire project length resides in Summit County and the White River National Forest. Other project partners include the United States Forest Service, who currently maintains the proposed trail alignment, and Colorado Department of Transportation that maintains Highway 91 and would be impacted by the connection of the trail to where an overpass pedestrian bridge is proposed. The primary goal of the project is to improve safety by reducing the number of bicycles on Highway 91 through providing a paved trail alternative. A project vicinity map is presented in Figure 1.

The new trail alignment is proposed to be aligned over an abandoned railroad bench that is east of and generally parallels Highway 91 and Ten Mile Creek. The proposed typical section is two-way 6-foot lanes with 2-foot shoulders generally matching the width of the existing railroad bench and therefore large cut and fill slopes for most the alignment are not anticipated. Minor proposed realignment at the south end near the overpass bridge will require some cuts into the talus slopes and possibly bedrock.

The proposed alignment passes through several shallow stream crossings, mapped wetland areas, and one fen restoration site approximately 460 feet long. At the stream crossings, pipe culverts ranging in diameter from 24 inches to 42 inches will be constructed. Rock aggregate embankment drains will be constructed where the alignment passes through wetlands to allow groundwater to pass through the embankment. A boardwalk approximately 460-feet long will be constructed over the fen restoration site to limit impacts to the fen. The boardwalk will be of fiberglass reinforced plastic and timber construction, have a clear space above the fen ranging from about 2 to 6 feet, and be founded on helical piles. Rockery walls up to approximately 4 feet in height will be constructed as the boardwalk wingwalls.

Rather than terminate the trail on the east side of Highway 91, the design team and project partners opted for an overpass bridge to terminate traffic on the west side of the highway near the entrance to Climax Mine at the south end of the project. The overpass bridge is proposed as a two-span prefabricated steel bridge with a total length of approximately 260 feet and a minimum vertical clearance of 19 feet. The abutments will consist of either drilled shafts or shallow spread footing foundations and be constructed at approximately the current grade with no significant cuts or fills anticipated. The center pier will consist of a single drilled shaft foundation located within an existing bench approximately 15 feet east of Highway 91 and 35 feet west of Ten Mile Creek.

1.2 SCOPE AND PURPOSE

The scope of work includes geotechnical investigation and analysis to estimate the subsurface profiles and ground water conditions at the foundation abutments, estimate material properties for foundation design, and provide AASHTO based design recommendations on foundation types/geometry, bearing resistance, settlement, global stability, and earth pressures. This report presents the findings of the subsurface investigation program and provides geotechnical recommendations for the design and construction of the Fremont Pass Trail project.

SECTION TWO – Field Exploration and Laboratory Testing

2.1 FIELD EXPLORATION

An engineer from the Central Federal Lands (CFL) Geotechnical Group observed the field exploration program that was completed in two phases. The first phase included characterization, seismic geophysical testing, drilling two borings at the north end of the proposed alignment and advancing two core holes at the proposed overpass location between October and December of 2017. The second phase included advancing one boring at the overpass bridge east abutment, one boring at each of the boardwalk abutments, two additional borings at culvert crossings, and three test pits at an off-site wetland mitigation site in September 2018. Five proposed borings from the initial exploration program were removed due to the consistency of the near surface soils. The exploration locations are presented on Figures 2, 3, and 4. Explorations logs are presented in Appendix A. A summary of the field exploration is provided in Table 2.1 and additional details are presented in the following paragraphs.

TABLE 2.1 - Summary of the Field Exploration Program

EXPLORATION

DESIGNATION

LOCATION

STATION OFFSET

GROUND

ELEVATION

TERMINATION

ELEVATION

EXPLORATION

DEPTH

BH-01 Rock Drain 9+75 CL 9,777 ft 9,766.5 ft 10.5 ft BH-02 Rock Drain 17+50 CL 9,793 ft 9,789 ft 4 ft

BH-03

Boardwalk Abutment

40+75 CL 9,884 ft 9,866 ft 18 ft

BH-04

Boardwalk Abutment

47+75 CL 9,914 ft 9,904 ft 10 ft

BH-05 Rock Drain 62+75 Boring Removed from Exploration Program BH-06 Rock Drain 73+50 Boring Removed from Exploration Program PH-02 Pavement 84+00 Boring Removed from Exploration Program BH-07 Rock Drain 92+50 Boring Removed from Exploration Program PH-03 Pavement 104+25 Boring Removed from Exploration Program BH-08 Culvert 113+50 CL 10,126 ft 10,105 ft 21 ft BH-09 Culvert 144+00 CL 10,236 ft 10,233 ft 3 ft

BH-10

Overpass East

Abutment 163+70 CL 10,272 ft 10,192 ft 80 ft

BH-11 Overpass Pier 165+05 10 ft RT 10,241 ft 10,157.25 ft 83.75 ft

BH-12

Overpass West

Abutment 166+50 CL 10,265 ft 10,206 ft 59 ft

S-1 Seismic Site

Class/ Overpass 165+05 CL 10,241 ft 10,141 ft 100 ft

TP-1

Wetland

Mitigation Site N/A N/A 8,655 ft 8,650.5 ft 4.5 ft

TP-2

Wetland

Mitigation Site N/A N/A 9,065 ft 9,058 ft 7 ft

TP-3

Wetland

Mitigation Site N/A N/A 8,652 ft 8,648.5 ft 3.5

Note: The exploration locations and elevations were estimated relative to features shown on the 95 percent project plans.

EXPLORATORY BORINGS:

Nine borings were completed to depths ranging from 3 feet to 83.75 feet below the ground surface. Vine Laboratories, Inc. of Denver, Colorado provided the drilling services using a CME-750 ATV-mounted drill rig. Borings BH-01 through BH-04, BH-08, and BH-09 were advanced using 6-inch outside diameter hollow stem augers, borings BH-10 and BH-11 using Robit casing advancer and NQ size core, and boring BH-12 using hollow-stem auger and NQ core. Upon completion of drilling activities, field personnel backfilled the borings with cuttings generated during the drilling. Drilling was completed in three phases. The first phase began on October 16th, 2017 and consisted of drilling boring BH-01, BH- 02, BH-12 and the upper 74 feet of boring BH-11, where boring BH-11 was terminated due to a damaged pump on the drill rig. Drilling resumed at BH-11 on January 8th, 2018, and the boring was completed to a depth of approximately 84 feet. The remainder of the borings were completed September 5th through September 7th, 2018.

Disturbed samples were obtained from the borings using the Standard Penetration Test (SPT), in accordance with AASHTO T-206. The SPT involves driving a 2-inch outside diameter, 1.375-inch inside diameter split spoon sampler 18 inches with a 140-pound hammer falling 30 inches and recording the blow counts per 6-inch of driving. An automatic-trip hammer with a measured efficiency of 85 percent was used to drive the samplers. The SPT N-value is the sum of the blow counts required to drive the sampler over the second and third 6-inch increments. The N-value provides a means for evaluating the relative density of cohesionless soils and consistency or stiffness of cohesive soils. Representative soil from the SPT split-spoon sampler were placed and sealed in plastic bags and were transported to the CFLHD Materials Laboratory for testing. Logs of the explorations are presented in Appendix A. Photos of drilling equipment and field exploration activities are included in Appendix C.

FEN EXPLORATION:

A conventional drill rig was not allowed within the fen. Attempts were made with a hand auger, but refusal was generally encountered within two feet due to either coarse gravel or the stiffness of the fine-grained material. The hand auger was attempted at the north end of the boardwalk near approximate Station 43+50.

A Wildcat Dynamic Cone Penetrometer (DCP) was also used to estimate penetration resistance. A total of 8 DCP tests were performed within the fen spaced at about 50 foot intervals from Station 43+50 to 47+00. Refusal was encountered in all DCP tests at depths ranging from 5 to 6 feet. The DCP refusal appeared to be due to the stiffness of the fine-grained material, and not on cobbles or boulders.

GEOPHYSICAL TESTING:

On October 24, 2017, CFLHD geotechnical engineers performed one surface seismic survey located near the center pier of the overpass bridge. The intention of the geophysical survey was to supplement the boring data and to provide additional information regarding the seismic Site Class determination.

The Multi-Channel Analysis of Surface Waves (MASW) method utilizing both active and passive sources was selected for the seismic testing.

The active source test consists of producing a seismic energy source at the ground surface and then recording the seismic waves travelling through the ground at each geophone location. The active source was generated by impacting a 14-pound sledgehammer on a steel plate located at various locations on the ground near both ends of the seismic line. A minimum of 9 records were collected at offsets ranging from 5 to 40 feet from the end of the survey line. A series of (24) 4.5-Hz geophones spaced 5-feet apart for a total line length of 115 feet were connected to a Geode seismograph manufactured by Geometrics, Inc. The passive source test used the same equipment setup while recording only naturally existing ambient seismic energy as the source for a total 30 second record.

The field data was reduced by CFLHD geotechnical engineers using SurfSeis 4.2.4 software developed by Kansas Geological Survey. The seismic survey results are presented in Appendix D.

2.2 LABORATORY TESTING

Soil samples recovered from the borings were tested in the laboratory to support the field classifications and to provide an estimated of the engineering characteristics and mechanical properties of the soil.

Laboratory tests included gradation (AASHTO T88), classification (AASHTO M145) and Atterberg limits (AASHTO T89 and T90). When the necessary tests were completed, samples were classified using the Unified Soil Classification System (USCS) and AASHTO soil classification system. A summary of the soil test results is shown in Table 2.2.

TABLE 2.2 - Summaries of Laboratory Index Test Results

BORING

NUMBER

SAMPLE

DEPTH, ft

PERCENT

SAND

PERCENT

PASSING 200

LL PI USCS

BH-02 0 to 2 54 12 NV NP SM BH-03 2 to 3.5 61 10 NV NP SP-SM

BH-03 3, 14, 18 56 17 26 3 SM

BH-08 10, 15 55 16 - - -

BH-10 10, 15 40 8 - - -

BH-10 25, 30, 35 78 9 NV NP SP-SM

BH-10 50, 55 48 16 24 8 SC

BH-11 11, 16, 21 39 10 NV NP GP-GM

BH-11 46, 51 39 16 NV NP GM

TP-1 2 to 3 26 3 NV NP GW TP-2 2 to 3 27 3 NV NP GW TP-3 1 to 2 42 7 NV NP GW-GM

FEN-1 1 to 2 62 16 NV NP SM

The uniaxial compression test of one select rock sample was performed on a core of the gneiss bedrock in general accordance with ASTM D 7012, Method C, Compressive Strength of Intact Rock Core Specimens. In this test, cylindrical rock specimens are tested in compression without lateral confinement. Test results measured a uniaxial compressive strength of 13,200 pounds per square inch.

The full results of the laboratory tests are presented in Appendix B.

SECTION THREE–Geology, Seismicity and Site Conditions

3.1 GEOLOGY

A 2004 geologic map prepared by Widmann and others was reviewed to evaluate the project geologic conditions. The trail alignment generally follows Ten Mile Creek which is surrounded by a combination of glacial and alluvial deposits. The majority of the alignment will be constructed on Pinedale Till (Qp) with the other areas being constructed on alluvial fan deposits (Qf). The alluvial deposits are likely to be underlain by till at shallow depths. Bedrock in the area is mapped as migmatite (Xm) consisting of gneiss and schist. The geotechnical exploration data generally agrees with mapped geological conditions. The aforementioned geology map is presented in Figure 2.

3.2 SEISMIC DESIGN PARAMETERS

Geophysical surveys performed near the center pier of the overpass bridge measured the top 100 feet (Vs100) weighted average shear wave velocity to be 2,191 feet per second corresponding to Site Class C as defined in AASHTO Section 3.10.3.1. Recommended seismic response parameters for use in design are based on the 7th Edition of AASHTO, ground motion corresponding to an exceedance probability of 7 percent in 75 years for an earthquake with an approximate 1000-year return period. The 1000-year return period uniform hazard spectrum for the soils at the overpass bridge site (latitude 39.4581, longitude -106.1439) for the probabilistic horizontal acceleration values corresponding to specific ground acceleration (PGA) and spectral coefficients, namely the short- and long-period ground acceleration (Ss and S1) for Site Class C and the corresponding Seismic Zone are summarized Table 3.1.

TABLE 3.1 - Summary of Seismic Parameters for Earthquake with 1000-Year Return Period for Site Class C

Horizontal Peak Ground Acceleration, (PGA) 0.077 g Horizontal Response Spectral Acceleration at Period of 0.2 sec, (Ss) 0.159 g Horizontal Response Spectral Acceleration at Period of 1.0 sec, (S1) 0.041 g

Site Factor at Zero-Period of Acceleration Spectrum, (Fpga) 1.2 Site Factor at Short-Period Range of Acceleration Spectrum, (Fa) 1.2 Site Factor at Long-Period Range of Acceleration Spectrum, (Fv) 1.7

Factored Horizontal Peak Ground Acceleration, (As) 0.093 g Factored Horizontal Response Spectral Acceleration at Period of 0.2 sec, (SDs) 0.191 g Factored Horizontal Response Spectral Acceleration at Period of 1.0 sec, (SD1) 0.069 g

Seismic Zone Zone 1

3.3 SUMMARY OF SITE CONDITIONS

Trail Alignment The ground surface is an abandoned railroad bench which generally consists of grass with sand, gravel, cobbles, and boulders. Adjacent to the bench are gentle slopes and occasional soft, wet soil mapped as wetlands. Several stream crossings are present along the bench. The borings along the trail consisted of till soils that included sand with low plasticity clayey or non-plastic silty fines, gravel, cobbles, and boulders from the ground surface to practical auger refusal. Explorations were not drilled through the boulders, so their size could not be determined in the exploration. However, boulders are present at the ground surface that are several feet in diameter and should be anticipated subsurface. Fill soils consisting of reworked native material are also likely present but could not be distinguished from the native till soils. Table 3.2 presents the depth to practical auger refusal for each boring along the trail alignment.

TABLE 3.2:- Depth to Practical Auger Refusal

BORING NUMBER APPROXIMATE STATION DEPTH TO PRACTICAL AUGER REFUSAL (FT)

BH-01 9+75 10

BH-02 17+50 4

BH-03 40+75 18

BH-04 47+75 10

BH-08 113+50 21

BH-09 144+00 3

Boardwalk The boardwalk will span a fen restoration site approximately 460 feet in length. The current conditions at the ground surface consist of loose/soft silty-clayey sand soils with grass and other vegetation typical of wetland environments.

Since no borings were allowed within the fen area, information was obtained from the DCP testing, a surficial sample, and from borings BH-03 and BH-04 that were drilled near the proposed abutments.

Boring BH-03 (North Abutment) appeared from the ground surface to be in similar conditions to the majority of the fen. The subsurface conditions consisted of gravelly lean clay with sand (CL) and silty sand with gravel (SM) within the upper 14 feet. Uncorrected field blow counts ranged from 6 to 20 blows per foot. Refusal was encountered at a depth of 18 feet on probable cobbles and boulders. Boring BH-04 was drilled approximately 75 feet south of the proposed abutment due to drill rig access. The soil conditions are not expected to be representative of the material at the abutment based on the surface observations. Boring BH-04 consisted of very dense sand, gravel, cobbles, and boulders. The boring was attempted three times and encountered refusal at depths of 4 feet, 7 feet, and 10 feet.

The moisture conditions within the fen appear to be variable. During some field visits, the ground surface contained standing water and other areas where water would be expressed by footprints. At other times, the ground surface was firmer with no standing water and the surficial soils being very moist. The site generally slopes and drains down grade from east to west.

Considering the information available, the subsurface conditions assumed for design purposes are a firm to stiff lean clay with sand to a medium dense silty sand with gravel. Coarse gravel, cobbles, and occasional boulders will likely be encountered, consistent in size with those exposed at the ground surface along the trail alignment. The groundwater level is assumed to be the ground surface.

Overpass Bridge Boring BH-11 drilled near the center pier location consisted of till soil classified in the field as well-graded gravel with silt and sand (GW-GM). Surface conditions at boring BH-11 consist of a fill embankment with a relatively level surficial cover of gravel and no vegetation between Highway 91 and Ten Mile Creek. The upper few feet is likely fill, but no distinction could be made between fill soil and the till soil. The encountered materials consisted of gravel, sand and generally less than 15 percent non-plastic fines. Abundant cobbles and boulders were inferred from the drill rig response and the side walls of the borings that were visible. Drill cuttings could not be used to determine the presence of cobbles and boulders since the drilling method pulverized the material. SPT blowcounts indicate an apparent density of medium dense to dense, but were highly variable due to the presence of cobbles and boulders.

In general, the apparent density increased to very dense or SPT refusal below a depth of approximately 35 feet. Groundwater was encountered at a depth of 23 feet, approximate elevation of 10,218 feet. The soils continued with varying density to a depth of approximately 60 feet where bedrock was encountered. The bedrock consisted of metamorphic gneiss that was generally medium strong with RQD ranging from 0 to 75 percent (very poor to fair) and an unconfined compressive strength measured in the lab of 13,200 pounds per square inch. The upper approximately 6 feet of the bedrock appears to be more weathered than the relatively fresh bedrock encountered below a depth of 66 feet, elevation 10,175 feet.

Boring BH-12 is located on a slope approximately 30 feet above the Highway 91 elevation, near the west abutment. The surface conditions included cobbles and boulders up to approximately 5 feet in diameter visible at the ground surface, a light cover of grass, and typical forest trees. Boring BH-12 drilled near the proposed west abutment consisted of glacial till soil classified in the field as poorly-graded gravel with silt and sand (GP-GM). The upper 11 feet consisted of silty sand with gravel (SM) that contained visible cobbles and boulders. Auger refusal was encountered at a depth of 11 feet and coring through the more granular till soil began. The till consisted of gravel, sand and generally less than 15 percent non-plastic fines. Abundant cobbles and boulders were inferred from the drill rig response and the side walls of the borings that were visible. Core recovery was 50 percent or less in all runs with an average of 28 percent. The recovered core was likely fragments of cobbles and boulders with a maximum recovered core length of 10 inches. The boring was abandoned at a depth of 59 feet, elevation 10,206 feet, when the core steel broke while coring through boulders.

Boring BH-10 is located on a slope approximately 30 feet above the Highway 91 elevation, near the east abutment. The boring was located on the existing bench, and therefore surface conditions are similar to the rest of the alignment. Abundant cobbles and boulders were inferred from the drill rig response and the side walls of the borings that were visible. Drill cuttings could not be used to determine the presence of cobbles and boulders since the drilling method pulverized the material. SPT blowcounts indicate an apparent density of medium dense to dense, but were highly variable due to the presence of cobbles and boulders. In general, the apparent density was very dense with SPT refusal. Groundwater was encountered at a depth of approximately 50 feet, approximate elevation of 10,222 feet. The boring was abandoned at a depth of 80 feet, elevation 10,192 feet, without encountering bedrock.

Site descriptions are based on field observations, subsurface characterization information, and may vary in material type and strength with distance away from boring locations. This variability is due to topography and land forming processes.

SECTION FOUR – Analysis and Recommendations

The recommendations presented in this report are based on the subsurface conditions observed in the field explorations, laboratory test results, engineering analysis, and experience with similar projects. The following sections include recommendations regarding foundations for the overpass bridge, foundations for the boardwalk, abutment design, embankments, rockery walls, construction considerations, and specifications.

4.1 FOUNDATIONS - OVERPASS BRIDGE

The proposed two span bridge is approximately 260 feet long and will be supported by a deep foundation at the pier and shallow foundations at the abutments. Based on the 95 percent design plans, the proposed abutment centerline stations and cap elevations are shown in Table 4.1.

TABLE 4.1:- Proposed Bridge Foundation Locations

FOUNDATION APPROXIMATE STATION FOOTING ELEVATION / TOP OF SHAFT ELEVATION*

Abutment 1 163+80.27 10,258 FT

Center Pier 165+10.27 10,239 FT

Abutment 2 166+38.52 10,259 FT

*Estimated from information presented on the 95% design plans.

4.1.1 Bridge Foundation Selection

A single drilled shaft foundation is proposed for the center pier due to the high axial and lateral loads. A wide shallow foundation was considered but determined unfeasible due to limited space available to construct the shallow foundation and the possibility for deep scour and erosion along the stream bank.

For the abutments, both shallow spread footings and drilled shafts are feasible from a geotechnical perspective. Shallow spread footings may be desired since the subsurface profile is very dense and contains numerous cobbles and boulders that will likely result in construction difficulty for drilled shafts. Additionally, there are no scour concerns and the design includes sufficient working space for the shallow foundations to be constructed. Design recommendations for both shallow and deep foundations are provided.

The following preliminary bridge loads factored for the strength limit state have been provided by the bridge engineer:

Abutments = 350 kips (spread footing or split between two shafts) Center pier = 600 kips (single shaft)

4.1.2 Drilled Shaft Axial Resistance

The drilled shaft axial resistance analysis was performed for 24 inch diameters at the abutments using the beta method for cohesionless soils presented in section 10.8.3.5.2a of AASHTO. The abutment foundations will obtain their resistance through side friction and the tip resistance was neglected. The factored resistance for the strength limit state was calculated by applying a 0.55 resistance factor for side resistance.

The drilled shaft axial resistance analysis was performed for a 48-inch diameter at the center pier using the analysis procedure for drilled shafts socketed in rock presented in section 10.8.3.5.4b of AASHTO.

The center pier will gain its axial resistance from side resistance in competent bedrock only and therefore the side resistance within the overburden has been neglected. The factored resistance for the strength limit state was calculated by applying a 0.50 resistance factor for side resistance applying a 20 percent reduction since the center pier is considered non-redundant. Based on the results of the analysis, a minimum socket of 5 feet into competent bedrock is required to resist the strength limit state load of 600 kips. If loads greater than 600 kips are anticipated, additional geotechnical analysis must be performed.

4.1.3 Group Effects on Axial Resistance

The resistance of a shaft group to the applied axial loads is not necessarily the sum of the axial resistance of individual piles within the group. The zone of influence from an individual shaft in a shaft group may overlap with other shafts, depending on spacing. Historically the axial efficiency of groups of shafts has not been a concern if the center-to-center spacing between shafts is greater than 3 times the shaft diameter.

4.1.4 Lateral Loads on Deep Foundations

Lateral load analysis will be performed by the bridge engineer using the software program Lpile developed by Ensoft, Inc. This program analyzes a single pile or shaft considering deflection as a function of design loads, foundation construction, and subsurface conditions. Table 4.2 provides preliminary Lpile input parameters for the foundation soils based on available subsurface information and generally accepted engineering correlations. It is also recommended that lateral support within five feet of the final road grade be neglected due to the potential loss of support from frost penetration or other shallow ground disturbance.

TABLE 4.2 – LPILE Parameters

APPROXIMATE

ELEVATION

(FT)

DEPTH BELOW

EXISTING GROUND

SURFACE (FT)

LPILE P-Y

MODEL

EFFECTIVE UNIT

WEIGHT (PCF)

FRICTION

ANGLE

(DEG)

SOIL

MODULUS

(PCI)

ABUTMENT No. 1 (BORING BH-10)

10,272 to 10,222 0 to 50 Sand (Reese) 130 38 225 10,222 to 10,192 50 to 80 Sand (Reese) 65 38 125

ABUTMENT No. 2 (BORING BH-12) 10,265 to 10,218 0 to 47 Sand (Reese) 130 38 225 10,218 to 10,206 47 to 59 Sand (Reese) 65 38 125

CENTER PIER (BORING BH-11)

10,241 to 10,218 0 to 23 Sand (Reese) 130 38 225 10,218 to 10,181 23 to 60 Sand (Reese) 65 38 125

TRANSITION TO ROCK PROPERTIES

LPILE P-Y

MODEL

EFFECTIVE UNIT

WEIGHT (PCF)

UNIAXIAL COMPRESSIVE

STRENGTH (PSI)

10,181 to 10,157 60 to 84 Strong Rock 140 13,200

Material properties provided are for single shafts and do not account for the reduced lateral resistance of shafts in a group. P-multipliers are a function of the number of rows of shafts and center-to-center shaft spacing in the direction of loading. P-multipliers are required even for a single row of shafts if the center-to-center spacing is less than 5 shaft diameters. P-multipliers are specified in Table 10-7.2.4-1 in AASHTO. When this analysis method is used, the resistances at the strength limit state as represented by the P-y curves should not be factored since they already represent the nominal conditions.

4.1.5 Shallow Foundations Factored Bearing Resistance - Strength Limit State

Factored bearing resistance at the strength limit state was estimated using the methods presented in section

10.6.3 of AASHTO. The factored bearing resistance is a function of the subsurface conditions and the proposed footing dimensions including length, width, sloping ground conditions, and embedment depth.

Based on the soils encountered in the bearing zone and guidance presented in AASHTO, a resistance factor of 0.45 has been applied. The sloping ground conditions as well as the offset from the slope varies between the east and west abutments. The following table presents the factored bearing resistances for each foundation.

TABLE 4.3:- Factored Bearing Resistance for Shallow Foundations

EFFECTIVE FOOTING WIDTH (FT) FACTORED BEARING RESISTANCE (KSF)

WEST ABUTMENT (FOOTING DEPTH = FOOTING WIDTH)

5.5 9.2

6.0 10.1

6.5 10.9

7.0 11.8

WEST ABUTMENT (FOOTING DEPTH = 1/2FOOTING WIDTH)

5.5 4.6

6.0 5.0

6.5 5.4

7.0 5.9

EAST ABUTMENT (FOOTING DEPTH = FOOTING WIDTH)

5.5 3.7

6.0 4.0

6.5 4.4

7.0 4.7

Lateral loads on the footings may be resisted by the friction developed between the concrete footing and soil. A nominal sliding resistance friction coefficient of 0.72 is recommended. A resistance factor of 0.8 should be applied for the strength limit state in accordance with AASHTO. Passive resistance should not be utilized to resist lateral loads due to the limited depth of soil below the design scour elevation.

4.1.6 Settlement Evaluation and Global Stability - Service Limit State

Settlement for footings on properly prepared subgrade with the dimensions and bearing pressures proposed on the bridge plans are estimated to be less than one inch. Any settlement is expected to occur quickly, generally within a few days of loading the structure, due to the granular nature of the soil and large depth to groundwater.

A global stability analysis was performed at the east abutment. The analysis included the service loads of 3,320 psf for the proposed abutment configuration. A limit equilibrium analysis (Spencer method) using the software Slide 7 by Rocscience was performed and a factor of safety of 1.3 was calculated.

4.2 FOUNDATIONS - BOARDWALK

The proposed bridge will be constructed from approximately station 43+05 to 47+65 over the fen restoration area. The proposed foundation system of helical piles will consist of the two abutments and 45 pier locations spaced 10 feet apart for a total length of 460 feet.

4.2.1 Bridge Foundation Selection

For ease of construction and to limit environmental impacts, the design team prefers to use helical piles for the boardwalk foundations. However, the presence of cobbles, boulders, and till soils below the softer fen soil may make installation difficult. Despite the possible difficulty of installation, we understand that other project considerations deem helical piles to be the preferred foundation alternative.

Other foundations that may result in easier construction include cased micropiles or pin piles.

The following preliminary boardwalk loads factored for the strength limit state have been provided by the bridge engineer:

Abutments = 7.8 kips per helical pile Pier = 10.9 kips per helical pile

4.2.2 Helical Pile Axial Resistance

Design analyses conducted using typical empirical methods based on an assumed SPT blowcount of 12 from boring BH-03 indicate that a nominal bearing pressure of approximately 16 kips per square foot (ksf) may be used for design. Although LRFD resistance factors for helical piles are not presented in the code, it is recommended that a resistance factor of 0.40 be used for design. This is based on the typical ASD factor of safety of 2.0 converted to a resistance factor of 0.50 and then reduced by 20 percent to account for the limited field exploration. Helix configurations (i.e., smaller diameter helixes with more flights) may result in more efficient installation. Installation torque measurements should be performed to verify pile capacity during construction in accordance with manufacturer recommendations. Note that installation torque may not correlate well to capacity where piles refuse on boulders.

Due to the presence of cobbles and boulders within the boardwalk area, it is likely that refusal will be encountered at some helical pile locations. The following procedure should be followed to meet the design requirements.

All piles should be installed to the minimum depth required by the approved helical pile design and also to the minimum depth required for lateral resistance presented on the bridge plans.

If refusal is encountered, the pile may be terminated at that depth provided it meets the minimum embedment depth required for lateral resistance presented on the plans and the helical pile designer approves of the early termination.

Where piles meet refusal prior to the minimum embedment depth required for lateral resistance, the holes should be pre-drilled such that the minimum embedment is achieved while also meeting axial requirements.

Offsetting the helical pile and re-attempting to install may be considered with the approval of the bridge engineer.

4.2.3 Helical Pile Lateral Resistance

Lateral load analysis will be performed by the bridge engineer using the software program Lpile developed by Ensoft, Inc. This program analyzes a single pile considering deflection as a function of design loads, foundation construction, and subsurface conditions. Table 4.4 provides preliminary Lpile input parameters for the foundation soils based on available subsurface information and generally accepted engineering correlations.

TABLE 4.4 - Lpile Input Parameters - Boardwalk

Depth (ft) P-Y Model Effective Unit Weight (pcf)

Friction Angle (deg)

Soil Modulus Constant (pci)

0 to 20 Sand (Reese) 65 34 125

4.3 ABUTMENT DESIGN

Abutments and wing walls should be designed to resist lateral earth pressures and other applicable lateral loads in accordance with AASHTO. Lateral earth pressure is influenced by the strength of the abutment backfill, the presence or absence of water, and the ability of the abutment or wall to move in response to lateral loads. Other loads, such as live loads, construction loads, and soil compaction loads should also be considered in the design.

Unbalanced water behind an abutment or wall adds significant lateral pressure and should be avoided by using free draining gravity outlets for water. Abutment and wing wall backfill should consist of structural backfill as specified in Section 704.04 of FP-14.

The coefficient of at-rest earth pressure should be used for design if the abutment is so restrained that it cannot be expected to rotate (deflect at the top) 0.002 times the wall height. Where deflection of the abutment can be expected, a coefficient of active earth pressure should be used for wall design. Active and at-rest lateral earth pressures of native materials, properly placed and compacted structural backfill, and unclassified borrow above the water table are presented in Table 4.5. The values are unfactored loads and assume that the surface of the soil slope behind the wall is horizontal.

TABLE 4.5 - Lateral Earth Pressures for Bridge Abutments

Backfill Type Assumed Backfill

Properties Case

Unfactored Equivalent Fluid Density (pcf)

Structural Backfill c = 0 psf = 34 deg.

γ = 125 pcf

Active 32

At-Rest 55

Unclassified Borrow c = 0 psf = 30 deg.

γ = 125 pcf

Active 37

At-Rest 63

Native Till Soil c = 0 psf = 38 deg γ = 130 pcf

Active 29

At-Rest 50

4.4 EMBANKMENT CONSTRUCTION

Construct permanent long-term embankments with a maximum slope ratio of 1V:2H to maintain slope stability and promote slope vegetation. The cut slopes planned at the south end of the alignment near the overpass bridge may be designed at 1:1.

On-site soils expected to be encountered within the project limits generally consist of alluvial sands and gravels with varying amounts of silt and cobbles/boulders. It is estimated that these soils will have a 10 percent shrink percentage, corresponding to a shrink/swell factor of 0.90. The recommended shrink/swell factor is based on a combination of standard tabled values for common materials in the FLH Technical Guidance Manual (2006) and experience with other CFLHD projects in similar materials.

4.5 ROCKERY WALLS

Using the data obtained from the subsurface investigation, the on-site soils were given presumptive material parameters. The rock for the rockery abutment is assumed to be imported during construction.

The material parameters used in design are presented in Table 4.6.

TABLE 4.6 - Presumptive Material Parameters for Design of Rockery Walls

Notes: (1) Cohesion of Rockery rock was artificially inflated to simulate assumed rigidity of rock face.

The rockery abutment design was checked for adequate factors of safety against sliding, overturning, and bearing capacity. The recommended and calculated factors of safety for static and seismic design are presented in Table 4.7 below:

TABLE 4.7 - Factors of Safety for Static and Seismic Design

Mode of Failure Static Factor of Safety Seismic Factor of Safety

Recommended Calculated Recommended Calculated

Sliding 1.5 2.2 1.1 1.8

Overturning 2.0 3.8 1.5 2.8 Bearing 3.0 8.4 2.0 6.7

Rockeries are earth retaining structures constructed of large interlocking, dry-stacked rocks typically placed without mortar or reinforcement. These structures rely on the weight, size, shape, and interface friction of the individual rocks to resist earth pressures and provide overall stability.

The rockery abutments were designed in accordance with the guidelines provided in the Central Federal Lands publication, “Rockery Design and Construction Guidelines”. The Rockeries were designed as gravity retaining walls and a surcharge load of 250 pounds per square foot to simulate low volume traffic loading. Static and seismic lateral earth pressures were applied to the back of the rockery;

however, passive earth pressure was neglected at the toe of the wall. Table 4.7 presents the recommended geometry and Figure 1 displays the relevant dimensions.

TABLE 4.8 - Recommended Geometry for Rockery Abutments

H Maximum Height (ft)

B Minimum Base

Width (ft)

D Embedment Depth at Toe

(ft) ß Backslope Angle

Wall Face Batter (V:H) φ Back Cut

Inclination (V:H)

9 5 3 0 4:1 8:1

Material Type Unit Weight, γ pcf.

Friction Angle, (deg) Cohesion, c (psf)

Existing Embankment (SM) 130 36 0

Wall Backfill (SM) 130 36 0

Rock for Standard Rockery Wall & Rockery Facing 150 40 500(1)

Figure 1: Schematic of Rockery and Notation for Recommended Geometry Layout

4.6 CONSTRUCTION CONSIDERATIONS

The contractor will be required to submit a drilled shaft construction plan as required in Section 565 of the FP-14 which includes the outlining of proposed methods to maintain borehole stability and manage the excavation of cobbles and boulders.

Groundwater was encountered in Boring BH-11 at approximate elevation 10,218 feet and should be similar to the stream elevation at the time of construction. Based on the groundwater conditions and the granular soils within the overburden alluvium, it is anticipated that temporary casing extended into bedrock will be needed to raveling and caving of the borehole. The contractor should be prepared for wet construction methods as groundwater can infiltrate even when temporary casing is used.

The contractor should be prepared to encounter cobbles and boulders along the entire project length. For the drilled shaft, the excavation of cobbles and boulders will likely result in difficult and slow drilling conditions. Modified single-helix augers and/or other specialized tools will likely be needed to extract boulders, or to core through large, solidly embedded boulders. The exact size of boulders could not be determined in the boreholes. The largest diameter estimated during drilling was approximately 3 feet, but based on the geologic conditions and observations at the ground surface near the stream, larger boulders should be expected.

4.7 SPECIFICATIONS

Special provisions were developed to be consistent with geotechnical recommendations stated above and should be incorporated into the special contract requirements (SCR) to amend the FHWA Standard

Specification for Construction of Roads and Bridges on Federal Highway Projects; known as FP-14.

4.8 LIMITATIONS

The subsurface explorations and tests described in the Subsurface Investigation Section on Procedures and Results have been conducted in accordance with standard practices and procedures (except as specifically noted). The results of these explorations and tests represent conditions at the specific locations indicated. Subsurface conditions between these locations may vary. The Analysis and Recommendations Section in this report include interpretations and recommendations developed by the Government in the process of preparing the design. These interpretations are not intended as a substitute for the personal investigation, independent interpretation, and judgment of the Contractor.

SECTION FIVE -References

AASHTO, 2017, Standard Specifications for Highway Bridges, Load and Resistance Factor Design, Section 10.

FHWA, 2008, Federal Lands Highway Project Development and Design Manual (PDDM).

FHWA, 2010, Drilled Shafts: Construction Procedures and LRFD Design Methods, Publication number

FHWA-NHI-10-016, May.

Federal Lands Highway, 2006, Technical Guidance Manual (TGM), A Geotechnical Supplement to the

FLH Project Development and Design Manual

Widmann, B.L., Bartos, P.J., McCalpin, J.P., and Jackson, J., 2004, Geologic Map of the Copper

Mountain Quadrangle, Summit, Eagle, Lake, and Park Counties, Colorado: Colorado. Geological Survey, scale 1:24,000

FIGURES-Vicinity, Geology, & Exploration Location Maps

APPENDIX A- Boring & Test Pit Logs

Use primary colors or hyphenated compound primary colors. Use "mottled" or "streaked" if necessary.

4. Color Dry Dry to touch, dusty Moist Damp but no visible water Wet Visible free water

5. Moisture Content

Flat Width/Thickness >3 Elongated Length/Width >3

Flat & Elongated Meets both of the above

Applies to sand, gravel, cobbles and boulders.

Length, width and thickness refer to the greatest, intermediate and least dimensions, respectively.

6c. Particle Shape

N Density N Consistency

Unconfined Compressive Strength qu

(tsf)

Undrained Compressive Strength su

(tsf)

Behavior

0-4 Very Loose 0-1 Very Soft <0.25 <0.125 Extrudes between fingers when squeezed

5-10 Loose 2-4 Soft 0.25-0.50 0.125-0.25 Remolded by light finger pressure

11-30 Medium Dense 5-8 Firm 0.50-1.00 0.25-0.50

Imprinted easily with fingers, remolded by strong finger pressure

31-50 Dense 9-15 Stiff 1.00-2.00 0.50-1.00 Imprinted with considerable finger pressure, indented by finger nail

>50 Very Dense 16-30 Very Stiff 2.00-4.00 1.00-2.00 Barely imprinted by fingers or indented by finger nail

>30 Hard >4.00 >2.00 Not imprinted by fingers or difficult to indent with finger nail

SAND & GRAVEL SILT AND CLAY

3. Consistency and Density:

Angular Sharp edges and relatively plane sides.

Subangular Same as angular with rounded edges.

Subrounded Nearly plane sides but well-rounded corners and edges.

Rounded Smooth curved sides and no edges.

Well-

Rounded Very Smooth surfaces, spherical or ovular, no edges.

6b. Particle Angularity Applies to coarse sand, gravel, cobbles and boulders.

Sieve < #200 Flour or smaller

Fine > #200 to #40 Flour to sugar Medium #40 to #10 Sugar to rock salt Course #10 to #4 Rock salt to pea-sized

Fine #4 to 3/4 in. Pea-sized to thumb Coarse 3/4 in. to 3 in. Tumb to fist Cobble 3 in. to 12 in. Fist to basketball

Boulders > 12 in. Larger than Basketball

Material

Gravel

6a. Particle Size

Silt or Clay

Sand

Particle Size Approximate Scale

Example, fine-grained soil: Lean CLAY with Sand (CL)- stiff, brown, moist, medium plasticity, laminated Example, coarse-grained soil: Silty SAND with Gravel (SM)-medium dense, gray, wet, fine to coarse grained, angular to subangular gravel, weakly cemented Example, fine-grained soil (Long Form):

Clayey GRAVEL with SAND (CL-ML)- loose to soft, dark brownish green to pale brownish gray, wet; fine to medium grained, angular, flat sand; coarse grained, rounded elongated gravel, some chert, trace coarse gravel, and cobbles, medium plasticity, dessicated, weak cementation, low dry strength, rapid dilatancy, moderate HCL reaction, hydrocarbon odor, iron oxide staining, alluvium fill, (Quaternary Alluvium), Additional Description.

1. Group Name (Pg. 3) 9.

2. Group Symbol (Pg. 3) 10.

3. Consistency / Relative Density (Pg. 1) 11.

4. Color (Pg. 1) 12.

5. Moisture (Pg. 1) 13.

6. Particle Size / Shape / Angularity (Pg. 1) 14.

7. Plasticity (Pg. 2) 15.

8. Structure (Pg. 2)

Odor (Pg. 2)

Use the following descriptive sequence when classifying soils, both in the field and when entering data into gINT:

Staining (Pg. 2)

Cementation (Pg. 2) Organics Dry Strength (Pg. 2) Dilatancy (Pg. 2) HCL Rxn (Pg. 2)

Weak Crumbles with little finger pressure Moderate Crumbles with considerable finger pressure Strong Will not crumble with finger pressure

Intact coarse-grained soil

9. Cementation

Nonplastic Thread cannot be rolled at any water content.

Low The thread can barely be rolled and the lump cannot be formed when drier than the plastic limit.

Medium The thread is easy to roll and not much time is required to reach the plastic limit. The thread cannot be rerolled after reaching the plastic limit. The lump crumbles when drier than the plastic limit.

High It takes considerable time kneading and rolling to reach the plastic limit. The thread can be rerolled several times after reaching the plastic limit. The lump can be formed without crumbling when drier than the plastic limit.

Toughness test: Shape the specimen into an elongated pat and rolled on a smooth surface or between the palms into a thread ~1/8”. If the sample is too wet to roll, it should be allowed to dry. Fold the thread and reroll repeatedly until the thread crumbles at a diameter of ~1/8”. This will be near the plastic limit. Note the pressure required to roll the thread near the plastic limit and the strength of the thread. After the thread crumbles, the pieces should be lumped together and kneaded until the lump crumbles. Note the toughness during kneading.

On the basis of observations made during the toughness test, describe plasticity.

7. Plasticity

None No visible reaction Weak Some reaction, bubbles forming slowly Strong Violent reaction, with bubbles forming immediately

13. HCL Reaction 14. Odor 15. Staining None None

Chemical Hyrodcarbon Hyrodcarbon Iron Oxide

Organic

None No visible change Slow Water appears slowly during shaking and does not disappear or disappears slowly during squeezing.

Rapid Water appears quickly during shaking and disappears quickly during squeezing.

12. Dilatancy

1. Mold soil, adding water if necessary, into ~1/2” diameter ball with soft but not sticky consistency.

2. Smooth in palm of one hand with knife blade. Shake horizontally, striking the side of the hand vigorously against the other hand several times. Note the reaction of water appearing on the surface.

4. After Dilatancy has been determined perform the Toughness test (see explanation in #7).

3. Squeeze by closing the hand or pinching the soil between fingers. The reaction is the speed with which water appears while shaking and disappears while squeezing.

None Crumbles into powder with mere pressure of handling.

Low Crumbles into powder with some finger pressure.

Medium Breaks into pieces with considerable finger pressure.

High Cannot be broken with finger pressure, will break between hard surface and thumb.

Very High Cannot be broken between hard surface and thumb.

1. Mold a ball ~1” diameter until it has the consistency of putty, adding water if necessary.

2. From the ball, make at least 3 ½” diameter balls. Allow to air dry.

3. If the specimen contains natural dry lumps, those that are ~1/2” diameter may be used in place of molded balls.

4. Test the strength of the dry balls or lumps by crushing between the fingers.

11. Dry…