10204 EXHIBIT C - Terracon Geotechnical Engineering Report.pdf

Report Cover Page

Prepared for:

Alfred Benesch & Company, Inc.

7979 East Tufts Avenue, Suite 800 Denver, Colorado 80237

Timberline Culvert Replacement Geotechnical Engineering Report Timberline Road and Mountain Vista Drive

August 27, 2024 | Terracon Project No. 20245020

1901 Sharp Point Drive, Suite C Fort Collins, Colorado 80525

P (970) 484-0359 Terracon.com

Facilities | Environmental | Geotechnical | Materials

Report Cover Letter to Sign

August 27, 2024

Alfred Benesch & Company, Inc.

7979 East Tufts Avenue, Suite 800 Denver, Colorado 80237

Attn: Justin Wierema P: (303) 517-0028 E: jwierema.benesch.com

Re: Geotechnical Engineering Report Timberline Culvert Replacement Timberline Road and Mountain Vista Drive Fort Collins, Colorado Terracon Project No. 20245020

Dear Mr. Wierema:

We have completed the scope of Geotechnical Engineering services for the project referenced above in general accordance with Terracon Proposal No. P20245020 dated April 1, 2024. This report presents the findings of the subsurface exploration and provides geotechnical recommendations concerning earthwork and the design and construction of foundations for the proposed project.

We appreciate the opportunity to be of service to you on this project. Materials testing and construction observation services are provided by Terracon as well. We would be pleased to discuss these services with you. If you have any questions concerning this report or if we may be of further service, please contact us.

Sincerely, Terracon

Andrea L. Wahls Eric D. Bernhardt, P.E.

Field Engineer Regional Geotechnical Manager https://na3.documents.adobe.com/verifier?tx=CBJCHBCAABAA094KqQ74snnBjmMHK84EDU3wvzJD0ZdA https://na3.documents.adobe.com/verifier?tx=CBJCHBCAABAA094KqQ74snnBjmMHK84EDU3wvzJD0ZdA

Geotechnical Engineering Report Timberline Culvert Replacement | Fort Collins, Colorado August 27, 2024 | Terracon Project No. 20245020

Facilities | Environmental | Geotechnical | Materials i

Table of Contents

Report Summary .............................................................................................. i Introduction Project Description Site Conditions Geotechnical Characterization Seismic Site Class Corrosivity Geotechnical Overview Earthwork Shallow Foundations Lateral Earth Pressures General Comments

Figures GeoModel

Attachments

Exploration and Testing Procedures Site Location and Exploration Plans Exploration and Laboratory Test Results Supporting Information

Note: This report was originally delivered in a web-based format. Blue Bold text in the report indicates a referenced section heading. The PDF version also includes hyperlinks which direct the reader to that section and clicking on the logo will bring you back to this page. For more interactive features, please view your project online at client.terracon.com.

Refer to each individual Attachment for a listing of contents.

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Report Summary

Topic 1 Overview Statement 2

Project Description

A geotechnical exploration has been performed for the proposed culvert and wing walls to be constructed near the intersection of Timberline Road and Mountain Vista Drive in Fort Collins, Colorado. Three borings were performed to depths of approximately 20 to 25½ feet below existing grades.

Geotechnical Characterization

Subsurface conditions encountered in our exploratory borings generally consisted of about 19 to 20 feet of lean clay with varying amounts of sand over about 5 feet of weathered claystone bedrock. Boring No. B-3 has about 9 feet of clayey sand, over sandy lean clay. Groundwater not observed during our field exploration.

Geotechnical Overview

■ Expansive soils and weathered claystone bedrock are present on this site; however, swell test results for samples obtained from our borings were generally low swelling to compressible. This report provides recommendations to help mitigate the effects of soil movement/heave associated with these materials. The risk can be mitigated by careful design, construction and maintenance practices;

however, it should be recognized these procedures will not eliminate risk. The owner should be aware and understand pavements and/or utilities may be affected to some degree by the expansive soils and bedrock on this site.

■ Comparatively loose, low relative density sand soils were encountered at a depth of approximately 7 feet below existing grade in Boring No. B-3. These materials present a risk for potential settlement of shallow foundations and other surficial improvements. These materials can also be susceptible to disturbance and loss of strength under repeated construction traffic loads and unstable conditions could develop. Rework or stabilization of loose soils may be required at some locations to provide adequate support for construction equipment and the proposed structure.

Terracon should be contacted if these conditions are encountered to observe the conditions exposed and to provide guidance regarding stabilization (if needed).

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Topic 1 Overview Statement 2

Earthwork

On-site soils typically appear suitable for use as engineered fill and backfill on the site provided they are placed and compacted as described in this report. Import materials (if needed) should be evaluated and approved by Terracon prior to delivery to the site.

Shallow Foundations

We believe the wing walls for the proposed box culvert can be supported on a shallow, spread footing foundation system, provided the soils beneath the wing walls and the box culvert are over-excavated to a depth of at least 1 foot below the bottom of footings and replaced with moisture conditioned, properly compacted engineered fill.

Allowable bearing pressure = 1,500 psf Expected settlements: 1-inch total, ½ to ¾-inch differential

General Comments

This section contains important information about the limitations of this geotechnical engineering report.

1. If the reader is reviewing this report as a pdf, the topics above can be used to access the appropriate section of the report by simply clicking on the topic itself.

2. This summary is for convenience only. It should be used in conjunction with the entire report for design purposes.

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Introduction

This report presents the results of our subsurface exploration and Geotechnical Engineering services performed for the proposed culvert to be located near the intersection of Timberline Road and Mountain Vista Drive in Fort Collins, Colorado. The purpose of these services was to provide information and geotechnical engineering recommendations relative to:

■ Subsurface soil and rock conditions

■ Groundwater conditions

■ Seismic site classification

■ Site preparation and earthwork

■ Demolition considerations

■ Dewatering considerations

■ Foundation design and construction

■ Lateral earth pressures

The geotechnical engineering Scope of Services for this project included the advancement of test borings, laboratory testing, engineering analysis, and preparation of this report.

Drawings showing the site and boring locations are shown in the Site Location and Exploration Plan section of this report. The results of the laboratory testing performed on soil and bedrock samples obtained from the site during our field exploration are included on the boring logs and as separate graphs in the Exploration Results section.

Project Description

Our initial understanding of the project was provided in our proposal and was discussed during project planning. A period of collaboration has transpired since the project was initiated, and our final understanding of the project conditions is as follows:

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Item Description

Information Provided

The project information described below is based on the following:

■ Email and phone communication with Benesch.

■ Mountain Vista and Timberline.kmz, provided by Benesch.

■ Timberline Mountain Vista Culvert Replacement

Scope.pdf, provided by Benesch and dated February 20, 2024.

■ Preliminary site plans provided by Benesch, dated June 7, Project Description

We understand a new culvert is planned to replace the existing 6-foot by 9-foot arch pipe for the Number 8 Outlet Ditch at the intersection of Mountain Vista Drive and Timberline Road. We understand the planned box culvert will be approximately 8 feet wide, 7 feet in height, and 160 feet long.

Grading/Slopes

Grading plans were not provided to Terracon at the time of this report. We anticipate cuts and fills on the order of about 5 feet or less will be required to achieve proposed grades. Deeper excavations could be required for some structural elements.

Free-Standing Retaining Walls

Wing walls are planned to be constructed at the ends of the culvert to achieve final grades. Wing walls are anticipated to be designed and constructed as reinforced concrete, cantilevered walls.

Pavements

No new pavements requiring design are anticipated. We anticipate existing roadway pavements/surfacing will be demolished as part of the installation of the culvert will be replaced per Larimer County specifications to a minimum thickness at least equal to the existing pavement/surfacing thickness. The asphalt core that was observed during this project was approximately 6 inches thick.

Design Code AASHTO LRFD Bridge Design Specifications

Terracon should be notified if any of the above information is inconsistent with the planned construction, especially the grading limits, as modifications to our recommendations may be necessary.

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Site Conditions

The following description of site conditions is derived from our site visit in association with the field exploration and our review of publicly available geologic and topographic maps.

Item Description

Parcel Information

The project site is located near the intersection of Timberline Road and Mountain Vista Drive in Fort Collins, Colorado.

Latitude/Longitude (approximate): 40.61022° N, 105.02905° W See Site Location

Existing Improvements

Mountain Vista Drive spans over the ditch and has two, asphalt-paved drive lanes, with approximately 6-foot-wide shoulders on each side.

Current Ground Cover

The existing roadways are paved with asphalt, and shoulders are vegetated with native grasses and weeds.

Existing Topography

The alignment slopes towards the Number 8 Outlet Ditch. The interpolated ground surface elevations at the boring locations range from about El. 5,001 to El. 5,003 feet AMSL.

Geotechnical Characterization

We have developed a general characterization of the subsurface conditions based upon our review of the subsurface exploration, laboratory data, geologic setting and our understanding of the project. This characterization, termed GeoModel, forms the basis of our geotechnical calculations and evaluation of the site. Conditions observed at each exploration point are indicated on the individual logs. The individual logs can be found in the Exploration Results and the GeoModel can be found in the Figures attachment of this report.

As part of our analyses, we identified the following model layers within the subsurface profile. For a more detailed view of the model layer depths at each boring location, refer to the GeoModel.

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Model Layer

Layer Name General Description

1 Lean Clay Lean clay with varying amounts of sand; medium stiff to very stiff

2 Clayey Sand Clayey sand; very loose to medium dense

Weathered

Bedrock Weathered claystone bedrock; medium stiff to stiff

Groundwater Conditions

The boreholes were observed while drilling and shortly after completion for the presence and level of groundwater. Groundwater was not observed during or after drilling.

Groundwater conditions may change because of water levels within Number 8 Outlet Ditch, seasonal variations in rainfall, runoff, and other conditions not apparent at the time of drilling. Long-term groundwater monitoring was outside the scope of services for this project.

Seismic Site Class

The seismic design requirements for buildings and other structures are based on Seismic Design Category. Site Classification is required to determine the Seismic Design Category for a structure. The Site Classification is based on the upper 100 feet of the site profile defined by a weighted average value of either shear wave velocity, standard penetration resistance, or undrained shear strength in accordance with Section 20.4 of ASCE 7 and the International Building Code (IBC). Based on the soil and bedrock properties observed at the site as described on the exploration logs and laboratory test results, our professional opinion is a Seismic Site Classification of D be considered for the project. Subsurface explorations at this site were extended to a maximum depth of 25½ feet. The site properties below the boring depth to 100 feet were estimated based on our experience and knowledge of geologic conditions of the general area. Additional deeper borings or geophysical testing may be performed to confirm the conditions below the current boring depth.

Corrosivity

Results of water-soluble sulfate testing indicate Exposure Class S0 according to ACI 318.

ASTM Type I, IL, or II portland cement should be specified for all project concrete on and below grade. Foundation concrete should be designed for low sulfate exposure in accordance with the provisions of the ACI Design Manual, Section 318, Chapter 4.

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Numerous sources are available to characterize corrosion potential to buried metals using the parameters above. ANSI/AWWA is commonly used for ductile iron, while threshold values for evaluating the effect on steel can be specific to the buried feature (e.g., piling, culverts, welded wire reinforcement, etc.) or agency for which the work is performed. Imported fill materials may have significantly different properties than the site materials noted above and should be evaluated if expected to be in contact with metals used for construction. Consultation with a NACE certified corrosion professional is recommended for buried metals on the site.

Mapping by the NRCS includes qualitative severity of corrosion to concrete and steel.

Based on this source, the near-surface materials are rated “Low” for corrosion to concrete and “Moderate” for corrosion of steel.

Geotechnical Overview

Based on subsurface conditions encountered in the borings, the site appears suitable for the proposed construction from a geotechnical point of view provided certain precautions and design and construction recommendations described in this report are followed. We have identified several geotechnical conditions that could impact design, construction and performance of the proposed structures and other site improvements. These included expansive soils and weathered claystone bedrock, and potentially loose, low relative density sand soils. These conditions will require particular attention in project planning, design and during construction and are discussed in greater detail in the following sections.

Expansive Soils and Bedrock

Expansive soils and weathered claystone bedrock are present on this site; however, swell test results for samples obtained from our borings were generally low swelling to compressible. This report provides recommendations to help mitigate the effects of soil shrinkage and expansion. However, even if these procedures are followed, some movement and cracking in the structures, pavements, and flatwork (if any) is possible.

The severity of cracking and other damage such as uneven flatwork will probably increase if modification of the site results in excessive wetting or drying of the expansive clays and/or weathered claystone bedrock. Eliminating the risk of movement and cosmetic distress is generally not feasible, but it may be possible to further reduce the risk of movement if significantly more expensive measures are used during construction.

It is imperative the recommendations described in section Grading and Drainage section of the Earthwork section of this report be followed to reduce potential movement.

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Low Strength Soils

Comparatively loose, low relative density sand soils were encountered at a depth of approximately 7 feet below existing grade in Boring No. B-3. Loose and/or soft soils could also exist at other locations on the site and extend to greater depths particularly within the ditch itself. These materials present a risk for potential settlement of shallow foundations and other surficial improvements. These materials can also be susceptible to disturbance and loss of strength under repeated construction traffic loads and unstable conditions could develop. Stabilization of soft soils may be required at some locations to provide adequate support for construction equipment and proposed structures. Terracon should be contacted if these conditions are encountered to observe the conditions exposed and to provide guidance regarding stabilization (if needed).

Foundation Recommendations

Based on the results of the borings and our understanding of the project, we believe the wing walls for the proposed box culvert can be supported by a shallow, spread footing foundation system provided the soils are over-excavated to a depth of at least 1 foot below the bottom of footings and replaced with moisture conditioned, properly compacted engineered fill. On-site soils free of any deleterious materials are suitable for use as engineered fill below foundations. Design recommendations for foundations for the proposed structure and related structural elements are presented in the following sections of this report.

To reduce the potential for differential movements between the culvert and wing walls due to the presence of potentially loose soils at/near the anticipated culvert floor elevation, we recommend the soils be over-excavated to a depth of at least 1 foot below the bottom of the culvert and replaced with moisture conditioned, properly compacted engineered fill.

The recommendations contained in this report are based upon the results of field and laboratory testing (presented in the Exploration Results), engineering analyses, and our current understanding of the proposed project. The General Comments section provides an understanding of the report limitations.

Earthwork

Earthwork is anticipated to include demolition, site preparation, excavations, subgrade preparation, soil stabilization (if needed), and engineered fill placement. The following sections provide recommendations for use in the preparation of specifications for the project. Recommendations include critical quality criteria, as necessary, to render the site in the state considered in our geotechnical engineering evaluation for foundations.

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Demolition

Demolition of the existing roadway and drainage structure should include complete removal of all foundation systems, any other below-grade structural elements, and existing roadway pavements/surfacing within the proposed construction area. This should include removal of any utilities to be abandoned along with any loose utility trench backfill or loose backfill found adjacent to existing foundations or structures. All materials derived from the demolition of existing structures and roadways should be removed from the site.

Site Preparation

Prior to placing fill, existing vegetation, topsoil, and root mats, and any other deleterious materials should be removed from the proposed construction area. Complete stripping of the topsoil should be performed in areas of new construction. While not encountered in our borings, any existing, undocumented fill if encountered should also be completed removed from within the proposed construction area.

Stripped organic materials should be wasted from the site or used to re-vegetate landscaped areas or exposed slopes after completion of grading operations. Prior to the placement of fills, the site should be graded to create a relatively level surface to receive fill, and to provide for a relatively uniform thickness of fill beneath proposed structures.

Where fill is placed on existing slopes steeper than 5H:1V (Horizontal:Vertical), benches should be cut into the existing slopes prior to fill placement. The benches should have a minimum vertical face height of 1 foot and a maximum vertical face height of 3 feet and should be cut wide enough to accommodate the compaction equipment. This benching will help provide a positive bond between the fill and natural soils and reduce the possibility of failure along the fill/natural soil interface.

Although no evidence of fill or underground facilities (such as septic tanks, cesspools, basements, and utilities) was observed during the exploration and site reconnaissance, such features could be encountered during construction. If unexpected fills or underground facilities are encountered, such features should be removed, and the excavation thoroughly cleaned prior to backfill placement and/or construction.

Excavation

We anticipate excavations for the proposed construction can be accomplished with conventional earthmoving equipment. Excavations into the on-site soils will encounter loose soil conditions with possible caving conditions. The bottom of excavations should be thoroughly cleaned of loose/disturbed materials prior to backfill placement and/or construction.

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Any over-excavation that extends below the bottom of culvert and wing wall foundation elevations should extend laterally beyond all edges of the foundations at least 8 inches per foot of over-excavation depth below the foundation base elevation. The over-excavation should be backfilled to the foundation and culvert base elevation in accordance with the recommendations presented in this report.

Depending upon depth of excavation and seasonal conditions, surface water infiltration and/or groundwater may be encountered in excavations on the site. We anticipate pumping from sumps may be utilized to control water within excavations. Well points may be required for significant groundwater flow, or where excavations penetrate groundwater to a significant depth. Groundwater was not encountered during the time of our field exploration; however, we anticipate groundwater levels in the Number 8 Outlet Ditch to rise in different times of the year.

The subgrade soil conditions should be evaluated during the excavation process and the stability of the soils determined at that time by the contractors’ Competent Person as defined by OSHA. Slope inclinations flatter than the OSHA maximum values may have to be used. The individual contractor(s) should be made responsible for designing and constructing stable, temporary excavations as required to maintain stability of both the excavation sides and bottom. All excavations should be sloped or shored in the interest of safety following local, and federal regulations, including current OSHA excavation and trench safety standards.

As a safety measure, we recommend all vehicles and soil piles be kept a minimum lateral distance from the crest of the slope equal to the slope height. The exposed slope face should be protected against the elements.

Subgrade Preparation

After site preparation, removal of existing, undocumented fill (if necessary), and completion of the recommended over-excavation, the top 10 inches of the exposed ground surface should be scarified, moisture conditioned, and compacted to at least 95 percent of the maximum dry unit weight as determined by AASHTO T99 before any new fill, roadway embankment, or foundation is placed or constructed.

Large areas of prepared subgrade should be proof rolled prior to new construction. Proof rolling is not required in areas which are inaccessible to proof rolling equipment.

Subgrades should be proof rolled with an adequately loaded vehicle such as a fully-loaded tandem-axle dump truck. Proof rolling should be performed under the observation of the Geotechnical Engineer or representative. Areas excessively deflecting under the proof roll should be delineated and subsequently addressed by the Geotechnical Engineer. Excessively wet or dry material should either be removed or moisture conditioned and compacted.

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After the bottom of the excavation has been prepared as recommended above, engineered fill can be placed to bring the subgrade to the desired grade. Engineered fill should be placed in accordance with the recommendations presented in subsequent sections of this report.

The stability of the subgrade may be affected by precipitation, repetitive construction traffic or other factors. If unstable conditions develop, workability may be improved by scarifying and drying. Alternatively, over-excavation of wet zones and replacement with granular materials may be used, or crushed gravel and/or rock can be tracked or “crowded” into the unstable surface soil until a stable working surface is attained. Use of geosynthetics could also be considered as a stabilization technique. Lightweight excavation equipment may also be used to reduce subgrade pumping.

Subgrade Stabilization

Methods of subgrade stabilization/improvement, as described below, could include scarification, moisture conditioning and compaction, removal of unstable materials and replacement with granular fill (with or without geosynthetics), and chemical treatment.

The appropriate method of improvement, if required, would be dependent on factors such as schedule, weather, the size of area to be stabilized, and the nature of the instability. More detailed recommendations can be provided during construction as the need for subgrade stabilization occurs. Performing site grading operations during warm seasons and dry periods would help reduce the amount of subgrade stabilization required.

If the exposed subgrade is unstable during proof rolling operations, it could be stabilized using one of the methods described below.

■ Scarification and Compaction - It may be feasible to scarify, dry, and compact the exposed soils. The success of this procedure would depend primarily upon favorable weather and sufficient time to dry the soils. Stable subgrades likely would not be achievable if the thickness of the unstable soil is greater than about 1 foot, if the unstable soil is at or near groundwater levels, or if construction is performed during a period of wet or cool weather when drying is difficult.

■ Crushed Stone - The use of crushed stone or crushed concrete is a common procedure to improve subgrade stability. Typical undercut depths would be expected to range from about 6 to 24 inches below finished subgrade elevation.

Crushed stone and/or concrete can be tracked or “crowded” into the unstable subgrade until a stable working surface is attained. The use of high modulus geosynthetics (i.e., geotextile or geogrid) could also be considered after underground work such as utility construction is completed. Prior to placing the geosynthetic, we recommend all below-grade construction, such as utility line installation, be completed to avoid damaging the geosynthetic. Equipment should

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■ Chemical Treatment - Improvement of subgrades with portland cement, or fly ash could be considered for improving unstable soils. Chemical treatment should be performed by a pre-qualified contractor having experience with successfully treating subgrades in the project area on similar sized projects with similar soil conditions. Results of chemical analysis of the chemical treatment materials should be provided to the Geotechnical Engineer for review prior to use. The hazards of chemicals blowing across the site or onto adjacent properties should also be considered. Additional testing would be needed to develop specific recommendations to improve subgrade stability by blending chemicals with the site soils. Additional testing could include, but not be limited to, determining the most suitable chemical treating agent, the optimum amounts required, the presence of sulfates in the soil, and freeze-thaw durability of the subgrade.

Further evaluation of the need and recommendations for subgrade stabilization can be provided during construction as the geotechnical conditions are exposed.

Fill Material Types

Fill for this project should consist of engineered fill. Engineered fill is fill that meets the criteria presented in this report and has been properly documented. On-site soils free of deleterious materials or approved granular and low plasticity cohesive imported materials may be used as fill material. Weathered bedrock excavated during site development and construction can be reused as fill provided the material is broken down and thoroughly processed to a “soil-like” consistency, with no particles greater than 2 inches in size. The earthwork contractor should expect significant mechanical processing and moisture conditioning of the site soils and/or bedrock will be needed to achieve proper compaction.

Imported fill materials (if required) should meet the following material property requirements. Regardless of its source, compacted fill should consist of approved materials that are free of organic matter and debris. Frozen material should not be used, and fill should not be placed on a frozen subgrade.

Gradation Percent Finer by Weight (ASTM C136)

3” 100

1” 70-100

No. 4 Sieve 30-100

No. 200 Sieve 60 (max.)

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Soil Properties Values

Liquid Limit 35 (max.)

Plasticity Index 15 (max.)

Other import fill material types may be suitable for use on the site depending upon proposed application and location on the site and could be tested and approved for use on a case-by-case basis.

Fill Placement and Compaction Requirements

Engineered fill should be placed and compacted in horizontal lifts, using equipment and procedures that will produce recommended moisture contents and densities throughout the lift.

Item Description

Maximum Lift Thickness

9 inches or less in loose thickness when heavy, self-propelled compaction equipment is used 4 to 6 inches in loose thickness when hand-guided equipment (i.e., jumping jack or plate compactor) is used

Minimum Compaction

Requirements 1

Engineered Fill: At least 95% of the maximum dry unit weight as determined by AASHTO T99.

Engineered Fill 8 Feet or Greater: At least 98% of the maximum dry unit weight as determined by AASHTO T99 for the entire depth of fill in areas receiving 8 feet of fill or greater.

Aggregate Base Course: At least 95% of maximum dry unit weight as determined by AASHTO T180.

Water Content

Range 2,3 Cohesive (clay): -1% to +3% of optimum moisture content Granular (sand): -3% to +3% of optimum moisture content

1. We recommend engineered fill be tested for moisture content and compaction during placement. If the results of the in-place density tests indicate the specified moisture or compaction limits have not been met, the area represented by the test should be reworked and retested as required until the specified moisture and compaction requirements are achieved.

2. Moisture conditioned clay materials should not be allowed to dry out. A loss of moisture within these materials could result in an increase in the material’s expansive potential.

Subsequent wetting of these materials could result in undesirable movement.

3. Specifically, moisture levels should be maintained low enough to allow for satisfactory compaction to be achieved without the fill material pumping when proof rolled.

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Utility Trench Backfill

Any loose, soft, or unsuitable materials encountered at the bottom of utility trench excavations should be removed and replaced with engineered fill or bedding material in accordance with public works specifications for the utility to be supported. This recommendation is particularly applicable to utility work where settlement control of the utility is critical.

On-site materials are considered suitable for backfill of utility and pipe trenches provided the material is free of organic matter and deleterious substances.

Utility trench backfill should be placed and compacted as discussed earlier in this report.

Compaction of initial lifts should be accomplished with hand-operated tampers or other lightweight compactors. Flooding or jetting for placement and compaction of backfill is not recommended.

We recommend a representative of the Geotechnical Engineer provide full-time observation and compaction testing of trench backfill within construction areas.

Grading and Drainage

Grades should be adjusted to provide effective drainage away from the proposed structure and roadway during construction. Infiltration of water into foundation excavations must be prevented during construction.

Backfill against foundations and walls should be properly compacted and free of all construction debris to reduce the possibility of moisture infiltration. After construction of the proposed structure and prior to project completion, we recommend verification of final grading be performed to document positive drainage, as described above, has been achieved.

Maximum grades practical should be used for roadways to prevent areas where water can pond. Care should be taken to prevent the infiltration of surface water into the roadway subgrade and behind the culvert wing walls.

Earthwork Construction Considerations

Upon completion of filling and grading, care should be taken to maintain the subgrade water content prior to construction. Construction traffic over the completed subgrades should be avoided. The site should also be graded to prevent ponding of surface water on the prepared subgrades or in excavations. Water collecting over or adjacent to construction areas should be removed. If the subgrade freezes, desiccates, saturates, or is disturbed, the affected material should be removed, or the materials should be scarified, moisture conditioned, and recompacted.

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Construction site safety is the sole responsibility of the contractor who controls the means, methods, and sequencing of construction operations. Under no circumstances shall the information provided herein be interpreted to mean Terracon is assuming responsibility for construction site safety or the contractor's activities; such responsibility shall neither be implied nor inferred.

Construction Observation and Testing

The earthwork efforts should be observed by the Geotechnical Engineer (or others under their direction). Observation should include documentation of adequate removal of surficial materials (vegetation, topsoil, and existing pavements), subgrade stabilization, as well as proof rolling and mitigation of unsuitable areas delineated by the proof roll.

Each lift of compacted fill should be tested, evaluated, and reworked, as necessary, as recommended by the Geotechnical Engineer prior to placement of additional lifts.

In areas of foundation excavations, the bearing subgrade and exposed conditions at the base of the recommended over-excavation should be evaluated by the Geotechnical Engineer. If unanticipated conditions are observed, the Geotechnical Engineer should prescribe mitigation options.

In addition to the documentation of the essential parameters necessary for construction, the continuation of the Geotechnical Engineer into the construction phase of the project provides the continuity to maintain the Geotechnical Engineer’s evaluation of subsurface conditions, including assessing variations and associated design changes.

Shallow Foundations

If the site has been prepared in accordance with the requirements noted in Earthwork, the following design parameters are applicable for shallow foundations.

Spread Footings – Design Recommendations

Item Description Maximum Net Allowable Bearing

Pressure 1 1,500 psf

Required Bearing Stratum 2 1 foot of moisture conditioned, properly compacted engineered fill

Minimum Foundation Dimensions Continuous: 18 inches

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Item Description

Lateral Earth Pressure Coefficients 3

On-site clay soil:

Active, Ka = 0.36 Passive, Kp = 2.77 At-rest, Ko = 0.53

Sliding Resistance 4 On-site clay soil:

μ = 0.43

Moist Soil Unit Weight On-site clay soil and engineered fill:

γ = 100 pcf Minimum Embedment Below

Finished Grade 5 36 inches

Estimated Total Movement 6 About 1 inch or less

Estimated Differential Movement 6 About ½ to ¾ of total movement

1. The maximum net allowable bearing pressure is the pressure in excess of the minimum surrounding overburden pressure at the footing base elevation. Values assume exterior grades are no steeper than 20% within 10 feet of structure. The design bearing pressure applies to a dead load plus design live load condition. The design bearing pressure may be increased by one-third when considering total loads that include wind or seismic conditions.

2. Unsuitable or soft/loose soils should be over-excavated and replaced with engineered fill per the recommendations presented in Earthwork.

3. Use of lateral earth pressures require the sides of the excavation for the spread footing foundation to be nearly vertical and the concrete placed neat against these vertical faces or the footing forms be removed and compacted engineered fill be placed against the vertical footing face. Assumes no hydrostatic pressure. The lateral earth pressure coefficients are ultimate values and do not include a factor of safety. The foundation designer should include the appropriate factors of safety.

4. For fine-grained materials, lateral resistance using cohesion should not exceed ½ the dead load.

5. Embedment necessary to minimize the effects of frost and/or seasonal water content variations. For sloping ground, maintain depth below the lowest adjacent exterior grade within 5 horizontal feet of the structure.

6. The estimated movements presented above assume the maximum footing width is 5 feet for continuous footings. Larger foundation footprints will likely require reduced net allowable soil bearing pressures to reduce risk for potential settlement.

Footings should be proportioned to reduce differential foundation movement. As discussed, total movement resulting from the assumed structural loads is estimated to be on the order of about 1 inch. Additional foundation movements could occur if water from any source infiltrates the foundation soils; therefore, proper drainage should be provided in the final design and during construction and throughout the life of the structure. Failure to maintain the proper drainage as recommended in the Grading and

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Facilities | Environmental | Geotechnical | Materials 15

Drainage section of the Earthwork section of this report will nullify the movement estimates provided above.

Any over-excavation that extends below the bottom of foundation elevation should extend laterally beyond all edges of the foundations at least 8 inches per foot of over-excavation depth below the foundation base elevation. The over-excavation should be backfilled to the foundation base elevation in accordance with the recommendations presented in this report.

Shallow Foundation Construction Considerations

As noted in Earthwork, foundation excavations should be evaluated under the observation of the Geotechnical Engineer. The base of all foundation excavations should be free of water and loose soil, prior to placing concrete. Concrete should be placed soon after excavating to reduce bearing soil disturbance. Care should be taken to prevent wetting or drying of the bearing materials during construction. Excessively wet or dry material or any loose/disturbed material in the bottom of foundation excavations should be removed/reconditioned before foundation concrete is placed.

To reduce the potential of “pumping” and softening of the foundation soils at the foundation bearing level and the requirement for corrective work, we suggest the foundation excavation be completed remotely with a track-hoe operating outside of the excavation limits.

Foundation elements should be reinforced as necessary to reduce the potential for distress caused by differential foundation movement.

Unstable subgrade conditions encountered in foundation excavations should be observed by Terracon to assess the subgrade and provide suitable alternatives for stabilization.

Typical methods of stabilization/improvement are presented in the Subgrade Stabilization section of Earthwork.

Lateral Earth Pressures

Design Parameters

Structures with unbalanced backfill levels on opposite sides should be designed for earth pressures at least equal to values indicated in the following table. Earth pressures will be influenced by structural design of the walls, conditions of wall restraint, methods of construction, and/or compaction and the strength of the materials being restrained. Two wall restraint conditions are shown in the diagram below. Active earth pressure is commonly used for design of free-standing cantilever retaining walls and assumes wall

August 27, 2024 | Terracon Project No. 20245020

Facilities | Environmental | Geotechnical | Materials 16 movement. The “at-rest” condition assumes no wall movement and is commonly used for basement walls, loading dock walls, or other walls restrained at the top. The recommended design lateral earth pressures do not include a factor of safety and do not provide for possible hydrostatic pressure on the walls (unless stated).

Lateral Earth Pressure Design Parameters

Earth Pressure

Condition 1

Coefficient for Backfill Type 2

Surcharge Pressure 3 p1 (psf)

Equivalent Fluid Pressures

(psf) 2,4

Unsaturated 5 Submerged 5

Active (Ka) Fine Grained - 0.36 (0.36)S (35)H (75)H

At-Rest (Ko) Fine Grained - 0.53 (0.53)S (50)H (80)H

Passive (Kp) Fine Grained – 2.77 --- --- ---

1. For active earth pressure, wall must rotate about base, with top lateral movements 0.002 H to 0.004 H, where H is wall height. For passive earth pressure, wall must move horizontally to mobilize resistance. Fat clay or other expansive soils should not be used as backfill behind the wall.

2. Uniform, horizontal backfill, with a maximum unit weight of 100 pcf for cohesive soils.

3. Uniform surcharge, where S is surcharge pressure.

4. Loading from heavy compaction equipment is not included.

5. To achieve “Unsaturated” conditions, follow guidelines in Subsurface Drainage for Below-Grade Walls below. “Submerged” conditions are recommended when drainage behind walls is not incorporated into the design.

To control hydrostatic pressure behind the walls, we recommend a drain be installed at the foundation and wing walls with a collection pipe leading to a reliable discharge. If this is not possible, then combined hydrostatic and lateral earth pressures should be calculated for backfill using the submerged equivalent fluid pressures presented in the above table for active and at-rest conditions. These pressures do not include the influence of surcharge, equipment, or traffic loading, which should be added. Heavy

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Facilities | Environmental | Geotechnical | Materials 17 equipment should not operate within a distance closer than the exposed height of retaining walls to prevent lateral pressures more than those provided.

The lateral earth pressure recommendations given in this section are applicable to the design of rigid retaining walls subject to slight rotation, such as cantilever, or gravity type concrete walls. These recommendations are not applicable to the design of modular block - geogrid reinforced backfill walls (also termed MSE walls). Recommendations covering these types of wall systems are beyond the scope of services for this assignment. However, we would be pleased to develop a proposal for evaluation and design of such wall systems upon request.

General Comments

Our analysis and opinions are based upon our understanding of the project, the geotechnical conditions in the area, and the data obtained from our site exploration.

Variations will occur between exploration point locations or due to the modifying effects of construction or weather. The nature and extent of such variations may not become evident until during or after construction. Terracon should be retained as the Geotechnical Engineer, where noted in this report, to provide observation and testing services during pertinent construction phases. If variations appear, we can provide further evaluation and supplemental recommendations. If variations are noted in the absence of our observation and testing services on-site, we should be immediately notified so that we can provide evaluation and supplemental recommendations.

Our Scope of Services does not include either specifically or by implication any environmental or biological (e.g., mold, fungi, bacteria) assessment of the site or identification or prevention of pollutants, hazardous materials or conditions. If the owner is concerned about the potential for such contamination or pollution, other studies should be undertaken.

Our services and any correspondence are intended for the sole benefit and exclusive use of our client for specific application to the project discussed and are accomplished in accordance with generally accepted geotechnical engineering practices with no third-party beneficiaries intended. Any third-party access to services or correspondence is solely for information purposes to support the services provided by Terracon to our client. Reliance upon the services and any work product is limited to our client and is not intended for third parties. Any use or reliance of the provided information by third parties is done solely at their own risk. No warranties, either express or implied, are intended or made.

Site characteristics as provided are for design purposes and not to estimate excavation cost. Any use of our report in that regard is done at the sole risk of the excavating cost estimator as there may be variations on the site that are not apparent in the data that

August 27, 2024 | Terracon Project No. 20245020

Facilities | Environmental | Geotechnical | Materials 18 could significantly effect excavation cost. Any parties charged with estimating excavation costs should seek their own site characterization for specific purposes to obtain the specific level of detail necessary for costing. Site safety and cost estimating including excavation support and dewatering requirements/design are the responsibility of others.

Construction and site development have the potential to affect adjacent properties. Such impacts can include damages due to vibration, modification of groundwater/surface water flow during construction, foundation movement due to undermining or subsidence from excavation, as well as noise or air quality concerns. Evaluation of these items on nearby properties are commonly associated with contractor means and methods and are not addressed in this report. The owner and contractor should consider a preconstruction/precondition survey of surrounding development. If changes in the nature, design, or location of the project are planned, our conclusions and recommendations shall not be considered valid unless we review the changes and either verify or modify our conclusions in writing.

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Facilities | Environmental | Geotechnical | Materials

Figures

Contents:

GeoModel

4,975

4,980

4,985

4,990

4,995

5,000

EL

EV

A

TI

O N

M

S L) fe et

Layering shown on this figure has been developed by the geotechnical engineer for purposes of modeling the subsurface conditions as required for the subsequent geotechnical engineering for this project.

Numbers adjacent to soil column indicate depth below ground surface.

NOTES:

Legend

This is not a cross section. This is intended to display the Geotechnical Model only. See individual logs for more detailed conditions.

GeoModel

North Timberline Road and Mountain Vista | Fort Collins, Colorado Terracon Project No. 20245020

Timberline Culvert Replacement

1901 Sharp Point Dr Ste C Fort Collins, CO

Sandy Lean Clay

Lean Clay with Sand Weathered Rock

Asphalt Aggregate Base Course

Clayey Sand

Model Layer Layer Name General Description

1 Lean clay with varying amounts of sand; medium stiff to very stiff, brown to light brown

3 Weathered claystone bedrock; medium stiff to stiff, gray, brown, red and orange

2 Clayey sand; very loose to medium dense, brown

B-1

B-2 B-3

Vegetative LayerLean Clay

Weathered Bedrock

Clayey Sand

25.5

August 27, 2024 | Terracon Project No. 20245020

Facilities | Environmental | Geotechnical | Materials

Attachments

August 27, 2024 | Terracon Project No. 20245020

Facilities | Environmental | Geotechnical | Materials

Exploration and Testing Procedures

Field Exploration

Number of Borings Approximate Boring

Depth (feet) Location

(Boring Nos. B-1 and B-3)

25 to 25.5 Near each end of the proposed culvert alignment

(Boring No. B-2)

20 Existing roadway

Boring Layout and Elevations: Terracon personnel provided the boring layout using handheld GPS equipment (estimated horizontal accuracy of…