Attachment 5 - Geotechnical Report 20211206.pdf

SUBMITTED TO:

DJ&A, P.C.

2000 Maple Street Missoula, Montana 59808

BY:

Shannon & Wilson 1321 Bannock Street, Ste 200 Denver, Colorado 80204

(303) 825-3800 www.shannonwilson.com

GEOTECHNICAL REPORT

USDA Forest Service, Region 2 Michigan River Bridge Replacement

JACKSON COUNTY, COLORADO

December 2021

Shannon & Wilson No: 105992-001

Michigan River Bridge Replacement

Geotechnical Report

105992-001 December 2021 ii

CO

NT

EN

TS

CONTENTS

1 Introduction

2 Site and Project Description

3 Field Explorations and Laboratory Testing

4 Regional Geology and Subsurface Conditions

4.1 Regional Geology

4.2 Subsurface Conditions

4.3 Groundwater

4.4 Subsurface Variation

5 Geologic Hazard Evaluation

5.1 Corrosion

5.2 Seismic Hazards

6 Geotechnical Recommendations

6.1 Design Ground Motion Parameters

6.2 Geosynthetic Reinforced Soil – Integrated Bridge System

6.2.1 GRS Backfill

6.2.2 Bearing Capacity

6.2.3 Lateral Resistance and Sliding

6.2.4 Settlement

6.3 Micropile Foundations

6.3.1 Micropile Design

6.3.2 Abutment Backfill

7 Construction Considerations

7.1 Dewatering

7.2 Subgrade Preparation

7.3 Fill Placement

7.4 Temporary Slopes

7.5 Micropile Installation

7.6 Construction Monitoring

8 Limitations iii

CO

NT

EN

9 References

Exhibits Exhibit 5-1: Soil and Rock Corrosion Testing Results Exhibit 6-1: Design Seismic Ground Motion Parameters for Michigan River Bridge Exhibit 6-2: Typical GRS-IBS cross section Exhibit 6-3: Recommended Soil Parameters for Abutment Wall Backfill

Figures Figure 1: Vicinity Map Figure 2: Site and Exploration Plan Figure 3: Geologic Map Figure 4: Recommended Surcharge Loading for Temporary and Permanent Walls

Appendices Appendix A: Subsurface Explorations Appendix B: Laboratory Test Results Appendix C: Drilling Photographs Important Information

Geotechnical Report

105992-001 December 2021

1 INTRODUCTION

This report presents the results of our subsurface exploration program and geotechnical engineering recommendations for the proposed bridge replacement over the South Fork of the Michigan River in Jackson County, Colorado. The report summarizes our subsurface explorations, laboratory testing, geotechnical engineering analyses, and presents conclusions and recommendations for design of bridge foundations. Our services were conducted in general accordance with DJ&A's Professional Services Agreement (PSA), dated March 16, 2021.

We understand this report will be used for design of the proposed Michigan River Bridge Replacement project. This report should not be used for other purposes without Shannon & Wilson’s review. Our scope of services included:

Coordinating a subsurface investigation program, which included observing and logging two geotechnical borings, clearing underground utilities, coordinating a drilling subcontractor, and measuring groundwater levels.

Completing geotechnical laboratory tests on selected soil samples and collecting a surface water sample from the Michigan River for analytical testing.

Performing geotechnical analyses for the proposed bridge foundations and associated structures.

Providing recommendations relevant to the geotechnical aspects of construction.

Preparing this geotechnical report.

The authorized scope of services was based on your objectives, budget, and report purpose.

The scope of our services did not include evaluating the presence of cultural resources at or around the site. The scope of our services also did not include any environmental assessment or evaluation regarding the presence or absence of wetlands or hazardous or toxic materials in the soil, groundwater, or air, on or below the site; or construction issues (e.g., disposal or restoration) related to these considerations. If a service is not specifically indicated in this report, do not assume it was performed.

2 SITE AND PROJECT DESCRIPTION

The site of the Michigan River Bridge replacement is located on National Forest System (NFS) Road 792.1 (Forest Route 792) on the Parks Ranger District of the Medicine Bow-Routt National Forest in Jackson County, Colorado (Figure 1). We understand the proposed

Geotechnical Report

105992-001 December 2021 replacement bridge will be a single-span structure over the South Fork of the Michigan River. At this location, the river flows approximately south-east to north-west. We understand that the existing bridge is a temporary single span single lane truss bridge that was installed in 2016 after a culvert at the crossing failed in 2015.

We evaluated several foundation alternatives with DJ&A. Based on the evaluation, we understand the replacement bridge will be supported on a Geosynthetic Reinforced Soil Integrated Bridge System (GRS-IBS) or micropiles. We understand the replacement bridge will be 53 feet long with a 16-foot clear width to accommodate access to timber sale units, snowmobile tours, fishing/hunting, and general forest use. The replacement bridge will also optimize the waterway opening and include bridge and approach railings.

3 FIELD EXPLORATIONS AND LABORATORY TESTING

Shannon & Wilson conducted a field exploration program to explore subsurface conditions at the project site. Two borings, designated SW-01 and SW-02, were drilled on July 23 and 24, 2021 at the locations shown on Figure 2. In addition to the subsurface exploration program, a surface water sample was collected from the Michigan River for analytical testing.

Appendix A describes the procedures used to complete the drilling and sampling used to complete the borings and presents the individual exploration logs. An explanation of the symbols and terminology used on the logs is included for reference.

Appendix B presents the results of geotechnical and environmental laboratory testing for select samples collected from the borings and Michigan River. The laboratory program included index and property testing for natural water content, Atterberg limits, grain size distribution, percent fines, and corrosion. Additionally, a surface water sample was collected from the Michigan River and tested for corrosion. The laboratory test results are presented in Appendix B along with a brief discussion of the laboratory testing procedures.

The natural water content, Atterberg limits, and percent fines are also indicated on the individual boring logs in Appendix A. Photographs of the drill rig setup are provided in Appendix C.

Geotechnical Report

105992-001 December 2021

4 REGIONAL GEOLOGY AND SUBSURFACE

CONDITIONS

We based our understanding of the geology and subsurface conditions at the project site on a review of regional geologic maps and the results of our field exploration program.

4.1 Regional Geology

Based on a review of geologic maps of the Gould Quadrangle (Kinney, 1970) and Craig Quadrangle (Tweto, 1976), the overburden consists of Quaternary-age surficial deposits of alluvium, younger terrace deposits consisting of outwash gravels, and glacial deposits consisting of poorly sorted unconsolidated sand, gravel, and granitic boulders up to 8 feet in diameter. Mapping indicates that overburden is underlain by Browns Park and North Park Formation bedrock consisting of claystone, siltstone, sandstone, limestone, conglomerate, and volcanic ash. Refer to Figure 3 for a geologic map at the project location.

4.2 Subsurface Conditions

Boring SW-01 and SW-02 were completed on the northeastern and southwestern sides of the Michigan River near the proposed bridge structure, respectively. Soil conditions consisted of approximately 11 to 12 feet of loose to very dense, clayey to silty sand (SC and SM) with occasional gravel, and soft to stiff, sandy lean clay (CL) with occasional gravel. The soils encountered in the upper 12 feet were characterized as fill likely imported during construction of the approach abutments for the original crossing. Underlying the fill, medium dense to very dense, well-graded gravel with silt, sand and cobbles (GW-GM) to well-graded sand with silt, gravel and cobbles (SW-SM) along with boulders were encountered above bedrock. Bedrock was encountered at depths of 41 and 42 feet and consisted of extremely weak, completely weathered North Park Formation claystone. The formation was characterized as stiff to hard, lean clay (CL) with varying percentages of sand and lignite nodules.

4.3 Groundwater

During drilling, groundwater was observed in both borings at elevations of 9,049.1 feet and 9,048.2 feet in boring SW-01 and SW-02, respectively. The groundwater level was observed during drilling and is noted on the individual boring logs included in Appendix A.

Groundwater fluctuations are likely and will depend on seasonal variations, local precipitation and runoff, and other factors, but will likely be influenced by water levels in the Michigan River.

Geotechnical Report

105992-001 December 2021

4.4 Subsurface Variation

Shannon & Wilson completed a subsurface exploration program consisting of two borings to evaluate geotechnical soil and bedrock conditions at the site. Our observations are specific to the locations, depths, and times noted on the logs and may not be applicable to all areas of the site. No amount of explorations or testing can precisely predict the characteristics, quality, or distribution of subsurface and site conditions. Potential variation includes, but is not limited to:

The conditions between explorations may be different.

The passage of time or intervening causes (natural and manmade) may result in changes to site and subsurface conditions.

Penetration test results in gravelly soils may be unrealistic. Actual soil density may be lower than estimated if the test was performed in gravel, cobbles, or boulders.

If conditions that are different from those described herein are encountered during construction, we should review our description of the subsurface conditions and reconsider our conclusions and recommendations.

5 GEOLOGIC HAZARD EVALUATION

5.1 Corrosion

The soil and rock encountered at the project site can be corrosive to substructure elements. To assist in estimating the corrosion potential at the site, selected samples were tested for pH, resistivity, water soluble sulfates, and chlorides. The results are presented in Exhibit 5-1 and in Table B-1 in Appendix B.

Exhibit 5-1: Soil and Rock Corrosion Testing Results

Boring Sample Material pH Resistivity (ohm-cm)

Water-Soluble Sulfates

Chlorides

SW-01 S-12 Bedrock 6.3 1204 <0.001 0.0002

SW-02 S-2 Fill 6.2 3486 <0.001 0.0002

NOTES:

Water-soluble sulfate and chloride values are shown as percent weight in dry soil.

-ohm-cm = ohm-centimeter

The resistivity measured in the samples ranged from 1,204 to 3,486 ohm-centimeters. Based on correlations developed by Roberge (2012), these values suggest highly corrosive to

Geotechnical Report

105992-001 December 2021 corrosive subsurface conditions for metal in contact with subsurface materials across the site.

The concentration of water-soluble sulfates measured in the samples was less than 0.001% by weight. Based on classifications as defined by ACI-318-21 (ACI, 2021), these test results suggest an exposure class S0 on concrete exposed to site soils.

The test results and the above discussion are provided to assist the designer in the selection of project materials, concrete type, or other features with respect to corrosion. As appropriate, the designer should consider protective measures, such as coatings, upsizing for section loss, or using alternative materials to reduce the corrosion potential.

A surface water sample was collected from the Michigan River and tested for corrosion.

The results of the testing are included in Appendix B.

5.2 Seismic Hazards

In general, the Colorado Rocky Mountains are characterized as an area of low potential for damaging earthquakes. It is not possible to accurately estimate the timing or location of future earthquakes as the occurrence of earthquakes is relatively infrequent and the historical earthquake record in Colorado is short (about 150 years).

Based on a recent geologic map by the U.S. Geological Survey (USGS, 2021), the nearest mapped fault to the proposed project is the Williams Fork Mountains fault which is located approximately 40 miles to the southwest (Widmann and Haller, 2015). The normal fault is approximately 24 miles long and generally dips to the northeast and southwest along the Williams Fork Mountain Range. The most recent prehistoric deformation is Quaternary in age and the fault currently exhibits less than 0.2 millimeters of movement per year. Based on the distance from the site to the Williams Fork Mountains fault and the age of the most recent movement, it is our opinion that the risk of fault rupture impacting the bridge is low.

6 GEOTECHNICAL RECOMMENDATIONS

As discussed in Section 2, we understand that the proposed bridge will be a single-span structure supported on GRS-IBS, or micropiles. Geotechnical design criteria for this project are based on the American Association of State Highway and Transportation Officials (AASHTO) Load and Resistance Factor Design (LRFD) Bridge Design Specifications (AASHTO, 2020) and applicable design standards as noted in the following sections.

Geotechnical Report

105992-001 December 2021

6.1 Design Ground Motion Parameters

Using the AASHTO LRFD Bridge Design Specifications (AASHTO, 2020) criteria, and based on subsurface conditions encountered in our borings (assuming that similar conditions are present from the maximum depth of our borings to a depth of 100 feet), Site Class D is recommended.

Ground motion parameters were determined for the Project site using the USGS AASHTO 2009 Web Service Documentation Web Application (USGS, 2009) and procedures recommended by AASHTO (2020). Exhibit 6-1 presents recommended seismic design ground motion parameters.

Exhibit 6-1: Design Seismic Ground Motion Parameters for Michigan River Bridge

Design Parameter Value

Peak Ground Acceleration (PGAB)1 0.072 g

Site Class D

Short-period Spectral Acceleration, SS 0.150 g

Long-period Spectral Acceleration, S1 0.038 g

Site Factor, FPGA 1.6

Site Factor, Fa 1.6

Site Factor, FV 2.4

Peak Design Spectral Acceleration, AS 0.114 g

Short-period Design Spectral Acceleration, SDS 0.239 g

Long-period Design Spectral Acceleration, SD1 0.090 g

TO 0.076 sec.

TS 0.378 sec.

NOTES:

PGAB = peak ground acceleration for a site underlain by Site Class B soil (soft rock) g = gravitational acceleration; sec. = seconds

6.2 Geosynthetic Reinforced Soil – Integrated Bridge System

The following recommendations are based on discussions with DJ&A that a geosynthetic reinforced soil GRS-IBS is an alternative to support the abutments for the bridge over the Michigan River. We followed the design guidance of the FHWA Design and Construction Guidelines for Geosynthetic Reinforced Soil Abutments and Integrated Bridge Systems (2018). We understand the internal stability analysis will be completed by DJ&A.

We based our design on a preliminary abutment configuration provided to us by DJ&A in an email dated August 17, 2021. We should be contacted if the abutment or bridge

Geotechnical Report

105992-001 December 2021 configurations change from what was provided to ensure the recommendations provided below remain applicable. Refer to Exhibit 6-2 for a typical GRS-IBS cross section.

Exhibit 6-2: Typical GRS-IBS cross section. From FHWA Design and Construction Guidelines for Geosynthetic Reinforced Soil Abutments and Integrated Bridge Systems (2018).

6.2.1 GRS Backfill

We recommend that backfill materials consist of an open-graded backfill in accordance with FHWA (2018) guidelines. The open-graded backfill generally consist of aggregate of one size to allow drainage and reduce the potential for water to accumulate in the backfill and induce hydrostatic pressures on the structure. The backfill should have a maximum aggregate size of 2-inches, less than 5% passing the #50 sieve and a friction angle of 38 degrees or greater. Utilizing such backfill will be critical due to the potential for the backfill to become saturated from fluctuations in the level of the adjacent river.

6.2.2 Bearing Capacity

Bearing capacity is governed by the strength and settlement characteristics of the underlying soil strata. We anticipate that the GRS abutments will include a reinforced soil

Geotechnical Report

105992-001 December 2021 foundation (RSF) as shown in Exhibit 6-2. The RSF should consist of GRS abutment fill encapsulated on all sides with a geotextile. The RSF should be constructed to bear on the medium dense gravels and sands at an estimated elevation of 9,048 feet or 3 feet below the maximum scour depth, whichever is deeper. The top of the RSF will serve as a leveling pad for the GRS abutment and facing elements.

We recommend a nominal bearing resistance of 10 kips per square foot (ksf) for the base of the RSF founded on the native medium dense gravels and sands. AASHTO (2020) recommends a resistance factor of 0.45 be applied to the nominal bearing resistance for the Strength Limit State. For the extreme event, a resistance factor of 1.0 should be used.

We recommend improving the subgrade in the zone 12 inches below the bottom of the RSF.

Improvement should consist of removing large cobbles or boulders from the soil matrix in this zone and replacing such materials with GRS backfill materials prior to construction of the RSF (see Section 7.2).

6.2.3 Lateral Resistance and Sliding

For structures founded on GRS, lateral loads are resisted by the frictional resistance between the base of the RSF and the underlying soil. We recommend neglecting the passive resistance from soils in front of the structure due to the proximity to the river and potential for scour.

Sliding resistance at the base of the GRS abutment will occur between the encapsulated RSF and the underlying soil. Sliding parameters and analyses should be evaluated considering the interface friction between the soil and reinforcement. We recommend a nominal friction factor of 0.4 for sliding between geosynthetic reinforcement and the native subgrade, in accordance with FHWA (2018).

6.2.4 Settlement

Due to the granular soils encountered in our borings, we anticipate most settlement will occur relatively quickly following construction of the bridge. We anticipate approximately 1 inch of settlement at the abutment locations. We anticipate that approximately 75% of this settlement will occur during construction, with the remaining 25% occurring over a period of one year at decreasing rate. We anticipate about one-half inch of differential settlement across the span of the bridge.

Geotechnical Report

105992-001 December 2021

6.3 Micropile Foundations

We understand that micropiles are being considered as a design alternative to support the abutments. Micropiles should be designed in accordance with the FHWA Micropile Design and Construction Manual (Sabatini and others, 2005) and AASHTO (2020).

6.3.1 Micropile Design

Additional design recommendations are provided below:

AASHTO (2020) specifies a minimum center-to-center spacing of 30 inches or 3 pile diameters, whichever is greater.

Micropiles should be designed to develop axial resistance in the overburden and/or bedrock. Axial resistance should be ignored along the cased section of the micropile.

Assuming a bond zone in gravel or claystone, the micropiles can be designed for a nominal bond strength of 2 kips per square foot (ksf) using Type A gravity grout methods. Higher bond strength could be obtained by using pressure grouting.

AASHTO (2020) recommends a strength limit resistance factor of 0.55, an extreme event resistance factor of 1.0 for compression and 0.80 for uplift, and a service limit resistance factor of 1.0 on the bond strength.

The contractor should complete verification testing on a sacrificial micropile to confirm the actual bond strength. Assuming load testing is completed, the resistance factor for compression at the strength limit state can be increased to 0.70. Additional considerations with respect to load testing are discussed in Section 7.5.

Depending on the results of the verification test, the bond length may need to be increased to provide adequate axial resistance. Conversely, the length of the bond zone could be decreased if testing indicates a bond strength that is greater than assumed.

We recommend a minimum bond length of 10 feet, regardless of the results of verification testing.

Micropiles may be battered to resist lateral loads.

For properly constructed micropiles, we estimate total settlement on the order of ½ inch and differential settlement of about one-half of the total settlement across the bridge span. These values exclude elastic compression of the micropile.

In accordance with Sabatini and others (2005), we recommend reducing the thickness of permanent casing by 0.063 inches for structural analyses to account for corrosion.

Additionally, we recommend the use of epoxy-coated or galvanized central reinforcing bars for corrosion protection.

Geotechnical Report

105992-001 December 2021

6.3.2 Abutment Backfill

To provide drainage and reduce potential pore pressures acting on the facing elements of the abutment walls or the back of pile caps, we recommend using Structural Backfill (FP-14 704.04) as backfill. Recommended design parameters for backfill are provided in Exhibit 6-

3. The provided active earth pressure coefficient assumes the wall is free to displace at least 1/1,000th the wall height (0.001H). If the walls are not free to displace this magnitude, at-rest earth pressures should be used for design. Surcharge loads on abutment walls can be calculated using Figure 4.

Exhibit 6-3: Recommended Soil Parameters for Abutment Wall Backfill

Material Parameter Design Value

Structural Backfill

Friction Angle, φ' 34 deg

Cohesion, c' 0 psf

Total Unit Weight, γ 135 pcf

At-Rest Earth Pressure Coefficient 0.44

Active Earth Pressure Coefficient 0.28

Nominal Passive Earth Pressure Coefficient 7.8

Resistance Factor for Passive Resistance 0.5

NOTES:

Parameters may be adjusted based on test results for material sources.

Earth pressure coefficients calculated assuming no wall friction (δ = 0 deg.)

Passive resistance should be ignored above the frost depth (3 feet).

deg = degrees; pcf = pounds per cubic foot; psf = pounds per square foot

The earth pressure parameters provided for the proposed retaining walls assume a free-draining backfill condition. As such, it will be important to control surface water and to provide drainage measures that reduce the potential for water to accumulate behind walls, particularly considering the proximity of abutment walls to Michigan River.

Surface water behind the wall should not be allowed to discharge directly into the wall backfill materials.

In general, materials with greater than about 3% fines content are not considered free draining. Backfill (FP-14 704.04) may have a maximum fines content of 15%, indicating the material may not be free draining. If the backfill is not free draining, a rapid drawdown condition could occur following flood events, i.e. water could be trapped in the backfill after flood waters recede. This condition could be addressed by designing the wall for appropriate water pressures or by including internal drainage features to reduce the

Geotechnical Report

105992-001 December 2021 potential for water to accumulate in the backfill. Appropriate drainage features could include:

Placement of a 12-inch thick drainage layer (CDOT Filter Material [CDOT, 2019]) on the back face of the wall, with a discharge system (e.g. weep holes or a perforated collector pipe at the base of the drainage layer, daylighting to a suitable discharge point).

Installation of geocomposite drainage boards on the back face of the wall, with a suitable discharge system (e.g. weep holes or a perforated collector pipe, daylighting to a suitable discharge point)

Limiting the fines content of the Structural Backfill to 3%.

7 CONSTRUCTION CONSIDERATIONS

The applicability of the design recommendations provided in this report is contingent on good construction practice. Poor construction techniques may alter conditions from those on which our recommendations are based and may result in reduced performance. Based on discussions with DJ&A, we understand the Federal Standard Specifications FP-14 (FHWA, 2014) will be used on this project. The following sections present additional construction and material considerations for this project.

7.1 Dewatering

Based on groundwater levels encountered in our explorations, the proposed excavation to construct the GRS abutments may extend below groundwater levels. The presence of groundwater may cause instability of open-cut excavations and subgrades. The contractor should consider these conditions in planning the work and the groundwater control measures.

The groundwater level should be drawn down prior to excavating and should be maintained in a dewatered state until the GRS abutment is constructed and backfilled. We anticipate that groundwater control can be accomplished using collection trenches coupled with trash pumps. In addition, a temporary diversion of the river may be required to accomplish dewatering needs.

Consistent with typical practice, the contractor will be responsible for control of surface water and groundwater. In this regard, slope protection, ditching, sumps, dewatering and other measures should be employed, as necessary, to direct water away from the excavations, to prevent ponding of water next to the work zone, and to permit proper completion of the work.

Geotechnical Report

105992-001 December 2021

7.2 Subgrade Preparation

All foundation subgrades should be stripped, scarified to a depth of 12 inches, moisture conditioned, and compacted in place to a dense and unyielding condition per the requirements described in Section 204 and 208 of FP-14 (2014). The compacted surface should be proof-rolled with a fully loaded, tandem-axle, 10-yard dump truck or equivalent. If it is not possible to perform a proof-roll, the subgrade should be probed and accepted by an experienced geotechnical engineer or materials testing technician. Areas that are identified as being loose, soft, or yielding during proof-rolling and/or probing should be removed, reconditioned and recompacted. Care should be taken during proof-rolling and subgrade preparation to avoid disturbing subgrade soils and supporting soils that will remain in place, as they can rut and pump under repeated construction traffic.

7.3 Fill Placement

All fill material should be placed in horizontal lifts and compacted to a dense and unyielding condition per the requirement described in Section 208. The thickness of loose lifts should not exceed 6 inches for heavy equipment compactors and 4 inches for hand-operated compactors, but may be less depending on the effort required to obtain the required relative compaction. Well graded granular soils should be moisture treated to within 2% of optimum moisture content and compacted to at least 95% of the maximum dry density per AASHTO T99 (standard compaction effort). Open-graded granular soils should be compacted to a firm and unyielding condition as evidenced by non-movement or no appreciable displacement beneath the compaction equipment.

The use of hand-operated compaction equipment is recommended within 3 feet of the wall face.

The onsite clayey fill soils encountered in the borings should not be used as backfill.

7.4 Temporary Slopes

We anticipate temporary excavations will be required to construct the project. Temporary, excavations should be sloped, as needed, to provide a safe, stable slope. Consistent with conventional construction practice, the Contractor should be responsible for temporary excavation slopes. The Contractor is continually at the site, is able to observe the nature and conditions of the subsurface materials encountered, and is responsible for the methods, sequence, and schedule of construction.

Geotechnical Report

105992-001 December 2021

For planning purposes only, we anticipate Type C soils will be encountered and 1.5H:1V (horizontal to vertical) slopes may be used. We recommend using the excavation criteria in OSHA 29 CFR, Part 1926, Subpart P, Excavations (1989).

7.5 Micropile Installation

Micropiles should be constructed in accordance with our recommendations, the guidelines in the Section 567 of FP-14 (FHWA, 2014), and the FHWA Reference Manual, “Micropile Design and Construction” (Sabatini and others, 2005). We recommend the project specifications be developed to encompass the following items related to micropile installation:

The drilling and grouting process should be continuous and completed expeditiously with minimal stoppage between the completion of drilling and placement of reinforcing bar(s) and grout. Individual micropiles should be required to be completed within one shift and under no circumstances should the excavation be allowed to be left open overnight.

Due to the proximity of the bridge to the river, water should be anticipated. If water is present in the excavation at the time of grout placement, the grout should be placed via tremie methods.

The micropile contractor selected for this project should be required to demonstrate their equipment and personnel have experience installing micropiles in similar subsurface conditions.

Due to anticipated presence of alluvial soils during installation and presence of water in the subsurface, the contractor should be prepared to encounter flowing conditions in the overburden. Permanent or temporary casing may be required.

The contractor should detail how they plan to address cobbles and boulders during installation.

7.6 Construction Monitoring

Geotechnical design recommendations are developed from a limited number of explorations and tests. We recommend that Shannon & Wilson be retained to monitor the geotechnical aspects of construction, particularly foundation installation. This monitoring would allow us to evaluate the subsurface conditions as they are exposed during construction and to determine that the work is accomplished in accordance with our recommendations and good construction practice.

Geotechnical Report

105992-001 December 2021

8 LIMITATIONS

This report was prepared for the exclusive use of DJ&A for use in design of the Michigan River Bridge Replacement project. It should be made available to prospective contractors and/or the Contractor for information on factual data only, and not as a warranty of subsurface conditions.

Our evaluations, analyses, conclusions, and recommendations are based on:

The limitations of our approved scope, schedule, and budget described in our Contract Agreement.

Our understanding of the project and information provided by DJ&A.

This report should not be used without our approval if any of the following occurs:

Conditions change due to natural forces or human activity under, at, or adjacent to the site.

Assumptions stated in this report have changed.

Project details change or new information becomes available such that our analyses, conclusions, and recommendations may be affected.

If the site ownership or land use has changed.

More than 5 years has passed since the date of this report.

If any of these occur, we should be retained to review the applicability of our analyses, conclusions, and recommendations.

Unanticipated soil conditions are commonly encountered and cannot be fully determined by a limited subsurface exploration and laboratory testing program. Such unexpected conditions frequently require that additional expenditures be made to attain a properly constructed project. Therefore, some contingency fund is recommended to accommodate such potential extra costs.

Within the limitations of scope, schedule, and budget, the analyses, conclusions, and recommendations presented in this report were prepared in accordance with generally accepted professional geotechnical and geological principles and practice in this area at the time this report was prepared. We make no other warranty, either express or implied.

Shannon & Wilson has prepared the attached document, "Important Information about Your Geotechnical Report," to assist you and others in understanding the use and limitations of our reports.

Geotechnical Report

105992-001 December 2021

9 REFERENCES

American Association of State Highway and Transportation Officials (AASHTO), 2020, AASHTO LRFD bridge design specifications, customary U.S. units, 9th edition:

Washington, D.C., American Association of State Highway and Transportation Officials.

American Concrete Institute (ACI), 2021, Building code requirements for structural concrete and commentary, Farmington Hills, Mich., ACI 318-21.

CDOT, 2019, Standard Specifications for Road and Bridge Construction; Available:

https://www.codot.gov/business/designsupport/cdot-construction-specifications/2019-construction-specifications/2019-specs-book/2019-standard-specifications

FHWA, 2018, Design and Construction Guidelines for Geosynthetic Reinforced Soil Abutments and Integrated Bridge Systems. Publication No. NHWA-HRT-17-080.

FHWA, 2014, Federal Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects, FP-14 English Units: U.S. Department of Transportation and Federal Highway Administration.

Kinney, D.M., 1970, Preliminary geologic map of the Gould quadrangle, North Park, Jackson County, Colorado, U.S. Geologic Survey, Open-File Report OF-70-182, 1:48,000.

OSHA 29 CFR, Part 1926, Subpart P, Excavations (1989).

Roberge, P.R., 2012, Handbook of corrosion engineering, Second Edition: McGraw-Hill, New York, New York.

Sabatini and others, 2005, Federal Highway Administration (FHWA) and National Highway Institute (NHI) Course No. 1320789, Micropile Design and Construction, Publication No. FHWA NHI-05-039, December.

Tweto, O., 1976, Geologic map of the Craig 1 degree x 2 degrees quadrangle, northwestern Colorado, U.S. Geologic Survey, Miscellaneous Investigations Series Map I-972, 1:250,000.

U.S. Geological Survey (USGS) AASHTO 2009 Web Service Documentation Web Application (USGS, 2009). Available from:

https://earthquake.usgs.gov/ws/designmaps/ https://www.codot.gov/business/designsupport/cdot-construction-specifications/2019-construction-specifications/2019-specs-book/2019-standard-specifications https://www.codot.gov/business/designsupport/cdot-construction-specifications/2019-construction-specifications/2019-specs-book/2019-standard-specifications https://www.codot.gov/business/designsupport/cdot-construction-specifications/2019-construction-specifications/2019-specs-book/2019-standard-specifications https://earthquake.usgs.gov/ws/designmaps/

Geotechnical Report

105992-001 December 2021

U.S. Geological Survey (USGS) Quaternary Fault and Fold Database of the United States, 2021. Available from: https://earthquake.usgs.gov/hazards/qfaults/

Widmann, B.L., and Haller, K.M., compilers, 2015, Fault Number 2301, Williams Fork Mountains fault, in Quaternary fault and fold database of the United States: U.S.

Geologic Survey website: https://earthquakes.usgs.gov/hazards/qfaults.

https://earthquake.usgs.gov/hazards/qfaults/

VICINITY MAP

FIG. 1

Geotechnical and Environmental Consultants

SHANNON & WILSON, INC.

Approximate Scale in Miles

0 6 12

December 2021 105992-001

F i l e n a m e

I E

F D

E N s

U

S F

S

M i c h i g a n

R

D r a f t i n g R

F

V i c i n i t y d w g

D a t e

L o g i n m l w

Denver

Project

Location

Colorado

Colorado

Springs

NOTE:

Map adapted from aerial imagery provided by

Google Earth Pro, reproduced by permission granted by Google Earth ™ Mapping Service.

Ft. Collins

USDA Forest Service, Region 2

Michigan River Bridge Replacement

Jackson County, Colorado

MT

BRIDGE

LOCATION

to Ft. Collins

Grand Lake

Walden

Gould

Rand to Estes Park to Kremmling

LEGEND

MT

Boring Designation and Approximate Location SW-01

Medicine Bow-Routt National Forest

0 100 200

Scale in Feet

SITE AND EXPLORATION PLAN

FIG. 2Geotechnical and Environmental Consultants

SHANNON & WILSON, INC.

December 2021 105992-001

Fi le na m e:

I:

\E F\

D

EN

\1

0s

\1

U

SF

S M ic hi ga n

R \D ra fti ng \1

2-

1_ R

1_ F2

Si te a nd E xp lo ra tio n.

dw at e:

7-

-2 og in : M

LW

USDA Forest Service, Region 2 Michigan River Bridge Replacement

Jackson County, Colorado

Map adapted from aerial imagery provided by Google Earth Pro, reproduced by permission granted by Google Earth ™ Mapping Service.

NOTE

Forest Route 740

Existing Temporary Bridge

South Fork Michigan River

SW-02

SW-01

Fo res t R ou te to Gould

GEOLOGIC MAP

FIG. 3

Geotechnical and Environmental Consultants

SHANNON & WILSON, INC.

USDA Forest Service, Region 2

Michigan River Bridge Replacement

Jackson County, Colorado

Qg - Young Gravels

Qgo - Old Gravels and Alluvium

Qd - Young Glacial Drift

Qdo - Old Glacial Drift

Ql - Landslide Deposits

Tnp - North Park Formation

Tv - Volcanic Rocks

Taf - Ash-flow Tuff

RELEVANT GEOLOGIC UNITS AND FEATURES

Approximate Scale in Miles

2 4

December 2021 105992-001

F i l e n a m e

I E

F D

E N s

U

S F

S

M i c h i g n

R

D r a f t i n g

R

F d t e o g i n m l w

Generated from Tweto, Ogden, 1979, Geologic Map of Colorado, U.S. Geologic Survey

BRIDGE

LOCATION

Qg

RECOMMENDED SURCHARGE

LOADING FOR TEMPORARY AND

PERMANENT WALLS

F i l e

I E

F

D

E N s

I

E x i t

I

C a s t l e

V a l l e y

R

D r a f t i n g R

F

S u r c h a r g e d t e

A u t h o r

M

L W

105992-001December 2021

USDA Forest Service, Region 2 Michigan River Bridge Replacement

Jackson County, Colorado

FIG. 4

For m ≤ 0.4: s H

= 0.28 (psf) (see Note 3)

W a l l

PLAN VIEW

2.0 q

1.5

D e p t h

F a c t o r

Z

B

1.0

0.0

0.5

W a l l

Z

Wall Line

UNIFORM SURCHARGE

a d

EARTH BERM

(see Note 4)

Note:

H z=nH

Bottom of

Excavation x = mH

Point Load in Pounds s H

(psf) s H

(psf)

ELEVATION VIEW

x = mH

H

Bottom of

Excavation

W a l l z=nH

Line Load in

Pounds/Foot

B) LATERAL PRESSURE DUE TO LINE LOAD

i.e. NARROW CONTINUOUS FOOTING

PARALLEL TO WALL

C) LATERAL PRESSURE DUE TO STRIP LOAD

R i g i d

W a l l

W a l l

W a l l

A) LATERAL PRESSURE DUE TO POINT LOAD

i.e. SMALL ISOLATED FOOTING OR WHEEL LOAD y

D) LATERAL PRESSURE DUE TO EARTH BERM

OR UNIFORM SURCHARGE

Bottom of

Excavation

Bottom of

Excavation

E) LATERAL PRESSURE DUE

TO ADJACENT FOOTING

(see Notes 5 and 6)

0 0.5 1.0

L

B

NOTES

1. Figures are not drawn to scale.

2. Applicable surcharge pressures should be added to appropriate permanent wall lateral earth and water pressure.

3. If point or line loads are close to the back of the wall such that m £ 0.4, it may be more appropriate to model the actual load distribution (i.e., Detail E) or use more rigorous analysis methods.

4. See text for recommended K values.

5. The stress is estimated on the back of the wall at the center of the length, L, of loading.

6. The estimated stress is based on a

Poisson's ratio of 0.5.

W a l l

Bearing

Pressure q (psf)

I p

, Influence Factor s H

= 2(I p

) q s

Lateral Footing

Pressure on Wall

(derived from NAVFAC DM 7.02, 1986; and Sandhu, Earth Pressure on Walls Due to Surcharge, 1974)

(derived from Poulos and Davis, Elastic Solutions for

Soil and Rock Mechanics, 1974; and Terzaghi and

Peck, Soil Mechanics in Engineering Practice, 1967)

(NAVFAC DM 7.02, 1986)

(AASHTO LRFD Bridge Design Specifications, 2020)

(NAVFAC DM 7.02, 1986)

Point Load in Pounds g = Unit Weight of Earth Berm

Bearing

Pressure

Earth

Berm s H

= Lateral Pressure in radians

ELEVATION VIEW

(see Note 3)

(see Note 3) q s

L

2B

Q p

H n

(0.16 + n

For m > 0.4: s H

= 1.77 (psf)

Q p

H m n

(m

+ n

Q p s H s'

H

= s H cos

(1.1q) (psf) s'

H s H

For m ≤ 0.4: s H

= 0.20 (psf) (see Note 3)

Q l

H n

(0.16 + n

For m > 0.4: s H

= 1.28 (psf)

Q l

H m n

(m

+ n

Q l

Q p y ≤ 33°

H s

≤ 15 Feet

(K)(g)(H s s H

H s s H

= (K)q s

(see Note 4) q s

(psf) s H

= [d - sin d cos (d + 2a)] (psf)

2q p

L

= 0.25

= 0.5

= 1 s H

L

2B

L

2B

2B

12/6/2021-105992-001_R1_AA.docx A-i

AP

PE

ND

IX

A : S

UB

SU

RF

AC

E

EX

PL

OR

AT

IO

NS

Appendix A: Subsurface Ex plorations

Appendix A

Subsurface Explorations

CONTENTS

A.1 Introduction ........................................................................................................................... A-1

A.2 Explorations ........................................................................................................................... A-1

A.2.1 Soil and Rock Classification System ...................................................................... A-1

A.2.2 Standard Penetration Test (SPT) ............................................................................ A-2

A.2.3 Pocket Penetrometer ................................................................................................ A-2

A.3 Surface Water Sampling ....................................................................................................... A-3

Figures Figure A-1: Soil Description and Log Key Figure A-2: Rock Classification and Log Key Figure A-3: Log of Boring SW-01 Figure A-4: Log of Boring SW-02

A-1

AP

PE

ND

IX

A : S

UB

SU

RF

AC

E

EX

PL

OR

AT

A.1 INTRODUCTION

The field exploration program for the Michigan River Bridge Replacement project was conducted on July 23 and 24, 2021. The program consisted of drilling two borings, designated as SW-01 and SW-02 at the locations shown on Figure 2. A representative from Shannon & Wilson observed the drilling and sampling, retrieved representative samples for laboratory testing, and prepared a descriptive field log of the borings. The methods used to conduct the field exploration program are described below.

A.2 EXPLORATIONS

The drilling was coordinated (including site access, subcontractor coordination and utility locates) and observed by our field representative. The individual boring logs are presented on Figures A-3 and A-4 and represent our interpretation of the subsurface conditions encountered at the time of drilling and the results of laboratory testing. The locations and elevations of the explorations were surveyed by DJ&A after drilling was completed.

Surveyed coordinates of the boring locations are indicated on the boring logs.

The borings were drilled by Vine Laboratories (Vine) of Commerce City, Colorado under subcontract to Shannon & Wilson using a CME-750X buggy drill rig. Boring SW-01 was initially advanced with an 8-inch outer diameter (4.25 inch inside diameter) hollow stem auger (HSA) to a depth of 23 feet. After encountering auger refusal at 23 feet, the crew offset two feet and used overburden drilling eccentric (ODEX) drilling methods to complete the boring through apparent dense cobbles and boulders. During drilling, a 5-inch diameter casing was advanced through the soil overburden and aided in preventing borehole collapse. After advancing the casing to sound rock, the drill string was advanced ahead allowing for sample retrieval in the rock. Boring SW-02 was completed with ODEX drilling techniques to the final depth.

When encountered, the Shannon & Wilson field representative measured the approximate depth to groundwater using an electronic water level indicator during and at the conclusion of drilling. The groundwater level measured during drilling is noted on the individual boring logs. At the conclusion of drilling, the borings were backfilled with drill cuttings.

A.2.1 Soil and Rock Classification System

During drilling, the Shannon & Wilson representative collected soil/bedrock samples and prepared a field log of each boring. Soil classifications were based on ASTM International

A-2

AP

PE

ND

IX

A : S

UB

SU

RF

AC

E

EX

PL

OR

AT

(ASTM) Designation: D2487, Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System), and ASTM Designation: D2488, Standard Practice for Description and Identification of Soils (Visual-Manual Procedure). The system is referred to as the Unified Soil Classification System and is summarized on Figure A-1.

The Shannon & Wilson representative classified rock samples in general accordance with the International Society of Rock Mechanics (IRSM) classification method. According to this system, rock is classified based on the stratigraphic structure, rock strength, degree of weathering, and other properties. The rock classification system is summarized on Figure A-2.

The bedrock encountered in the borings was generally found to be stiff to hard when considered as a lithified soil material. However, when compared to other types of bedrock using the ISRM classification of rock strength, the rock resembles an extremely weak rock.

Therefore, for completeness, the boring log contains dual descriptions of the bedrock using the USCS and rock classification system, where appropriate.

A.2.2 Standard Penetration Test (SPT)

Disturbed samples were obtained in the borings in general accordance with the Standard Penetration Test (SPT) (ASTM Designation: D1586). The SPT consists of driving a 2-inch outside diameter, 1.375-inch inside diameter split-spoon sampler a distance of 18 inches with a 140-pound hammer free-falling a distance of 30 inches. An automatic hammer system was used to advance the samplers. During sampling, the Shannon & Wilson field representative recorded the number of blows for each 6-inch increment of penetration and summed the blow counts for the last two 6-inch increments. This sum is recorded as the penetration resistance number, or N-value. If high penetration resistance prevented driving the entire 18 inches, the Shannon & Wilson field representative recorded the partial penetration depth and blow count. The N-values provide a means for evaluating the relative density or compactness of cohesionless (granular) soils and consistency or stiffness of cohesive (fine-grained) soils (see Figure A-1). The raw N-values are shown on the individual boring logs. Representative portions of the split-spoon sample obtained in conjunction with the SPT were placed in a screw-top plastic jar and transported to our laboratory.

A.2.3 Pocket Penetrometer

Select cohesive soil samples were also tested in the field using a pocket penetrometer. The penetrometer estimates the unconfined compressive strength of clay soil samples by penetrating the clay with a one-quarter-inch diameter cylindrical penetrometer and measuring the resistance (in units of tons per square foot [tsf]) with a calibrated spring.

A-3

AP

PE

ND

IX

A : S

UB

SU

RF

AC

E

EX

PL

OR

AT

Measurements can be taken to the nearest 0.25 tsf increment. The field measurements from the pocket penetrometer are listed on the boring logs.

A.3 SURFACE WATER SAMPLING

A water sample was collected from the South Fork Michigan River near the proposed bridge site on July 24, 2021. The sample was retrieved from the river channel by dipping a sample container into a portion of the flowing portion of the stream. After retrieving the water sample, a portion was transferred into a sample jar containing a preservative and sealed.

An additional sample was collected and placed in a sample jar without preservative. The samples were placed in a cooler with ice and transported to Origins Laboratory of Denver, Colorado for further testing (see Appendix B).

December 2021 105992-001

Michigan River Bridge Replacement

Jackson County, Colorado

1Gravel, sand, and fines estimated by mass. Other constituents, such as organics, cobbles, and boulders, estimated by volume.

2Reprinted, with permission, from ASTM D2488 - 09a Standard Practice for Description and Identification of Soils (Visual-Manual Procedure), copyright ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428.

A copy of the complete standard may be obtained from ASTM International, www.astm.org.

140 pounds with a 30-inch free fall.

Rope on 6- to 10-inch-diam. cathead 2-1/4 rope turns, > 100 rpm

NOTE: If automatic hammers are used, blow counts shown on boring logs should be adjusted to account for efficiency of hammer.

10 to 30 inches long Shoe I.D. = 1.375 inches Barrel I.D. = 1.5 inches Barrel O.D. = 2 inches

Sum blow counts for second and third 6-inch increments.

Refusal: 50 blows for 6 inches or less; 10 blows for 0 inches.

RELATIVE

CONSISTENCY

N, SPT,

BLOWS/FT.

5% to 12% fine-grained:

with Silt or with Clay 3

15% or more of a second coarse-grained constituent:

with Sand or with Gravel 5

< 5%

5 to 10%

15 to 25%

30 to 45%

50 to 100%

Surface Cement Seal

Asphalt or Cap

Slough

Inclinometer or Non-perforated Casing

Vibrating Wire Piezometer

N, SPT,

BLOWS/FT.

< 4 4 - 10

10 - 30 30 - 50

> 50

DESCRIPTION

< #200 (0.075 mm = 0.003 in.)

#200 to #40 (0.075 to 0.4 mm; 0.003 to 0.02 in.)

#40 to #10 (0.4 to 2 mm; 0.02 to 0.08 in.)

#10 to #4 (2 to 4.75 mm; 0.08 to 0.187 in.)

SIEVE NUMBER AND/OR APPROXIMATE SIZE

#4 to 3/4 in. (4.75 to 19 mm; 0.187 to 0.75 in.)

3/4 to 3 in. (19 to 76 mm)

3 to 12 in. (76 to 305 mm)

> 12 in. (305 mm)

Fine Coarse

Fine Medium Coarse

BOULDERS

COBBLES

GRAVEL

FINES

SAND

Sheet 1 of 3

CONSTITUENT2

SOIL DESCRIPTION

AND LOG KEY

SHANNON & WILSON, INC.

Geotechnical and Environmental Consultants

Absence of moisture, dusty, dry to the touch

Damp but no visible water

Visible free water, from below water table

FIG. A-1

Shannon & Wilson, Inc. (S&W), uses a soil identification system modified from the Unified Soil Classification System (USCS). Elements of the USCS and other definitions are provided on this and the following pages. Soil descriptions are based on visual-manual procedures (ASTM D2488) and laboratory testing procedures (ASTM D2487), if performed.

STANDARD PENETRATION TEST (SPT)

SPECIFICATIONS

Hammer:

Sampler:

N-Value:

Dry

Moist

Wet

MOISTURE CONTENT TERMS

Modifying (Secondary)

Precedes major constituent

Major

Minor Follows major constituent

1All percentages are by weight of total specimen passing a 3-inch sieve.

2The order of terms is: Modifying Major with Minor.

3Determined based on behavior.

4Determined based on which constituent comprises a larger percentage.

5Whichever is the lesser constituent.

COARSE-GRAINED

SOILS

(less than 50% fines)1

NOTE: Penetration resistances (N-values) shown on boring logs are as recorded in the field and have not been corrected for hammer efficiency, overburden, or other factors.

PARTICLE SIZE DEFINITIONS

RELATIVE DENSITY / CONSISTENCY

Sand or Gravel 4

30% or more coarse-grained:

Sandy or Gravelly 4

More than 12% fine-grained:

Silty or Clayey 3

15% to 30% coarse-grained:

with Sand or with Gravel 4

30% or more total coarse-grained and lesser coarse-grained constituent is 15% or more:

with Sand or with Gravel 5

Very soft Soft Medium stiff Stiff Very stiff Hard

Very loose Loose Medium dense Dense Very dense

RELATIVE

DENSITY

FINE-GRAINED SOILS

(50% or more fines)1

COHESIVE SOILS

< 2 2 - 4 4 - 8

8 - 15 15 - 30

> 30

COHESIONLESS SOILS

Silt, Lean Clay, Elastic Silt, or

Fat Clay 3

PERCENTAGES TERMS 1, 2

Trace

Few

Little

Some

Mostly

WELL AND BACKFILL SYMBOLS

Bentonite Cement Grout

Bentonite Grout

Bentonite Chips

Silica Sand

Perforated or Screened Casing

S&W INORGANIC SOIL CONSTITUENT DEFINITIONS

S O

IL

_C