CO FLAP LAR 10(1) Horsetooth and Carter Lake - Final Pavement Report with Addendum.pdf

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SUBMITTED TO:

DJ&A

2000 Maple Street Missoula, Montana, 59808

BY:

Shannon & Wilson, Inc.

1321 Bannock St., Suite 200 Denver, Colorado 80204

(303)825-3800 www.shannonwilson.com

PAVEMENT DESIGN REPORT

CO FLAP LAR 10(1) Horsetooth and Carter Lake Reservoirs

LARIMER COUNTY, COLORADO

November 2021

Shannon & Wilson No: 106316-001

CO FLAP LAR 10(1) Horsetooth and Carter Lake Reservoirs Pavement Design Report

106316-001 November 2021 ii

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CONTENTS

1 Introduction

2 Project and Site Description

3 Subsurface Investigation

4 Geotechnical Laboratory Testing

5 Subsurface Conditions

5.1 Regional Geology

5.2 Existing Pavement and Aggregate Surfacing Thicknesses

5.3 Pavement Subgrade

5.4 Groundwater

6 Pavement Design Recommendations

6.1 Design Methodology

6.2 Traffic Loading

6.3 Larimer County Minimum Pavement Sections

6.4 Design Subgrade R-value

6.5 Full Depth Reclamation (FDR)

6.6 Recommended Pavement Sections

6.7 Additional Considerations

6.7.1 Expansive Soil

6.7.2 PDDM Subexcavation Requirements

6.7.3 Frost Damage

6.7.4 Corrosion Testing

7 Construction and Material Specifications

7.1 Site Preparation

7.2 Earthwork

7.2.1 General

7.2.2 Subgrade Preparation and Fill Placement

7.2.3 Proof-Rolling

7.3 Paving Materials

8 Closures

9 References iii

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Exhibits Exhibit 5-1: Summary of the Existing Pavement Sections Exhibit 6-1: Summary of Traffic Data Exhibit 6-2: Summary of R-value Test Results Exhibit 6-3: Recommended Pavement Sections Exhibit 6-4: Larimer County Subgrade Expansion Potential (2017) Exhibit 7-1: Recommended Materials for Pavements

Figures Figure 1: Vicinity Map Figure 2: Site and Exploration Plan, Horsetooth Reservoir Figure 3: Site and Exploration Plan, Carter Lake Reservoir

Appendices Appendix A: Subsurface Explorations Appendix B: Laboratory Test Results Appendix C: Exploration Photographs Appendix D: Pavement Design Calculations Important Information

1 INTRODUCTION

This report summarizes the results of our subsurface exploration and laboratory testing program and presents pavement design recommendations and construction considerations for the proposed Horsetooth and Carter Lake Reservoirs, CO FLAP LAR 10(1) (the Project) in Larimer County, Colorado. Our scope of services was specified in Task Order 6982AF21F0000067 of Contract Number 6982AF-19-D-000006 with DJ&A dated April 26, 2021. Our conclusions and recommendations in this report are based on:

The limitations of our approved scope, schedule, and budget described in our contract;

Our understanding of the Project and information provided by DJ&A;

Subsurface conditions observed in the borings at the time our explorations were completed; and

The results of testing performed on samples collected from the explorations.

The objective of our geotechnical studies was to provide recommendations and construction considerations, as presented herein, for the proposed roadway reconstruction. The authorized scope of services was based on this objective and this report should not be used for other purposes without Shannon & Wilson’s review. If a service is not specifically indicated in this report, do not assume that it was performed.

2 PROJECT AND SITE DESCRIPTION

Horsetooth Reservoir & Carter Lake Reservoir are federally owned reservoirs managed by the U.S. Bureau of Reclamation (USBR) and maintained by Larimer County (Figure 1). The Project scope of work consists of improving the vehicular, pedestrian and bicycle access at Horsetooth and Carter Lake Reservoirs while simultaneously addressing safety concerns to these areas which serve over one million visitors annually. The proposed facility improvements primarily consist of paving and/or reconstruction of the existing parking lot pavement and access road pavement at the following sites:

Widening of W. County Road 38E (CR 38E) to incorporate a designated turn lane at the Horsetooth Reservoir, South Bay entrance. We understand that no evaluation or improvements to the existing CR 38E pavement was scoped.

Horsetooth Reservoir, South Bay parking areas;

Horsetooth Reservoir, Inlet Bay parking area;

Carter Lake Reservoir, Big Thompson Parking Area and proposed Overflow Lot;

Carter Lake Reservoir, Gate House South pullout area;

Carter Lake Reservoir, Gate House North pullout area;

Carter Lake Reservoir, South Shore parking area;

Carter Lake Reservoir, North Pines Eagle Campground parking and access roads;

Carter Lake Reservoir, North Pines Northwest and Southwest Lots and Access Road;

and

Carter Lake Reservoir, North Pines Marina Lot.

All existing parking lots and access roads are paved with either asphalt concrete pavement (ACP) or an aggregate surfacing (AS). The project consists of new ACP in each of the above parking lots and access roads. Parking lots located at the Cater Lake Reservoir, Big Thompson Parking Areas will be widened and the Big Thompson Overflow Lot will be newly constructed. Refer to Figures 2 and 3 for the Horsetooth and Carter Lake Reservoirs site plans, respectively.

3 SUBSURFACE INVESTIGATION

Shannon & Wilson conducted a field exploration program on July 7 and 8, 2021, to explore subsurface conditions withing the proposed parking lots and access roads. The subsurface exploration program consisted of drilling and sampling a total of 21 borings designated as borings SW-01 through SW-21. Refer to Figures 2 and 3 for the approximate boring locations. The borings were advanced to a depth of approximately 4 to 10 feet, except at boring SW-10 where sandstone bedrock was encountered at 2.5 feet, and the boring advancement was terminated.

Appendix A presents a discussion of the drilling and sampling procedures used to complete the borings. Appendix A also presents the individual exploration logs and an explanation of the symbols and terminology used.

4 GEOTECHNICAL LABORATORY TESTING

We completed geotechnical laboratory tests on selected samples retrieved from the borings to estimate index and engineering properties. The index tests included natural water content, grain size analysis, and Atterberg limits. The engineering property tests included corrosion, swell/collapse, and Hveem stabilometer (R-value). The laboratory test results and a discussion of the testing procedures are included in Appendix B. The natural water content, fines content, and Atterberg limits are also shown on the individual boring logs included in Appendix A.

5 SUBSURFACE CONDITIONS

5.1 Regional Geology

A Geologic Map of the Horsetooth Reservoir Quadrangle, Larimer County, Colorado (Braddock and others, 1989) indicates that bedrock units in the Project area are, from youngest to oldest, the Lykins Formation, Lyons Sandstone, and Fountain Formation. Their mapping indicates that the Lykins Formation is approximately 600 feet thick and consists of red and reddish-brown siltstone and sandstone and occasionally contains thin carbonate beds. The Lyons Sandstone is an orange to pinkish-gray, quartzose sandstone approximately 50 feet thick. The Fountain Formation is up to 1100 feet thick and is composed of reddish-brown to purplish-gray, arkosic conglomerate, feldspathic sandstone, siltstone and shale, and has minor thin limestone layers. While not shown on the geologic map in the vicinity of the Horsetooth Reservoir portion of the project area, our borings typically encountered alluvial deposits in this area.

A Geologic Map of the Carter Lake Reservoir Quadrangle, Boulder and Larimer Counties, Colorado (Braddock and others, 1988) indicates that surficial deposits in the project area are composed of silt, clay, sand, gravel, and boulders. Bedrock units in the vicinity of the project area consist of the Lykins Formation, Lyons Sandstone, and Owl Canyon Formation.

Descriptions for the Lykins Formation and the Lyons Sandstone can be found above. The Owl Canyon Formation is described as red siltstone and fine-grained sandstone up to 200 feet thick.

5.2 Existing Pavement and Aggregate Surfacing Thicknesses

The existing pavement or aggregate surfacing thicknesses at each boring location are summarized in Exhibit 5-1. In general, many of the site parking lots and access roads have a relatively thin existing AS or ACP section.

Exhibit 5-1: Summary of the Existing Pavement Sections

Location Boring Pavement

Section Location Boring Pavement

Section

County Road 38E SW-01 9-1/2 in. ACP CLR Gate House North SW-12 < 1 in. AS

Horsetooth Reservoir South Bay

SW-02 4 in. ACP CLR South Shore

SW-13 ~4 in. AS

SW-03 5 in. ACP SW-14 ~4 in. AS

SW-04 4 in. ACP CLR North Pines (NP)

Eagle Campground

SW-15 ~2 in. AS

SW-05 ~5 in. AS SW-16 ~2 in. AS

Horsetooth Reservoir Inlet Bay

SW-06 ~4 in. AS SW-17 1 in. ACP

SW-07 < 1 in. AS CLR NP Access Road SW-18 < 1 in. AS

Carter Lake Reservoir (CLR) Big Thompson Parking Area

SW-08 ~3 in. AS CLR NP Southwest Lot SW-19 ~9 in AS

SW-09 ~3 in. AS CLR NP Northwest Lot SW-20 ~4 in. AS

CLR Big Thompson Overflow SW-10 NA CLR NP Marina Lot SW-21 ~3 in. AS

CLR Gate House South SW-11 3 in. ACP

NOTE:

1. CLR Big Thompson Overflow boring completed in proposed parking lot.

AS = Aggregate Surfacing; ACP = Asphalt Concrete Pavement; in. = inches; NA = Not Applicable

5.3 Pavement Subgrade

Subsurface conditions below the existing pavements varied between (and within) each parking lot. Individual boring logs of our exploration locations are presented in Appendix A. Generalized subsurface conditions consisted of:

Horsetooth Reservoir South Bay: Sandy lean clay and clayey sand [AASHTO soil classification A-6 soils]. Borings SW-02 and SW-04 encountered loose to medium dense, poorly graded sand with clay and gravel and silty sand [AASHTO A-2-4 and A-4 soils].

Horsetooth Reservoir Inlet Bay: Surficial soils consisted of clayey sand and sandy lean clay [AASHTO A-4 and A-6 soils].

Carter Lake Reservoir Big Thompson Parking Area and Overflow Lot: Loose to very dense, silty to clayey sand [AASHTO A-2-4, A-4, and A-6 soil] overlying Lyons Sandstone bedrock at depths of 1.5 to 4.5 feet.

Carter Lake Reservoir Gate House South: Boring SW-11 indicated very loose, poorly graded sand [AASHTO A-1-b soil] overlying lean clay with sand [AASHTO A-7-6 soil].

Carter Lake Reservoir Gate House North: Boring SW-12 indicated soft to medium stiff, lean clay with sand [AASHTO A-7-6 soil].

Carter Lake Reservoir South Shore: Borings SW-13 and SW-14 indicate medium stiff to very stiff, lean clay with sand to sandy lean clay [AASHTO A-7-6 soil] and clayey sand with gravel [AASHTO A-2-4 soil], respectively.

Carter Lake Reservoir North Pines Eagle Campground: Medium dense to dense, poorly graded sand to silty sand [AASHTO A-2-4 soil] overlying Lyons Sandstone bedrock at depths of 2.8 to 4.0 feet.

Carter Lake Reservoir North Pines Access Road: Boring SW-18 indicated soft to medium stiff, lean clay with sand [AASHTO A-6 soil] overlying Lyons Sandstone bedrock at a depth of 2.5 feet.

Carter Lake Reservoir North Pines Southwest Lot: Boring SW-19 indicated loose, clayey sand with gravel [AASHTO A-2-6 soil].

Carter Lake Reservoir North Pines Northwest Lot: Boring SW-20 indicated medium dense, clayey sand [AASHTO A-2-6 soil] overlying Lyons Sandstone bedrock at a depth of 2.5 feet.

Carter Lake Reservoir North Pines Marina Lot: Boring SW-21 indicated medium dense to dense, poorly graded gravel with silt and sand [AASHTO A-1-b soil] overlying Lyons Sandstone bedrock at a depth of 4.5 feet.

Our observations are specific to the locations, depths, and dates noted on the exploration logs in Appendix A and may not be applicable to all areas of the site. There is no amount of explorations or laboratory testing that can precisely predict the characteristics, quality, or distribution of subsurface conditions at every location throughout the site. Variations in the subsurface conditions may occur between and below the borings. Also, the passage of time or intervening causes (natural and humanmade) may result in changes to the conditions of the site and subsurface conditions.

5.4 Groundwater

Groundwater was only encountered in boring SW-09 at a depth of 4 feet. However, fluctuations of groundwater levels at the site are possible and will depend on many factors, including seasonal variations, local precipitation, and flood events. Nevertheless, groundwater is not anticipated to affect the design or construction of the Project.

6 PAVEMENT DESIGN RECOMMENDATIONS

Based on discussions with CFLHD and DJ&A, we understand each parking lot will be reconstructed with either a composite pavement section consisting of ACP over crushed aggregate base (CAB) or ACP over a full depth reclamation (FDR); refer to Section 6.5. As indicated in Exhibit 5-1, several of the existing parking lots are surfaced with aggregate.

Incorporating the existing AS into the proposed pavement sections was initially considered, but due to the relatively thin thicknesses of AS in several parking lots (less than 3 inches thick) and variability from adjacent borings, in our opinion there is a potential for overestimating the existing AS thickness and, therefore, underestimating the quantity of CAB that will be required.

In our opinion, the existing AS can remain in place and paved over with the proposed pavement section. If grades need to be maintained, the AS can be overexcavated and the proposed pavement section can be installed.

6.1 Design Methodology

Our pavement design and results are based on the design procedures presented in the 1993 AASHTO Guide for the Design of Pavement Structures with guidance from the U.S.

Department of Transportation and Federal Highway Administration (FHWA) Federal Lands Highway Project Development and Design Manual (PDDM), dated March 2008; the Colorado Department of Transportation (CDOT) 2021 M-E Pavement Manual; and Larimer Counties pavement design guidelines (2007). Pavement design inputs and calculations are presented in Appendix D.

6.2 Traffic Loading

The PDDM does not provide guidance for pavement design for parking lots but does indicate a minimum analysis traffic loading of 50,000 18-kip equivalent single axle loading (ESAL) for roadways. In our opinion the design traffic values (ESALs) are difficult to determine for parking lots, as the traffic patterns are difficult to predict. Further, the overall traffic frequency within individual parking stalls are typically less than roadways, and the sustained tire loading from parked vehicles can be more damaging and difficult to quantify.

We were provided a study of the 2018 annual number of visitors for each parking lot (not vehicles accessing the parking lots), which is summarized in Exhibit 6-1. Based on discussions with DJ&A and CFLHD, a design traffic loading of either 30,000 or 50,000 ESALs are to be used for the parking lot and access road pavements approximately based on total number of annual visitors. Exhibit 6-1 summarizes the traffic loading provided to us for each parking lot.

For CR 38E, we accessed the data provided in the Larimer County GIS Enterprise Portal (2021), which provides the 2015 average annual daily traffic value of 4,700 vehicles per day and a major collector roadway classification. No additional information was provided on percentage of trucks or vehicle types. We then reviewed the County design standards for 20-year design life ESALs for major collector (2007), which indicates a minimum value design ESAL value of 365,000. With no additional information provided, we used the County minimum design ESAL for our pavement analysis for CR 38E.

Exhibit 6-1: Summary of Traffic Data

Site Parking Lot / Location Annual Visitors

18-kip Design ESALs

CR 38E South Bay Parking Lot Turn Lane - 365,000(1)

Horsetooth Reservoir

South Bay 198,000 50,000

Inlet Bay 129,500 30,000

Carter Lake

Big Thompson Parking Area - 30,000

Big Thompson Overflow Lot - 30,000

Gate House South 100,000 50,000

Gate House North 100,000 50,000

South Shore Parking Area 60,000 30,000

North Pines Eagle Campground 30,000 30,000

North Pines Parking Areas 10,000 30,000

North Pines Marina Lot 50,000 30,000

NOTES:

20 year design life is based on Larimer County minimum ESAL requirement for major collectors.

ADT = Average Daily Traffic, CR = County Road; ESAL = Equivalent Single Axle Loading

6.3 Larimer County Minimum Pavement Sections

Based on discussions with Larimer County, only a composite pavement section with a minimum of 4 inches of ACP was considered for the proposed parking lots and access roads.

6.4 Design Subgrade R-value

Exhibit 6-2 summarizes the subsurface soils and laboratory test results encountered at each of the parking lots.

Exhibit 6-2: Summary of R-value Test Results

Site Parking Lot Boring

Fines Content

(%) LL PI USCS

AASHTO

Soil

Class.

R-value Test

Result

South Bay SW-05 58 28 15 CL A-6 (5) 15.7

Inlet Bay SW-07 38 26 11 SC A-6 (1) 12.4

Gate House North SW-12 70 41 25 CL A-7-6 (15) 5.1

South Shore SW-13 57 45 28 CL A-7-6 (13) 5.4

North Pines Eagle Campground SW-15 26 NV NP SM A-2-4 (0) 73.6

North Pines Southwest Lot SW-19 32 30 18 SC A-2-6 (1) 16.1

North Pines Marina Lot SW-21 12 NV NP GP-GM A-1-b (0) 79.9

LL = Liquid Limit; NP = Non-Plastic; NV = No Value; PI = Plasticity Index; USCS = Unified Soil Classification System

Based on the design traffic loading for each parking lot as well as the Larimer County minimum ACP pavement thicknesses requirements, we evaluated the proposed parking lot and access road pavements for the lowest R-value to satisfy the structural section. Our analysis indicates that the lowest R-value of 5 will provide the minimum sections for the parking lots. For County Road 38E, we assumed a design R-value of 15 corresponding to the test result obtained in boring SW-05 (located in the South Bay parking lot). To estimate a design resilient modulus, we used a Colorado Department of Transportation (CDOT) correlation between R-value and resilient modulus (2020). The correlation used is provided on the individual flexible pavement worksheet provided in Appendix D.

6.5 Full Depth Reclamation (FDR)

In our opinion, only the East Lot of Horsetooth Reservoir’s South Bay is suitable for FDR, which consists of in-place pulverization that becomes the base of a new ACP paving subgrade. The surface of the FDR can accommodate minor grading adjustments provided a minimum FDR thickness is maintained. A new ACP surface layer is then paved over the FDR prepared roadway typically resulting in an overall raise to the roadway grades.

Rehabilitating the park roads with FDR effectively provides a new pavement and eliminates the potential for reflective cracking from the existing pavement. As the existing pavement thickness varied from 4 to 5 inches, we recommend placing a minimum of 2 inches of CAB over the parking lot and performing a 6-inch-thick FDR.

We do not recommend utilizing FDR at the remaining ACP parking lots as the existing ACP is relatively thin (approximately 3 inches or less) and no base course was observed underlying the ACP. Performing FDR on these parking lots will mix the relatively thin ACP with the clay subgrade and result in a poor FDR material.

6.6 Recommended Pavement Sections

Using the AASHTO procedures and the parameters outlined in Appendix D, we recommend the following pavement sections:

Exhibit 6-3: Recommended Pavement Sections

Site Parking Lot / Location Recommended Pavement Section

CR 38E South Bay Parking Lot Turn Lane

7.5 in. Full Depth ACP

5.5 in. ACP over 6 in. CAB

All Lots 4 in. ACP over 6 in. CAB

East & West Lot 4 in. ACP over 6 in. FDR (Requires 2 in. CAB)(1)

Inlet Bay 4 in. ACP over 6 in. CAB

Big Thompson Lot 4 in. ACP over 6 in. CAB

Big Thompson Overflow Lot 4 in. ACP over 6 in. CAB

Gate House South 4 in. ACP over 6 in. CAB

Gate House North 4 in. ACP over 6 in. CAB

South Shore Parking Area 4 in. ACP over 6 in. CAB

North Pines Eagle Campground 4 in. ACP over 4 in. CAB(2)

North Pines Southwest Lot 4 in. ACP over 6 in. CAB

North Pines Northwest Lot 4 in. ACP over 6 in. CAB

North Pines Marina Lot 4 in. ACP over 4 in. CAB(2)

2 inches of CAB placed over the existing asphalt prior to FDR is required.

Larimer County minimum pavement thicknesses for local, two-lane residential road is 4 inches of ACP over 6 inches of CAB. In our opinion, if subgrade soils consist of A-1 or A-2-4, the CAB thickness can be reduced to 4 inches.

ACP = Asphalt Concrete Pavement; CAB = Crushed Aggregate Base, ESAL = Equivalent Single Axle Loading;

FDR = Full Depth Reclamation

For CR 38E, we recommend installing a full depth ACP pavement section. Boring SW-01, located in the CR 38E westbound lane indicated full depth pavement section with ACP 9-1/2 inches thick. Installing a composite pavement section adjacent to the existing roadway will likely result in the CAB layer unable to drain unless the CAB layer in the composite pavement section can be daylighted.

6.7 Additional Considerations

6.7.1 Expansive Soil

High plasticity soils in Colorado are susceptible to volume change by swelling/shrinking.

This geologic phenomenon has the potential to cause substantial damage to lightly loaded structures, such as pavements, when exposed to water. To provide an indication of the swell potential of near surface soils at the site, we performed Atterberg limits, grain size distributions, moisture contents, and swell tests on pavement subgrade soil samples encountered in our explorations. This data is summarized in Table B-1 in Appendix B.

Swell testing was completed on three subgrade samples from the South Bay and Inlet Bay at Horsetooth Reservoir. All swell tests indicated no expansive volume change.

Guidance from Larimer County indicates expansive subgrade mitigation measures are only required if the observed swell exceeds 2%, but Larimer County also provides the guidance summarized in Exhibit 6-4 for expansive subgrade characteristics. As noted, several properties can provide an indication of potential volume change, but assessing the swell potential based on these properties (liquid limit [LL] and fines content [% passing no. 200 sieve]) does not consider the current moisture content regime for pavements that have been in place for several years. For clays, the standard penetration test (SPT) does not directly correlate with moisture content, but low blow counts (SPT resistance) typically are an indication of higher moisture contents (moisture contents above the plastic limit [PL]) and very stiff clays (SPT blow counts exceeding 15 blows per foot) typically have lower moisture contents (moisture contents below the PL) and are more susceptible to volume changes. For clayey sands, in our opinion, SPT blow counts are a poor indication of swell potential.

Exhibit 6-4: Larimer County Subgrade Expansion Potential (2017)

Based on the above Larimer County expansion potential guidance, the clayey sand subgrade (AASHTO A-2-6 soils) generally have low to medium expansive characteristics. The lean clay encountered in boring SW-11 through SW-13 (AASHTO A-7-6 soils) are an indication of high expansive characteristic (as indicated by the fines content and LL), but the SPT blow counts indicate soft to medium stiff clays and, therefore, have low expansive characteristics.

In our opinion, pavements typically block evapotranspiration from occurring and the subgrade below the pavement (including aggregate surfaced roads) have an overall elevated in-situ moisture content after paving. Comparing the in-situ moisture content of the subgrade to the PL provides a better indication of the in-situ expansive characteristics.

For the clayey soils (AASHTO A-6 and A-7-6 soils) observed beneath the proposed parking lots, the in-situ moisture contents are near or greater than the PL, and in our opinion, likely have a low swell susceptibility provided the subgrade soils are not exposed and allowed to dry during construction.

Based on the above, we recommend the following for expansive soil mitigation:

Areas with Existing AS or Proposed FDR Areas: Proof roll the existing AS or compacted FDR and place proposed pavement section on existing AS or FDR.

Areas of Exposed Subgrade Below Existing AS or ACP: Remove existing ACP where applicable. Scarify the exposed subgrade and moisture treat subgrade. Then proof roll the moisture treated surface.

Widening or Roadway Realignment Areas (Areas with No Existing Pavement): Complete a minimum of 16 inches of subgrade moisture treatment. Then proof roll the moisture treated surface. Proposed parking lots at the Eagle Campground Lot and Marina Lot encountered granular materials (AASHTO A-1-b and A-2-4). For these parking lots we recommend scarifying and recompacting 8 inches below the proposed pavement section.

Refer to Section 7.2.2 for compaction requirements and Section 7.2.3 for discussion on proof rolling. Areas of failing proof rolls may require subexcavation (removal and import of non-swell susceptible material). Other methods of expansive subgrade treatment can be completed, such as lime or cement stabilization, but given the project budgetary constraints, these stabilization methods are not anticipated to be economically feasible.

6.7.2 PDDM Subexcavation Requirements

To mitigate against potential constructability issues (pumping subgrade and or achieve compaction of subgrade), as well as long-term swell potential, Section 11.3.2.1.3 of the PDDM indicates:

2 feet of subexcavation (overexcavation and removal of excavated material from the site) for roadways with a plastic index (PI) ranging from 15 to 25, 2 to 4 feet of subexcavation for subgrades with a PI ranging from 25 to 35 or LL ranging from 50 to 60, and

4 to 6 feet of subexcavation for subgrades with a PI greater than 35 or a LL greater than 60.

To reduce project costs, we suggest the CFLHD consider subexcavation only in areas of failing proof rolls. Refer to Section 6.7.1 for our expansive subgrade treatment recommendations.

6.7.3 Frost Damage

Frost susceptible soils can lead to pavement performance issues due to heaving or deformation from ice lenses in the underlying soil and pavement fatigue damage due to thaw-weakened subgrade of the springtime freeze-thaw cycle (PDDM, 2008). Frost susceptible soils typically include fine grained soils such as silts and fine sand. In accordance with the PDDM, typical treatment for frost susceptible soils consists of assuring there is an adequate pavement layer structure to account for the loss of bearing capacity during the spring thaw and removing or replacing highly frost susceptible soil for a portion of the expected frost depth. Based on our experience on past CFLHD projects and recommendations in the PDDM, we understand it is cost prohibitive to provide protection against frost heave on smaller projects. If this is not the case, we recommend removal and replacement to a depth of 2 feet (up to about 70% of the 3-foot-deep frost depth).

6.7.4 Corrosion Testing

The soil encountered at the project site can be corrosive to substructure elements. To assist in estimating the corrosion potential at the site, three samples consisting of a sandy lean clay, clayey sand, and poorly graded gravel with silt and sand from borings SW-05, SW-19, and SW-21, respectively, were each tested for pH, resistivity, water soluble sulfates, and chlorides. The results are presented in Table B-1 in Appendix B.

The resistivity measured in the samples was 2,307 to 9,187 ohm-centimeters. Based on correlations developed by Roberge (2012), these values suggest highly corrosive to moderately corrosive subsurface conditions for metal in contact with subsurface materials across the site.

The concentration of water-soluble sulfates measured in the samples were all less than 0.01 percent by weight. Based on classifications as defined by the American Concrete Association (2019), these test results suggest an exposure class S0 on concrete exposed to site soils.

The test results and the above discussion are provided to assist the designer in the selection of project materials, concrete type, or other features with respect to corrosion.

7 CONSTRUCTION AND MATERIAL SPECIFICATIONS

The applicability of the design parameters in Section 6 is contingent on good construction practice. Poor construction techniques may alter conditions from those upon which our recommendations are based, and therefore result in poor performance. Our analyses assumed that this project is constructed according to FP-14 U.S. Customary Units (USDOT and FHWA, 2014) construction standards. The following sections provide additional construction considerations for this project.

7.1 Site Preparation

All surface and subsurface structures associated with current development of the site, including utility poles, fence poles, underground utilities and other deleterious material, should also be removed. Any existing surficial topsoil and soil containing visible organics should be stripped and removed from all areas.

7.2 Earthwork

7.2.1 General

Earthwork, including placement of fill and subgrade preparation, should conform to the requirements provided in the FP-14 U.S. Customary Units, (USDOT and FHWA, 2014) and the recommendations provided in the following sections.

7.2.2 Subgrade Preparation and Fill Placement

Proper subgrade preparation is required for adequate pavement performance. Areas of exposed subgrade should be prepared in accordance with FP-14 Section 204.11. All subgrade material and fill should be compacted to a dense/firm and unyielding condition.

On-site subgrade and fill materials should be compacted to at least 95 percent maximum density, as determined by AASHTO T 180 or T 99, as presented in FP-14 Section 204. Fill should be placed in uniform, horizontal layers not exceeding 8 inches in loose thickness for heavy, self-propelled compactors, or 4 inches for hand-operated mechanical compactors.

The appropriate lift thickness will depend on the Contractor’s equipment as well as the moisture content and quality of the fill material.

7.2.3 Proof-Rolling

We recommend proof-rolling or probing excavation subgrade areas to determine suitability of the subgrade. In areas that are identified as being loose, soft, or yielding during proof-rolling or probing, we recommend:

1. Overexcavating the pavement section a depth up to 16 inches below the pavement section.

2. Place a stabilization and separator geotextile at the base of the overexcavation.

3. Backfilling and moisture treating with granular import material with a fines content between 15% and 35%.

7.3 Paving Materials

The following exhibit summarizes our recommendations for pavement material selection using the FP-14 specifications. Based on the project estimated paving quantities, we understand that the ACP material will utilize FP-14 Section 403, which uses the local department of transportation mix design. Exhibit 7-1 below reflects a CDOT (2019) mix design for the ACP.

Exhibit 7-1: Recommended Materials for Pavements

Material FP-14 Specification Additional Requirements/Comments

CAB Section 301 Use Gradation C, D, or E

FDR Section 304 -

ACP Section 403 (FP-14) Using CDOT Section 401

(2019)

Aggregate Gradation: SX (1/2-inch NMAS) PG Binder: PG 64-28 Gyratory Number (N): 75

Subgrade Stabilization and Separator Geotextile

Section 714.01 Class 1 Separation and Stabilization Geotextile Maximum AOS 0.3 mm

AOS = Apparent Opening Size; ACP = Asphalt Concrete Pavement; CAB = Crushed Aggregate Base; CDOT = Colorado Department of Transportation; FDR = Full Depth Reclamation; mm = Millimeters; NMAS = Nominal Maximum Aggregate Size; PG = Performance Grade;

To determine the recommended SuperPave Binder, we used the long-term pavement performance (LTPP) Bind Version 3.1Beta (2008) using data from the Waterdale weather station data (CO8839) . We recommend using a performance grade (PG) 64-28 binder for the ACP with a SX gradation (½-inch nominal maximum aggregate size [NMAS]) for all lifts. We recommend that lift thicknesses for the ½-inch NMAS ACP should be between 1.5 and 3.0 inches. A tack coat should be placed between subsequent lifts if more than 24 hours has occurred between paving.

8 CLOSURES

This report has been prepared for the exclusive use of DJ&A and the Central Federal Lands Highway Division for the purpose of providing pavement recommendations for the Horsetooth and Carter Lake Reservoirs improvements project. This pavement design report should not be used without our approval if any of the following occurs:

Assumptions stated in this report have changed.

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

A substantial period of time has passed since the date of this report.

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

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

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

9 REFERENCES

American Concrete Institute (ACI), 2019, Building code requirements for structural concrete and commentary, Farmington Hills, Mich., ACI 318-19.

American Association of State Highway and Transportation Officials (AASHTO), 1993, AASHTO guide for design of pavement structures: Washington, D.C., AASHTO, 2 v.

Braddock, W.A., Nutalay, Prinya, and Colton, R.B., 1988, Geologic map of the Carter Lake Reservoir quadrangle, Boulder and Larimer Counties, Colorado: U.S. Geological Survey, scale 1:24,000.

Braddock, W.A., Caltert, R.H., O’Connor, J.T., and Swann, G.A., 1989, Geologic map of the Horsetooth Reservoir quadrangle, Larimer County, Colorado: U.S. Geological Survey, scale 1:24,000.

Colorado Department of Transportation (CDOT), 2020, 2021 M-E Pavement Design Manual;

available: https://www.codot.gov/business/designsupport/materials-and-geotechnical/manuals/2021-m-e-pave-design-manual

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

https://www.codot.gov/business/designsupport/cdot-construction-specifications

Larimer County, 2021, Geographic Information Systems (GIS) Enterprise GIS Portal;

accessed August, 2021; available: https://www.larimer.org/it/gis

Larimer County, 2007, Urban Area Street Standards, Adopted January 2, 2001; Chapter 10 – Pavement Design and Report; available:

https://www.larimer.org/engineering/standards-and-guides/prior-urban-area-street-standards

Federal Highway Administration (FHWA), 2008, LTPPBind 3.0/3.1; available:

https://infopave.fhwa.dot.gov/Page/Index/LTPP_BIND

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

U.S. Department of Transportation (USDOT), FHWA, 2014, Federal standard specifications for construction of roads and bridges on Federal Highway projects, FP-14 English Units: U.S. Department of Transportation and Federal Highway Administration, available: https://highways.dot.gov/federal-lands/specs https://www.codot.gov/business/designsupport/materials-and-geotechnical/manuals/2021-m-e-pave-design-manual https://www.codot.gov/business/designsupport/materials-and-geotechnical/manuals/2021-m-e-pave-design-manual https://www.codot.gov/business/designsupport/cdot-construction-specifications https://www.larimer.org/it/gis https://www.larimer.org/engineering/standards-and-guides/prior-urban-area-street-standards https://www.larimer.org/engineering/standards-and-guides/prior-urban-area-street-standards https://infopave.fhwa.dot.gov/Page/Index/LTPP_BIND

USDOT, FHWA, 2008, Federal Lands Highway project development and design manual (PDDM): U.S. Department of Transportation and Federal Highway Administration; available: http://flh.fhwa.dot.gov/resources/manuals/pddm/ http://flh.fhwa.dot.gov/resources/manuals/pddm/

Horsetooth and Carter Lake Reservoirs

Drake

Mountain View

Coyote Ridge Natural Area

Mountain Park

Lory State Park

Heights

Masonville

Carter Lake

Welch

Boedecker

Mariana Butte Golf Course

Little Dam

Cedar Cove

Buckingham

Sinnard

Black Hollow Junction

Fort Collins

Fo ss il C re ek

R es er vo ir In le t

Fossil Creek

Running Deer

Cathy Fromme Prairie Natural

Area

Colorado State University

Omega

Harmony

Drakes

McClellands

Trilby Corner

Arrowhead

Redmond

Horseshoe

Boyd Lake

W E isen h o w e r B lvd isenhow er lvd

Campion

Kings Corner

Welty

Loveland

Berthoud

Esri, HERE, Garmin, SafeGraph, METI/NASA, USGS, EPA, NPS, USDA, Maxar

Colorado Springs Pueblo

Fort Collins

Project Location

25 0 1 2 3 4 5

Miles

CO FLAP LAR 10(1)

Larimer County, Colorado

VICINITY MAP

FIG. 1

106316-001

LEGEND

Parking Areas

SHANNON & WILSON, INC.

Geotechnical and Environmental Consultants

Denver

To Loveland

To Fort Collin s

Maxar, Microsoft

SITE AND EXPLORATION PLAN

HORSETOOTH RESERVOIR

FIG. 2

November 2021 106316-001

Sheet 1 of 3

South Bay Parking Areas

0 200 400 600

Approximate Scale In Feet

SW-02

S T a ft

H il l d

City of Fort Collins, Esri, HERE, Garmin, SafeGraph, METI/NASA, USGS, EPA, NPS, USDA

County Road 38E

< Boring Designation and Approximate Location

Proposed Improvement Parking Areas

South Bay Entrance Road

M at ch lin h ee

SW-03

SW-01

Sheet 2 of 3

&< Boring Designation and Approximate Location

Access Road (East Lot to North Lot)

South Bay Parking Areas ch h ee

SW-05

SW-04

Sheet 3 of 3

Access Road (Shoreline Drive to Inlet Bay Lot)

Shoreline Drive

Shoreline Drive

Sh or el in

Dr ive

Inlet Bay Parking AreasInlet Bay

SW-06

SW-07

CARTER LAKE RESERVOIR

SW-08

FIG. 3

Sheet 1 of 4

CR-8E

Esri, HERE, Garmin, SafeGraph, METI/NASA, USGS, EPA, NPS, USDA

Big Thompson Parking Areas & Proposed Overflow Lot Gate House North

Gate House South

South Shore

Big Thompson Parking Marina Lot

North Pines Lots

SW-09

SW-10

Sheet 2 of 4

0 200 400

Gate House North Gate House South South Shore

C ounty R oad 31 ounty R oad 38E

Co un ty oa

N

SW-13

SW-14

SW-11SW-12

Sheet 3 of 4

SW-17

S County Road 31

North Pines Marina Lot & Eagle Campground

SW-21

SW-15 SW-16

Sheet 4 of 4

SW-18

North Pines Road

Co ty oa

North Pines Northwest and Southwest Lots

SW-20

SW-19

CO FLAP LAR 10(1), CFL Horsetooth and Carter Lake Reservoirs

11/2/2021-106316-R1-AA.docx A-i

AP

PE

ND

IX

A : S

UB

SU

RF

AC

EX

PL

OR

AT

IO

NS

Appendix A: Subsurface Ex plorations

Appendix A

Subsurface Explorations

CONTENTS

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

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

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

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

A.2.3 Modified California (MC) Sampling ..................................................................... A-2

A.2.4 Pocket Penetrometer ................................................................................................ A-2

A.2.5 Bulk Sampling .......................................................................................................... A-3

Figures Figure A-1: Soil Classification and Log Key Figure A-2: Rock Classification and Log Key Figures A-3 through A-9: Log of Borings SW-01 through SW-07, Horsetooth Reservoir Figures A-10 through A-23: Log of Borings SW-08 through SW-21, Carter Lake Reservoir

A-1

A.1 INTRODUCTION

Shannon & Wilson’s field exploration program was conducted on July 7 and 8, 2021 and consisted of drilling 21 borings at the locations shown on Figures 2 and 3. The methods used to conduct the field exploration program are described below. Laboratory testing procedures and results are presented in Appendix B.

A.2 EXPLORATIONS

The borings were coordinated (including subcontractor coordination, utility locates, permitting, and traffic control) and observed by Shannon & Wilson. Individual boring logs are presented in Figures A-3 to A-23. The exploration logs represent our interpretation of the contents of the field log and select results of laboratory testing.

The borings were drilled by Vine Laboratories, Inc. of Commerce City, Colorado (under subcontract to Shannon & Wilson) using a CME 55 truck mounted drill rig. The borings were advanced to depths of approximately 4.3 to 11.5 feet. The borings were advanced with 6-inch diameter solid-stem auger. Where groundwater was encountered, our field representative measured the approximate depth to groundwater using an electronic water level indicator at the time of drilling. Boring SW-01 was backfilled with flow fill and the pavement was repaired with asphalt cement pavement. All other borings were backfilled with drill cuttings. Repairs to the existing pavement were made with cold-patch asphalt.

A.2.1 Soil Classification System

During exploration, our representative collected samples and prepared field logs of the explorations. Soil classification for this project was based on ASTM International (ASTM) Designation: D2487, Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System), and ASTM Designation: D2488, Standard Practice for Description and Identification of Soils (Visual-Manual Procedure). The Unified Soil Classification System (USCS) is summarized in Figure A-1. The Shannon & Wilson representative classified rock samples in general accordance with the International Society of Rock Mechanics (ISRM) classification method. According to this system, rocks are classified based on the stratigraphic structure, rock strength, degree of weathering, and other properties. The rock classification system is summarized in Figure A-2.

Consistent with other locations in Colorado, the bedrock encountered in the borings was found to be hard when considered as a lithified soil material. However, when compared with other types of bedrock using the ISRM classification of rock strength, the material

A-2 resembles a very low strength rock. Therefore, for completeness, the boring logs included in Appendix A contain dual descriptions of the bedrock using the Unified Soil Classification System and the ISRM classification system.

A.2.2 Standard Penetration Test (SPT)

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

A.2.3 Modified California (MC) Sampling

Samples were also obtained using a modified California (MC) barrel sampler. The MC test procedure is similar to the SPT, except a larger diameter barrel sampler (2½-inch O.D., lined with 2-inch-diameter brass tubing) is used and only driven 12 inches. During sampling, the Shannon & Wilson field representative recorded the number of blows for each 6-inch increment of penetration. As a result of the larger diameter, the MC sampler yields slightly higher raw blow count numbers when compared to SPT N-values for similar soils.

However, this is counted by not ignoring a seating depth like is done for the SPTs. Because the difference in blow counts does not significantly impact our evaluation, we used the field MC blow counts over the 12-inch increment to define the relative density and consistency/stiffness of the subsurface materials following SPT terminology. Representative samples were sealed in the brass liner tubes with plastic caps and transported to our laboratory.

A.2.4 Pocket Penetrometer

Select cohesive soil samples were also tested in the field using a pocket penetrometer. The penetrometer estimates the unconfined compressive strength of clay soil samples by penetrating the clay with a one-quarter-inch-diameter penetrometer and measuring the

A-3 resistance (in units of tons per square foot [tsf]) with a calibrated spring. Measurements can be taken to the nearest 0.25 tsf increment. The field measurements from the pocket penetrometer are included on the boring logs.

A.2.5 Bulk Sampling

Bulk soil samples were obtained by collecting the drill cuttings from select borings.

Approximately 20 to 30 pounds of cuttings from each location were placed in a plastic bag and transported to our laboratory for further evaluation and testing. The bulk samples are composite samples sometimes spanning over several soil layers.

106316-001November 2021

COBBLES

GRAVEL

FINES

SAND

Sheet 1 of 3

S&W INORGANIC SOIL CONSTITUENT DEFINITIONS

CONSTITUENT2

COHESIONLESS SOILS

Silt, Lean Clay, Elastic Silt, or

Fat Clay 3

PERCENTAGES TERMS 1, 2

Trace

Few

Little

Some

Mostly

WELL AND BACKFILL SYMBOLS

Bentonite Cement Grout

Bentonite Grout

Bentonite Chips

Silica Sand

Perforated or Screened Casing

Absence of moisture, dusty, dry to the touch

Damp but no visible water

Visible free water, from below water table

FIG. A-1

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

STANDARD PENETRATION TEST (SPT)

SPECIFICATIONS

Hammer:

Sampler:

N-Value:

Dry

Moist

Wet

MOISTURE CONTENT TERMS

Modifying (Secondary)

Precedes major constituent

Major

Minor Follows major constituent

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

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

3Determined based on behavior.

4Determined based on which constituent comprises a larger percentage.

5Whichever is the lesser constituent.

COARSE-GRAINED

SOILS

(less than 50% fines)1

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

PARTICLE SIZE DEFINITIONS

RELATIVE DENSITY / CONSISTENCY

Sand or Gravel 4

30% or more coarse-grained:

Sandy or Gravelly 4

More than 12% fine-grained:

Silty or Clayey 3

15% to 30% coarse-grained:

with Sand or with Gravel 4

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

with Gravel 5

Very soft Soft Medium stiff Stiff Very stiff Hard

Very loose Loose Medium dense Dense Very dense

RELATIVE

DENSITY

FINE-GRAINED SOILS

(50% or more fines)1

COHESIVE SOILS

< 2 2 - 4 4 - 8

8 - 15 15 - 30

> 30

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

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

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

140 pounds with a 30-inch free fall.

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

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

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

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

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

RELATIVE

CONSISTENCY

N, SPT,

BLOWS/FT.

5% to 12% fine-grained:

with Silt or with Clay 3

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

with Gravel 5

< 5%

5 to 10%

15 to 25%

30 to 45%

50 to 100%

Surface Cement Seal

Asphalt or Cap

Slough

Inclinometer or Non-perforated Casing

Vibrating Wire Piezometer

N, SPT,

BLOWS/FT.

< 4 4 - 10

10 - 30 30 - 50

> 50

DESCRIPTION

< #200 (0.075 mm = 0.003 in.)

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

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

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

SIEVE NUMBER AND/OR APPROXIMATE SIZE

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

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

3 to 12 in. (76 to 305 mm)

> 12 in. (305 mm)

Fine Coarse

Fine Medium Coarse

BOULDERS

SOIL CLASSIFICATION

AND LOG KEY

_B O

IN

G _C

LA

F L

.G P

J

D

/2 2/

GC

SC

Inorganic

Organic

(more than 50% of coarse fraction retained on No. 4 sieve)

MAJOR DIVISIONS GROUP/GRAPHIC

SYMBOL

CH

OH

ML

CL

TYPICAL IDENTIFICATIONS

Gravel

Sand

Silty Sand; Silty Sand with Gravel

Clayey Sand; Clayey Sand with Gravel

Clayey Gravel; Clayey Gravel with Sand

Sheet 2 of 3

Gravels

Primarily organic matter, dark in color, and organic odor

SW

(more than 12% fines)

Silts and Clays

Silts and Clays

(more than 50% retained on No.

200 sieve)

(50% or more of coarse fraction passes the No. 4 sieve)

(liquid limit less than 50)

(liquid limit 50 or more)

Organic

Inorganic

FINE-GRAINED

SM

Sands

Silty or Clayey Gravel

Silt; Silt with Sand or Gravel; Sandy or Gravelly Silt

Organic Silt or Clay; Organic Silt or Clay with Sand or Gravel; Sandy or Gravelly Organic Silt or Clay

HIGHLY-

ORGANIC

COARSE-

GRAINED

GW

(less than 5%

PT

(more than 12%

MH

SP

GP

GM

Silty or Clayey Sand

Silty Gravel; Silty Gravel with Sand

(50% or more passes the No.

200 sieve) Elastic Silt; Elastic Silt with Sand or Gravel; Sandy or Gravelly Elastic Silt

Fat Clay; Fat Clay with Sand or Gravel;

Sandy or Gravelly Fat Clay

Organic Silt or Clay; Organic Silt or Clay with Sand or Gravel; Sandy or Gravelly Organic Silt or Clay

Poorly Graded Sand; Poorly Graded Sand with Gravel

Well-Graded Sand; Well-Graded Sand with Gravel

Well-Graded Gravel;…