39.45-H75 Geotech Report.pdf

FINAL REPORT

Geotechnical Investigation

Mesa County, Colorado

Rapid Creek Roadway and Drainage Improvements Palisade, Mesa County, Colorado

February 12, 2025

Prepared For:

River City Consultants Attn: Craig Rothluebber, P.E.

215 Pitkin Avenue, Unit 201 Grand Junction, CO 81501

GeoStrata Job No. 1812-002

Office – 2487 Industrial Boulevard #1, Grand Junction, Colorado 81505

Phone (801) 501-0583 | info@geostrata-llc.com

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TABLE OF CONTENTS

1.0 EXECUTIVE SUMMARY

2.0 INTRODUCTION

2.1 Purpose and Scope of Work

2.2 Project Description

2.3 Site Reconnaissance and Observations

3.0 METHOD OF STUDY

3.1 Field Investigation – Subsurface Conditions

3.2 Laboratory Testing and Subsurface Classifications and Conditions

3.2.1 Undocumented Fill – Gravel and Sand

3.2.2 Native Sand and Gravels

3.3 Subsurface Conditions

4.0 GEOLOGIC CONDITIONS

4.1 Geologic Setting

4.2 Seismicity and Faulting

5.0 ENGINEERING ANALYSIS AND RECOMMENDATIONS

5.1 Box Culvert Recommendations

5.2 Standard Construction Requirements

5.2.1 Earthwork

5.2.2 General Site Preparation

5.2.3 Excavations

5.2.4 Structural Fill and Compaction

5.2.5 Moisture Protection and Surface Drainage

5.3 Roadway Widening and Retaining Wall Discussion

5.4 Recommended Additional Investigations

6.0 CLOSURE

6.1 Limitations

6.2 Additional Services

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APPENDIX

Appendix A Plate A-1 ......................................... Site Vicinity Map

Plate A-2 ........................................ Exploration Location Map

Appendix B Plates B-1 to B-2 ............................. Borehole Logs

Plate B-3 ......................................... Key to Soil Symbols and Terms

Appendix C Plate C-1 ......................................... Lab Summary Table

Plate C-2 ......................................... Atterberg Limits Test Results

Plate C-3 ......................................... Grain Size Distribution Test Results

Appendix D Plates D-1 to D-13 .......................... Project Photographs

Appendix E Plates E-1 and E-2 ........................... Important Information About This Report

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1.0 EXECUTIVE SUMMARY

This report presents the results of our geotechnical engineering services performed for the Rapid

Creek Roadway Culvert Replacement Project in Palisade, Mesa County, Colorado.

Geotechnical engineering services included a site reconnaissance for preliminary field observations, a field study including subsurface drilling to retrieve samples of the subgrade for laboratory testing and engineering analysis for the replacement of the corrugated steel pipe with a concrete box culvert along Rapid Creek Road at the Rapid Creek overpass. It has been requested that we provide our opinion for potential widening and realignment of the existing Rapid Creek

Road. Roadway realignment would potentially straighten the existing sharp bends to accommodate truck traffic by shifting the road north and south of the overpass to the west. We understand a retaining wall may be required depending on the alignment and our findings.

Based on information gathered during our investigation, replacement of the existing corrugated metal culvert with either another corrugated metal culvert or concrete box culvert (CBC) appears feasible. Additionally, widening and realignment of the roadway with potential use of a retaining wall also appears feasible. At the time of this report, we understand the current design will not require a retaining wall for realignment. Preliminary recommendations are provided in case the proposed roadway realignment is changed.

This report presents our ‘FINAL’ summary for replacement of the existing corrugated metal culvert with either another corrugated metal culvert or CBC and optional preliminary retaining wall design criteria based from our field reconnaissance and adjacent study. Existing pavement evaluation and pavement section options and recommendations were not requested as part of the

Scope of Work for this project.

NOTICE: The scope of services provided within this report is limited to the assessment of the existing subsurface conditions for the proposed roadway improvement. In the event that existing conditions change, results and recommendations contained in this report may need to be modified. This executive summary is not intended to replace the report of which it is part and should not be used separately from the report. The executive summary is provided solely for purposes of overview. The executive summary omits a number of details, any one of which could be crucial to the proper application of this report.

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2.0 INTRODUCTION

2.1 Purpose and Scope of Work

This report presents the results of GeoStrata’s geotechnical engineering services for the foundation of a proposed box culvert to replace the existing corrugated metal pipe along Rapid Creek Road at the Rapid Creek overpass, Palisade, Mesa County, Colorado. A preliminary site observation for potential roadway widening and realignment is also included. Realignment would potentially straighten the existing sharp bends in the road to accommodate truck traffic by shifting the road north and south of the overpass to the west. We understand a retaining wall may be required depending on the alignment and our findings.

The scope of work completed for this study included a field site reconnaissance, subsurface exploration including limited borehole advancement, soil sampling, laboratory testing, engineering analyses and foundation recommendations, and preparation of this report. Our services were performed in accordance with the Mesa County Request for Qualification (RFQ-20-03148) dated

September 9, 2024 for MM-39.45-H.75 Rapid Creek Culvert Replacement, our proposal dated

October 17, 2024, and the executed authorization email received December 9, 2024 and contract

January 17, 2025. This report represents a ‘FINAL’ summary of services completed and recommendations based on the proposed construction, subsurface exploration, and site reconnaissance performed as part of our investigation and to fulfill the requests of the project team.

The recommendations contained in this report are subject to the limitations presented in the

"Limitations" section of this report (Section 7.1).

2.2 Project Description

Mesa County has proposed the replacement of the existing 10-foot diameter corrugated metal culvert with a concrete box culvert at the Rapid Creek Road overpass of Rapid Creek in Palisade, Mesa County, Colorado. We understand the existing roadway alignment causes turning difficulty for trucks and Mesa County has requested a re-alignment be evaluated to straighten the existing sharp bends in the road to accommodate truck traffic by shifting the road north and south of the overpass to the west. This was made known to us after our cost estimate and only a preliminary discussion is provided herein based on our site observations as a subsurface study for this scope was not performed. This study was performed in accordance with improvements known to us at this time. Geotechnical engineering services included a site reconnaissance for preliminary field

© 2025 GeoStrata 3 R1812-002 FINAL REPORT observations, a field study including subsurface drilling to retrieve samples of the subgrade for laboratory testing and engineering analysis of replacing the corrugated metal pipe. A test pit was hand dug to retrieve a bucket of material for scour analysis. An additional study may be required to confirm our assumptions regarding construction associated with roadway realignment and widening if a retaining wall is required.

The project site is located at MM-39.45-H.75 of Rapid Creek Road, approximately 0.7 miles north from its intersection with the Interstate 70 Frontage Road shown on Plate A-1, Appendix A. North of the overpass Rapid Creek Road narrows as it turns from the north to the west to a one lane road cut into steep hillsides to the north which steeply drop to the south down to Rapid Creek as shown in project site photos, Plates D-1 through D-13, Appendix D. Interstate 70 is located approximately

0.16 miles to the west and the Colorado River is approximately 0.2 miles to the west. From our site reconnaissance, our field study was adjusted to two locations, which were the only accessible locations for drilling without roadway closures. Drilled locations and the proposed roadway widening area are shown on Plate A-2 of Appendix A.

This report provides foundation recommendations for the proposed box culvert based on our field information at this time. Additional discussion of roadway widening is provided based on our observations and experience with similar projects, however subsurface drilling was not available due to access limitations which would require roadway closures to perform. Further study may be required to confirm retaining wall feasibility and our assumptions.

2.3 Site Reconnaissance and Observations

We understand our scope of service was to provide geotechnical recommendations for the construction of a box culvert overpass at the Rapid Creek Road crossing of Rapid Creek.

Photographs of the project site are found in Appendix D. The existing project site has a two-lane gravel roadway with a ‘staging’ area to the south for driveway access which narrows to a one lane road at the crossing and as the road continues to the north and east. After crossing Rapid Creek, the road turns from the north to the east as a one lane road cut into steep hillsides to the north and steeply dropping shoulder and hillside to the south down to Rapid Creek. The narrow roadway appears to be difficult to navigate for utility trucks where southbound trucks appear to utilize the shoulder to properly angle the turn at the overpass.

The steep hillside to the north and valley bottom at the project site consists of gravels, cobbles and boulders as shown on Plates in Appendix D. The gravel road appears to be in good condition and

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Creek runs from the east down to the west toward the Colorado River and crosses under Rapid

Creek Road through a 10-foot diameter round corrugated pipe shown on Plates D-3, D-4 and D-6.

Approximately 3 inches of water was observed running through the pipe during our site visit. The existing culvert appears to be in good working condition with some observed corrosion, see Plate

D-6. Due to site access limitations, two (2) locations were selected by GeoStrata, one northwest and one southwest of the existing overpass, for a field investigation to evaluate existing subsurface information. Due to roadway narrowing further study locations to the north and west may be required during a road closure to complete a proposed retaining wall design. Prior to our field study, utility locates were performed and marked.

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3.0 METHOD OF STUDY

3.1 Field Investigation – Subsurface Conditions

It is our understanding that construction will match existing roadway pavement section conditions and that a pavement design and recommended pavement sections are not required. Prior to our field investigation utilities were located using Colorado 811 UNCC notification center.

As a part of this investigation, subsurface soil conditions were explored by completing two investigatory boreholes within areas of proposed construction. Location BH-01 was drilled southwest of the overpass in a shoulder ‘staging’ area for existing driveway access and was not drilled in the existing roadway. Location BH-02 was drilled in the shoulder northwest of the existing overpass to allow traffic to utilize the roadway. Due to subsurface cobbles and boulders, several drill attempts were made at each location which terminated in auger refusal when subsurface penetration ceased. The boreholes were advanced by Colorado Drilling and Sampling and HRL Compliance to an approximate depth between 4½ and 5½ feet below surface grades. All exploration locations were in shoulder areas of Rapid Creek Road within proposed construction and were completed on December 30, 2024. Our excavation encountered fill and native granular material below surface grades as explained in sections below. A pit was hand dug at the streambed at the northwest area of the overpass where a bucket sample of material was excavated and laboratory tested for scour evaluation by the design team. Bedrock was not encountered. The approximate locations of the boreholes and test pit are shown on Plate A-2, (Exploration Location

Maps) in Appendix A, with photos showing our exploration found in Appendix D, Plates D-1 through D-13.

Subsurface soil conditions encountered in borehole explorations were logged at the time of our investigation by GeoStrata staff and are presented on the enclosed logs, Plates B-1 and B-2 in

Appendix B. A key to these logs can be found on Plate B-3 (Soil Symbols Description Key). The boreholes were advanced using a Simco 2800 truck-mounted drill rig with 4-inch solid stem continuous flight auger. Drilling was advanced to pre-determined depths for sampling. At selected intervals, a modified California sampler with a 2-inch interior diameter (ID) and 2.5-inch outside diameter (OD), or standard split spoon sampler with a 1 3/8-inch ID and 2-inch OD was driven into subsurface materials to record blow counts and obtain samples. The sampler was seated at the bottom of the borehole, then advanced by a 140-pound hydraulic automatic, or “auto”, hammer falling a distance of 30 inches. The number of blows required to drive the sampler two 6-inch

© 2025 GeoStrata 6 R1812-002 FINAL REPORT intervals or a fraction thereof, constituted the N-value. The N-value, when properly evaluated, is an index of the consistency or relative density of the material tested. Drill cuttings or bulk samples were not available for collection or testing due to the large size of rock in the subsurface and surrounding hillsides. Limited samples obtained during the field explorations were examined by

GeoStrata staff and representative samples were submitted for laboratory testing to evaluate the engineering characteristics of the materials encountered. Borehole logs and legend are presented in Appendix B.

The soils were classified according to the Unified Soil Classification System (USCS) by a staff geotechnical engineer and geologist. Classifications for the individual soil units encountered are shown on the attached Borehole Logs (Plates B-1 to B-2).

3.2 Laboratory Testing and Subsurface Classifications and Conditions

Geotechnical laboratory tests were conducted on soil samples obtained during our field investigation. The laboratory testing program was designed to evaluate the engineering characteristics of on-site earth materials for the limited samples retrieved. Laboratory tests conducted during this investigation include:

- Sieve Analysis of Fine and Coarse Aggregates (ASTM C136) and Hydrometer (ASTM D7928)

- Liquid Limit, Plastic Limit, and Plasticity Index of Soils (ASTM D4318)

The results of the laboratory testing are presented in Appendix C attached to the end of this report

(Plates C-1 to C-3).

According to our observations and laboratory testing, subsurface conditions generally consisted of approximately 12 inches of fill over sand, gravels, cobbles and boulders to the depth explored.

Boulders observed on the hillsides adjacent to our drilling were measured up to 7 foot by 5 foot by 4 foot as shown on Plate D-13, Appendix D.

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3.2.1 Undocumented Fill – Gravel and Sand

One sample of granular fill material tested had 26 percent fines (material passing the No. 200 sieve). Atterberg limits testing indicated a liquid limit of 29 and a plasticity index of 15. The fill material is classified as GC by the Unified Soil Classification System (USCS).

3.2.2 Native Sand and Gravels

Four native sand and gravel samples tested had 12 and 50 percent fines with one of the samples indicating 7.3 percent silt based off hydrometer test results. Three samples tested for Atterberg limits indicated liquid limits of between non-plastic and 31 and plasticity indices of between non-plastic and 13. The native sand and gravel classified as SM, SC, SP-SC, and GM by the USCS.

3.3 Subsurface Conditions

As previously discussed, the subsurface soil conditions were explored at the site by advancing test boreholes to depths between 4½ and 5½ feet below existing site grades and excavation of a pit at the stream level. The soils encountered were visually classified and logged during our field investigation and are included on the borehole logs (Plates B-1 to B-2). A Key to Soil Symbols and

Terminology is presented on Plate B-3. The subsurface conditions are mapped on the Palisade

Quadrangle 1:24,000 scale Geologic Map (Carrara, P.E., 2000) as consisting of near-surface exposures of Cretaceous-aged Mesaverde Group (Kmv) bedrock. The soil observed in our explorations largely consisted of mixed fill material over alluvium silts and clays overlying sands and gravels over bedrock.

3.3.1 General Soils Summary

Based on our observations and geologic literature review, roadway conditions consist of a layer of asphalt over base course and undocumented fill material. Native surficial deposits of basalt cobbles and boulders in a clayey to very clayey sand matrix overlie possible sandstone bedrock as was observed in roadway cuts and nearby outcroppings.

The overburden GRAVEL with sand and clay (GM-GC), poorly graded GRAVEL with sand (GP), poorly graded GRAVEL with silt and sand (GP-GM), SAND with silt and clay (SM-SC) and poorly graded SAND with clay (SP-SC) (based on field observation and laboratory testing) consisted of several cobbles and boulders of large size measured up to 7 foot by 5 foot by 4 foot which may create difficulty during construction as boulders may need to be removed and the divot from removal filled to create a flat and firm working surface.

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3.3.2 Groundwater

Groundwater was not encountered in the boreholes drilled at the site due to shallow refusal on basalt cobbles and boulders. We anticipate groundwater to be at the existing creek bed level near approximately 12 feet below existing road grade. It is likely that bedrock below the creek creates perched water conditions down the valley bottom.

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4.0 GEOLOGIC CONDITIONS

4.1 Geologic Setting

The project site is located in the foothills to the extreme eastern end of the Grand Valley and east of the Colorado River floodplain in west-central Colorado. The Grand Valley is marked by De

Beque Canyon to the east and the Uncompahgre Plateau to the west and represents a sediment-filled erosional valley located on the southern side of the Book Cliffs in westernmost Colorado.

Water from the Grand Mesa (and beyond) to the east of carried west across the Grand Valley and into the Colorado National Monument to the west by the Colorado River and to a lesser degree, the Gunnison River. The elevation of the Grand Mesa and Colorado National Monument relative to the elevation of the Grand Valley has caused the impedance of the flow of the Colorado River, resulting in a low stream gradient and causing the river to meander. Lateral planation of the valley by the meandering river eroded the softer Cretaceous Mancos shale bedrock, resulting in the widening evolution of the Grand Valley. The same impedance has likewise led to the deposition of large quantities of quaternary fluvial sediments that now fill much of the Grand Valley (Scott and others, 2002).

The near-surface geology of the Grand Valley (and nearby geologically similar valleys) is dominated by sediments which were deposited approximately within the last 15,000 years. These sediments are predominately colluvial, alluvial, and eolian in origin. The Gunnison River combines with the Colorado River near the central portion of the Grand Valley, with both rivers depositing significant amounts of sediment as part of the erosional process. Deltaic sediments were transported and deposited into the Grand Valley and consist primarily of clay, silt, and sand.

Sediments at the subject property are mapped as consisting of near-surface exposures of

Cretaceous-aged Mesaverde Group (mapped symbol Kmv) overlain by fluvial and debris flow deposits associated with Rapid Creek (mapped symbol Qdu). These bedrock deposits are described by Caraara (2001) as consisting of, “The rocks of Mesaverde age, which overlie the Mancos shale, are from 1,000 to 1,200 feet thick in the vicinity of Grand Junction. They are described as composed of a massive sandstone at the base, called the “Lower Escarpment”, a middle coal-bearing group, and a massive sandstone at the top, called the “Upper Escarpment”. The upper part is buff, sandy shale and thinly laminated, buff sandstone; lower part is massive, light-buff, ledge-forming sandstone containing thin lenticular coal beds.”

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As per the mapping completed by Caraara (2000) for the subject site, the near-surface earth materials may be corrosive to buried metal and concrete. Soil corrosivity test results for soils sampled at the project site can be found in Appendix C of this report.

4.2 Seismicity and Faulting

The site lies within the north-south trending belt of seismicity known as the Rio Grande Rift, which cuts north/south across the mountainous, central part of Colorado. Colorado’s high mountains are a result of uplift on faults (with associated earthquakes) that are part of the rift system, which features over 90 potentially active faults and more than 700 recorded earthquakes of magnitude

2.5 or higher since 1867 (Widmann et al, 2019). An active fault is defined as a fault that has had activity within the Holocene (<11ka). No active faults are mapped through or immediately adjacent to the roadway alignment (Haller et al, 2014). The site is located approximately 51 miles to the north of the nearest mapped portion of the Roubideau Creek fault. The Roubideau Creek fault is located on the east flank of the Uncompahgre Uplift, where it is marked by a northeast facing 80-meter-high scarp. Additional studies have revealed Holocene-aged landslide deposits that have been offset along this fault near Roubideau Creek. The site is also located approximately 114½ miles southwest of the nearest mapped portion of the Gore Range frontal fault. The Gore Range frontal fault bounds the eastern margin of the Gore Range and is also known as the Blue River fault. Evidence for quaternary surface rupture is incomplete because much of the fault is under heavy forest cover and is generally inaccessible except on foot. LiDAR data has revealed evidence of faulting at a number of locations resulting in the current interpretation that the fault has experienced multiple episodes of late Quaternary movement. Each of the faults listed above show evidence of Holocene-aged movement and are therefore considered active.

Seismic hazard maps depicting probabilistic ground motions and spectral response have been developed for the United States by the U.S. Geological Survey as part of NEHRP/NSHMP

(Frankel et al, 1996). These maps have been incorporated into both NEHRP Recommended

Provisions for Seismic Regulations for New Buildings and Other Structures (FEMA, 1997) and the International Building Code (IBC) (International Code Council, 2018). Spectral responses for the Maximum Considered Earthquake (MCER) are shown in the table below. These values generally correspond to a two percent probability of exceedance in 50 years (2PE50) for a “firm rock” site. To account for site effects, site coefficients which vary with the magnitude of spectral acceleration are used. Based on our field exploration and geologic information about the site, it is our opinion that this location is best described as a Site Class C, due to the bedrock encountered at relatively shallow depths. The spectral accelerations are shown in the table below. The spectral

© 2025 GeoStrata 11 R1812-002 FINAL REPORT accelerations are calculated based on the site’s approximate latitude and longitude of 38.9905° and

-108.4711° respectively.

Description Value

Site Class Ct

Ss - MCER ground motion (period – 0.2s) 0.240

S1 - MCER ground motion (period – 1.0s) 0.065

Fa - Site amplification factor at 1.0s 1.600

Fv - Site amplification factor at 1.0s 2.400

PGA - MCEG peak ground acceleration 0.133

PGAM – Site modified peak ground acceleration 0.204

It should be noted that our investigation did not include a site-specific ground motion hazard analysis and a Site Class C has been used to determine the seismic parameters presented above.

The seismic parameters presented herein may be used for design of the proposed structures provided that structural design allows for the ground motion hazard analysis exception in ASCE

7-16 Section 11.4.8. Alternatively, GeoStrata may be contacted to complete a ground motion hazard analysis in accordance with ASCE 7-16 Chapter 21.

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5.0 ENGINEERING ANALYSIS AND RECOMMENDATIONS

The recommendations presented herein are governed by the physical properties of the materials encountered in the field exploration and our understanding of the project as discussed in previous sections of this report.

5.1 Box Culvert Recommendations

The existing crossing of Rapid Creek Road over Rapid Creek consists of a 10-foot diameter corrugated steel pipe estimated to be bearing in native gravel material. Sandstone bedrock may be encountered at excavation during construction depending on depth. Approximately 3 inches of water was running through the base of the pipe at the time of our site visit which was approximately

12½ feet below top of roadway grade. The culvert appeared in good working condition with the exception of areas of corrosion observed in the pipe. It has been proposed that the existing pipe be removed and recommendations for replacement with a box culvert be provided.

Two boreholes were drilled in the Rapid Creek Road approach shoulder to the northwest and southwest of the existing culvert at the Rapid Creek crossing. In general, the subsurface conditions at the borehole locations consisted of approximately 1 foot of undocumented sand and gravel fill over native sand with cobbles and boulders with a clayey sand matrix material. Poorly graded

GRAVEL with silt and sand was encountered at approximately 12 feet below grade from a pit excavated at the creek level. Scour concerns should be evaluated for the box culvert. The existing culvert appears to be bearing on native gravel material and performing in good working condition with areas of corrosion observed as previously discussed and shown in Plate D-6 of Appendix D.

One sample of granular fill material tested had 26 percent fines (material passing the No. 200 sieve). Atterberg limits testing indicated a liquid limit of 29 and a plasticity index of 15. The fill material classified as GC by the Unified Soil Classification System (USCS).

Four native sand and gravel samples tested had 12 and 50 percent fines with one of the samples indicating 7.3 percent silt based off hydrometer test results. Three samples tested for Atterberg limits indicated liquid limits of between non-plastic and 31 and plasticity indices of between non-plastic and 13. The native sand and gravel classified as SM, SC, SP-SC, and GM by the USCS.

The concentrations of water-soluble sulfates measured in the laboratory from a sample of

GRAVEL with silt at 12 feet in TP-01 were 51 ppm. Based on the American Concrete Institute

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(ACI) Building Code, these concentrations represent a “Severe” degree of sulfate attack on concrete structures. Sulfate resistant concrete is recommended when concrete elements are in contact with the onsite soils. The pH, electrical resistivity and water-soluble chloride were also determined for the same sample. Test results measured a pH value of 7.82, a resistivity measurement of 2,000 ohm-centimeters, and water-soluble chlorides were not detected at the corresponding Minimum Reporting Limit. We anticipate in-situ soil to be highly corrosive and appropriate levels of corrosion protection be utilized.

Culvert Pipe Replacement. The existing culvert appears operational although some corrosion damage to the pipe was observed. In our opinion, the existing pipe may remain operational for up to 10 years before the effects of corrosion become damaging to the pipe. We anticipate replacing the existing pipe with a new pipe of similar dimensions is feasible. Based on observed pipe corrosion and laboratory testing, we recommend the corrugated steel replacement pipe be protected against corrosion.

Replacement with Box Culvert. The replacement of the existing corrugated steel pipe with a box culvert structure appears feasible. We anticipate the box culvert to bear below the existing creek bed in native granular materials. We anticipate a bearing capacity of 3,500 psf for the box culvert structure bearing on native granular material be used for design. Additional factors used in design are as follows:

1. Shallow foundations for CBC and wing wall strip footings with a minimum width of 3 feet may be designed using a nominal (unfactored) bearing resistance of 7.0 ksf. The bearing resistance factor for shallow foundations is 0.50 in accordance with AASHTO LRFD

(2017) Table 10.5.5.2.2-1.

2. Wingwall foundations should bear below frost depth, at least 24 inches below nearest adjacent finished grade, or deeper depending on scour requirements.

3. Fill material should be placed in accordance with recommendations in Section 6.2.5

Structural Fill Placement and Compaction of this report.

4. Backfill against structure foundations should consist of granular material such as CDOT

Class 1 Structure Backfill and compacted as discussed in Section 6.2.5 Structural Fill

Placement and Compaction.

5. An unfactored coefficient of friction of 0.60 may be used for the calculation of sliding resistance when performing an external stability check in accordance with AASHTO

(2020) Section 10.6.3.4. The recommended sliding resistance factor is 0.80 for shallow foundations per AASHTO (2020) Table 10.5.5.2.2-1.

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6. Global stability of the culvert wing walls can be performed when more detailed design information becomes available.

7. Total settlement is estimated to be 1 inch or less when constructed as discussed above.

Differential settlement, as measured along a horizontal distance of 50 feet, is estimated to be ½ to ¾ of the total settlement.

8. Design of the CBC structure should incorporate a toe wall or turned-down edges at the ends of the structure to mitigate underseepage and potential soil piping within foundation and fill section materials. It is recommended that the cutoff extend to the base of the imported aggregate layer at a minimum.

9. Contractor should plan for dewatering activities during construction at this location and may choose to schedule foundation work during winter months when groundwater tables are expected to be lowest. Methods to divert, pump and dispose of this water, including permits, should be finalized prior to the initiation of construction.

Approach Settlement. Subsurface soils adjacent to the culvert exhibited very dense conditions and auger refusal. We recommend that subsurface soil in approach areas be removed and replaced to match existing conditions. Structural fill should be properly processed and compacted in accordance with our recommendations in Section 5.2.4.

5.2 Standard Construction Requirements

Our above recommendations are based on subgrade materials, our experience, and design criteria obtained from Section 8.06 of the Mesa County Design Standards. The recommendations given in this section are the necessary requirements to satisfy our pavement designs for the roadway segments outlined in the preceding section. Where the County requirements exceed our requirements, those of the County should be followed. Additionally, contractors selected to perform the pavement reconstruction should review Title 29 of the Design Standards for specific

City construction requirements.

5.2.1 Earthwork

General site grading is recommended to provide proper support for construction of new asphalt pavement sections. Roadway grading is also recommended to provide proper drainage and moisture control. If proper grading is not achieved and surficial moisture is allowed to penetrate below the pavement section and saturate base course, granular borrow or native subgrade, a shortened pavement design life can be expected.

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5.2.2 General Site Preparation

Within the areas of the roadway to be reconstructed, the existing asphalt should be milled off. If at this time the existing base course is to be considered for use in the reconstructed pavement section, further investigation must be performed in order to assess the characteristics of the existing base course. This includes performing gradation size distribution (ASTM C136), Atterberg Limit

(ASTM D4318), and California Bearing Ratio (ASTM D1883) testing. Should leaving the material in place be considered, further verification of base course thickness by pot holing throughout the segment length should be performed. If the existing base course is not suitable for the new roadway segment, it should be removed. Following the removal of base course, grading may be conducted to bring the site to grade. Undocumented fill, debris, vegetation, roots, loose, soft or other deleterious materials (if encountered) should also be removed and replaced with granular borrow meeting the recommendations made in the previous sections of this report. The exposed subgrade should be proof-rolled with heavy equipment to identify loose, soft or otherwise disturbed soils.

We recommend that a GeoStrata representative be on-site during all excavations to assess the exposed subgrade soils. If soft soils are observed, they should be stabilized in accordance with our recommendations in Section 5.2.4 Structural Fill and Compaction Section.

5.2.3 Excavations

It is unknown if utilities are to be placed in the new roadways. The following paragraphs are provided in case such work is required. All anticipated utility work should be completed prior to reconstruction of the roadways.

Based on Occupational Safety and Health Administration (OSHA) guidelines for excavation safety, trenches with vertical walls up to 5 feet in depth may be occupied, however, the presence of fill soils, loose soils, or wet soils may require that the walls be flattened to maintain safe work conditions. When the trench is deeper than 5 feet, we recommend a trench-shield or shoring be used as a protective system to workers in the trench. Based on our soil observations, laboratory testing, and OSHA guidelines, native soils at the site mostly classify as Type C soils. Deeper excavations, if required, should be constructed with side slopes no steeper than one and one-half horizontal to one vertical (1.5H:1V). If wet conditions are encountered, side slopes should be further flattened to maintain slope stability. Alternatively shoring or trench boxes may be used to improve safe work conditions in trenches. The contractor is ultimately responsible for trench and site safety. Pertinent OSHA requirements should be met to provide a safe work environment. If

© 2025 GeoStrata 16 R1812-002 FINAL REPORT site specific conditions arise that require engineering analysis in accordance with OSHA regulations, GeoStrata can respond and provide recommendations as needed.

The temporary excavation stability information presented above is based on the native soil and groundwater conditions encountered within our boreholes and test pits. It is possible that existing utility trenches or other features will be encountered during construction which will present different conditions than those encountered within our boreholes and test pits. Such conditions may include soft and loose soils or localized areas of high or perched groundwater. Caving of trenches in these areas where such conditions are encountered is possible. The contractor should be aware of this possibility and make necessary plans for proper shoring.

We recommend that a GeoStrata representative be on-site during all excavations to assess the exposed foundation soils. We also recommend that the Geotechnical Engineer be allowed to review the grading plans when they are prepared in order to evaluate their compatibility with these recommendations.

5.2.4 Structural Fill and Compaction

All fill placed for the support of structures, concrete flatwork/wingwalls or pavements should consist of structural fill. Structural fill may consist of native, on-site granular soils that have been screened to remove all material exceeding 4-inches in nominal diameter. All structural fill should be free of vegetation, debris or frozen material. Onsite clayey soils should not be utilized as structural fill due to their elevated potential to demonstrate moisture sensitivity. Alternatively, an imported structural fill meeting the specifications below may be used. If imported structural fill is needed, it should be a relatively well graded granular soil with a maximum of 50 percent passing the No. 4 mesh sieve and a maximum fines content (minus No.200 mesh sieve) of 25 percent.

Structural fill for the backfilling of structures (box culverts, retaining walls, etc.) should meet the requirements of CDOT Class 1 Structure Backfill. Soils not meeting the aforementioned criteria may be suitable for use as structural fill. These soils should be evaluated on a case-by-case basis and should be approved by the Geotechnical Engineer prior to use. The contractor should have confidence that the anticipated method of compaction will be suitable for the type of structural fill used, and should anticipate testing all soils used as structural fill frequently to assess the maximum dry density, fines content, and moisture content, etc.

All structural fill should be placed in maximum 6-inch loose lifts if compacted by small hand-operated compaction equipment, maximum 8-inch loose lifts if compacted by light-duty rollers, © 2025 GeoStrata 17 R1812-002 FINAL REPORT and maximum 10-inch loose lifts if compacted by heavy duty compaction equipment that is capable of efficiently compacting the entire thickness of the lift. If cobbles or boulders are removed during construction, remaining pits/divots will need to be filled and compacted. We recommend that all structural fill be compacted on a horizontal plane, unless otherwise approved by the geotechnical engineer. Structural fill should be compacted to at least 95% of the MDD, as determined by ASTM D-1557. The moisture content should be at or slightly above the OMC at the time of placement and compaction. Also, prior to placing any fill, the excavations should be observed by the geotechnical engineer to observe that any unsuitable materials or loose soils have been removed.

Fill soils placed for subgrade below wing walls and pavements, should be within 3% of the OMC when placed and compacted to at least 95% of the MDD as determined by ASTM D-1557. All utility trenches backfilled below the proposed structure, pavements, and flatwork concrete, should be backfilled with structural fill that is within 3% of the OMC when placed and compacted to at least 95% of the MDD as determined by ASTM D-1557. All other trenches, in landscape areas, should be backfilled and compacted to at least 90% of the MDD (ASTM D-1557).

The gradation, placement, moisture, and compaction recommendations contained in this section meet our minimum requirements but may not meet the requirements of other governing agencies such as city, county, or state entities. If their requirements exceed our recommendations, their specifications should override those presented in this report.

5.2.5 Moisture Protection and Surface Drainage

Precautions should be taken during and after construction to eliminate saturation of subgrade soils.

Over-wetting the soils prior to or during construction may result in softening and pumping, causing equipment mobility problems and difficulty in achieving compaction. Diversion of the existing stream will likely be required for construction. Additionally, every effort should be taken to see that positive drainage occurs away from roadway areas to reduce the potential for water to migrate below pavements. Moisture should not be allowed to infiltrate the soil in the vicinity of, or upslope from, the roadway.

© 2025 GeoStrata 18 R1812-002 FINAL REPORT

5.3 Roadway Widening and Retaining Wall Discussion

Based on our observations made during our field investigations along the Coffman Road alignment, it was noted that the overburden material consisted of basalt cobbles and boulders in a clayey sand matrix at slope conditions greater than 70 degrees. In our opinion, a retaining wall to accommodate extension of the existing road to the south and east after the overpass is feasible. We understand at this time that the roadway realignment as previously discussed to fit the current topography such that retaining walls will not be required. The following discussion is provided in case the proposed roadway realignment is changed and the possibility of a retaining wall is again considered.

Potential retaining wall options may include a gravity or mechanically stabilized earth (MSE) retaining wall system from Keystone Retaining Wall Systems, Redi-Rock, or similar type manufacturer.

Additional preliminary recommendations for retaining wall design are as follows:

1. Retaining walls should be constructed on undisturbed natural clayey sand and gravel soils or properly compacted CDOT Class 1 Structure Backfill. Loose, disturbed soils encountered in the wall excavation should be removed and replaced with compacted structure backfill or the foundation should be extended to undisturbed gravels. Retaining walls should be backfilled with CDOT Class 1

Structure Backfill.

2. A nominal (unfactored) bearing resistance of 8 ksf was calculated for the planned retaining walls. A resistance factor of 0.55 for gravity walls and 0.65 for MSE walls per table 11.5.7-1 in AASHTO (2020) should be applied for the strength limit state.

Settlements are anticipated to be one (1) inch or less under service loading.

3. For walls that are backfilled with CDOT Class 1 Structure Backfill and can deflect or rotate about 0.1 to 0.2 percent of the wall height, the wall can be designed for

“active” earth pressure conditions. For a very rigid wall where negligible deflection can occur, an “at-rest” lateral earth pressure condition should be used. Backslope behind walls should be considered in calculating earth pressures.

4. Lateral earth pressure for backfill, existing fill, and overburden material should be calculated in accordance with Section 3.11.5 of the AASHTO LRFD specifications

© 2025 GeoStrata 19 R1812-002 FINAL REPORT

(2020). A soil unit weight of 135 pcf and an internal friction angle of 34 degrees can be used for backfill consisting of compacted Class 1 Structure Backfill.

5. Passive pressure in front of the wall should be neglected.

6. A nominal coefficient of friction of 0.50 may be used for the calculation of sliding resistance. A sliding resistance factor of 1.0 should be applied for the strength limit state.

7. The walls should be designed with appropriate surcharge pressures.

8. The final wall design should meet the minimum requirements of AASHTO LRFD

Bridge Design Specifications (AASHTO, 2020) for global stability conditions.

9. Retaining walls should be constructed with a drainage system to drain away any excess water immediately behind the wall. Drainage systems such as free-draining gravel, pipes, drain board and/or weep holes are commonly used for the wall drainage.

Global stability of the planned retaining structure should be performed after preliminary design is completed. A minimum required factor of safety of 1.5 is recommended for static long-term conditions.

5.4 Recommended Additional Investigations

As mentioned previously, an additional study would be required to complete a wall design if plans change. It is likely that a field investigation for retaining wall design would require a road closure and likely specialty ODEX drilling to achieve necessary depths and sampling. A cost estimate for these services can be provided upon request. We understand these services not to be required for the existing design.

© 2025 GeoStrata 20 R1812-002 FINAL REPORT

6.0 CLOSURE

6.1 Limitations

The recommendations contained in this report are based on limited field exploration and our understanding of the proposed construction. This investigation was completed for the proposed construction and should not be used for other projects. The subsurface data used in the preparation of this report were obtained from the explorations made for this investigation. It is likely that variations in the soil and groundwater conditions will exist. The nature and extent of variations may not be evident until construction occurs. If any conditions are encountered at this site that are different from those described in this report, our firm should be immediately notified so that we may make any necessary revisions to the recommendations contained in this report. In addition, if the scope of the proposed construction changes from that described in this report, our firm should also be notified.

This report was prepared in accordance with the generally accepted standard of practice at the time the report was written. No other warranty, expressed or implied, is made.

It is the Client's responsibility to see that all parties to the project including the Designer, Contractor, Subcontractors, etc. are made aware of this report in its entirety.

6.2 Additional Services

In addition to additional engineering services discussed above, the recommendations made in this report are based on the assumption that an adequate program of tests and observations will be made during the construction. GeoStrata staff should be on site to document compliance with these recommendations and to verify geologic conditions are as anticipated. Our services should include, but not necessarily be limited to, the following:

• Observations and testing before and during construction.

• Consultation as may be required during construction, including verification that the geologic conditions are as anticipated during excavation and design of shoring if deemed necessary based on actual geologic conditions encountered during construction.

© 2025 GeoStrata 21 R1812-002 FINAL REPORT

We appreciate the opportunity to be of service on this project. Should you have any questions regarding the report or wish to discuss additional services, please do not hesitate to contact us at your convenience.

APPENDIX A

APPENDIX B

NP

26.0

40.0

12.0

NP

UNDOCUMENTED FILL; Clayey GRAVEL with sand - moist, brown, very dense

Silty SAND with gravel - moist, brown, very dense

Poorly graded SAND with clay and gravel - moist, dark brown, very dense boulders and cobbles auger refusal at 5 feet

Bottom of Boring @ 5.5 Feet

GC

SM

SP-

SC

11.2

9.5

6.9

50/6"

50/4"

50/2"

50/4"

11.2

9.5

6.9

102030405060708090S A

M P

L E

S

DEPTH

MATERIAL DESCRIPTION

Liquid Limit

N

Moisture Content

P la st ic it y

In de x

Project Number 1812-002

102030405060708090

12/30/24

12/30/24

12/30/24

Plate

Copyright (c) 2025, GeoStrata

GeoStrata Rep:

Rig Type:

Boring Type:

- ESTIMATED

L iq ui d

L im it

U N

IF

IE

D S

O

IL

C L

A S

S

IF

IC

A

T

IO

N

N - OBSERVED UNCORRECTED BLOW COUNT

SAMPLE TYPE

G R

A P

H

IC

A L

L O

G

B - 1

Moisture Content and

Atterberg Limits

M oi st ur e C on te nt

Sheet 1 of 1

Plastic Limit

N*

P er ce nt m in us

M E

T E

R S

STARTED:

COMPLETED:

BACKFILLED:

WATER LEVEL

- MEASURED

N* - CORRECTED N1(60) EQUIVALENT SPT BLOW COUNT

LOCATION

LATITUDE

LONGITUDE

ELEVATION

-108.3208 deg

4,821.0 ft

Notes: Elevation Estimated Using Google Earth

SPT BLOW COUNTF

E

E T

BORING NO:

BH-01D

A

T E

D ry

D en si ty

(p cf

W A

T E

R L

E V

E L

NOTES:

River City Consultants Mesa County Rapid Creek Road Improvements Palisade, Mesa County, Colorado

Ryan Hogge Simco 2800 4" Solid Stem

39.1312 deg

- 2" O.D./1.38" I.D. SPLIT SPOON SAMPLER

- 2.5" O.D./2" I.D. SAMPLER

- 3" O.D. THIN-WALLED SHELBY SAMPLER

- GRAB SAMPLE

- 3" O.D./2.48" I.D. SAMPLER

L O

G O

F B

O R

IN

G

P

L A

T E

B

G

IN

T U

P D

A T

E T

E M

P L

A T

E B

O R

E H

O L

E S

.G P

J G

E O

S T

R A

T A

.G D

T

/2 2/

UNDOCUMENTED FILL; Clayey GRAVEL with sand - moist, brown

Silty SAND with gravel - slightly moist to moist, brown to light brown, dense, gravels and cobbles auger refusal at 4 and 4.5 feet

Bottom of Boring @ 4.5 Feet

GC

SM

102030405060708090S A

M P

L E

S

DEPTH

MATERIAL DESCRIPTION

Liquid Limit

N

Moisture Content

P la st ic it y

In de x

Project Number 1812-002

102030405060708090

12/30/24

12/30/24

12/30/24

Plate

Copyright (c) 2025, GeoStrata

GeoStrata Rep:

Rig Type:

Boring Type:

- ESTIMATED

L iq ui d

L im it

U N

IF

IE

D S

O

IL

C L

A S

S

IF

IC

A

T

IO

N

N - OBSERVED UNCORRECTED BLOW COUNT

SAMPLE TYPE

G R

A P

H

IC

A L

L O

G

B - 2

Moisture Content and

Atterberg Limits

M oi st ur e C on te nt

Sheet 1 of 1

Plastic Limit

N*

P er ce nt m in us

M E

T E

R S

STARTED:

COMPLETED:

BACKFILLED:

WATER LEVEL

- MEASURED

N* - CORRECTED N1(60) EQUIVALENT SPT BLOW COUNT

LOCATION

LATITUDE

LONGITUDE

ELEVATION

-108.3209 deg

4,845.0 ft

Notes: Elevation Estimated Using Google Earth

SPT BLOW COUNTF

E

E T

BORING NO:

BH-02D

A

T E

D ry

D en si ty

(p cf

W A

T E

R L

E V

E L

NOTES:

River City Consultants Mesa County Rapid Creek Road Improvements Palisade, Mesa County, Colorado

Ryan Hogge Simco 2800 4" Solid Stem

39.1312 deg

- 2" O.D./1.38" I.D. SPLIT SPOON SAMPLER

- 2.5" O.D./2" I.D. SAMPLER

- 3" O.D. THIN-WALLED SHELBY SAMPLER

- GRAB SAMPLE

- 3" O.D./2.48" I.D. SAMPLER

L O

G O

F B

O R

IN

G

P

L A

T E

B

G

IN

T U

P D

A T

E T

E M

P L

A T

E B

O R

E H

O L

E S

.G P

J G

E O

S T

R A

T A

.G D

T

/2

Copyright GeoStrata, 2025

Soil Symbols Description Key

Plate B-3

River City Consultants Mesa County Rapid Creek Road Improvements Palisade, Mesa County, Colorado Project Number: 1812-002

DESCRIPTION OF SYMBOLS

3” OD “California” Style Split Barrel…