West Milner_FinalGeotechnicalReport.pdf

FINAL GEOTECHNICAL REPORT

GR 16-20

ID CASSIA 972(1)

West Milner Road 700W to 1100W

IDIQ Contract No. DTFH70-15-D-00007 Task Order No. 69056719F000129

Modification No. P00001 Project 19-3829G

Submitted by

2511 Holman Avenue P. O. Box 80190

Billings, Montana 59108-0910

Prepared for

Robert Peccia & Associates P. O. Box 5653

Helena, Montana 59604-5653

June 18, 2020

June 18, 2020 Project 19-3829G

Mr. Brad Thompson, PE Robert Peccia & Associates P. O. Box 5653 Helena, Montana 59604-5653 Via Email: brad@rpa-hln.com

Dear Mr. Thompson:

Re: Final Geotechnical Report, ID CASSIA 972(1), West Milner Road 700W to 1100W, IDIQ

Contract No. DTFH70-15-D-00007, Task Order No. 69056719F000129, Modification No.

P00001

The final geotechnical report for the above referenced project has been completed. This report presents the findings of our geotechnical evaluation for the West Milner Road project.

Introduction

Background. The Western Federal Land Highway Division (WFLHD) is planning to improve West Miler Road between the roads of 700W to 1100W. SK Geotechnical was originally tasked to perform a field reconnaissance, literature review, and preliminary geotechnical report for the preliminary phase (30% design) of the project. The results of this work were submitted in our preliminary geotechnical memorandum report dated November 20, 2019.

For the current phase of the project (70% design), SK Geotechnical was tasked to develop a plan for and perform the final site investigation based off of our preliminary work, develop a material testing plan for the recovered samples to be performed by WFLHD Vancouver, Washington office, analyze the field and laboratory data, and prepare a geotechnical evaluation report presenting the results of the investigation and providing geotechnical recommendations for the project.

Project Description. The anticipated project is improving the 4-mile long portion of West Milner Road between county roads 700W to 1100W. This section of road serves to allow outdoor recreationalist access to the Milner Historic Recreation Area, about seven miles west of Burley, Idaho. The Federal Lands Access Program (FLAP) permit submitted for the proposed project indicates the road is also used by the local fire department, school district, farmers, bass club, trap club, hunters, and fishermen. At this time, some of the proposed improvements for the roadway include the following:

Construct a paved surface designed to meet vertical and horizontal standards that can accommodate the recreational and commercial users of West Milner Road.

Increase the roadway width from the existing 24 feet to 28 feet to accommodate two 12-foot wide lanes with 2-foot shoulders.

Realign the intersection of West Milner Road and 700W to intersect perpendicularly.

Provide a realignment curve for a portion of West Milner Road located approximately 0.8 miles west of 700W.

2511 Holman Avenue P. O. Box 80190

Billings, Montana 59108-0190 p: 406.652.3930; f: 406.652.3944 www.skgeotechnical.com

Robert Peccia & Associates June 18, 2020 Project 19-3829G Page 2

During our site visit, we observed that West Milner Road is primarily an aggregate surfaced road with a small paved segment towards the beginning of the project near the intersection of West Milner Road and 700W. The western two-thirds of the project are bordered to the south by small hills into which the existing road is cut. The eastern one-third of the project and north of West Milner Road is gently sloping rangeland and farmland. We also observed that additional aggregate surfacing had been placed on the roadway since our reconnaissance performed last fall.

Scope of Work. Our scope of work is outlined in the Mod 70% statement of work for the ID CASSIA 972(1), West Milner Road, 700W to 1100W project. The purpose of this evaluation was to assist FHWA and the contractor in evaluating subsurface soil and bedrock conditions along the project, as well as to provide geotechnical recommendations related to preparing plans and specifications for the project.

Results

General Site Topography and Geology. As can be seen on the attached partial Topographic Sketch, roughly the first quarter of the project increases 80 feet in elevation as it goes from east to west. The project then gradually slopes downward across a natural drainage and then parallels an existing small hill, remaining relatively flat until the end of the project. The surrounding area generally slopes down to the north towards the Snake River, and West Milner Road has generally been cut into small hills to the south for the western most two-thirds of the project.

A portion of the geologic map of Idaho compiled by Reed S. Lewis, Paul K. Link, Loudon R. Stanford, and Shawn P. Long, 2012, is attached. The map indicates the majority of the project is underlain by basalt. However, the map indicates the eastern portion of the project could extend into Lake Bonneville Deposits, consisting of lacustrine silt, clay, sand, and gravel. A portion of the Idaho Miocene-Quaternary Fault Map from the Idaho Geologic Survey is attached, as well. This map indicates that lesser tertiary and major tertiary faults are present in the Pike Mountain and Mount Harrison areas to the south of the project. The closest fault is just over 9 1/2 miles from the western end of the project.

Other Available Soils and Geologic Information. SK Geotechnical also previously reviewed soil survey data collected across the project by the United States Department of Agriculture (USDA) as part of our preliminary design work. Based on the soil survey, the majority of the alignment consists of sandy and silty loam, associated with alluvial lacustrine and aeolian deposits. Sandy loam and silty loam are not classifications under the United Soil Classification System (USCS), however, generally this type of soil consists of intermixed sands, silts, and clays typically having USCS symbols of SM, ML, ML-CL, and CL. Portions of the soil survey indicated shallow bedrock present at depths of 2 feet or slightly less.

Several well logs from the area were also reviewed for our preliminary report. Well logs adjacent to the project generally showed 1 1/2 to 4 feet of topsoil over what was typically called basalt by the well drillers (one well log identifies this material as lava, possibly referencing to its geologic origins).

We were not provided with as-builts or boring logs for the previous development of West Milner Road.

However, a memo prepared by WFLHD, dated February 25, 2014, for the Final Structural Pavement Recommendation of the Milner Loop Road Network was provided to us. In this memo under the Subgrade Evaluation section, it is noted that the subgrade material was assumed to be silt, clay, and silty clay. Further, a nearby trail project for the Bureau of Land Management (BLM) began construction prior to our fieldwork, and it our understanding that shallow basalt was encountered that was too difficult to excavate, and blasting to loosen the bedrock was necessary.

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Drilling and Sampling. Twelve subgrade borings (SB20-01 through SB20-12) were performed on or adjacent to West Milner Road to depths ranging from approximately 2 1/4 to 5 1/2 feet. Three cut slope borings (CS20-13 through CS20-15) were also performed off West Milner Road near the anticipated realignment to depths of approximately 15 feet. Photographs of the staked boring locations are attached, and provide a good visual representation of the alignment.

The test borings were performed on the dates indicated on the boring logs with a CME-75 truck mounted drilling rig by Haz-Tech Drilling Inc. Sampling for the borings was conducted in general accordance with ASTM D 1586, "Penetration Test and Split-Barrel Sampling of Soils." Using this method, we advanced the borehole with hollow-stem auger to the desired test depth. Then a 140-pound hammer falling 30 inches drove a standard, 2-inch OD, split-barrel sampler a total penetration of 1 1/2 feet below the tip of the hollow-stem auger. The blows for each 6-inch increment of penetration were recorded on the logs and are an index of soil strength characteristics. Shelby tube samples, surfacing samples, bulk subgrade samples, and bedrock core samples were collected from the borings. Photographs of these samples are available upon request.

Dynamic Cone Penetrometer (DCP) tests were also performed on the subgrade through our boreholes.

The hollow-stem auger was advanced through the surfacing to the underlying soil, was withdrawn, and then cuttings in the hole were carefully removed to expose the underlying subgrade. The Dual Mass DCP was positioned in the hole with the disposable cone tip on the surface of the subgrade. Then a 10.1-pound hammer and/or or a 17.6-pound hammer falling approximately 22.6 inches drove the cone tip into the subgrade. The heavier hammer was used in soils where the DCP appeared to have a harder time penetrating the subgrade. The cumulative depth driven per five blows of the hammer were recorded, and input into a spreadsheet provided by the manufacturer which converts the cumulative penetration and blow count to a California Bearing Ratio (CBR) value for the in-place subgrade. The results of these tests are attached and are discussed in more detail later in this report.

Surface and Subsurface Conditions. Borings SB20-02 through SB20-12 performed in West Milner Road encountered approximately 4 1/2 to 16 1/2 inches of existing aggregate surfacing that classified as silty sand, well graded sand with silt, and poorly graded sand with clay. Boring SB20-01, which was performed adjacent to the road for the intersection realignment with 700W, encountered approximately 3 inches of silty clay topsoil. Below the surfacing and topsoil, the borings performed in West Milner Road encountered a subgrade generally consisting of silt, sandy silt, silty clay, clayey sand, and poorly graded sand with silt to the termination depths of the borings ranging from 2 1/4 to 5 1/2 feet. The shallower termination depths of some of the borings was associated with auger refusal encountered during drilling, most likely due to subsurface boulders. Borings SB20-02 and SB20-03 towards the east extent of the project encountered layers of poorly graded gravel with silt below the surfacing and Borings SB20- 10 and SB20-12 towards the west extent of the project encountered layers of existing fill consisting of silty, clayey sand below the surfacing.

Borings CS20-13 through CS20-15 performed near the realignment cut area encountered approximately 3 to 3 1/2 inches of silty clay topsoil underlain by poorly graded gravel with silt, silt, and silty clay to depths ranging from approximately 1 1/2 to 6 feet. The borings then encountered basalt bedrock to the termination depth of the borings at 15 1/2 feet.

Standard penetration tests (SPT) values in the fine-grained subgrades ranged from 3 blows per foot (BPF) to 50 blows for 0 inches of penetration, but generally ranged from 3 to 13 BPF, indicating the fine-grained soils were generally soft to stiff with some areas having a very stiff to hard consistency. SPT values in

Project 19-3829G Page 4 the coarse-grained subgrades ranged from 13 BPF to 50 blows for 4 inches of penetration, indicating the coarse-grained subgrades were medium dense to very dense. The higher SPT values indicate the presence of cobbles and boulders in the subgrade.

HQ3 coring was performed in the basalt bedrock in Borings CS20-13 through CS20-15 below depths ranging from 1 1/2 to 6 feet. Core recovery ranged from 80 to 100 percent. Rock quality designation (RQD) ranged from 35 to 88 percent. We judged the relative strength of the intact rock specimens to range from R3 to R4, medium strong to strong rock by International Society for Rock Mechanics (ISRM) standards.

Table 1 below shows a summary of the surfacing encountered and anticipated subgrade for the subgrade borings.

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Table 1. Existing Surfacing and Anticipated Subgrade Conditions along West Milner for PTW Borings

Boring SB20-01 SB20-02 SB20-03 SB20-04 SB20-05 SB20-06 SB20-07 SB20-08

Date Drilled 3/3/2020 3/3/2020 3/3/2020 3/3/2020 3/3/2020 3/3/2020 3/3/2020 3/3/2020

Station, Offset 10+30, 20'R 31+15, 5'R 66+40, 5'L 83+70, 5'R 101+20, 5'L 119+00, 5'R 136+70, 5'L 154+00, 5'R

Existing Surfacing N/A 6" 4 1/2" 8 1/2" 11 1/2" 13 1/2" 16 1/2" 4 1/2"

Subgrade(1)

Description Silty Clay

Poorly Graded Gravel with Silt/Poorly

Graded Sand with Silt

Poorly Graded Gravel with Silt/Poorly

Graded Sand with Silt

Sandy Silt Silt/Silty Clay

Sandy Silty Clay Silty Clay Silty Clay

ASTM Class CL-ML GP-GM/SP-

SM

GP-GM/SP-

SM

ML ML/CL-ML CL-ML CL-ML CL-ML

N-Values 7, 5 50-4"/27 50/17 13, 10 4 3, 4 22, 50-1" 50-0"

Consistency Firm Very

Dense/Medium Dense

Dense/Medium Dense Stiff Soft Soft Very Stiff to

Hard Hard

Moisture Content, % 9.8, 12.6 13.0, 16.1 10.0, 11.4 4.7 17.0, 22.3 19.5, 20.7 10.6, 23.5 ---

Groundwater Depth N/E N/E N/E N/E N/E N/E N/E N/E

Risk of Subgrade Failure Due to Heavy Equipment

High Low Low Medium High High Low Low

Geosynthetic Recommended(2) Yes No No No Yes Yes No No

(1)Anticipated subgrade that will be present beneath the proposed pavement section.

(2)Beneath sections.

(3)Borings SB20-01, SB20-07, SB20-09 excluded from average based on typical gravel thicknesses.

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Table 1. Existing Surfacing and Anticipated Subgrade Conditions along West Milner for PTW Borings

Boring SB20-09 SB20-10 SB20-11 SB20-12

Average(3) Date Drilled 3/3/2020 3/3/2020 3/4/2020 3/4/2020

Station, Offset 171+50, 5'L 188+70, 5'R 206+60, 5'L 223+70, 5'R

Existing Surfacing 6 1/2" 15 1/2" 5 1/2" 8" 7 1/2"

Subgrade(1)

Description Silt Fill: Silty, Clayey Sand/Silt

Silt/Poorly Graded Sand with Silt

Silty, Clayey Sand

ASTM Class ML SC-SM/ML ML/SP-SM SC-SM

N-Values 4, 7 14/27 7/50-4" 17, 10

Consistency Soft to Firm Medium

Dense/Very Stiff

Firm/Very Dense

Medium Dense to

Loose

Moisture Content 13.8, 16.7 9.9 16.6, 18.3 9.1

Groundwater Depth N/E N/E N/E N/E

Risk of Subgrade Failure Due to Heavy Equipment

High Medium Medium Medium

Geosynthetic Recommended(2) Yes No No No

(1)Anticipated subgrade that will be present beneath the proposed pavement section.

(2)Beneath sections.

(3)Borings SB20-01, SB20-07, SB20-09 excluded from average based on typical gravel thicknesses.

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Groundwater. Groundwater was not encountered in the subgrade borings up to a depth of 5 feet at the time of the fieldwork. Water was added during the coring for the deeper cut slope borings to 15 feet, and therefore groundwater levels were unable to be measured. Fluctuations in the groundwater levels can occur due to rainfall, snow melt, run-off, flooding, irrigation, and many other seasonal factors.

Laboratory Testing Program and Procedures

General. After completion of the borings, the samples were returned to our office for inventory before being sent to the WFLHD Vancouver laboratory for material testing. The samples included Standard Penetration Test (SPT) samples, surfacing samples, bulk subgrade samples, Shelby tube samples, and bedrock core samples. Table 2 below shows the test methods requested and performed by the WFLHD laboratory for the provided samples.

Table 2. Laboratory Test Methods.

Test Method

Natural Moisture Content AASHTO T 255/T 265 Plasticity Index (Atterberg Limits) AASHTO T 89/T 90 Sieve Analysis AASHTO T 11/T 27 Apparent Specific Gravity AASHTO T100 Moisture Density (Standard Proctor) AASHTO T 99 R-Value (300 PSI Exudation) WL1901 Consolidation Test AASHTO T 216/ASTM D2435 Unconfined Compression of Rock Cores DL221 (1)Modified from standard AASHTO tests methods

Moisture Content Tests. Moisture content tests were performed on the SPT samples from the borings, as well as the Shelby tube, bulk subgrade, and rock core samples. The results of these tests are indicated on the attached Log of Boring Sheets.

Subgrade and Surfacing Classification and Proctor Tests. Classification tests consisting of full sieve and Atterberg limits were performed on the SPT, surfacing, bulk subgrade, and Shelby tube samples obtained from various borings at various depths. Tables 3 and 4 below provide summaries of the test results. Several of these samples were also selected for Proctor tests. However, the results of the Proctor tests are only for informational purposes, as laboratory moisture density testing (AASHTO T-99) will be required during construction. As such, these Proctor results have been excluded from these tables.

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Table 3. Summary of Subgrade Classification Tests

Boring Station, Offset

Depth (feet)

Atterberg Limits P200

ASTM

Classification

AASHTO

Classification LL PL PI

SB20-01 10+30, 20'R ½ – 4 25 21 4 72.6 CL-ML A-4

SB20-04 83+70, 5'L 1 – 4 NP NP NP 54.6 ML A-4

SB20-07 136+70, 5'L 1½ – 4 25 19 6 68.3 CL-ML A-4

SB20-10 188+70, 5'R 1½ – 4 23 19 4 48.1 SC-SM A-4

SB20-11 206+60, 5'L 3 – 4 NP NP NP 72.2 ML A-4

SB20-12 223+70, 5'R 1 – 4 25 19 6 42.5 SC-SM A-4

Table 4. Summary of Composite Surfacing Samples Classification Tests

Borings Depth (feet)

Atterberg Limits P200

ASTM

Classification

AASHTO

Classification LL PL PI

SB20-02, SB20-03,

SB20-04, SB20-05 Varies NP NP NP 14.5 SM A-1-b

SB20-06, SB20-07,

SB20-08 Varies NP NP NP 11.7 SW-SM A-1-a

SB20-09, SB20-10,

SB20-11, SB20-12 Varies 23 19 4 11.4 SP-SC A-1-b

The surfacing sample results indicated in Table 4 were composed of composite samples from several borings to provide sufficient quantity. Surfacing samples from borings SB20-02, SB20-03, SB20-04, and SB20-5 made up the first composite sample, surfacing samples from borings SB20-06, SB20-07, and SB20-08 made up the second composite sample, and surfacing samples from borings SB20-09, SB20-10, SB20-11, and SB20-12 made up the third composite sample. The depths of these composite samples vary due to the variations in the thickness of aggregate surfacing.

Based on the subgrade test results, two of the samples classified as silty clay (CL-ML), two as silt (ML), and two as silty, clayey sand (SC-SM). The composite surfacing samples classified as silty sand (SM), well graded sand with silt (SW-SM), and poorly graded sand with clay (SP-SC).

Consolidation Test. Consolidation tests were run on the three Shelby tube samples collected from borings SB20-01, SB20-07, and SB20-11. The resulting consolidation graphs are attached to this report.

The samples were inundated at the start of the test, and consolidated about 1/2 to 1 1/2 percent under a load increase of 1500 psf. This is a minor amount, generally indicating the silt clay and silt subgrade is not collapsible. Dry densities of these samples ranged from 91.8 to 104.1 pounds per cubic foot (pcf).

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Unconfined Compressive Strength Test. Several rock core samples were selected for unconfined compressive strength tests. These samples were selected from borings CS20-13, CS20-14, and CS20-15, and the results are summarized in Table 5 below.

Table 5. Summary of Unconfined Compressive Strength Tests

Boring Station, Offset Depth (feet)

Unconfined Compressive Strength

(psi)

CS20-13 37+40, 150'R

6.3 – 7 3660 7 – 7.5 3190 9 – 10 4490

10.8 – 11.5 6830

11.5 – 12.8 6439

13.8 – 14.7 7120

CS20-14 48+00, 270'R

2.3 – 2.7 3080

5.3 – 6 5120

6.3 – 8 9010

8.8 – 9.4 2670

9.8 – 10 7070

10 – 10.7 6570

11.3 – 12.1 10910

CS20-15 59+80, 120'R

5 – 6 5358

6.2 – 7 5740

10.5 – 11.5 2970

11.5 – 12.3 5380

R-Value Test. R-value tests were performed on bulk subgrade samples collected from various borings.

The results of these R-value tests are shown in Table 6 below. We wish to point out that these R-values are unusually high for soils having a silty consistency, in our opinion. Additionally, the composite surfacing sample consists of all surfacing samples from borings SB20-02 through SB20-12.

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Table 6. Summary of R-Value Tests

(feet) R-Value

SB20-01 10+30, 20'R ½ – 4 60

SB20-04 83+70, 5'R 1 – 4 63

SB20-07 136+70, 5'L 1½ – 4 35

SB20-10 188+70, 5'R 1½ – 4 64

SB20-12 233+70, 5'R 1 – 4 54

Various (Composite Gravel Sample) Varies Varies 70

Specific Gravity. Specific gravity tests were performed on bulk samples collected from various borings.

The results of these specific gravity tests are shown in Table 7 below.

Table 7. Summary of Specific Gravity Tests

(feet) Specific Gravity

SB20-01 10+30, 20'R ½ – 4 2.648

SB20-04 83+70, 5'R 1 – 4 2.663

SB20-07 136+70, 5'L 1½ – 4 2.653

SB20-10 188+70, 5'R 1½ – 4 2.628

SB20-12 233+70, 5'R 1 – 4 2.637

Pavement Analysis and Recommendations

Traffic Analysis. Traffic data for the project was not provided for our pavement analysis. However, the FLAP application submitted in 2015 for the project provides an average annual daily traffic (AADT) value of 250 with 12 percent trucks, which were anticipated during the peak season. As discussed in our preliminary report, the Burley Highway District (BHD) notes that they believe a growth value of 12 percent is warranted, as the anticipated growth along West Milner Road is expected to be larger than the typical growth due to increased recreational use. We have therefore used these values in our traffic analysis.

Pavement Analysis. DARWin®, a computer program based on the 1993 AASHTO Guide for Design of Pavement Structures, was used for a simple equivalent single axle load (ESAL) calculation of West Milner Road. Based on the provided AADT, percent trucks, and percent growth, the simple traffic

Project 19-3829G Page 11 analysis indicates an ESAL value of 234,490. This value equates to a daily ESAL value of approximately 32.

Alternative pavement sections were analyzed using an Excel spreadsheet based on the 1993 AASHTO Guide for Design of Pavement Structures. The following parameters were used for design.

Change in serviceability of 2.2, Standard deviation of 0.49, An inherent reliability of 75 percent, A design lift of 20 years.

Laboratory tests were performed on five subgrade samples to determine R-values for each. As previously shown in Table 6, these values ranged from 35 to 64 for the bulk subgrade samples. Using these values, an R-value analysis was performed to plot all R-values on a percent greater than curve. At the 85th percentile, a value of 49.3 was obtained.

These R-values ranging from 35 to 64 are unusually high, in our opinion. The DCP results indicate the lowest minimum in-place CBRs typically range from 1.2 to 12.1 at the subgrade borings. The AASHTO Pavement Design Manual is based on a modulus of subgrade reaction (MR). Based on these results, MR values range from 1,800 to greater than 20,000 psi. This range of subgrade strengths obtained from the DCP tests is too extreme, in our opinion.

We therefore are basing design MR on our experience with similar silty clay soils. We have found CBR values range from about 3 to 6 for these types of clay soils. We recommend using a design MR of 6,000 psi, which is equivalent to a CBR of 4 and an R-value of 11. This value was used in our analysis to determine recommended pavement sections.

Pavement Design. Based on a design MR of 6,000 psi and the traffic analysis, the AASHTO pavement design guide indicates a required structural number (SN) of 2.64 is needed for the pavement section. We developed five pavement sections for the project; three for total reconstruction and two assuming the existing surfacing was to be left-in-place as a “dirty” subbase. The existing surfacing contains approximately 11 to 15 percent fines, and is therefore not free draining and should be considered frost-susceptible. It is our opinion the existing surfacing can be left-in-place as subbase, assuming the risk of frost heave is considered acceptable by stake holders. Table 8 below shows our five pavement sections.

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Table 8. Recommended Pavement Sections

Typical Section Total Reconstruction Existing Surfacing Reused as

“Dirty” Subbase

1 2 3 4 5 Hot Asphalt

Concrete Pavement

(HACP)

4" 3" 3" 4" 3"

Crushed Base Course (CBC) 8" 11" 4" 4" 7"

Subbase --- --- 11" --- --- Existing “Dirty”

Subbase --- --- --- 7 ½" 7 ½"

6-ounce Nonwoven Fabric Yes Yes Yes No No

Total Thickness 12" 14" 18" 15 ½" 17½"

The analysis for these sections is included in the attached spreadsheet which is based on the 1993 AASHTO Guide for Design of Pavement Structures. Typical Sections 1 and 4 address providing 4 inches of HACP while Typical Sections 2, 3, and 5 address providing 3 inches of HACP. When considering the relatively high percentage of truck traffic, providing 4 inches of HACP is slightly preferred due to the ability to mill and overlay in the future. As indicated above, we recommend providing a nonwoven fabric on top of the subgrade for long term separation and protection of new aggregate base where total reconstruction is planned. Separation fabric in roadway widening areas does not appear justified.

Unstable Subgrade. As shown in Table 1, several borings encountered soft subgrades, and there is a risk of unstable subgrades could be encountered during construction. Eight of the twelve borings are noted as having a medium to high risk of subgrade failure. The actual extent of unstable subgrades is dependent on numerous variables. If construction occurs during spring thaw, heavy snow melt, or rainy weather, then more subgrade areas are likely to be unstable. Also, construction equipment has a relatively high axle loading and repetitive construction traffic can weaken subgrades causing them to become unstable.

Assuming fair to good summer weather during construction, we recommend providing the thicker stabilized pavement section for the following extents.

Station 10+00 to 12+00, and Station 95+00 to 125+00.

The actual extent of stabilized pavement sections should be determined by the Project Manager during construction. We recommend the existing aggregate surfacing (dirty subbase) and exposed subgrade be proof rolled with a loaded tandem axle dump truck. The following deflection criteria should then be used to identify unstable conditions requiring the installation of the thicker stabilized pavement section.

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Deflection of ¼-inch or more of existing aggregate surfacing.

Deflection of ¾-inch or more of subgrade.

Two options for stabilized pavement sections are presented in Table 9 below.

Table 9. Stabilized Pavement Sections

Option 1 Option 2

Asphalt Concrete Pavement 4" 4" Crushed Base Course 18" 4"

Select Borrow --- 14" Stabilization Geogrid Yes Yes

6-ounce Nonwoven Fabric Yes Yes Total Thickness 2 " 2 "

While either option above will generally be feasible for a stabilized pavement section, Option 1 is generally preferred due to the better quality aggregate and its ability to bridge over soft subgrades.

Stabilization geogrid should meet the requirements of Table 714-4 and nonwoven separation fabric should meet the requirements of Table 714-1, Class 1, Type C according to Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects, FP-14.

Geotechnical Analysis and Recommendations

Shallow Bedrock Concerns. Bedrock was encountered in Borings CS20-13 through CS20-15 at depths ranging from 1 1/2 to 6 feet below existing grade and extending to the termination depth of the borings of 15 feet. The unconfined compressive strength tests on core samples ranged from about 3,000 to 11,000 psi, indicating the basalt was medium strong to strong bedrock, Grade R3 and R4 rock. The curve realignment from about Station 48+50 to 52+00 has cuts up to 6 feet in the basalt bedrock. In our opinion, bedrock will likely be encountered primarily in this portion of the project, as other cuts are anticipated to be minimal. We anticipate production blasting to loosen the bedrock and expedite cutting will be beneficial to the project. If the basalt bedrock is not blasted, then ripping with a D9 or bigger dozer having a single shank ripper will be necessary.

Geologic and Geotechnical Hazards. Besides the potential of strong bedrock in cuts, we did not observe any other significant geologic hazards that will affect the project. The planned roadway improvements will result in better surface water drainage and distinct ditches. Improvements at culvert crossings are recommended and should consider potential erosion at outlets. From a geotechnical perspective, the silt and silty clay are considered highly erodible soils, and flat as possible slopes are recommended to reduce erosion and help establish vegetation, as well as safety.

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Cut Slopes. Numerous existing cut slopes up to 5 feet deep were present along the alignment due to the hilly terrain. These existing cut slopes generally ranged from 1V:1.5H to 1V:3H, or flatter. We did not observe signs of slope failures, but some erosion was apparent. The existing slopes were observed to have some minor vegetation established on them, particularly along the western extent of the project along the taller cut slopes.

In general, we recommend 1:3 cut slopes wherever possible for maintenance, better establishment of vegetation, reduced erosion, and roadside safety. It is our understanding that the majority of the cuts are 2 feet or less, and are needed primarily to establish roadside ditches to improve drainage. The only other cut needed for the project is the curve realignment from Station 48+50 to 52+00, which is up to 6 feet deep. Basalt bedrock is anticipated in this area.

It is our understanding that 1:1.5 cuts are desired in some locations along the project, to avoid having to acquire additional right of way (ROW). Additionally, based on the provided cross sections, 1:2 cut slopes are planned for the curve realignment. When considering the shallow ditches for most of the cuts, no observed instability of the existing 1:1.5 cut slopes, and basalt bedrock likely present in the only other cut, it is our opinion 1:1.5 cut slopes can be used for the ditches and 1:2 cut slopes can be used for the curve realignment. However, if sufficient ROW is available, we recommend flatter 1:3 cut slopes to establish vegetation and reduce erosion, as discussed above.

A summary of these recommendations and cut slopes along the project are shown in Table 10 below.

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Table 10. Cut Slope Recommendations

Cut Slope Description and Location

Existing and Proposed Cut Slope (V:H)2

Anticipated Subsurface

Conditions on Cut Side of Roadway3, 4

Cut Slope Recommendations

(V:H)2

Cut Slopes up to 4 feet.1 Station 32+00 to 34+00 Station 38+50 to 44+50 Station 59+50 to 64+00 Station 71+50 to 87+00 Station 92+00 to 94+00 Station 97+00 to 98+50 Station 101+50 to 105+00 Station 121+00 to 134+00 Station 147+50 to 150+00 Station 166+50 to 167+50 Station 172+00 to 175+00 Station 183+00 to 186+00 Station 193+50 to 195+00 Station 199+00 to 201+00 Station 207+00 to 212+50 Station 222+50 to 224+00

Existing cut:

Generally 1:3 to

1:4

Proposed cut:

1:1.5 to 1:2

Silty Clay/Sandy Silt/Silty Clayey Sand [Suitable only for 1:3

Fill Slopes]

1:2 cut slopes5

Bedrock Cut

Station 48+50 to Station 52+50

Existing cut: None

Proposed cut: 1:2 up to 6 feet.

Basalt Bedrock [Suitable]6

1:2 cut slope for bedrock cut

1. Slope height measured vertical from ditch line to crest. Majority of cuts are 2 feet or less, and are primarily to create roadside ditches.

2. Slope inclinations listed as Vertical:Horizontal (V:H).

3. Subsurface conditions listed are those observed in the existing cuts and interpreted from subsurface investigation locations. The actual soil/rock profile in the cuts will be variable, and the description is intended to describe the predominant soil/rock observed for the cuts.

4. An assessment was made as to the materials’ suitability for embankment considering both the soil/rock type and the in-situ moisture relative to optimum moisture for compaction.

5. Where necessary due to limited ROW along the project, consideration can be given to a 1:1.5 cut slope for shallow ditch cuts along the project. Where it is critical to establish vegetation, consider using turf reinforcement mats (TRM).

6. Basalt bedrock from cut can be used as embankment in accordance with project specifications.

Fill Slopes. Numerous existing fill slopes up to 5 feet deep were present along the alignment due to the hilly terrain, mostly along native drainages and culvert crossings. The slopes of these existing cut slopes generally ranged from 1:2 or flatter. We did not observe signs of slope failures. The existing slopes were

Project 19-3829G Page 16 generally observed to have some vegetation established on them, however the larger fill slopes near Station 88+00 had very little to no vegetation. Minor erosion of fill slopes was also apparent.

As previously discussed, it has been indicated to us that most fill for the project will be imported due to minimal cuts planned along the project. We recommend imported unclassified borrow consist of A-1, A-3, and/or A-2-4 soils as outlined in section 704.06 Unclassified Borrow FP-14.

In general, we recommend 1:3 fill slopes wherever possible for maintenance, better establishment of vegetation, reduced erosion, and roadside safety. However, like the cut slopes, it is our understanding that steeper 1:2 fills are desired in some limited locations along the project to avoid acquiring additional ROW. When considering we did not observe instability of the existing 1:2 fill slopes, it is our opinion 1:2 fill slopes can be used for the project provided fill consists of imported A-1, A-3, or A-2-4 Unclassified Borrow. However, if sufficient ROW is available, we recommend flatter 1:3 fill slopes be used as often as possible to establish vegetation and reduce erosion as discussed above.

Material for embankments having 1:3 slopes or flatter can also be taken from the cuts along the project.

Based on our borings, we anticipate that this material will generally consist of A-4 silt, silty clay, and silty, clayey sand. Basalt bedrock that has to be blasted or ripped is also anticipated in the curve realignment. This broken up basalt bedrock from cut areas can also be used as embankment in accordance with project specifications.

Earthwork. Based on our borings, we anticipate silt, silty clay, and poorly graded sand with silt make up the majority of the subgrade along the project. Silts and silty soils are generally moisture sensitive and difficult to compact to specification. Silty soils have relatively flat moisture and density curves, and moisture has to be very close to optimum moisture content to meet the required percent compaction.

Project 19-3829G Page 18

Attachments:

Site Location Sketch Plan and Profile Sheets with Approximate Boring Locations (8 Pages) Partial Topographic Sketch Geologic Sketch Partial Geologic Fault Sketch Log of Boring Sheets (15 Pages) DCP Results (11 Pages) Boring Location Photographs (8 Pages) Pavement Design Spreadsheet Atterberg Limits Tests Summary Sheet Laboratory Test Results Sheets (108 Pages)

North

SB20-01

STA ~10+30

Offset ~20'R

Elevation ~4160'

SB20-02

STA ~31+15

Offset ~5'R

Elevation ~4188

CS20-13

STA ~37+40

Offset ~150'R Elevation ~4215

Note: Boring Locations Should Be Considered

Approximate

CS20-14

STA ~48+00

Offset ~270'R Elevation ~4235

CS20-15

STA ~59+80

Offset ~120'R Elevation ~4230

SB20-03

STA ~66+40

Offset ~5'L

Elevation ~4226

SB20-04

STA ~83+70

Offset ~5'R

Elevation ~4191

SB20-05

STA ~101+20

Offset ~5'L

Elevation ~4181

SB20-06

STA ~119+00

Elevation ~4193

SB20-07

STA ~136+70

Offset ~5'L

Elevation ~4196

SB20-08

STA ~154+00

Elevation ~4189

SB20-09

STA ~171+50

Elevation ~4179

SB20-10

STA ~188+70

Offset ~5'R

Elevation ~4175

SB20-11

STA ~206+60

Elevation ~4174

SB20-12

STA ~222+40

Elevation ~4167

Reference: Burley SW Quadrangle – Idaho (2017), U.S. Geological Survey, North American Datum of 1983 (NAD83)

Reference: Idaho Geological Survey, Idaho Miocene-Quaternary Fault Map

DCP TEST DATA

File Name: SB20-01

Project: West Milner ID CASSIA 972(1) Date: 3-Mar-20 Location: SB20-01 Soil Type(s): Low plasticity Clay with CBR<10

No. of Accumulative Type of Blows Penetration Hammer

(mm) 0 0 2 5 130 2 5 183 2 5 215 2 5 245 2 5 276 2 5 304 2 5 333 2 5 359 2 5 383 2 5 414 2 5 445 2 5 475 2 5 504 2 5 535 2 5 572 2 5 610 2 5 657 2 5 705 2 5 767 2 5 767 2 3 810 2

0.1 1.0 10.0 100.0

0.1 1.0 10.0 100.0

DE

PT

H , i n.

CBR

DE

PT

H , m m

10.1 lbs.

17.6 lbs.

Both hammers used

Soil Type

CH

CL

All other soils

10.1 lbs.

17.6 lbs.

Both hammers used

Hammer

0 1000 2000 3000 4000 5000 6000 7000

DE

PT

H , i n

BEARING CAPACITY, psf

Based on approximate interrelationships of CBR and Bearing values (Design of Concrete Airport Pavement, Portland

Cement Association, page 8, 1955)

File Name: SB20-02

Project: West Milner ID CASSIA 972(1) Date: 3-Mar-20 Location: SB20-02 Soil Type(s): GP-GM/SP-SM

No. of Accumulative Type of Blows Penetration Hammer

(mm) 0 0 2 5 24 2 5 42 2 5 57 2 5 72 2 5 89 2 5 107 2 5 124 2 5 142 2 5 173 2 5 221 2 5 278 2 5 333 2 5 359 2 5 367 2 5 373 2 5 382 2 5 389 2 5 397 2 5 402 2 5 407 2 5 408 2

0.1 1.0 10.0 100.0

0.1 1.0 10.0 100.0

DE

PT

H , i n.

CBR

DE

PT

H , m m

10.1 lbs.

17.6 lbs.

Both hammers used

Soil Type

CH

CL

All other soils

10.1 lbs.

17.6 lbs.

Both hammers used

Hammer

-2500000 -2000000 -1500000 -1000000 -500000 0 500000

DE

PT

H , i n

BEARING CAPACITY, psf

Based on approximate interrelationships of CBR and Bearing values (Design of

File Name: SB20-04

Project: West Milner ID CASSIA 972(1) Date: 3-Mar-20 Location: SB20-04 Soil Type(s): Sandy ML

No. of Accumulative Type of Blows Penetration Hammer

(mm) 5 0 2 5 38 2 5 69 2 5 98 2 5 126 2 5 153 2 5 175 2 5 195 2 5 228 2 5 268 2 5 308 2 5 348 2 5 383 2 5 423 2 5 458 2 5 497 2 5 540 2 5 573 2 5 608 2 5 650 2

0.1 1.0 10.0 100.0

0.1 1.0 10.0 100.0

DE

PT

H , i n.

CBR

DE

PT

H , m m

10.1 lbs.

17.6 lbs.

Both hammers used

Soil Type

CH

CL

All other soils

10.1 lbs.

17.6 lbs.

Both hammers used

Hammer

0 1000 2000 3000 4000 5000 6000

DE

PT

H , i n

BEARING CAPACITY, psf

Based on approximate interrelationships of CBR and Bearing values (Design of

File Name: SB20-05

Project: West Milner ID CASSIA 972(1) Date: 3-Mar-20 Location: SB20-05 Soil Type(s): Low plasticity Clay with CBR<10

No. of Accumulative Type of Blows Penetration Hammer

(mm) 5 0 2 5 45 2 5 75 2 5 104 2 5 135 2 5 160 2 5 184 2 5 213 2 5 242 2 5 273 2 5 305 2 5 333 2 5 353 2 5 378 2 5 393 2 5 421 2 5 458 2 5 523 2 5 634 2 5 738 2 4 828 2

0.1 1.0 10.0 100.0

0.1 1.0 10.0 100.0

DE

PT

H , i n.

CBR

DE

PT

H , m m

10.1 lbs.

17.6 lbs.

Both hammers used

Soil Type

CH

CL

All other soils

10.1 lbs.

17.6 lbs.

Both hammers used

Hammer

0 2000 4000 6000 8000 10000 12000

DE

PT

H , i n

BEARING CAPACITY, psf

Based on approximate interrelationships of CBR and Bearing values (Design of

File Name: SB20-06

Project: West Milner ID CASSIA 972(1) Date: 3-Mar-20 Location: SB20-06 Soil Type(s): Low plasticity Clay with CBR<10

No. of Accumulative Type of Blows Penetration Hammer

(mm) 5 0 2 5 42 2 5 73 2 5 107 2 5 143 2 5 194 2 5 264 2 5 373 2 5 489 2 5 582 2 5 653 2 5 723 2 5 793 2

0.1 1.0 10.0 100.0

0.1 1.0 10.0 100.0

DE

PT

H , i n.

CBR

DE

PT

H , m m

10.1 lbs.

17.6 lbs.

Both hammers used

Soil Type

CH

CL

All other soils

10.1 lbs.

17.6 lbs.

Both hammers used

Hammer

0 1000 2000 3000 4000 5000

DE

PT

H , i n

BEARING CAPACITY, psf

Based on approximate interrelationships of CBR and Bearing values (Design of

File Name: SB20-07

Project: West Milner ID CASSIA 972(1) Date: 3-Mar-20 Location: SB20-07 Soil Type(s): Low plasticity Clay with CBR<10

No. of Accumulative Type of Blows Penetration Hammer

(mm) 5 0 2 5 30 2 5 54 2 5 84 2 5 125 2 5 180 2 5 235 2 5 262 2 5 281 2 5 297 2 5 315 2 5 331 2 5 351 2 5 362 2 5 380 2 5 392 2 5 406 2 5 417 2 5 429 2 5 439 2

0.1 1.0 10.0 100.0

0.1 1.0 10.0 100.0

DE

PT

H , i n.

CBR

DE

PT

H , m m

10.1 lbs.

17.6 lbs.

Both hammers used

Soil Type

CH

CL

All other soils

10.1 lbs.

17.6 lbs.

Both hammers used

Hammer

-400000 -300000 -200000 -100000 0 100000

DE

PT

H , i n

BEARING CAPACITY, psf

Based on approximate interrelationships of CBR and Bearing values (Design of

File Name: SB20-08

Project: West Milner ID CASSIA 972(1) Date: 3-Mar-20 Location: SB20-08 Soil Type(s): Low plasticity Clay with CBR<10

No. of Accumulative Type of Blows Penetration Hammer

(mm) 0 0 2 5 29 2 5 69 2 5 101 2 5 139 2 5 179 2 5 224 2 5 261 2 5 288 2 5 314 2 5 337 2 5 355 2 5 374 2 5 389 2 5 404 2

0.1 1.0 10.0 100.0

0.1 1.0 10.0 100.0

DE

PT

H , i n.

CBR

DE

PT

H , m m

10.1 lbs.

17.6 lbs.

Both hammers used

Soil Type

CH

CL

All other soils

10.1 lbs.

17.6 lbs.

Both hammers used

Hammer

-50000 -40000 -30000 -20000 -10000 0 10000 20000

DE

PT

H , i n

BEARING CAPACITY, psf

Based on approximate interrelationships of CBR and Bearing values (Design of

File Name: SB20-09

Project: West Milner ID CASSIA 972(1) Date: 3-Mar-20 Location: SB20-09 Soil Type(s): ML

No. of Accumulative Type of Blows Penetration Hammer

(mm) 5 0 2 5 37 2 5 71 2 5 106 2 5 144 2 5 185 2 5 228 2 5 284 2 5 338 2 5 392 2 5 449 2 5 509 2 5 578 2 5 663 2 5 756 2 4 832 2

0.1 1.0 10.0 100.0

0.1 1.0 10.0 100.0

DE

PT

H , i n.

CBR

DE

PT

H , m m

10.1 lbs.

17.6 lbs.

Both hammers used

Soil Type

CH

CL

All other soils

10.1 lbs.

17.6 lbs.

Both hammers used

Hammer

0 500 1000 1500 2000 2500 3000 3500

DE

PT

H , i n

BEARING CAPACITY, psf

Based on approximate interrelationships of CBR and Bearing values (Design of

File Name: SB20-10

Project: West Milner ID CASSIA 972(1) Date: 3-Mar-20 Location: SB20-10 Soil Type(s): ML/Sandy ML w/ Grav

No. of Accumulative Type of Blows Penetration Hammer

(mm) 0 0 2 5 30 2 5 46 2 5 58 2 5 75 2 5 92 2 5 111 2 5 135 2 5 158 2 5 190 2 5 228 2 5 266 2 5 320 2 5 385 2 5 455 2 5 511 2 5 555 2 5 580 2 5 601 2 5 622 2

0.1 1.0 10.0 100.0

0.1 1.0 10.0 100.0

DE

PT

H , i n.

CBR

DE

PT

H , m m

10.1 lbs.

17.6 lbs.

Both hammers used

Soil Type

CH

CL

All other soils

10.1 lbs.

17.6 lbs.

Both hammers used

Hammer

0 2000 4000 6000 8000 10000

DE

PT

H , i n

BEARING CAPACITY, psf

Based on approximate interrelationships of CBR and Bearing values (Design of

File Name: SB20-11

Project: West Milner ID CASSIA 972(1) Date: 4-Mar-20 Location: SB20-11 Soil Type(s): ML with Sand

No. of Accumulative Type of Blows Penetration Hammer

(mm) 5 0 2 5 36 2 5 58 1 5 90 1 5 137 1 5 221 1 5 328 1 5 438 1 5 496 1 5 540 1 5 590 1 5 627 1

0.1 1.0 10.0 100.0

0.1 1.0 10.0 100.0

DE

PT

H , i n.

CBR

DE

PT

H , m m

10.1 lbs.

17.6 lbs.

Both hammers used

Soil Type

CH

CL

All other soils

10.1 lbs.

17.6 lbs.

Both hammers used

Hammer

0 1000 2000 3000 4000 5000 6000 7000

DE

PT

H , i n

BEARING CAPACITY, psf

Based on approximate interrelationships of CBR and Bearing values (Design of

File Name: SB20-12

Project: West Milner ID CASSIA 972(1) Date: 4-Mar-20 Location: SB20-12 Soil Type(s): Silty Clayey Sand

No. of Accumulative Type of Blows Penetration Hammer

(mm) 0 0 2 5 15 2 5 26 1 5 36 1 5 41 1 5 50 1 5 55 1 5 70 1 1 72 1 5 77 1 5 82 1 5 88 1 5 95 1 5 103 1 5 112 1 5 116 1 5 126 1 5 136 1 5 145 1

0.1 1.0 10.0 100.0

0.1 1.0 10.0 100.0

DE

PT

H , i n.

CBR

DE

PT

H , m m

10.1 lbs.

17.6 lbs.

Both hammers used

Soil Type

CH

CL

All other soils

10.1 lbs.

17.6 lbs.

Both hammers used

Hammer

-40000 -30000 -20000 -10000 0 10000 20000

DE

PT

H , i n

BEARING CAPACITY, psf

Based on approximate interrelationships of CBR and Bearing values (Design of

Project 19-3829G

Route West Milner Road

Name ID CASSIA 972(1) Date of Run 5/11/2019 by B. Western, SK Geotechnical

Typical Section 1 2 3 4 5

Daily ESAL 32 32 32 32 32

Yearly ESAL 11680 11680 11680 11680 11680

20 Year ESAL 233600 233600 233600 233600 233600

Demand

Reliability 75 75 75 75 75

So 0.49 0.49 0.49 0.49 0.49

DeltaPSI 2.2 2.2 2.2 2.2 2.2

Mr 6000 6000 6000 6000 6000

SNDES 2.64 2.64 2.64 2.64 2.64

W18 233600 233600 233600 233600 233600

Zr -0.674 -0.674 -0.674 -0.674 -0.674

ESAL 32 32 32 32 32

Life 20.0 20.0 20.0 20.0 20.0

Capacity a1 0.41 0.41 0.41 0.41 0.41 Layer 1

D1 (in) 4 3 3 4 3 a1 = 0.41

SN1 1.6 1.2 1.2 1.6 1.2

a2 0.14 0.14 0.14 0.14 0.14 Layer 2 m2 0.95 0.95 1.0 1.0 1.0 a2 = 0.14 for CBC

D2 (in) 8.0 11.0 4.0 4.0 7.0

SN2 1.1 1.5 0.6 0.6 1.0

a3 0.09 0.07 0.07 Layer 3 m3 0.9 0.9 0.9 0.9 0.9 a3 = 0.09 for new

D3 (in) 11.0 7.5 7.5 subbase, 0.07 for

SN3 0.0 0.0 0.9 0.5 0.5 existing subbase

Sntot =

SN1+SN2+SN3+SN4

2.70 2.69 2.68 2.67 2.68

Traffic Chk W18=20 Yr

ESAL

OK OK OK OK OK

SN Check OK OK OK OK OK Design Check DESIGN OK DESIGN OK DESIGN OK DESIGN OK DESIGN OK

Existing Surfacing reused as "Dirty" SubbaseTotal Reconstruction

Note

CBC Section

HACP = 4"

CBC Section

HACP = 3"

Subbase Section

HACP = 3"

Existing Surfacing used as "Dirty" Subbase

HACP = 4"

Existing Surfacing used as "Dirty" Subbase

HACP = 3"