5-03. Geotechnical Report.pdf

REPORT COVER PAGE

Geotechnical Engineering Report

S. 120th Street Widening & Improvements Boulder County, Colorado

December 23, 2020 Terracon Project No. 22205027

Prepared for:

Otak, Inc.

Louisville, Colorado

Prepared by:

Terracon Consultants, Inc.

Longmont, ColoradoDRAFT

Terracon Consultants, Inc. 1831 Lefthand Circle, Suite C Longmont, Colorado 80501 P (303) 776 3921 F (303) 776 4041 terracon.com

REPORT COVER LETTER TO SIGN

December 23, 2020

Otak, Inc.

371 Centennial Parkway, Suite 210 Louisville, Colorado 80027

Attn: Ms. Christine Bisio, P.E. – Colorado Regional Manager P: (720) 758-7753 E: chris.bisio@otak.com

Re: Geotechnical Engineering Report S. 120th Street Widening & Improvements Overlook Drive and Dillon Road Boulder County, Colorado Terracon Project No. 22205027

Dear Ms. Bisio:

Terracon Consultants, Inc. (Terracon) has performed geotechnical engineering services for the project referenced above. This study was performed in general accordance with Terracon Proposal No. P22205027 dated June 16, 2020. This report presents the findings of the subsurface exploration and provides geotechnical recommendations concerning earthwork and the design and construction of pavements 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 Consultants, Inc.

Alec N. Strassburg, P.E. (KS, OK) Eric D. Bernhardt, P.E.

Project Engineer Geotechnical Department ManagerDRAFT

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REPORT TOPICS

INTRODUCTION

SITE CONDITIONS

PROJECT DESCRIPTION

GEOTECHNICAL CHARACTERIZATION

GEOTECHNICAL OVERVIEW

EARTHWORK

PAVEMENTS

ADDITIONAL DESIGN AND CONSTRUCTION CONSIDERATIONS

GENERAL COMMENTS

Note: This report was originally delivered in a web-based format. Orange 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 GeoReport logo will bring you back to this page. For more interactive features, please view your project online at client.terracon.com.

ATTACHMENTS

EXPLORATION AND TESTING PROCEDURES

SITE LOCATION AND EXPLORATION PLANS

EXPLORATION RESULTS

SUPPORTING INFORMATION

Note: Refer to each individual Attachment for a listing of contents.

DRAFT

http://client.terracon.com/

S. 120th Street Widening & Improvements ■ Boulder County, Colorado December 23, 2020 ■ Terracon Project No. 22205027

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REPORT SUMMARY

Topic 1 Overview Statement 2

Project Overview

A geotechnical exploration has been performed for the proposed S. 120th Street Widening & Improvements planned between Overlook Drive and Dillon Road in Boulder County, Colorado. Six (6) borings were performed to depths of approximately 10 to 25 feet below existing site grades. Terracon also performed borings along the project alignment in 2013 (see our Geotechnical Engineering Report, Terracon Project No. 22135025E, dated November 18, 2013). The data from the 2013 study along with the borings and laboratory testing performed under the current study were used to develop our recommendations for project.

Subsurface Conditions

Subsurface conditions encountered in our exploratory borings generally consisted of about 2 to 10 feet or greater of existing fill consisting of sand and clay soils with varying amounts of sand, clay, and gravel over about 3 feet of native clay soils.

Claystone/siltstone/sandstone bedrock was encountered below the overburden soils in most of the borings at depths of approximately 2 to 9 feet below existing site grades.

Boring logs are presented in the Exploration Results section of this report.

Groundwater Conditions

Groundwater was not encountered in any our test borings at the time of drilling.

Groundwater levels can fluctuate in response to site development and to varying seasonal and weather conditions, irrigation on or adjacent to the site, and fluctuations in nearby water features.

Geotechnical Concerns

■ Existing fill was encountered in the borings performed on this site to depths ranging from about 2 to 10 feet or greater below existing site grades. We anticipate the fill has been in place for a relatively long period of time and supporting existing pavements with variable success. Therefore, at a minimum, we recommend thoroughly proof-rolling the prepared subgrade prior to placing aggregate base course and/or pavement materials in areas planned for full-depth reconstruction.

■ Expansive clays and bedrock are present on this site. 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 that pavements may be affected to some degree by the expansive soils and bedrock on this site. We recommend that high swell potential soils encountered within two feet of the top of subgrade elevation be removed and replaced with approved engineered fill. As an alternative, the high swell potential materials could be chemically treated with lime to reduce their potential to swell in the future.DRAFT

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Topic 1 Overview Statement 2

Geotechnical Concerns (con’t)

■ We understand a cut section is planned from approximately Sta. 109+40 to Sta.

117+00. Cut depths in this area will range from about 5 feet to 15 feet. We encountered relatively shallow claystone/siltstone/sandstone at depths up to about 5½ feet or less below existing ground surface in this area. Based on the hardness of the bedrock encountered, excavations penetrating the claystone/siltstone/sandstone bedrock may require the use of specialized heavy-duty equipment, together with ripping or jack-hammering.

Earthwork

On-site soils typically appear suitable for use as general 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. Earthwork recommendations are presented in the Earthwork section of this report.

Pavements Recommended Pavement thicknesses for this project include 8 inches of asphalt over 12 inches of aggregate base course. Pavement section discussion and design parameters are presented in the report.

Construction Observation and Testing

Close monitoring of the construction operations and implementing drainage recommendations discussed herein will be critical in achieving the intended pavement performance. We therefore recommend that Terracon be retained to monitor this portion of the work.

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 (bold orange font) 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 making and design purposes. It should be recognized that specific details were not included or fully developed in this section, and the report must be read in its entirety for a comprehensive understanding of the items contained herein.

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INTRODUC TION

Geotechnical Engineering Report S. 120th Street Widening & Improvements

Boulder County, Colorado Terracon Project No. 22205027

December 23, 2020

INTRODUCTION

This report presents the results of our subsurface exploration and geotechnical engineering services performed for the proposed roadway widening project planned along S. 120th Street between Overlook Drive and Dillon Road in Boulder County, Colorado. The purpose of these services is to provide information and geotechnical engineering recommendations relative to:

■ Subsurface conditions ■ Pavement design and construction

■ Site preparation and earthwork

The geotechnical engineering scope of services for this project included the advancement of six test borings, designated TB-12 to TB-17, to depths ranging from approximately 10 to 25 feet below existing site grades.

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

Terracon performed a subgrade evaluation and pavement thickness design for rehabilitation/ reconstruction of a portion of the project alignment in 2013 (see our Geotechnical Engineering Report, Terracon Project No. 22135025E, dated November 18, 2013). This project included drilling six (6) borings, designated TB-1 to TB-6 along the south one-third of the project alignment and five (5) borings, designated TB-7 to TB-11, along the north one-third. The middle one-third of the project alignment was not included in the 2013 study. Laboratory testing was performed on samples from the borings to characterize the subgrade conditions and included moisture content, dry unit weight, volumetric swell potential, Atterberg limits, percent fines/grain size distribution, and R-value. The data from the 2013 study along with the borings and laboratory testing performed under the current study will be used to develop our recommendations for pavement design and construction.

SITE CONDITIONS

The following description of site conditions is derived from our site visit in association with the field exploration.

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Item Description

Project Location The project alignment is located along S. 120th Street between Overlook Drive and Dillon Road in Boulder County, Colorado (Approx. latitude/longitude 39.965131° N, 105.071922° W; See Site Location).

Existing Improvements

S. 120th Street is a two-lane asphalt paved road and is classified as a minor arterial street. For the most part, there is no curb and gutter adjacent to either side of the street. However, concrete curb, gutter and sidewalk are located on the west side of the street from Overlook Drive to about 1,600 feet to the south.

In general, the project area is surrounded by open space and/or undeveloped land. However, commercial/light-industrial development is located adjacent to the north end of the site. A two-span bridge also exists near the middle of the project alignment where S. 120th Street crosses over Northwest Parkway.

Existing Topography

The ground surface along the project alignment can be characterized as hilly terrain with some ravines/gullies which are located primarily in the central part of the project area. Overall, the ground surface slopes down to the north and west. A difference in elevation of roughly 200 feet is estimated along the project alignment. Cut and fill construction was used at some locations along the street alignment, particularly on the southern ½ of the alignment.

PROJECT DESCRIPTION

Our final understanding of the project conditions is as follows:

Item Description

Information Provided Project plans dated September 22, 2020 provided by Otak, Inc. (Otak).

Project Description

We understand the project will include new pavement construction and/or rehabilitation for the subject street. In addition, widening of the existing street will likely be included at some locations. We understand the project may involve some vertical realignment (cuts/fills) in order to improve safety and sight distance. We expect asphalt pavement will be used for this project. Total length of street improvements will be on the order of about 6,800 lineal feet.

We anticipate pavement improvements/rehabilitation will consist of completely reconstructed sections. We assume no major pavement improvement/ rehabilitation efforts have been completed on the subject street segment since our original exploration was completed in 2013.

Grading/Slopes

Plan and profile sheets indicate up to about 15 feet of cut (near Sta. 113+00) and about 9 feet of fill (near Sta. 150+80) will be required to develop final grades along the project alignment. Final slope angles of no steeper than 3H:1V (Horizontal: Vertical) are expected.DRAFT

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Item Description

Pavements

We expect flexible (asphalt) pavement sections will be used for the project.

Based on our discussions with the project design team, we anticipated traffic is as follows:

■ Current AADT of 7,000 vehicles per day (vpd)

■ 2040 AADT of 15,000 (vpd)

■ Annual growth rate: about 3.9 percent (based on the ADT values)

■ Truck percent: 9 percent (7 percent single unit trucks and 2 percent combination trucks) The pavement design period is 20 years.

If project information or assumptions vary from what is described above or if location of construction changes, we should be contacted as soon as possible to confirm and/or modify our recommendations accordingly.

GEOTECHNICAL CHARACTERIZATION

Subsurface Profile

Specific conditions encountered at each boring location are indicated on the individual boring logs.

Stratification boundaries on the boring logs represent the approximate location of changes in material types; in situ, the transition between materials may be gradual. Details for each of the borings can be found in Exploration Results. A discussion of field sampling and laboratory testing procedures and test results are presented in Exploration and Testing Procedures. Based on the results of the borings, subsurface conditions on the project site can be generalized as follows:

Stratum Approximate Depth to Bottom of Stratum Material Description Consistency/Density/

Hardness

Surface 11 to 14 inches

Asphalt pavement over granular base

(No granular base encountered in borings TB-15 to TB-17)

2 to 9 feet in borings TB-12, TB-13, and TB-15 to TB-17 To the maximum depth of the exploration of 10 feet in boring TB-14

Existing fill materials consisting of silty sand, clayey sand, and silty clayey sand with varying amounts of gravel; and lean clay with varying amounts of sand and gravel

2 8 feet in boring TB-15 only Lean to fat clay Stiff

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Stratum Approximate Depth to Bottom of Stratum Material Description Consistency/Density/

Hardness

To the maximum depths of the exploration ranging from 10 to 25 feet in borings TB-12, TB-13, and

TB-15 to TB-17

Claystone/Siltstone/Clayey Sandstone Weathered to very hard

Groundwater Conditions

The boreholes were observed while drilling and shortly after completion for the presence and level of groundwater. Groundwater was not observed in the borings while drilling, or for the short duration the borings could remain open. These observations represent short-term groundwater conditions at the time of and shortly after the field exploration and may not be indicative of other times or at other locations.

Groundwater level fluctuations occur due to seasonal variations in the water levels present in nearby water features, amount of rainfall, runoff and other factors not evident at the time the borings were performed. Therefore, groundwater levels during construction or at other times in the life of the roadway may be higher or lower than the levels indicated on the boring logs. The possibility of groundwater level fluctuations should be considered when developing the design and construction plans for the project.

Fluctuations in groundwater levels can best be determined by implementation of a groundwater monitoring plan. Such a plan would include installation of groundwater piezometers, and periodic measurement of groundwater levels over a sufficient period of time.

Laboratory Testing

Representative soil and bedrock samples were selected for swell testing and exhibited 0.1 percent compression to 5.6 percent swell when wetted. The sand soils are considered to have none to low “probable swell damage risk” based on the Colorado Department of Transportation (CDOT) Pavement Design Manual. The claystone/siltstone bedrock is considered to have low to high “probable swell damage risk”. Samples of soil and bedrock tested for swell during our 2013 study also exhibited similar swell potential. Swell values on the samples tested in 2013 ranged from about 0 to 8.8 percent (swell) when wetted.

Samples of site soils and bedrock selected for plasticity testing exhibited low to high plasticity with liquid limits ranging from 19 to 46 and plasticity indices ranging from 3 to 32. Laboratory test results are presented in the Exploration Results section of this report.

In addition, a bulk sample of the subgrade soils consisting of clayey sand existing fill material from

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GEOTECHNICAL OVERVIEW

Based on subsurface conditions encountered in the borings, the alignment 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 and the owner understands the inherent risks associated with construction on sites underlain by expansive soils and bedrock. We have identified several geotechnical conditions that could impact design, construction and performance of the proposed pavements and other site improvements. These included existing fill, subgrade stability, expansive soils and bedrock, and cut areas to depths of up to about 15 feet. These conditions will require particular attention in project planning, design and during construction and are discussed in greater detail in the following sections.

Existing Fill

As previously noted, existing fill was encountered to depths up to about 10 feet or greater in borings TB-12 to TB-17 and also encountered in many of the 2013 borings. Existing fill could exist at other locations along the project alignment and extend to greater depths. We do not possess any information regarding whether the fill was placed under the observation of a geotechnical engineer. Undocumented fill can present a greater than normal risk of post-construction movement of pavements and other site improvements supported on or above these materials.

However, we anticipate the fill along the project alignment has been in place for a relatively long period of time and supporting existing pavements with variable success. Therefore, at a minimum, we recommend thoroughly proof-rolling the prepared subgrade prior to placing aggregate base course and/or pavement materials in areas planned for full-depth reconstruction. Areas exhibiting significant deflection and or pumping during the proof roll will need to be repaired prior to pavement construction.

Subgrade Stability

Penetration resistance measurements indicate the near surface soils are not unusually soft at our boring locations. However, the boring data and our experience indicate subgrade materials below existing pavements typically have relatively high moisture content and could deflect and deform (pump) under construction traffic wheel loads and create subgrade stability issues. After removal of the existing pavement, the contractor should expect some areas of unstable subgrades that will need to be stabilized prior to construction of new pavements. Consequently, Terracon recommends a contingency be provided in the construction budget to stabilize and correct weak/unstable subgrade.

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Expansive Soils and Bedrock

Expansive soils and bedrock are present on this site and these conditions constitute a geologic hazard. 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 pavements and flatwork 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 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 the Earthwork section of this report be followed to reduce potential movement.

Based on guidance provided in the CDOT Pavement Design Manual, areas where the pavement will be supported directly on high swell potential clay soils or claystone bedrock can be over-excavated and replaced with approved engineered fill to reduce the potential for movement below the pavement. Based on CDOT recommendations, the depth of treatment, or over-excavation, is based on the Plasticity Index of the material. In our borings, we encountered high swell potential materials with Plasticity Index values of up to 32. Therefore, CDOT recommends the depth of treatment should be up to 3 to 4 feet in areas along the project alignment where the Plasticity Index value of the subgrade soil and/or bedrock is between 20 and 40 (such as near the locations of borings TB-16 and TB-17, and TB-3 and TB-5 from our 2013 study). Based on our experience, we recommend the over-excavation depth be no greater than 3 feet in these areas. The over-excavation and replacement recommendation need only be implemented in areas along the alignment where high swell potential materials are or will be present within 2 feet of the top of subgrade elevation, such as in the planned cut area between approximately Sta. 109+40 to Sta.

117+00.

As an alternative to an over-excavation and replacement program, consideration could also be given to chemically treating the subgrade soils and/or bedrock with lime to reduce the potential for future movement. We recommend that a chemical mix design be performed to determine the percentage of lime needed to treat the subgrade materials. Terracon can perform this service if needed.

Cut Area

Based on the provided project plan and profile sheets, we understand a cut section is planned from approximately Sta. 109+40 to Sta. 117+00. Cut depths in this area will range from about 5 feet to 15 feet. We encountered relatively shallow claystone/siltstone/sandstone at depths up to about 5½ feet or less below existing ground surface in this area (see borings TB-16 and TB-17, and TB-3 and TB-5 from our 2013 study). Based on the hardness of the bedrock encountered, excavations penetrating the claystone/siltstone/sandstone bedrock may require the use of specialized heavy-duty equipment, together with ripping or jack-hammering.

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Groundwater was not encountered in the borings performed within the planned cut section during the relatively short time that the boreholes could be left open. However, this does not necessarily mean that the borings terminated at depths above groundwater. Therefore, the contractor should be prepared to control groundwater in the area of the planned cut in the case that groundwater is encountered during excavation or once the area has been excavated to the planned maximum cut depth.

EARTHWORK

The following presents recommendations for site preparation, excavation, subgrade preparation, placement of engineered fill materials, and utility trench construction. Earthwork on the project should be observed and evaluated by Terracon. Evaluation of earthwork should include observation and/or testing of over-excavation, removal of existing fill, subgrade preparation, placement of engineered fills, subgrade stabilization, pavement materials and other geotechnical conditions exposed during the construction of the project.

Site Preparation

Site preparation should commence with removal of the existing pavement, surface vegetation, topsoil, trees, and any other deleterious material. Any organic soils removed during site preparation should not be used as fill. Loose/soft, or otherwise unsuitable material should be removed at this time as well along with any required cuts or over-excavations due to the presence high swell potential material at the subgrade elevation. Subgrade treatment should be performed as described in Subgrade Preparation of the Pavements section of the report.

Although evidence of underground facilities, other than existing utilities in or adjacent to the street, was not observed during site reconnaissance, other underground features could be encountered during construction. If unexpected underground facilities are encountered, they should be removed, and the excavation thoroughly cleaned. Terracon should observe the excavation prior to backfill placement and/or construction.

Exposed surfaces should be free of mounds and depressions that could prevent uniform compaction. Following completion of any required cuts or over-excavations but prior to placement of new fill, the exposed ground should be scarified, moisture conditioned as needed and recompacted. The subgrade should then be proof-rolled to help delineate weak or disturbed areas at or near the ground surface. Unsuitable areas should be improved by moisture adjustment and compaction or by undercutting and placement of suitable engineered fill.DRAFT

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Excavation

It is anticipated that excavations into the on-site soil for the proposed construction can be accomplished with conventional earthmoving equipment. Excavations into the on-site soils may encounter weak and/or saturated soil conditions with possible caving conditions.

We understand a cut section is planned from approximately Sta. 109+40 to Sta. 117+00. Cut depths in this area will range from about 5 feet to 15 feet. We encountered relatively shallow claystone/siltstone/sandstone at depths up to about 5½ feet or less below existing ground surface in this area (see borings TB-16 and TB-17, and TB-3 and TB-5 from our 2013 study). Relatively shallow bedrock could be encountered at other locations along the project alignment. Excavation penetrating the claystone/siltstone/sandstone bedrock may require the use of specialized heavy-duty equipment, together with ripping or jack-hammering. Consideration should be given to obtaining a unit price for difficult excavation in the contract documents for the project.

The soils to be excavated can vary significantly across the site as their classifications are based solely on the materials encountered in widely-spaced exploratory test borings. The contractor should verify that similar conditions exist throughout the proposed area of excavation. If different subsurface conditions are encountered at the time of construction, the actual conditions should be evaluated to determine any excavation modifications necessary to maintain safe conditions.

Although evidence of underground facilities such as grease pits, septic tanks, vaults, and utilities was not observed during the site reconnaissance, such features could be encountered during construction. If unexpected underground facilities are encountered, such features should be removed, and the excavation thoroughly cleaned prior to backfill placement and/or construction.

Depending upon depth of excavation and seasonal conditions, surface water infiltration and/or groundwater may be encountered in excavations on the site. It is anticipated that 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 seepage should be anticipated for excavations approaching the level of bedrock.

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. 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. If any excavation, including a utility trench, is extended to a depth of more than 20 feet, it will be necessary to have the side slopes and/or shoring system designed by a professional engineer.

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

Fill Materials

The on-site soils or approved granular and low plasticity cohesive imported materials may be used as fill material. 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 high plasticity, high swell potential claystone bedrock encountered on this site may not be re-used as fill. 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.

In general, imported materials (if needed) meeting the properties presented below should be acceptable for use on the site. However, imported soils should be evaluated and approved by the geotechnical engineer prior to delivery to the site.

Gradation Percent finer by weight (ASTM C136)

4” 100

3” 70-100

No. 4 Sieve 50-100

No. 200 Sieve 50 (max.)

Soil Properties Values

Liquid Limit 35 (max.)

Plasticity Index 15 (max.)

R-value 10 (min.)

Other import fill materials types may be suitable for use on the project depending upon proposed application and location along the project alignment, and could be tested and approved for use on a case-by-case basis. For instance, the use of select imported sand and gravel fill would be prudent for stabilization applications since these materials are better able to “bridge” soft/weak soils as compared to fine-grained soils.

Fill Compaction Requirements

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

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Item Description

Fill 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 At least 95 percent of the material’s maximum standard Proctor dry unit weight as determined by AASHTO T99

Moisture content on-site or imported cohesive soil (clay) 2

-1 to +3 % of the material’s optimum moisture content determined in accordance with AASHTO T99 (standard Proctor)

Moisture content cohesionless soil (sand/gravel) 3

-3 to +3 % of the material’s optimum moisture content determined in accordance with AASHTO T99 (standard Proctor)

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.

Utility Trench Backfill

All trench excavations should be made with sufficient working space to permit construction including backfill placement and compaction. Trench backfill should consist of clean on-site soils or approved imported materials. Backfill should be placed and compacted as described under Fill Compaction Requirements. It is strongly recommended a representative of Terracon provide full-time observation and compaction testing of trench backfill within pavement areas.

Earthwork Construction Considerations

Based on our experience, relatively high moisture content, unstable soils are often found beneath existing pavements after demolition. Therefore, we anticipate the contractor could encounter isolated, unstable areas during construction and could experience difficulties obtaining a satisfactory proof roll. The amount of unstable soils that may have to be over-excavated and replaced cannot be determined at this time.

Upon completion of filling and grading, care should be taken to maintain the subgrade water content prior to construction of pavements. 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

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Construction Observation and Testing

The earthwork efforts should be monitored under the direction of Terracon. Monitoring should include documentation of adequate removal of vegetation, topsoil, loose/soft soils and high swell potential materials, proof rolling, and mitigation of areas delineated by the proof roll to require mitigation. Each lift of compacted fill should be tested, evaluated, and reworked as necessary until approved by Terracon prior to placement of additional lifts or pavement.

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

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.

PAVEMENTS

General Pavement Comments

Pavement designs are provided for the traffic conditions and pavement life conditions noted in Project Description and/or in the following sections of this report. A critical aspect of pavement performance is site/subgrade preparation, surface drainage, and the use of quality paving materials. We recommend pavement subgrades for any full-depth pavement sections be prepared a discussed in the Earthwork section and in the following sections of this report.

Subgrade Preparation

Penetration resistance measurements indicate the near surface soils are not unusually soft at our boring locations. However, the boring data and our experience indicate subgrade materials below existing pavements typically have relatively high moisture content and could deflect and deform (pump) under construction traffic wheel loads and create subgrade stability issues. This condition is most likely due to moisture collecting in the subgrade through cracks or seams in the pavement and lack of natural evaporation which would allow for drying of the subgrade. After removal of the existing pavement, the contractor should expect some areas of unstable subgrades that will need

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There are a number of stabilization methods that can be used to improve the subgrade and depend, in part, on the extent and severity of the unstable soils exposed during construction as well as other factors. For isolated or small areas requiring stabilization, moisture conditioning, processing and compaction or mechanical stabilization with granular materials and/or geogrid may be effective. If large areas require stabilization, chemical treatment of the soils may be a more effective alternative. In any event, we feel the appropriate method and level of stabilization should be evaluated and can best be determined on a case-by-case basis during construction once the entire subgrade and overall conditions are exposed. We are available to provide specific stabilization recommendations during construction upon your request.

The use of low ground pressure (LGP) or lightweight earth moving equipment could be used over weak subgrades, but repetitive construction traffic on the subgrade soils should be minimized to help reduce subgrade disturbance and destabilization.

On most project sites, the site grading is accomplished relatively early in the construction phase.

Fills are typically placed and compacted in a uniform manner. However, as construction proceeds, the subgrade may be disturbed due to utility excavations, construction traffic, desiccation, or rainfall/snow melt. As a result, the pavement subgrade may not be suitable for pavement construction and corrective action will be required. The subgrade should be carefully evaluated at the time of pavement construction for signs of disturbance or instability. We recommend the pavement subgrade be thoroughly proof rolled with a loaded tandem-axle dump truck prior to final grading and paving. All pavement areas should be moisture conditioned and properly compacted to the recommendations in this report immediately prior to paving.

Based on guidance provided in the CDOT Pavement Design Manual, areas where the pavement will be supported directly on high swell potential clay soils or claystone bedrock can be over-excavated and replaced with approved engineered fill to reduce the potential for movement below the pavement. Based on CDOT recommendations, the depth of treatment, or over-excavation, is based on the Plasticity Index of the material. In our borings, we encountered high swell potential materials with Plasticity Index values of up to 32. Therefore, CDOT recommends the depth of treatment should be up to 3 to 4 feet in areas along the project alignment where the Plasticity Index value of the subgrade soil and/or bedrock is between 20 and 40 (such as near the locations of borings TB-16 and TB-17, and TB-3 and TB-5 from our 2013 study). Based on our experience, we recommend the over-excavation depth be no greater than 3 feet in these areas. The over-excavation and replacement recommendation need only be implemented in areas along the alignment where high swell potential materials are or will be present within 2 feet of the top of subgrade elevation, such as in the planned cut area between approximately Sta. 109+40 to Sta.

117+00.

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As an alternative to an over-excavation and replacement program, consideration could also be given to chemically treating the subgrade soils and/or bedrock with lime to reduce the potential for future movement. We recommend that a chemical mix design be performed to determine the percentage of lime needed to treat the subgrade materials. Terracon can perform this service if needed.

Prior to any new fill placement, the exposed subgrade should be scarified to a minimum depth of 12 inches, moisture conditioned as needed and compacted as described in the Earthwork section.

Placement and compaction of subgrade soils, new fills, base course and other pavement materials should be observed and tested by a representative of Terracon to verify the specified moisture and/or compaction requirements have been achieved.

Asphalt Pavement Design Parameters

Based on our boring data and anticipated grading, we expect lean clays with varying amounts of sand, silty and/or clayey sands will be present at subgrade elevation. Laboratory testing indicates the subgrade materials classify as A-6, A-7-6, A-4, and A-1-b soils according to the American Association of State Highway and Transportation Officials (AASHTO) classification system.

AASHTO group indices of these soils ranged from 0 to 30. These subgrade materials have a “CL”, “CL-ML”, “SC”, or “SM” classification in accordance with the Unified Soil Classification System (USCS). Based on physical properties, the R-value tests from the current study and the 2013 study and our experience, we judge the subgrade soils at the site to generally offer poor to fair pavement support.

Design of asphalt pavements for the project was based on the procedures outlined in the 1993 Guideline for Design of Pavement Structures prepared by the American Association of State Highway and Transportation Officials (AASHTO), the Boulder County Multimodal Transportation Standards, and local pavement construction practice. Design parameters or other data used for determining pavement thickness for this project are summarized in the following table.

The Equivalent Daily Load Application (EDLA) value was determined using the projected AADT value of the mid-point of the design period and applying flexible pavement 18-kip equivalency factors and the lane distribution factor provided in the CDOT Pavement Design Manual.

Pavement Design Parameter/Data Value 2020 AADT Value 7,000 2040 AADT Value 15,000

R-value (determined in laboratory) 1 7

Correlated Soil Resilient Modulus (MR) 3,500 psi Reliability 85%

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Pavement Design Parameter/Data Value Overall Standard Deviation (So) 0.45

Design Serviceability Loss (ΔPSI) 2.0

18-kip Equivalent Daily Load Application (EDLA) 305 3

Design Period 20 years Total 18-kip Equivalent Single Axle Load (ESAL) 2,226,500 Design Structural Number (SN) 4.69

Pavement Layer Strength Coefficient

Asphaltic Concrete (AC) 0.44 Aggregate Base Course (ABC) 0.11

1. The R-value is representative of the lean clay soils encountered along the alignment in the 2013 study and the resulting R-value testing of that material.

2. The soil resilient modulus (MR) was estimated using a CDOT correlation equation based on the estimated R-value of the subgrade soils.

3. EDLA value was increased by 10 percent to account for construction traffic per Boulder County Multimodal Transportation Standards.

Asphalt Pavement Section Thickness

Using the values presented above, environmental criteria and other factors, the design structural number (SN) of the pavement section was determined on the basis of the 1993 AASHTO design equation.

Asphalt Pavement Section Thickness

Traffic Area Alternative Asphaltic Concrete

Surface Course (AC) 1

(inches)

Aggregate Base Course (ABC) 2

(inches)

Total Thickness (inches)

120th Street AC + ABC 8 12 20

1. Superpave asphalt mixtures with aggregates meeting CDOT Grading SX, S or SG specification is recommended for asphalt concrete (AC). Asphalt should be placed in maximum 2½ to 3-inch lifts (4-inch max. for SG) and compacted to a density of 92 to 96 percent of the maximum theoretical density, determined in general accordance with ASTM D2041 (Colorado Procedure 51).

2. Aggregate base course (ABC) should consist of a blend of sand and gravel which meets strict specifications for quality and gradation and should have an R-value of at least 72. Use of materials meeting Colorado Department of Transportation (CDOT) Class 5 or 6 specification is recommended. ABC should be placed in maximum 6-inch lifts and compacted to at least 95 percent of the modified Proctor maximum dry density

(ASTM D1557/AASHTO T180).

Design parameters such as serviceability loss, reliability, EDLA values and other factors may vary with specific project requirements. Variation of these parameters may change the thickness of the pavement sections presented. Terracon is prepared to discuss the details of these parameters and their effects on pavement design and re-evaluate pavement thickness, as appropriate.

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Pavement Drainage

Pavements should be sloped to provide rapid drainage of surface water. Adequate slopes adjacent to the roadway should be provided and maintained to promote surface water drainage away from the pavement system. Water allowed to pond on or adjacent to the pavements could saturate the subgrade and contribute to premature pavement deterioration. Collection and diversion of surface drainage away from paved areas is critical to satisfactory performance of pavements.

Pavement Performance and Maintenance

Our experience indicates longitudinal cracking is common for asphalt pavements. The cracking normally occurs parallel to the interface between asphalt and concrete features such as curbs, gutters or drain pans and/or several feet in from pavement edges. The mechanism for this cracking is not fully understood but seems to be most prevalent for expansive and/or cohesive subgrade soils. Distress of this type is likely to occur even if the subgrade has been properly prepared and the asphalt has been compacted properly.

Pavement performance is affected by its surroundings. In addition to providing preventive maintenance, the civil engineer should consider the following recommendations in the design and layout of pavements:

■ Site grades should slope a minimum of 2 percent away from the pavements;

■ The subgrade and the pavement surface have a minimum 2 percent slope to promote proper surface drainage;

■ Consider appropriate edge drainage and pavement under drain systems;

■ Install pavement drainage surrounding areas anticipated for frequent wetting;

■ Install joint sealant and seal cracks immediately;

■ Seal all landscaped areas in, or adjacent to pavements to reduce moisture migration to subgrade soils; and

■ Placing compacted, low permeability backfill against the exterior side of curb and gutter.

The pavement sections provided in this report represent minimum recommended thicknesses and, as such, periodic maintenance should be anticipated. Therefore, preventive maintenance should be planned and provided for through an on-going pavement management program.

Preventive maintenance activities are intended to slow the rate of pavement deterioration and to help preserve the pavement investment.

Preventive maintenance consists of both localized maintenance (e.g., crack and joint sealing and patching) and global maintenance (e.g., surface sealing). Preventive maintenance is usually the priority when implementing a pavement maintenance program. Prior to implementing any maintenance, additional engineering observation is recommended to determine the type and

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ADDITIONAL DESIGN AND CONSTRUCTION CONSIDERATIONS

Soluble Sulfates Test Results (Concrete)

Samples of the soils that will likely be in contact with project concrete were tested for soluble sulfate concentrations. The sulfate concentrations measured in the samples varied from about

0.038 to 0.064 percent. Sulfate concentrations of less than 0.1 percent indicate Class 0 exposure to sulfate attack for concrete in contact with the subsoils, according to the American Concrete Institute (ACI) 318. For this level of sulfate concentrations, ACI indicates any type of cement can be used for concrete in contact with the subsoils.

Therefore, Type I Portland cement should be suitable for project concrete. However, if there is no, or minimal cost differential, use of Type II Portland cement (or equivalent) should be considered for additional sulfate resistance of construction concrete.

Sulfate concentrations above 0.2 percent can cause an adverse reaction between the sulfates and calcium based stabilizing agents, resulting in heaving of the subgrade if lime or fly ash stabilization is performed. Based on the soluble sulfate test results (about 0.038 to 0.064 percent), we believe risk of increased swelling due to sulfate reaction to chemical stabilizing agents is low for this site.

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. Natural 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…