58766GE LPC Fairgrounds Senior Center Pavement_FINAL (04-28-25).pdf

Durango and Cortez, Colorado 970.259.5095

GEOTECHNICAL ENGINEERING STUDY

FOR PAVEMENT SECTION THICKNESS DESIGN

LA PLATA COUNTY FAIRGROUNDS AND SENIOR CENTER

DURANGO, COLORADO

April 28, 2025

PREPARED FOR:

Blaine Montgomery

Assistant Director

La Plata County General Services

235 Bodo Drive

Durango, Colorado 81303 bmontgomery@lpcgov.org

PROJECT NO. 58766GE

mailto:bmontgomery@lpcgov.org

Project No. 58766GE

Table of Contents

1.0 REPORT INTRODUCTION

1.1 Proposed Construction

2.0 FIELD STUDY

2.1 Site Description and Geomorphology

2.2 Subsurface Soil and Water Conditions

3.0 LABORATORY STUDY

4.0 PAVEMENT SECTION THICKNESS DESIGN RECOMMENDATIONS

4.1 General Existing Asphalt Pavement Evaluation

4.2 Entrance Roadway Lot Existing Pavement Evaluation

4.2.1 Entrance Road Asphalt Pavement Design Recommendations

4.3 Senior Center Parking Lot Existing Pavement Evaluation

4.3.1 Senior Center Parking Lot Asphalt Pavement Design Recommendations

4.4 Central Parking Lots Existing Pavement Evaluation

4.4.1 Central Parking Lots Asphalt Pavement Design Recommendations

4.5 Northeast Parking Lot Existing Pavement Evaluation

4.5.1 Northeast Parking Lot Asphalt Pavement Design Recommendations

4.6 Pavilion Parking Lot Existing Pavement Evaluation

4.6.1 Pavilion Parking Lot Asphalt Pavement Design Recommendations

4.7 Western Parking Lots Existing Pavement Evaluation

4.7.1 Western Parking Lots Asphalt Pavement Design Recommendations

4.7.2 Reflection Crack Control

4.8 New Pavement Areas

4.8.1 New Western Parking Lots Asphalt Pavement Design Recommendations

4.8.2 New Flexible Trailer Parking Asphalt Pavement Design Recommendations

4.9 General Asphalt Pavement Recommendations

5.0 SUBGRADE STABILIZATION RECOMMENDATIONS

6.0 CONSTRUCTION CONSIDERATIONS

6.1 Fill Placement Recommendations

6.1.1 Natural Soil Fill

6.1.2 Granular Compacted Structural Fill

6.2 Excavation Considerations

6.3 Grading and Drainage Comments

7.0 CONSTRUCTION MONITORING AND TESTING

8.0 CONCLUSIONS AND CONSIDERATIONS

9.0 LIMITATIONS

LABORATORY TEST RESULTS ....................................................................................................... Appendix A

Sieve and Atterberg Limit Test Results

Moisture/Density Relationship Test Results

California Bearing Ratio Test Results

ATTACHMENT

Table 1 – Subsurface Conditions Summary Table

1.0 REPORT INTRODUCTION

This report presents our geotechnical engineering recommendations for the proposed drive lane and parking lot pavement rehabilitation project at the La Plata County Fairgrounds and Senior

Center in Durango, Colorado. This report was requested by Blaine Montgomery of the La Plata

County General Services Department and was prepared in accordance with our proposal dated

February 26, 2025, Proposal No. 25073P.

As outlined within our proposal for services for this project, the client is responsible for appropriate distribution of this report to other design professionals and/or governmental agencies unless specific arrangements have been made with us for distribution.

This geotechnical engineering report is the beginning of a process involving the geotechnical engineering consultant on any project. It is imperative that the geotechnical engineer be consulted throughout the design and construction process to verify the implementation of the geotechnical engineering recommendations provided in this report. Often the design has not been started or has only been initiated at the time of the preparation of the geotechnical engineering study. Changes in the proposed design must be communicated to the geotechnical engineer so that we have the opportunity to tailor our recommendations as needed based on the proposed site development and structure design.

All recommendations provided within this report must be followed in order to achieve the intended performance of the foundation system and other components that are supported by the site soil.

1.1 Proposed Construction

We understand that the proposed project will consist of evaluation of current and proposed future parking areas and drive lanes at the La Plata County Fairgrounds and La Plata County Senior

Center. The project will also include construction observations and materials testing in 2025 for the Fairgrounds entrance drive lane and the “G Lot.” Construction in other areas will be completed at a later date; however, the site characterization and geotechnical recommendations will cover the entire area identified in the Fairgrounds Master Plan.

2.0 FIELD STUDY

2.1 Site Description and Geomorphology

The project site is located in the North Main area of Durango, Colorado. A site location schematic is provided below as Figure 1.

Figure 1: Site Location Schematic. Adapted from Google Earth (Imagery Date 8/1/2024).

The La Plata County Fairgrounds and Senior Center complex consists of a variety of structures, parking areas and drive lanes. The facilities are used for a variety of user groups and events. The pavement can be generally characterized as being in poor condition with moderate to severe cracking of various types, pothole patching and rutting in some locations.

2.2 Subsurface Soil and Water Conditions

We advanced 19 test borings throughout the study area. A schematic showing the approximate boring locations is provided below as Figure 1. The logs of the soils encountered in our test borings are presented in the attached Table 1.

Figure 2: Locations of Exploratory Borings. Adapted from Google Earth (Imagery Date 8/1/2024).

The schematic presented above was prepared using notes and field measurements obtained during our field exploration and is intended to show the approximate test boring locations for reference purposes only.

The subsurface conditions encountered in our test borings consisted of 2½ to 4½ inches of asphalt except for test borings (TB) TB-18 and TB-19 that were advanced in landscape areas. Asphalt millings were found in some borings beneath the pavement surface. Approximate ¾ inch aggregate base course (ABC) ranged in thickness from about 5 to 14½ inches in borings advanced in existing pavement areas. Aggregate sub-base that ranged from approximate 1½ inch to 3 inch minus material was also encountered beneath the ABC in some borings.

The logs of the subsurface soil conditions encountered in our test borings are presented in the attached Table 1. The logs present our interpretation of the subsurface conditions encountered in the test borings at the time of our field work. Subsurface soil and water conditions are often variable across relatively short distances. It is likely that variable subsurface soil and water conditions will be encountered during construction. Laboratory soil classifications of samples obtained may differ from field classifications.

We did not encounter free subsurface water in our test borings at the time of the advancement.

We suspect that the subsurface water elevation and soil moisture conditions will be influenced by snow melt and/or precipitation and local irrigation.

3.0 LABORATORY STUDY

The laboratory study included tests to estimate the strength, swell and consolidation potential of the soils tested. We performed the following tests on select samples obtained from the test borings.

The laboratory test results are provided in Appendix A.

• Moisture Content and Dry Density

• Sieve Analysis (Gradation)

• Atterberg Limits, Liquid Limit, Plastic Limit and Plasticity Index

• Moisture Content Dry Density Relationship Test

• California Bearing Ratio Test

A synopsis of some of our laboratory data for some of the samples tested is tabulated below.

Sample

Designation

Percent

Passing

#200

Sieve

Atterberg

Limits

LL/PI

Moisture

Content

Maximum

Modified

Proctor Dry

Density

(pcf)

Optimum

Moisture

Content

California

Bearing

Ratio

TB-1 @ 4.5-

19” (ABC)

13.6 23/5 6.2 - - -

TB-4 @ 5-11”

(ABC)

15.2 NV/NP 4.1 - - -

TB-7 @ 5-10”

(ABC)

3.6 NV/NP 3.6 - - -

TB-8 @ 18-

50” (SG)

24.6 23/8 5.7 - - -

TB-13 @ 13-

48” (SG)

64.4 24/10 14.9 - - -

TB-14 @ 5-

11” (ABC)

13.0 20/NP 3.6 - - -

TB-6 @ 4.5-

17” (ABC)

19.7 25/8 6.9 - - -

TB-17 @ 4-

16”

14.8 20/7 8.0 - - -

TB-18 @ 0-

48”

62.9 27/13 14.5 - - -

TB-19 @ 0-

48”

61.8 26/11 14.1 - - -

Bulk SC-GC

Subgrade

45.8 27/13 8.8 135.4 8.3 4.4

Bulk SC-CL

Subgrade

59.8 24/10 2.6 132.6 8.7 12.2

Notes:

1. ABC=Aggregate Base Course; SG=Subgrade

2. Modified Proctor Dry Density and Optimum Moisture based on rock corrected test results.

3. California Bearing Ratio based on 90% of the Maximum Proctor Dry Density.

4.0 PAVEMENT SECTION THICKNESS DESIGN RECOMMENDATIONS

Based on the subsurface information and surface observations, we do not feel a mill and overlay, or fabric with overlay are feasible while keeping the existing asphalt elevation the same. Milling of the existing asphalt would likely result in excessive breakdown. Further, the overlay would have an increased risk of reflection cracking and premature deterioration.

We have provided a general analysis of the existing condition and current structural number values of the existing pavement section in Section 4.1 below. Our asphalt pavement recommendations are presented in Section 4.2. General asphalt pavement recommendations are presented in Section 4.3.

4.1 General Existing Asphalt Pavement Evaluation

This section of the report provides an analysis for the structural number of the existing asphalt pavement section. Our analysis is intended to be general in nature as it is based on limited field and laboratory data and is intended to provide a baseline structural number for the subsurface conditions encountered that can be utilized for the design recommendations provided in Section

4.2 below. The following analysis should be considered valid only for the portion of the project area outlined in Section 1.1 of this report.

Based on our observations of the existing asphalt pavement surface, we feel that the existing asphalt pavement in the project area may be considered as being in fair to locally poor condition.

We observed localized areas on the roadway surface where moderate to severe alligator cracking, potholing/patching, rutting and longitudinal and transverse cracking has occurred in the pavement surface. We suspect that the distressed area is related to the age of the pavement section, poor drainage, freeze-thaw, and structurally deficient sections of asphalt pavement for the existing traffic loading.

We observed variability in the pavement condition, existing asphalt pavement structural support materials and subgrade materials throughout the project area. We have provided a general assessment and breakdown of different project areas below.

Traffic projections and corresponding 18,000-pound (18k) equivalent single axel load (ESAL) factors were not available at the time of this report. We have provided conceptual pavement section thickness recommendations based on anticipated traffic loading estimates. If higher ESAL values are anticipated or if alternative recommendations are required, the pavement sections presented in this report should be re-evaluated. The table below provides a general outline of the estimated traffic volume and general assessment of the rehabilitation or reconstruction methods recommended.

Assessment Area Estimated 18K ESAL Average Daily Traffic (ADT) Percent Trucks Assessment

Main Entry Drive Lane 400,000 1,000 5% Full Reconstruction

Pavilion Parking Lot 400,000 1,000 5% Full Reconstruction

Senior Center 40,000 250 2% Asphalt Removal and Replacement

Central Parking Lots 200,000 500 5% Asphalt Removal and Replacement

NE Parking Lot 400,000 1,000 5% Asphalt Removal and Replacement

Existing Western Parking Lots 37,000 240 2% Mill and Overlay

New Western Parking Lots 87,000 575 2% New Construction

Ward Field New Trailer Parking 200,000 500 5% New Construction

4.2 Entrance Roadway Lot Existing Pavement Evaluation

The approximate location of the Entrance Roadway study area is shown below outlined in red.

Figure 3: Entrance Road Study Area. Adapted from Google Earth (Imagery Date 8/1/2024).

The Entrance Road Study Area analysis was based on subsurface information obtained from TB-

10, -11 and -12. The asphalt was observed to be in poor condition in this area. Other assumptions are outlined below.

• Existing Asphalt Thickness = 2 to 4 inches

• Avg. ABC Thickness = 8 inches (including asphalt milling)

• ABC Structural Coefficient = 0.11

• Avg. Aggregate Sub-Base Thickness = 4 inches (none encountered in TB-9)

• Aggregate Sub-Base Structural Coefficient = 0.09

• Structural Number (SN) of Existing Support Aggregate = 1.24

• Approximate Assumed 18k ESAL = 400,000 o 20-year design period o Two-way average daily traffic (ADT) = 1,000 o Percent Trucks = 5% o Truck Factor = 1.7 ESAL/Truck

• Minimum Structural Number = 3.61

We performed a California Bearing Ratio (CBR) test on a composite sample of soil obtained from the project site. Based on the laboratory analysis, a CBR value of 4.4 was obtained for the on-site soils, which roughly correlates to an R-Value of 10 and a resilient modulus of 3,562 pounds per square inch. The CBR results are provided in Appendix A.

Based on the relatively thin section and deterioration of the existing asphalt, a mill and overlay is not considered feasible. We understand that grades cannot be raised; therefore, a total reconstruction is necessary to support the anticipated traffic loading at the site as the existing support materials are not sufficient to support a new asphalt mat which meets the necessary traffic loading. Our design recommendations are provided below.

4.2.1 Entrance Road Asphalt Pavement Design Recommendations

We have provided multiple options for pavement sections for the entry roadway assuming a total reconstruction. Assumptions used for our design recommendations include:

• Reliability Factor R(%) = 80%

• Overall Standard Deviation, So = 0.44

• Estimated Total 18K-ESAL value(s) = 400,000

• Minimum Structural Number = 3.42

• Effective Roadbed Soils Resilient Modulus, Mr = 3,025

• Change is serviceability index, Delta PSI = 2.5

• Structural Coefficient of Asphalt Pavement = 0.44

• Structural Coefficient of Aggregate Base Course = 0.11

• Structural Coefficient of Aggregate Sub-Base Course = 0.09

• Modifying Structural Layer Coefficients for aggregate base course and aggregate sub-base course layers, mi = 1.0 (fair drainage conditions with 5%-25% saturation frequency)

Pavement Section Design Thickness – 400,000 ESAL (Minimum SN = 3.42)

Pavement Section

Component Alternative Thickness of Each Component (inches)

Asphalt Concrete 5 5 5.5 5.5

Class 6 Roadbase 4 11 4 10

Class 2 Sub-Base 9 0 7 0

Structural Number 3.45 3.52 3.49 3.52

If desired, the existing asphalt, existing millings and existing ABC may be repurposed/mixed and utilized as structural equivalent support to new ABC.

4.3 Senior Center Parking Lot Existing Pavement Evaluation

The approximate location of the Senior Center Parking Lot study area is shown below outlined in red.

Figure 4: Senior Center Parking Lot Study Area. Adapted from Google Earth (Imagery Date 8/1/2024).

The Senior Center Parking Study Area analysis was based on subsurface information obtained from TB-13 and TB-14. The asphalt was observed to be in poor condition in this area. Other assumptions are outlined below.

• Existing Asphalt Thickness = 2½ to 3 inches

• Avg. ABC Thickness = 8¾ inches (including asphalt milling)

• ABC Structural Coefficient = 0.11

• Avg. Aggregate Sub-Base Thickness = No Sub-Base Encountered

• Structural Number (SN) of Existing Support Aggregate = 0.9625

• Approximate Assumed 18k ESAL = 40,000 o 20-year design period o Two-way average daily traffic (ADT) = 250 o Percent Trucks = 2%

• Minimum Required Structural Number = 2.44 is not considered feasible. Based on the structural number of the existing aggregate support materials, removal of the existing asphalt surface and reworking of the existing materials followed by placement of a new asphalt mat should be sufficient based on the anticipated traffic loading.

Our design recommendations are provided below.

4.3.1 Senior Center Parking Lot Asphalt Pavement Design Recommendations

Based on the anticipated traffic and existing subsurface conditions, we feel removal of the existing asphalt, reworking of the support aggregate and replacement with a new 4 inch thick asphalt mat is suitable for the Senior Center Parking Lot. The new 4 inch mat will be roughly 1 to

1½ inches thicker than the existing asphalt; therefore, the grades of the underlying aggregate should be graded to allow for no change in the surface grade. Assuming a final ABC thickness of about 7 inches followed by a new 4 inch thick asphalt mat, final structural number of 2.53 has been calculated which exceeds the minimum of 2.44. Assumptions used for our design recommendations include:

• Estimated Total 18K-ESAL value(s) = 40,000

• Minimum Structural Number = 2.44

• Effective Roadbed Soils Resilient Modulus, Mr = 3,562

• Change is serviceability index, Delta PSI = 2.5

• Structural Coefficient of Asphalt Pavement = 0.44

• Structural Coefficient of Existing Aggregate Base Course = 0.11

4.4 Central Parking Lots Existing Pavement Evaluation

The approximate location of the Central Parking Lots study area is shown below outlined in red.

Figure 5: Central Parking Lots Study Area. Adapted from Google Earth (Imagery Date 8/1/2024).

The Central Parking Lots pavement analysis was based on subsurface information obtained from

TB-4, -5, -6 and -7. The asphalt was observed to be in fair condition in this area. Other

• Existing Asphalt Thickness = 2½ to 4½ inches

• Avg. ABC Thickness = 7 inches (including asphalt milling)

• ABC Structural Coefficient = 0.11

• Avg. Aggregate Sub-Base Thickness = 8¾ inches

• Aggregate Sub-Base Structural Coefficient = 0.09

• Structural Number (SN) of Existing Support Aggregate = 1.56

• Approximate Assumed 18k ESAL = 200,000 o 20-year design period o Two-way average daily traffic (ADT) = 500 o Percent Trucks = 5%

• Minimum Required Structural Number = 3.10 is not considered feasible. Based on the structural number of the existing aggregate support materials, removal of the existing asphalt surface and reworking of the existing materials followed by placement of a new asphalt mat should be sufficient based on the anticipated traffic loading.

Our design recommendations are provided below.

4.4.1 Central Parking Lots Asphalt Pavement Design Recommendations existing asphalt, reworking of the support aggregate and replacement with a new 4 inch thick asphalt mat is suitable for the Central Parking Lots area. The new 4 inch mat will be roughly 1½ inches thicker than the existing asphalt in some areas; therefore, the grades of the underlying aggregate should be graded to allow for no change in the surface grade. Assuming a final ABC thickness of about 6 inches followed by a new 4 inch thick asphalt mat, final structural number of

3.21 has been calculated which exceeds the minimum of 3.10. Assumptions used for our design recommendations include:

• Estimated Total 18K-ESAL value(s) = 200,000

• Minimum Structural Number = 3.10

• Effective Roadbed Soils Resilient Modulus, Mr = 3,562

• Change is serviceability index, Delta PSI = 2.5

• Structural Coefficient of Asphalt Pavement = 0.44

• Structural Coefficient of Existing Aggregate Base Course = 0.11

4.5 Northeast Parking Lot Existing Pavement Evaluation

The approximate location of the Northeast Parking Lot study area is shown below outlined in

Figure 6: Northeast Parking Lot Study Area. Adapted from Google Earth (Imagery Date 8/1/2024).

The Northeast Parking Lot pavement analysis was based on subsurface information obtained from TB-15 and TB-16. The asphalt was observed to be in fair to poor condition in this area.

Other assumptions are outlined below.

• Existing Asphalt Thickness = 4 to 4½ inches

• Average ABC Thickness = 8¾ inches (including asphalt millings)

• ABC Structural Coefficient = 0.11

• Average Aggregate Sub-Base Thickness = 7½ inches

• Aggregate Sub-Base Structural Coefficient = 0.09

• Structural Number (SN) of Existing Support Aggregate = 1.56

• Approximate Assumed 18k ESAL = 400,000 o 20-year design period o Two-way average daily traffic (ADT) = 1,000 o Percent Trucks = 5%

Based on the deterioration of the existing asphalt, a mill and overlay is not considered feasible.

Based on the structural number of the existing aggregate support materials, removal of the existing asphalt surface and reworking of the existing materials followed by placement of a new asphalt mat should be sufficient based on the anticipated traffic loading. Our design recommendations are provided below.

4.5.1 Northeast Parking Lot Asphalt Pavement Design Recommendations existing asphalt, reworking of the support aggregate and replacement with a new 4½ inch thick asphalt mat is suitable for the Northeast Parking Lot. The new 4½ inch mat may be slightly thicker than the existing asphalt in some locations; therefore, the grades of the underlying aggregate should be graded to allow for no change in the surface grade. The existing combined support aggregate ranged from about 14 to 18½ inches. The structural number of the existing aggregate materials is assumed to be about 1.64. Following removal of the existing asphalt and reworking of the existing gravel, a new 4½ inch thick asphalt mat should be placed which will yield a final structural number of 3.62 which should be suitable for support given the traffic loading assumptions. Assumptions used for our design recommendations include:

• Estimated Total 18K-ESAL value(s) = 40,000

• Minimum Structural Number = 3.42

• Effective Roadbed Soils Resilient Modulus, Mr = 3,562

• Change is serviceability index, Delta PSI = 2.5

• Structural Coefficient of Asphalt Pavement = 0.44

• Structural Coefficient of Existing Aggregate Base Course = 0.11

4.6 Pavilion Parking Lot Existing Pavement Evaluation

The approximate location of the Pavilion Parking Lot study area is shown below outlined in red.

Figure 7: Pavilion Lot Study Area. Adapted from Google Earth (Imagery Date 8/1/2024).

The Pavilion Parking Area analysis was based on subsurface information obtained from TB-8, TB-9. The asphalt was observed to be in poor condition in this area. Other assumptions are outlined below.

• Existing Asphalt Thickness = 3½ to 4 inches

• Avg. ABC Thickness = 7.75 inches (including asphalt milling)

• ABC Structural Coefficient = 0.11

• Avg. Aggregate Sub-Base Thickness = 6 inches (only encountered in TB-8)

• Aggregate Sub-Base Structural Coefficient = 0.09

• Structural Number (SN) of Existing Support Aggregate = 0.8525 (No Sub-Base Incl.)

• Approximate Assumed 18k ESAL = 200,000 o 20-year design period o Two-way average daily traffic (ADT) = 500 o Percent Trucks = 5%

• Minimum Structural Number = 3.27

Based on the relatively thin section of support aggregate and deterioration of the existing asphalt, a mill and overlay is not considered feasible. We understand that grades cannot be raised;

therefore, a total reconstruction is necessary to support the anticipated traffic loading at the site as the existing support materials are not sufficient to support a new asphalt mat which meets the necessary traffic loading. Our design recommendations are provided below.

4.6.1 Pavilion Parking Lot Asphalt Pavement Design Recommendations

We have provided multiple options for pavement sections for the pavilion parking lot assuming a total reconstruction. Assumptions used for our design recommendations include:

• Estimated Total 18K-ESAL value(s) = 200,000

• Minimum Structural Number = 3.09

• Effective Roadbed Soils Resilient Modulus, Mr = 3,562

• Change is serviceability index, Delta PSI = 2.5

• Structural Coefficient of Asphalt Pavement = 0.44

• Structural Coefficient of Aggregate Base Course = 0.11

• Structural Coefficient of Aggregate Sub-Base Course = 0.09

Pavement Section Design Thickness – 200,000 ESAL (Minimum SN = 3.09)

Pavement Section

Component Alternative Thickness of Each Component (inches)

Asphalt Concrete 4 4 4.5 4.5

Class 6 Roadbase 4 13 4 11

Class 2 Sub-Base 10 0 8 0

Structural Number 3.10 3.19 3.14 3.19

If desired, the existing asphalt, existing millings and existing ABC may be repurposed/mixed and utilized as structural equivalent support to new ABC.

4.7 Western Parking Lots Existing Pavement Evaluation

The approximate location of the existing Western Parking Lots pavement study area is shown below outlined in red.

Figure 8: Western Parking Lots Study Area. Adapted from Google Earth (Imagery Date 8/1/2024).

The Western Parking Lots pavement analysis was based on subsurface information obtained from

TB-1. -2, -3 and TB-17. The asphalt was observed to be in fair condition in this area. Other

• Existing Asphalt Thickness = 3 to 4½ inches o SW Parking Lot Avg. = 3½ inches o NW Parking Lot = 4 inches

• Existing Asphalt Structural Coefficient = 0.25

• ABC Thickness = 8 to 14½ inches (including asphalt milling) o SW Parking Lot Avg. = 12½ inches o NW Parking Lot = 8 inches

• ABC Structural Coefficient = 0.11

• Average Aggregate Sub-Base Thickness = 9 inches (only encountered in TB-3)

• Aggregate Sub-Base Structural Coefficient = 0.09

• Structural Number (SN) of Existing Support Aggregate = 0.88 to 1.595 (not including

Sub-Base)

• Approximate Assumed 18k ESAL = 37,000 o 20-year design period o Two-way average daily traffic (ADT) = 240 o Percent Trucks = 2%

• Existing Structural Number = 2.64

Based on the surface and subsurface conditions encountered in our study a mill and overlay may be a feasible alternative for the existing parking lot in this area if the final structural number of the asphalt surface meets the County’s anticipated traffic loading for this area. Our design recommendations are provided below.

4.7.1 Western Parking Lots Asphalt Pavement Design Recommendations

Based on the anticipated traffic and existing subsurface conditions, we feel a 1½ to 2½ inch mill over overlay may be feasible for these parking areas. We recommend a 1 ½ inch mill and overlay for the Southwest Parking Lot and a 2½ inch mill and overlay for the Northwest Parking Lot

Following the mill and overlay the final pavement surface will have an approximate structural number ranging from 2.36 to 2.41 which should be suitable for support of traffic loading given the assumptions outlined in Section 4.7. Assumptions used for our design recommendations include:

• Estimated Total 18K-ESAL value(s) = 30,000

• Minimum Structural Number = 2.33

• Effective Roadbed Soils Resilient Modulus, Mr = 3,562

• Change is serviceability index, Delta PSI = 2.5

• Structural Coefficient of New Asphalt Pavement = 0.44

• Structural Coefficient of Existing Milled Asphalt Pavement = 0.25 o NW Parking Lot Milled Asphalt Remining = 2½ inches o SW Parking Lot Milled Asphalt Remining = 1½ to 3 inches

• Structural Coefficient of Existing Aggregate Base Course = 0.11

• Structural Coefficient of Existing Aggregate Sub-Base = 0.09

Asphalt overlays should begin with pre-overlay repairs such as removal and patching of existing low areas and areas of severely damaged or yielding asphalt. Some portions of the roadway may require removal of large areas to establish proper surface grades and drainage of the asphalt surface. In these damaged areas, we recommend milling and removal of an additional 2 inches of existing asphalt and replacement with new asphalt material.

There are numerous variables from traffic considerations as well as variable material thicknesses and strengths which influence the proposed overlay project. Minor variations in the anticipated traffic volume and minor changes in the material thickness will not have a significant influence on the needed overlay thickness. Minor changes in the strength characteristics of the support materials may have a significant influence on the overlay thickness needed. We based our analysis on the project traffic loads to reduce the potential variables associated with this assessment.

4.7.2 Reflection Crack Control

The basic mechanism of reflection cracking is strain concentration in the overlay due to movement in the vicinity of cracks in the existing surface. This movement may be bending or shear induced by loads or may be horizontal contraction induced by temperature changes. Load induced movements are influenced by the thickness of the overlay and the thickness and stiffness of the existing pavement. Temperature induced movements are influenced by daily and seasonal temperature variations, the coefficient of thermal expansion of the existing pavement, and the spacing of cracks.

Reflection cracks are a frequent cause of the overlay deterioration. Additional steps must be taken to reduce the occurrence and severity of reflection cracking. Some overlays are less susceptible to reflection cracking than others because of their materials and design. Similarly, some reflection crack control measures are more effective with some pavement and overlay types than with others.

Pre-overlay repair (i.e., patching and crack filling, heater scarifying) may help delay the occurrence and deterioration of reflection cracks. Additional reflection crack control measures include the following:

• Removal of the existing pavement by milling an additional 2 inches. Specific distresses are reduced or eliminated by removal of some of the pavement.

• Crack filling at least one year prior to the overlay.

• Fabric overlays.

The long-term benefits of non-woven synthetic fabrics have been shown to be ineffectual as a crack resistance interlayer between the old pavement and the new overlay. They generally retard the cracks from propagating into the new overlay; however, the cracks reappear within a few years.

Encountering the non-woven synthetic fabric interlayer has caused production problems in most subsequent rehabilitation strategies (e.g. cold planning, hot-in-place recycling processes, FDR, etc.). Due to these adverse effects, it is not recommended to use non-woven synthetic fabrics as a pre-overlay repair method.

Fiberglass grids and composite interlayers (a combination of fiberglass grid and nonwoven fabric) offer significant benefits for controlling reflection cracking in asphalt overlays. Fiberglass grids provide high tensile strength and excellent resistance to crack propagation by intercepting and distributing tensile stresses caused by underlying pavement movement. Their low elongation properties ensure that cracks are arrested before they reflect through to the surface layer.

Composite systems further enhance performance by integrating the structural reinforcement of fiberglass grids with the moisture barrier and stress absorption capabilities of paving fabrics. This dual-functionality not only improves fatigue resistance and structural integrity but also prevents water infiltration, which can degrade pavement layers over time. Together, fiberglass and composite interlayers may offer a robust, long-term solution for pavement preservation, particularly in high-traffic areas and regions subject to thermal cycling or heavy loading conditions.

4.8 New Pavement Areas

The Fairgrounds Master Plan includes three new pavement areas. The western parking lots, west of the baseball fields will be connected in the vicinity of TB-18. Another new parking lot will be constructed south of the Fairgrounds entrance in the vicinity of TB-19. A final new parking flexible trailer parking area will be constructed on the outside (southeast) of the southern baseball field. The approximate location of the new pavement areas are shown below outlined in red.

Figure 9: New Pavement Areas. Adapted from Google Earth (Imagery Date 8/1/2024).

4.8.1 New Western Parking Lots Asphalt Pavement Design Recommendations

Two new parking areas are planned on the western side of the Fairgrounds complex. One area will essentially connect the two existing parking lots on the west side of the baseball fields and one new parking area is planned just south of the Fairgrounds entrance. These new areas are shown on Figure 9 above. The analysis for these parking areas was based on subsurface information obtained from TB-18 and TB-19. Other assumptions are outlined below.

• Approximate Assumed 18k ESAL = 87,000 o 20-year design period o Two-way average daily traffic (ADT) = 575 o Percent Trucks = 2%

• Minimum Structural Number = 2.74

We have provided multiple options for pavement sections for the new parking areas assuming new construction. Assumptions used for our design recommendations include:

• Reliability Factor R(%) = 80%

• Estimated Total 18K-ESAL value(s) = 87,000

• Minimum Structural Number = 2.74

• Effective Roadbed Soils Resilient Modulus, Mr = 3,562

• Change is serviceability index, Delta PSI = 2.5

• Structural Coefficient of Asphalt Pavement = 0.44

• Structural Coefficient of Aggregate Base Course = 0.11

• Structural Coefficient of Aggregate Sub-Base Course = 0.09

Pavement Section Design Thickness – 87,000 ESAL (Minimum SN = 2.74)

Pavement

Section

Component

Alternative Thickness of Each Component (inches)

Asphalt

Concrete 4 4 4.5 4.5

Class 6

Roadbase 4 9 4 7

Class 2 Sub-

Base 7 0 6 0

Structural

Number

2.83 2.75 2.96 2.75

We do not recommend use of ¾-inch aggregate base course in layers less than 4 inches or the use of 3-inch minus sub-base in layers less than 6 inches. This may result in total structural numbers that are in excess of the minimum required by the anticipated traffic loading as can be seen in the tables above.

4.8.2 New Flexible Trailer Parking Asphalt Pavement Design Recommendations

Two new parking areas are planned as shown on Figure 9 above. The analysis for these parking areas was based on subsurface information obtained from TB-2, -3, -4, -5, -7 and -15. Other

• Approximate Assumed 18k ESAL = 200,000 o 20-year design period o Two-way average daily traffic (ADT) = 500 o Percent Trucks = 5%

• Minimum Structural Number = 3.09

We have provided multiple options for pavement sections for the flexible trailer parking assuming new construction. Assumptions used for our design recommendations include:

• Estimated Total 18K-ESAL value(s) = 200,000

• Minimum Structural Number = 3.09

• Effective Roadbed Soils Resilient Modulus, Mr = 3,562

• Change is serviceability index, Delta PSI = 2.5

• Structural Coefficient of Asphalt Pavement = 0.44

• Structural Coefficient of Aggregate Base Course = 0.11

• Structural Coefficient of Aggregate Sub-Base Course = 0.09

Pavement Section Design Thickness – 200,000 ESAL (Minimum SN = 3.09)

Pavement

Section

Component

Alternative Thickness of Each Component (inches)

Asphalt

Concrete 4 4 5 5

Class 6

Roadbase 4 13 4 9

Class 2 Sub-

Base 10 0 6 0

Structural

Number

3.10 3.19 3.18 3.19

We do not recommend use of ¾-inch aggregate base course in layers less than 4 inches or the use of 3-inch minus sub-base in layers less than 6 inches. This may result in total structural numbers that are in excess of the minimum required by the anticipated traffic loading as can be seen in the tables above.

4.9 General Asphalt Pavement Recommendations

We have estimated a pavement reliability factor (R) of 80 percent for all of the pavement sections outlined above. The Federal Highway Administration defines R as “the probability that a pavement section will perform satisfactorily over the design period. It must account for uncertainties in traffic loading, environmental conditions, and construction materials. The

AASHTO design method accounts for these uncertainties by incorporating a reliability level R to provide a factor of safety into the pavement design and thereby increase the probability that the pavement will perform as intended over its design life.” A higher R will result in thicker pavement section materials; however, may lead to a greater reliability in the pavement performance. The designer or project civil engineer should evaluate the desired R factor for the intended use. We can provide alternate reliability factors for the proposed pavement section upon request.

The asphalt pavement used on this project should be mixed in accordance with a design prepared by a licensed professional engineer, or an asphalt pavement specialist. We should be contacted to review the mix design prior to placement at the project site. We recommend that the asphalt pavement be compacted to between 92 and 96 percent of the maximum theoretical density.

We suspect that the subgrade soils will be well above the optimum moisture content in many areas of the project. We anticipate that conventional scarification and drying of the subgrade soils will be sufficient for most areas of the roadway subgrade provided warm and preferably breezy weather conditions are present during the project construction, and there is adequate time to perform scarification and drying construction procedures. However, it is likely that some areas of the subgrade will require specialty stabilization techniques. We have provided cursory recommendations for stabilization of severely yielding soil materials in Section 5.0 below.

The existing support soils should be scarified to a depth of about 6 inches, moisture conditioned and compacted to at least 90 percent of the maximum dry density as defined by ASTM D1557 or

AASHTO T180 (Modified Proctor). Proof rolling observations should then be performed over the prepared subgrade surface. Any areas of significant yielding should be stabilized as needed prior to placement of the overlying aggregate base course materials. The surface of the subgrade soil should be graded and contoured to be approximately parallel to the finished grade of the asphalt surface.

The aggregate materials used within the pavement section should conform to the requirements outlined in the current Specifications for Road and Bridge Construction, Colorado Department of

Transportation (CDOT). The aggregate base material should be a ¾ inch minus material that conforms to the CDOT Class 6 aggregate base course specifications and have an R-value of at least 78. The aggregate sub-base course should conform to the CDOT specifications for Class 2 material and should have a minimum R-value 70. Other material may be suitable for use in the pavement section, but materials different than those listed above should be tested and observed by us prior to inclusion in the project design or construction. Aggregate sub-base and base-course materials should be compacted to at least 95 percent of maximum dry density as defined by the modified Proctor test, ASTM D1557.

Thorough proof rolling with a fully loaded tandem axle water truck should be performed across the prepared aggregate surface prior to placement of the asphalt cement. Any areas that are observed to yield should be stabilized as necessary. We should be contacted to observe the proof rolling operations and provide recommendations for stabilization if necessary.

Water intrusion into the pavement section support materials will negatively influence the performance of the parking lot surface. Water from irrigation, water from natural sources that migrates into the soils beneath landscapes surface and water from any source that gains access to the support materials can all decrease the life of the parking lot surface. Care should be taken along curbs and any edge of the parking lot to develop an interface between the material that will reduce subsurface and surface water migration into the support soil and pavement section materials. Landscape islands and other irrigated features often promote water migration since no surface flow from these features typically occurs. The same can occur along perimeter cub areas.

Water will often migrate along the interface of concrete curbs and gutter areas early in the life of any parking area. The tendency for this type of migration often decreases with time but can be reduced by compaction of materials along the outside base of curb areas adjacent to the interface of the concrete curb and the underlying soil prior to placement of landscaping soil above this interface. The drainage characteristics of the roadway should be addressed by the project civil engineer.

5.0 SUBGRADE STABILIZATION RECOMMENDATIONS

We anticipate that stabilization of the subgrade materials may be required in some areas of the project. The actual stabilization design will need to be determined on a case-by-case basis as the design will be influenced on the degree of yielding at a particular area. We have provided a stabilization technique for subgrade soils that exhibit severe yielding below. This stabilization technique may be useful as a bid item for the project; however, we should provide continued consultation and observations during construction to provide site-specific recommendations.

A general construction concept for the stabilization of severely yielding subgrade soils is provided below.

• The subgrade soils should be over-excavated below the project subgrade elevation by one

1 foot. The soils should be proof-compacted with non-vibratory methods if excessive moisture is present.

• A layer of geogrid material such as Tensar BX 1100 or TX 140 should be placed in the bottom of the excavation.

• A 1-foot layer of crushed, relatively clean aggregate material should then be placed over the grid in at least two lifts. The crushed aggregate material should pass through a 2-inch sieve screen and have less than 5 percent passing the #4 sieve screen. The clean aggregate material should be compacted with non-vibratory methods.

• A geotextile fabric such as Mirafi 140N should then be placed over the clean crushed aggregate material to limit the potential for fines migration into the crushed aggregate layer.

• The initial approximate 6- to 8-inch-thick lift of the actual pavement design support section should then be placed and compacted per our recommendations.

We anticipate that less robust stabilization techniques may be applicable for many areas of the project. As discussed above, the actual stabilization technique will need to be based on a case-by-case basis.

6.0 CONSTRUCTION CONSIDERATIONS

The section of the report provides comments, considerations, and recommendations for aspects of the site construction which may influence, or be influenced by the geotechnical engineering considerations discussed above. The information presented below is not intended to discuss all aspects of the site construction conditions and considerations that may be encountered as the project progresses. If any questions arise as a result of our recommendations presented above, or if unexpected subsurface conditions are encountered during construction we should be contacted immediately.

6.1 Fill Placement Recommendations

There are several references throughout this report regarding both natural soil and compacted structural fill recommendations. The recommendations presented below are appropriate for the fill placement considerations discussed throughout the report above.

All areas adjacent to the existing asphalt roadway that will receive fill or other site improvements should be properly prepared and grubbed at the initiation of the project construction. The grubbing operations should include scarification and removal of organic material and other unsuitable materials.

We observed evidence of previous site use and existing man-placed fill during our field work.

Existing embankment fills or existing utility trench backfill material should be thoroughly proof rolled to ensure that the existing fill material is suitable to support the new roadway section.

6.1.1 Natural Soil Fill

Any natural soil used for any fill purpose should be free of all deleterious material, such as organic material and construction debris. Natural soil fill includes excavated and replaced material or in-place scarified material.

The natural soils should be moisture conditioned, either by addition of water to dry soils, or by processing to allow drying of wet soils. The proposed fill materials should be moisture conditioned to between about optimum and about 2 percent above optimum soil moisture content. This moisture content can be estimated in the field by squeezing a sample of the soil in the palm of the hand. If the material easily makes a cast of soil which remains in-tact, and a minor amount of surface moisture develops on the cast, the material is close to the desired moisture content.

Material testing during construction is the best means to assess the soil moisture content.

Moisture conditioning of clay or silt soils may require many hours of processing. If possible, water should be added and thoroughly mixed into fine grained soil such as clay or silt the day prior to use of the material. This technique will allow for development of a more uniform moisture content and will allow for better compaction of the moisture conditioned materials.

The moisture conditioned soil should be placed in lifts that do not exceed the capabilities of the compaction equipment used and compacted to at least 90 percent of maximum dry density as defined by ASTM D1557, modified Proctor test. We recommend a maximum fill lift thickness of

6 inches for hand operated equipment and 8 to 10 inches for larger equipment. Care should be exercised in placement of utility trench backfill so that the compaction operations do not damage the underlying utilities.

6.1.2 Granular Compacted Structural Fill

Granular compacted structural fill or aggregate base course is referenced in numerous locations throughout the text of this report. We recommend that aggregate base course materials conform to the Colorado Department of Transportation (CDOT) “Class 6” aggregate road base material, or

“Class 2” aggregate sub-base course material. This specification can include an option for testing and approval in the event the contractor’s desired material does not conform to the Class 6 or Class

2 aggregate specifications. All aggregate base course materials should be moisture conditioned and compacted to at least 95 percent of maximum dry density as defined by ASTM D1557, modified Proctor test.

6.2 Excavation Considerations

Unless a specific classification is performed, the site soils should be considered as an

Occupational Safety and Health Administration (OSHA) Type C soil and should be sloped and/or benched according to the current OSHA regulations. Excavations should be sloped and benched to prevent wall collapse. Daily observations of the excavations should be conducted by OSHA competent site personnel to assess safety considerations.

We did not encounter free subsurface water in our test borings. If water is encountered during construction, it may be necessary to dewater excavations to provide for suitable working conditions.

If possible, excavations should be constructed to allow for water flow from the excavation the event of precipitation during construction. If this is not possible it may be necessary to remove water from snowmelt or precipitation from the foundation excavations to help reduce the influence of this water on the soil support conditions and the site construction characteristics.

6.3 Grading and Drainage Comments

Many areas adjacent to the existing pavement alignment were observed to have relatively poor drainage characteristics and currently collect or pool water. The project civil engineering consultant should develop a drainage scheme for the project.

7.0 CONSTRUCTION MONITORING AND TESTING

Engineering observation of subgrade bearing conditions, compaction testing of fill material and testing of foundation concrete are equally important tasks that should be performed by the geotechnical engineering consultant during construction. We should be contacted during the construction phase of the project and/or if any questions or comments arise as a result of the information presented below. It is common for unforeseen, or otherwise variable subsurface soil and water conditions to be encountered during construction. As discussed in our proposal for our services, it is imperative that we be contacted to verify that the conditions encountered in our field exploration were representative of those encountered during construction. Our general recommendations for construction monitoring and testing are provided below.

• Consultation with design professionals during the design phases: This is important to ensure that the intentions of our recommendations are properly incorporated in the design, and that any changes in the design concept properly consider geotechnical aspects.

• Observation and monitoring during construction: A representative of the Geotechnical engineer from our firm should observe pavement subgrade to determine that subsurface conditions are compatible with those used in the analysis and design and our recommendations have been properly implemented. Placement of backfill should be observed and tested to judge whether the proper placement conditions have been achieved.

Compaction tests should be performed on each lift of material placed in areas proposed for support of structural components.

8.0 CONCLUSIONS AND CONSIDERATIONS

While we feel that it is feasible to improve and reconstruct the existing asphalt pavement as planned using relatively conventional techniques we feel that it is prudent for us to be part of the continuing design of this project to review and provide consultation in regard to the proposed development scheme as the project progresses to aid in the proper interpretation and implementation of the recommendations presented in this report. This consultation should be incorporated in the project development prior to construction at the site.

9.0 LIMITATIONS

This study has been conducted based on the geotechnical engineering standards of care in this area at the time this report was prepared. We make no warranty as to the recommendations contained in this report, either expressed or implied. The information presented in this report is based on our understanding of the proposed construction that was provided to us and on the data obtained from our field and laboratory studies. Our recommendations are based on limited field and laboratory sampling and testing. Unexpected subsurface conditions encountered during construction may alter our recommendations. We should be contacted during construction to observe the exposed subsurface soil conditions to provide comments and verification of our recommendations.

The recommendations presented above are intended to be used only for this project site and the proposed construction which was provided to us.