mta-dot236-2_WinifredNorth_Final Geotechnical Report-Signed.pdf

FINAL GEOTECHNICAL REPORT

GR 07-24

MT FLAP DOT 236(2)

Winifred - North

Winifred, Montana 22-4208G

Submitted by

2511 Holman Avenue P. O. Box 80190

Billings, Montana 59108-0910

Prepared for

Robert Peccia & Associates PO Box 5653

Helena, Montana 59604

Date June 13, 2024

June 13, 2024 Project 22-4208G

Mr. Neal Bell, PE Robert Peccia & Associates P. O. Box 5653 Helena, Montana 59604

Dear Mr. Bell:

Re: Final Geotechnical Report, MT FLAP DOT 236(2), Winifred – North, IDIQ Contract No.

69056721D000008, Task Order No. 69056722F00105N

The final gravel surfacing report subject project has been completed. The purpose of the report was to evaluate the existing gravel surfacing and subgrade conditions along the project and provide recommendations related to alternative gravel surfacing improvements for Secondary Highway 236 (PN Bridge Road) between MP 28.5 and 47.0. This report presents:

A brief summary of the existing gravel surfacing conditions, geotechnical, and geological features, Findings of our geotechnical subsurface drilling program, Laboratory test results performed by FHWA, Recommended gravel surfacing section alternatives, and Recommendations related to unstable subgrades.

Please refer to the attached report for more detailed results of our fieldwork, gravel surfacing analyses and recommendations.

Thank you for using SK Geotechnical. If you have any questions regarding this report or require our services during the construction phase, please contact Dustin Hutzenbiler at (406) 652-3930.

Sincerely, Dustin P. Hutzenbiler, PE Geotechnical Engineer

Gregory T. Staffileno, PE Reviewing Engineer

Attachment:

Final Geotechnical Report

2511 Holman Avenue P. O. Box 80190

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

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Table of Contents

Description Page

A. Introduction

A.1. Project Description and Purpose A.2. Scope of Services and Background A.3. Available Information

B. Reconnaissance and Review of Available Information B.1. General B.2. General Site Conditions and Geology B.3. Existing Cut and Fill Slopes B.4. Drainage B.5. Existing Gravel Surfacing

C. Soil Boring Results C.1. General C.2. Boring Locations and Elevations C.3. Soil Boring Results C.4. Dynamic Cone Penetrometer Testing

D. Laboratory Testing Program and Procedures D.1. General D.2. Subgrade Samples D.3. Gravel Surfacing Samples D.4. Corrosion Test Results D.5. R-Value Test Results

E. Analysis and Recommendations E.1. Drainage E.2. Cut and Fill Slopes E.3. Shallow Bedrock Concerns E.4. Gravel Surfacing E.5. Unstable Areas and Digouts E.6. Chemical Subgrade Treatment Alternatives E.7. High Performance Geosynthetics E.8. Gravel Surfacing Treatment E.9. Specifications

F. Procedures F.1. Drilling and Sampling F.2. Soil Classifications F.3. Groundwater Observations

G. Qualifications G.1. Basis of Recommendations G.2. Review of Design G.3. Groundwater Fluctuations G.4. Use of Report G.5. Level of Care

Appendix Site Location Sketch

Partial Topographic Sketches Geologic Map Plan Sheets Boring Photographs Geotechnical Feature Photographs Descriptive Terminology – Soil and Rock Log of Boring Sheets ST-1 through ST-47

Laboratory Test Results Summary Table Laboratory Test Results – FWHA Results DCP Test Results Gravel Surfacing Analyses MDT Moisture Sensitive Soils Special Provision RPA Material Sources

A. Introduction

A.1. Project Description and Purpose The project is intended to improve approximately 18 1/2 miles of Secondary Highway 236, between milepost (MP) 28.5 and 47.0. This roadway primarily serves the local agricultural community, but also serves as a connecting route between the towns of Winifred and Big Sandy, Montana. The roadway is a Montana Department of Transportation (MDT) controlled corridor and is maintained by Fergus County.

The current scope of proposed improvements for the roadway includes the following:

Provide subgrade stabilization for soft areas throughout the alignment, Improve and restore surface water drainage characteristics, particularly through the Claggett Hill area, Provide new aggregate surfacing for a minimum finished roadway top width of 22 feet, Provide new signage and install and/or repair cattle guards and guardrails.

No major changes to horizontal alignment or vertical grades are included in the project scope at this time.

However, if the budget allows, it may be desired to improve some of the more extreme horizonal and vertical curves to improve sight distance and overall roadway safety.

The purpose of the geotechnical work was to characterize and evaluate the existing gravel surfacing as well as subsurface soil and groundwater conditions at the proposed boring locations, and to assist Robert Peccia and Associates in design of the new gravel surfacing improvements along the project.

A.2. Scope of Services and Background Our scope of work was outlined in a Statement of Work for the MT FLAP DOT 236(2), Winifred – North project documents. Our scope of work generally included

Literature and map review of the area.

Conducting an initial field reconnaissance of the roadway, paying particular attention to existing surfacing and geotechnical features.

Performing a follow-up reconnaissance by a geotechnical engineer(s).

Painting the proposed boring locations and setting offset stakes to the nearest shoulder. During the staking, recording the latitude and longitude coordinates with a handheld, consumer-grade Garmin GPS. These coordinates are indicated on the attached Log of Boring sheets and are also summarized in a table included in the appendix. Reference points related to mile markers on the project were also indicated on the Log of Boring Sheets and the attached table.

Coordinating the location of underground utilities near the boring locations.

Robert Peccia & Associates June 13, 2024 Project 22-4208G Page 2

Conducting 38 penetration test borings to a depth of 5 feet at approximately 1/2-mile intervals along the project. Conducting 9 additional test borings at various locations throughout the alignment to better evaluate soft subgrades and/or bedrock hardness. As the borings were performed, our geotechnical engineer observed the existing gravel surfacing and recorded the thickness to the nearest 1/4-inch.

Conducting dynamic cone penetrometer (DCP) testing in all of the borings performed for the project. DCP tests were used to evaluate in-place subgrade strength during the summer/fall time frame, when the work was performed.

Collecting bag samples of the existing gravel surfacing, where present, as well as select subgrade samples for various laboratory tests by FHWA.

Classifying the samples and preparing boring logs by a geotechnical engineer.

Selecting various samples for classification (sieve analysis and Atterberg limits), specific gravity, moisture content and R-value tests by FHWA.

Discussing the project with Mr. Neal Bell of RPA.

Preparing 30 Percent, 70 Percent and Final Geotechnical Reports containing logs of the soil borings, our analysis of the field and laboratory testing data, and recommendations for gravel surfacing improvements.

We wish to point out evaluation of potential material borrow sources was completed by RPA and their results are attached.

A.3. Available Information Several documents were reviewed as the project developed. The pertinent documents are summarized below.

Permitted Materials Sources Map by RPA, not dated.

Statement-of-Work document including Appendices A and B, dated July 8, 2022.

Boring Locations Table including mile post (MP), GPS, and state plane coordinates, not dated.

MT Secondary Highway 236 – Plan Sheets prepared by RPA, dated January 31, 2023.

Winifred to Big Sandy Corridor Study by DKS Associates, dated May 2011.

Final plans for Claggett Hill Slide Repair by MDT, Project STPS 236-1(13)45, dated 2011.

Project 22-4208G Page 3

B. Reconnaissance and Review of Available Information

B.1. General During the 30 percent design phase, Mr. Dustin P. Hutzenbiler, PE, and Mr. Chad C. Binstock with our firm, performed an initial field reconnaissance of the alignment to observe the existing gravel surfacing conditions, as well as geotechnical and geological features along the alignment. At this time, Mr. Hutzenbiler also reviewed topographic and geologic maps of the area. Portions of these maps are included in the appendix. Following the reconnaissance, a Preliminary Geotechnical Reconnaissance Memorandum and Field Investigation Plan dated September 23, 2022, was submitted containing the results of our reconnaissance, our preliminary recommended gravel surfacing section, and proposed alignment boring plan.

During the 70 Percent Plan-in-Hand Phase, a subsequent geotechnical reconnaissance was performed by Mr. Hutzenbiler and Mr. Gregory T. Staffileno, PE, both geotechnical engineers with our firm. The desired boring locations were staked and photographed at this time. Photographs of the borings are included in the Appendix.

B.2. General Site Conditions and Geology

B.2.a. Site Topography. As can be seen on the attached Site Location Sketch (Figure 1) and Partial Topographic Sketches (Figures 2 through 7), the initial one-third of the roadway from mile post MP 28.5 to about MP 34.5 traverses primarily moderately sloped rolling terrain, with five larger drainages, several smaller drainages, and gently sloping farmlands. The mileposts are shown in red on the topographic figures. Profile 1 below obtained from Google Earth® illustrates the approximate road elevation profile along the initial one-third of the roadway.

Profile 1. Approximate road surface elevation MP 28.5 to ~34.5

At about MP 34.5, the project transitions into the Missouri River Breaks which is considered relatively rugged terrain with notoriously unstable, moderately steep to steep slopes having significant landslides.

The roadway itself is generally moderately sloped as it traverses along, or adjacent to, ridgelines associated with the Missouri River Breaks. The slopes adjacent to the roadway are moderately steep to very steep and

Project 22-4208G Page 4 lead down into the Judith River and Missouri River Drainages. Profile 2 below obtained from Google Earth® illustrates the approximate road elevation profile along the remaining two-thirds of the roadway.

Profile 2. Approximate road surface elevation profile MP ~34.5 to ~46.0

B.2.b. Geology. Attached Figure 8 is a portion of the Geologic Map of Montana, Edition 1.0 (2007) by the Montana Bureau of Mines and Geology. The geologic map suggests the initial approximately 13 miles of the project primarily travels across sedimentary deposits of the Judith River Formation (Kjr) with some areas of variable gravel outcrop deposits (Qtgr). The remaining project length then primarily transitions into sedimentary deposits of the Claggett Formation (Kcl).

The supporting text for the geologic map describes the Judith River Formation (Kjr) as having “Light brown to light gray, fine- to coarse-grained sandstone with interbeds of gray to black carbonaceous shale, silty shale, and thin coal.” The text describes the Claggett Formation (Kcl) as having “Dark gray to gray shale that weathers brown, with thin, gray sandstone laminae and beds in upper or middle part and calcareous concretions in lower part.”

We also reviewed the Geologic Map of the Winifred 30’ x 60’ Quadrangle Central Montana by Edith M. Wilde and Karen W. Porter, 2001. The supporting text to this map includes the following description related to landslides in the area, “Landslide deposits are extensively developed within the Claggett Formation where often most or all of the outcrop is slumped and contorted. These massive landslides, not separately mapped, are readily recognized across the slopes beneath broad benches developed on the Judith River Formation.”

Several fault lines are also indicated on the Geologic Map. We reviewed the Interactive Fault Map available on the United States Geologic Survey (USGS) website to evaluate if any of these faults are considered “recent.” The USGS website indicates the faults predate the Undifferentiated Quaternary Era and are greater than 1.6-million years old, and therefore are not considered “recent”.

B.2.c. Climate. The area generally receives an average of about 13 to 14 inches of precipitation each year, with the wettest months being in May and June. Below freezing temperatures are observed on average, from October to the beginning of April. The climate graph below obtained from the Nation Weather Service (NWS) indicates the monthly average temperature and precipitation for Big Sandy, Montana, the nearest station to the project.

Project 22-4208G Page 5

B.3. Existing Cut and Fill Slopes Multiple fill slopes are present along the project. The majority of the fill slopes were estimated to be about 8 feet in height or less. These fill slopes were primarily observed to be constructed at slopes of 1V:2H, or flatter, and no apparent unstable areas were observed. The exception to this is the larger fill slopes associated with the Claggett Hill project completed between 2006 and 2011. One shallow slope failure was observed in the fill slope near MP 45.3. The head scarp of the failure was observed to be approximately 8 feet from the existing guardrail. Select photos of the observed fill slopes are attached.

Several existing cut slopes are also present throughout the project limits. The existing cut slopes were generally measured to be about 20 feet in height or less. Slope configurations varied from as steep as 1V:1H to about 1V:3H, but the majority of the slopes appeared stable. The largest cut along the project was also observed within the Claggett Hill area. This slope was constructed in 2011 at a slope of 1V:2.5H by MDT to stabilize a large landslide. A shallow slump was observed within the cut slope approximately 150 feet above the road edge. The slump appeared to be about 40 feet in height and width. The remainder of the cut slope appeared stable. Photos of several of the observed cut slopes are attached.

Project 22-4208G Page 6

B.4. Drainage We judged drainage along the majority of the roadway to be poor, primarily due to erosion rutting in wheel paths, washboards, and gravels berms along the road edge from maintenance over the years impeding the road’s ability to effectively shed water from the road surface. Erosion channels within the active roadway surface have developed throughout the project extents, the most severe of which are concentrated along the steep grades within the Claggett Hill area. These erosion channels have primarily developed due to grading operations blading material to the road edges, creating earthen berms, which redirects water down the roadway rather than off the edges. This has resulted in relatively deep and wide erosion channels directly within the driving lanes. These erosion channels are likely accelerating deterioration of the road surface requiring additional, more frequent, maintenance. Several photos of these erosion channels are attached.

Therefore, we judged drainage along the Claggett Hill area to be very poor.

B.5. Existing Gravel Surfacing In general, we judged the existing gravel surface to be in overall poor condition. Several areas exist where gravel surfacing has been lost due to grading material to road edges during maintenance or has been lost into the clay subgrade due to traffic. Several areas of exposed clay subgrades appear unstable when wet due to readily apparent rutting. This is particularly the case on most of the steeper grades approaching and departing knobs along the roadway. Based on the soil borings and our observations, about one-third of the roadway has exposed subgrades with very little (less than 1 inch) to no gravel surfacing present.

Additionally, the existing gravel surfacing is very highly compacted, mixed with clay, and when dry, is very hard. This condition, in our opinion, acts like a hard crust and when wet, becomes a slip plane. When new surfacing is placed on top of this harder crust, we anticipate the new gravel barely penetrates it, and is therefore highly susceptible to sliding towards the edges due to vehicular traffic. When it rains and this crust is wet, the surface is slick, creating a slip plane, exacerbating the loss of new gravel surfacing.

Drainage along the corridor was considered poor due to rutting cause by flowing water trapped in the roadway and gravel being graded (or slipping) to edges impeding surface water from readily draining off the surfacing. Gravel surfacing is also segregated the entire project length, resulting in loose, coarse aggregates with no sands or binders along the road. Loose rocks are likely causing the vehicle damage discussed in the corridor study report. Several photos of the road surface are attached.

We wish to point out, when the Claggett Hill project was constructed, the gravel surfacing section included 9 inches of MDT Crushed Top Surfacing, Type B, Grade 3. While there are some distressed areas and gravel segregation, it is our opinion the overall roadway surfacing is in fair condition from MP 45.0 to 47.0.

There is some loose aggregate on the surface due to segregation from poor drainage and grading operations, but the surfacing section is largely intact and performing as originally designed. Washboards and erosion observed in the surface is likely the cause of poor surface water drainage. Segregation resulting in lack of fines to act as binder is also contributing to poor drainage. Additionally, the borings generally indicate gravel loss of about 2 1/2 inches since placement in 2006.

Project 22-4208G Page 7

C. Soil Boring Results

C.1. General A total of 47 soil borings were completed for the project on the dates indicated on the boring logs. Borings ST-1 through ST-38 were performed at approximate 1/2-mile intervals from the beginning of the project near MP 28.5 to the end of the project near MP 47.0. The remaining 9 soil borings (ST-39 through ST-47), were performed at various locations selected during the subsequent reconnaissance along the alignment specifically targeting soft spots, bedrock and other geotechnical features observed within the existing roadway.

Log of boring sheets indicating the depth and identification of various pavement, soil, bedrock, penetration resistances, laboratory test data and water level information are included in the Appendix. It should be noted the depths shown as boundaries between the strata are only approximate. The actual changes may be transitioned and the depths of changes may vary between borings.

C.2. Boring Locations and Elevations The approximate boring locations are shown on the attached Plan Sheets in the Appendix. Boring locations were selected and staked in the field by our personnel and photographs of each of the borings are also included in the Appendix. Approximate coordinates were collected using an un-corrected, handheld consumer-grade Garmin GPS. These coordinates were provided to RPA to provide us with plan drawings showing the approximate locations of the borings. Boring surface elevations were not determined.

C.3. Soil Boring Results

C.3.a. Summary Table. Table 1 following this page summarizes the existing gravel surfacing and subgrade conditions at the boring locations along the project. The existing gravel surfacing and subgrade conditions are discussed in more detail following this page.

Project 22-4208G Page 8

Table 1. Existing Gravel Surfacing and Anticipated Subgrade Conditions

Boring ST-1 ST-39 ST-2 ST-3 ST-4 ST-40 ST-5 ST-6 ST-41 ST-7 ST-42 ST-8

Date Drilled 10/17/2022 10/17/2022 10/17/2022 10/17/2022 10/17/2022 10/17/2022 10/17/2022 10/18/2022 10/18/2022 10/18/2022 10/18/2022 10/18/2022

Mile Post (MP) 28.28 28.41 28.78 29.28 29.78 30.01 30.26 30.84 31.18 31.29 31.48 31.64

Existing Gravel Surfacing Thickness (inches)

1.5” 3” 3.5” 4” 0.75” 4.5” 2” 0” 0” 2.5” 0” 1.5”

ASTM Class SM SM SC SC SC SC SC N/a N/a SC N/a SC

AASHTO Class A-2-4 A-2-4 A-2-6 A-2-6 A-2-6 A-2-6 A-2-6 N/a N/a A-2-6 N/a A-2-6

Subgrade(1)

Description Sandy Lean Clay

Lean Clay with Sand

Lean Clay with Sand

Lean Clay with Sand

Lean Clay with Sand

Lean Clay with Sand

Lean Clay with Sand

Lean Clay with Sand Sandstone Lean Clay with Sand Lean Clay with Sand

Lean Clay with Sand

ASTM Class CL CL CL CL CL CL CL CL SM CL CL CL

AASHTO Class A-7-6 A-6 A-6 A-6 A-6 A-6 A-6 A-6 A-2 A-6 A-6 A-6

SPT value

(BPF) 13 11 11 9 16 9 11 14 30 10 9 10

Consistency Stiff Stiff Stiff Firm Very Stiff Firm Stiff Stiff Dense Stiff Firm Stiff

Risk of Subgrade Failure During Construction

Low Low Low Low Low Low Low Moderate Low Low Low Low

(1)Anticipated subgrade that will be present beneath the gravel surfacing section;

Project 22-4208G Page 9

Boring ST-43 ST-9 ST-10 ST-11 ST-12 ST-44 ST-13 ST-14 ST-45 ST-15 ST-16

Date Drilled 10/18/2022 10/18/2022 10/18/2022 10/18/2022 10/18/2022 10/18/2022 10/18/2022 10/18/2022 10/18/2022 10/19/2022 10/19/2022

Mile Post (MP) 31.94 32.28 32.77 33.29 33.78 34.21 34.29 34.79 35.13 35.29 35.79

Existing Gravel Surfacing Thickness (inches)

3” 2.75” 3.5” 0” 2.5” 3.25” 3.5” 0” 0.75” 0” 0.75”

ASTM Class SC SC SC N/a SC GC SC N/a SC N/a SC

AASHTO Class A-2-6 A-2-6 A-2-6 N/a A-2-6 A-2-6 A-2-6 N/a A-2-6 N/a A-2-6

Subgrade(1)

Description Sandy Lean Clay

Lean Clay with Sand

Lean Clay with Sand

Fat Clay with Sand

Sandy Lean Clay

Lean Clay with Sand

Sandy Lean Clay

Lean Clay with Sand

Lean Clay with Sand Sandstone Lean Clay with Sand

ASTM Class CL CL CL CH CL CL CL CL CL SM CL

AASHTO Class A-6 A-6 A-6 A-7-6 A-6 A-6 A-7-6 A-6 A-6 A-2 A-6

SPT value (BPF) 9 18 10 14 14 15 17 13 18 24 17

Consistency Firm Very Stiff Stiff Stiff Stiff Stiff Very Stiff Stiff Very Stiff Medium Dense Very Stiff

Risk of Subgrade Failure During Construction

Low Low Low Low Low Low Low Low Low Low Low

Project 22-4208G Page 10

Boring ST-17 ST-46 ST-18 ST-19 ST-20 ST-21 ST-47 ST-22 ST-23 ST-24 ST-25 ST-26

Date Drilled 10/19/2022 10/19/2022 10/19/2022 10/19/2022 10/19/2022 10/19/2022 10/19/2022 10/19/2022 10/19/2022 10/20/2022 10/20/2022 10/20/2022

Mile Post (MP) 36.28 36.45 36.76 37.29 37.77 38.30 38.45 38.79 39.28 39.78 40.29 40.79

Existing Gravel Surfacing Thickness (inches)

3.5” 0” 0” 3.5” 2” 3” 0” 3” 3.5” 4” 1” 2.5”

ASTM Class SC N/a N/a GC SC SC N/a SC SM SC SM SM

AASHTO Class A-2-4 N/a N/a A-2-6 A-2-6 A-2-6 N/a A-2-4 A-1-b A-2-6 A-2-4 A-2-4

Subgrade(1

Description Sandy Lean Clay Silty Sand Lean Clay with Sand Lean Clay with Sand

Lean Clay with Sand Fat Clay Sandy Lean

Clay

Sandy Lean Clay with

Gravel

Sandy Lean Clay Silty Sand Clayey Sand Sandy Lean

Clay

ASTM Class CL SM CL CL CL CH CL CL CL SM SC CL

AASHTO Class A-6 A-2-4 A-6 A-6 A-6 A-7-6 A-6 A-6 A-6 A-2 A-6 A-6

SPT value (BPF) 13 26 13 18 12 12 25 22 13 32 9 9

Consistency (Top

1.5 feet) Stiff Medium

Dense Stiff Very Stiff Stiff Stiff Very Stiff Very Stiff Stiff Dense Loose Stiff

Risk of Subgrade Failure During Construction

Low Low Low Low Low Low Low Low Low Low Moderate Low

Project 22-4208G Page 11

Boring ST-27 ST-28 ST-29 ST-30 ST-31 ST-32 ST-33 ST-34 ST-35 ST-36 ST-37 ST-38

Date Drilled 10/20/2022 10/20/2022 10/20/2022 10/20/2022 10/20/2022 10/20/2022 10/20/2022 10/20/2022 10/20/2022 10/20/2022 10/20/2022 10/20/2022

Mile Post

(MP) 41.29 41.78 42.28 42.79 43.29 43.79 44.28 44.78 45.25 45.79 46.29 46.67

Existing Gravel Surfacing Thickness (inches)

2.5” 4.5” 9” 6.25” 5” 6.5” 6.25” 5.5” 9” 7” 5.5” 5.5”

ASTM Class SC SC SC SC SC SC SC SC GP-GC GP-GC GP-GC GP-GC

AASHTO

Class A-2-6 A-2-6 A-2-6 A-2-6 A-2-6 A-2-4 A-2-6 A-2-6 A-1-a A-1-a A-1-a A-1-a

Description Clayey Sand Fat Clay Lean Clay with Sand

Sandy Lean Clay

Sandy Lean Clay

Sandy Lean Clay

Sandy Lean Clay Fat Clay

Clayey Gravel with

Sand

Sandy Lean Clay

Clayey Gravel with

Sand

Clayey Sand with Gravel)

Subgrade(1)

ASTM Class SC CH CL CL CL CL CL CH GC CL GC SC

AASHTO

Class A-6 A-7-6 A-6 A-6 A-6 A-6 A-6 A-7-6 A-2-6 A-6 A-2-6 A-2-6

SPT value

(BPF) 11 17 12 17 9 20 24 17 18 13 16 21

Consistency (Top 1.5 feet)

Medium Dense Very Stiff Stiff Very Stiff Stiff Very Stiff Very Stiff Very Stiff Medium

Dense Stiff Medium Dense

Medium Dense

Risk of Subgrade Failure During Construction

Low Low Low Low Low Low Low Low Low Low Low Low

Highlighted Borings Located within MDT Claggett Hill Project Limits

Project 22-4208G Page 12

C.3.b. Existing Gravel Surfacing. To evaluate the existing gravel surfacing section, we considered the project in two separate segments. The first segment includes all borings within the existing roadway up to the portion along the recent Claggett Hill project. Based on the results of the soil borings, the average existing gravel surfacing thickness was about 2 inches.

The 2006 Claggett Hill project typical section provided 9 inches of MDT’s Crushed Top Surfacing, Grade 3, Type B. Based on the borings performed in this area, the average existing gravel surfacing thickness was about 6 1/2 inches. This indicates a gravel surface loss of about 2 1/2 inches since placement, or about 0.2 inches per year.

C.3.c. Subgrade. The attached boring logs outline more details about the soil stratum encountered by each boring. The primary subgrade encountered by the borings was sandy lean clay, lean clay with sand, and fat clay with some areas of silty and clayey sand. Penetration resistances in the clayey soils indicated it generally was firm to very stiff, but primarily stiff. Penetration resistances in the sandy soils indicated it was loose to medium dense.

C.3.d. Groundwater Observations. Groundwater was not encountered in the borings to their termination depth of about 5 1/2 feet while drilling. Based on our observations of the alignment and boring results, we do not anticipate groundwater will be encountered. Even so, surface water runoff still must be considered, especially when considering the moisture sensitivity of the clayey soils. If the subgrade clay soils get wet during construction, they will become very soft to soft causing materials to pump and rut, requiring additional subexcavation and stabilization.

C.4. Dynamic Cone Penetrometer Testing We performed Dynamic Cone Penetrometer (DCP) tests at almost all boring locations at the existing surface to assist in evaluating in-situ subgrade strength. We used a dual-mass DCP which consists of a shaft assembly with a sacrificial 0.790-inch diameter, 60-degree hardened steel cone tip attached to the end. The cone was then driven into the soil using a 17.6-pound hammer dropping 22.6 inches.

A graduated scale is used during driving to measure the set-per-hammer blow, which has been correlated to in-place California bearing ratio (CBR) values by the US Army Corps of Engineers (USACE). Using these published correlations and the manufacturers software, in-place CBR values generally ranged from 2 to 15 with an average value of 6.8 and a standard deviation of 3.2. We wish to emphasize tests were performed in the fall when subgrades are drier and stronger than the wetter spring months.

Project 22-4208G Page 13

D. Laboratory Testing Program and Procedures

D.1. General After completion of the soil borings, the samples collected were returned to our office. Twenty of the samples (9 subgrade, 11 existing gravel surfacing) were selected for various laboratory testing including classification (Sieve analysis and Atterberg limits), specific gravity, moisture content and R-value. The selected samples were then packaged and shipped to FHWA’s testing laboratory in Vancouver, Washington, to perform the requested testing. The test methods used by FHWA are indicated in Table 2 below.

Table 2. FHWA Test Methods

Test Name AASHTO Standard Plasticity Index (Atterberg Limits) T89/T90 R-Value T190 Sieve Analysis T11/T27 Natural Moisture Content T265 Corrosion Tests T288, T289 Specific Gravity at 20° C T100

D.2. Subgrade Samples Table 3 below summarizes the results of the laboratory tests performed on the nine subgrade samples selected for testing.

Table 3. Summary of Subgrade Laboratory Tests

Boring Depth (feet)

Atterberg Limits

P200 ASTM

Class Natural Moisture

Content LL PL PI

ST-1 0.1-4.0 44 16 28 65.7% CL 14.1%

ST-11 0.5-4.0 56 17 39 77.2% CH NT

ST-13 0.5-4.0 48 15 33 62.5% CL 13.3%

ST-17 0.5-4.0 40 16 24 51.7% CL NT

ST-23 0.5-4.0 34 14 20 56.4% CL 13.1%

ST-25 0.5-4.0 29 17 12 49.0% SC NT

ST-27 0.5-4.0 29 18 11 45.1% SC 9.6%

ST-37 0.5-4.0 33 15 18 58.2% CL 14.6%

ST-43 0.5-4.0 35 15 20 59.2% CL 17.6%

NT- Not Tested

Project 22-4208G Page 14

As can be seen above, six samples classified as lean clay (CL), two samples classified as clayey sand (SC), and one samples classified fat clay (CH).

D.3. Gravel Surfacing Samples Table 4 below summarizes the results of the laboratory tests performed on ten selected gravel surfacing samples for the project.

Table 4. Summary of Gravel Surfacing Samples

Boring Depth (feet)

ASTM

Class

Atterberg Limits Percent Passing

LL PL PI 1 1/2 No. 4 No. 10 No. 200

ST-2 0-0.25 SC 25 12 13 100 60 47 17.8

ST-7 0-0.167 SM NP NP NP 100 78 62 13.9

ST-17 0-0.29 SC 24 14 10 100 68 51 18.1

ST-22 0-0.25 SC 23 15 8 88 54 45 16.1

ST-23 0-0.25 SM NP NP NP 100 79 64 20.1

ST-30 0-0.53 SC 25 14 11 100 63 48 14.1

ST-32 0-0.50 SC 23 14 9 100 68 55 15.4

ST-34 0-0.50 SC 25 14 11 100 63 47 14.7

ST-37 0-0.46 GP-GC 22 18 6 100 52 41 11.1

ST-40 0-0.375 SC 24 12 12 100 63 50 16.1

NP- Non-Plastic

The gravel surfacing samples primarily classify as clayey sand (SC) and silty sand (SM), with ASSHTO classification primarily of A-2-4 and A-2-6. We reviewed the gravel surfacing requirements in FP-14, Table 703-3, and the classification test results indicate the existing gravel surfacing does not meet the FP-14 specification. We also reviewed MDT’s Standard Specifications for Crushed Top Surfacing, Grade B, which is typically specified on MDT controlled corridors with gravel surfacing. The results indicate 7 of the 10 samples generally meet the gradation and plasticity requirements to be considered Crushed Top Surfacing, Grade B.

D.4. Corrosion Test Results Corrosion testing was also performed by the FHWA laboratory consistency of resistivity and pH. Sulfate testing was not performed. The results of the corrosion tests are attached and are summarized in Table 5 following this page.

Project 22-4208G Page 15

Table 5. Corrosion Test Results

(feet) pH Resistivity

(ohm-cm)

ST-1 0.1-4.0 6.8 490

ST-13 0.5-4.0 7.7 550

ST-23 0.5-4.0 8.4 400

ST-27 0.5-4.0 8.0 1,400

ST-37 0.5-4.0 7.9 420

ST-43 0.5-4.0 7.9 2,000

The resistivity results are generally considered moderately to highly corrosive to buried metallic conduits.

Sulfate testing was not performed. Therefore, assessment for concrete pipes could not be completed.

Additionally, typical subgrade treatments such as cement, lime, etc. generally require knowledge of subgrade sulfate level to evaluate their effects to the treatment alternatives.

D.5. R-Value Test Results R-value testing was performed on six subgrade samples along the alignment. Results are summarized in Table 6 below.

Table 6. R-Value Test Results

(feet)

ASTM

Soil Class

Specific Gravity

Natural Moisture

Test Density

(pcf)

Test Moisture R-Value

ST-1 0.1-4.0 CL 2.73 14.1% 108.7 19.2% 7

ST-13 0.5-4.0 CL 2.74 13.3% 103.0 20.6% 4

ST-23 0.5-4.0 CL 2.75 13.1% 113.8 18.6% 3

ST-27 0.5-4.0 SC 2.71 9.6% 116.6 14.2% 28

ST-37 0.5-4.0 CL 2.78 14.6% 113.5 17.9% 5

ST-43 0.5-4.0 CL 2.76 17.6% 119.8 11.6% 10

E. Analysis and Recommendations

E.1. Drainage As previously discussed, we judged drainage along the majority of the roadway to be poor, primarily due to erosion rutting in wheel paths, washboards, and gravels berms along the road edge from maintenance over the years impeding the road’s ability to effectively shed water. Erosion channels within the active roadway surface have developed throughout the project extents, the most severe of which are present in steeper graded sections of the roadway and especially within the Claggett Hill area. These erosion channels have primarily developed due to grading operations blading material to the road edges, creating earthen berms, which redirects water down the roadway rather than off the edges. Vehicle traffic may

Project 22-4208G Page 16 also be causing gravel to slip towards the edges. This has resulted in relatively deep and wide erosion channels that extend within the active driving lanes.

The overall project scope includes restoring the road drainage characteristics throughout the project limits, particularly in the Claggett Hill area. While restoring the roadway to provide positive drainage off the surface is good for the majority of the improvements, it is our opinion more detailed evaluation of the drainage characteristics in the Claggett Hill area is needed due to the high to very high risk of activating or aggravating landslides within this portion of the corridor. In particular, carefully planned improvements are needed in the area around approximate MP 44.9, where a head scarp is in a fill slope about 8 feet behind the current guardrail. If surface water is allowed to drain directly into the head scarp, it most likely will reactivate the slide, causing damage to the road. Historically, large landslides in this area have resulted in significant earthwork and costly repairs to the road.

Therefore, we recommend restoring grades and drainage along the roadway from the beginning of the project to the beginning of the guardrail at approximate MP 44.8. From about MP 44.8 to about MP 45.0, we recommended surface water drainage characteristics be further evaluated to prevent water from being shed off the roadway directly into the existing slide. Additionally, cuts and fills in this area, even minor, are not recommended and should be avoided. From MP 45.0 to the end of the project, restoring the roadway to its original condition is likely acceptable. Even so, we wish to emphasize, due to the inherent instability of the Claggett Hill area and previous experience with landslides in this area, there is risk that any earthwork in this area, even minor grading operations, could have adverse effects to overall stability.

We recommend stakeholders be informed of these risks prior to roadway improvements.

E.2. Cut and Fill Slopes It is our understanding this is a surfacing only project, and no cut or fill slopes are anticipated for the proposed Winifred-North improvements. As previously indicated, multiple fill slopes are present along the project. If minor cut or fill slopes are anticipated, we recommend slopes be constructed at 1V:3H, or flatter, but no steeper than 1V:2H. Any cut or fill slopes associated with the project will need to be carefully reviewed to avoid adverse effects to local stability, especially in the Claggett Hill area.

E.3. Shallow Bedrock Concerns The project scope currently does not include improvements to horizontal alignment or vertical grades.

However, if these become part of the scope as the project develops, shallow bedrock could be a concern.

Moderately hard sandstone caprock over poorly lithified, slope forming formation material is anticipated throughout the alignment. If cuts into these materials become part of the project, the bedrock will need to evaluated to determine general excavatability, or if ripping and/or blasting is necessary.

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E.4. Gravel Surfacing

E.4.a. Traffic Analysis. The scope of work document for the project indicates this corridor is considered a low volume road and has an ADT of less than 400 vehicles per day. However, detailed traffic information was not provided, such as percent trucks, annual growth rate, etc. Traffic data was developed as part of the Claggett Hill project which projected traffic levels through 2019. A design 20-year daily ESAL value of 4.62 was indicated in the original plans. Based on the traffic information and a growth rate of 1 percent, we projected forward for a design life of 10 years from 2024 to 2034, which is typical for a gravel-surfaced road. The traffic analysis indicates 5.3 ESALs per day for design.

This equates to total ESAL’s for a 10-year design life of 19,345. Appendix A, Section 2 of the SD LTAP document indicates low volume roads are those with design ESAL’s between 10,000 and 30,000.

Therefore, we considered the projected traffic reasonable for development of gravel surfacing recommendations.

E.4.b. Methodology. Gravel surfacing sections were analyzed following the design methods in Appendix A of the Gravel Roads Maintenance and Design Manual by FHWA and SD LTAP, November 2000.

E.4.c. Input Parameter Assumptions. The following design input parameters were used to evaluate gravel surfacing thickness.

18-kip ESAL’s over Design Life: 19,345 Change in Serviceability: 2.5 Allowable Rut Depth: 2 inches Base (gravel surfacing) Elastic Modulus: 30,000 psi Roadbed Resilient Modulus, MR

-Winter (frozen): 20,000 psi -Spring/Thaw (saturated): 2,800 psi -Spring/Fall (wet): 5,600 psi -Summer (dry): 6,150 psi

As indicated above, we broke the roadbed resilient modulus into four different categories per the design method. The design method indicates season lengths for projects within climatic region VI for winter, spring/thaw, spring/fall, and summer are 3, 1 1/2, 3, and 4 1/2 months, respectively.

The spring/thaw (saturated) condition was derived by taking one standard deviation below the mean of the R-value testing performed by FHWA. We wish to point out, this value did not include the results from Boring ST-27 since that was performed on clayey sand, which was not the predominate subgrade and was considered unrepresentative. The spring/fall (wet) and summer (dry) conditions were derived from field DCP tests performed during the drilling in October 2022, which we considered representative of these conditions. Similarly, we developed the design CBR based on one standard deviation below the

Project 22-4208G Page 18 average, which was 3.6. The CBR and R-values were correlated using equations from MDT’s Pavement Design Manual based on the 1993 AASHTO Guide for Design of Pavement Structures. During the winter months when the ground is frozen, we assumed a maximum roadbed resilient modulus of 20,000 psi.

To select an appropriate gravel surfacing elastic modulus for design, we referred to report No. UT-16.12 developed for the Utah Department of Transportation Research Division by Brigham Young University, December 2015. Table 2.3 indicates the measured Resilient Modulus Values by Soil Classification using ASSHTO and USCS methods. This reference is attached. Based on the anticipated AASHTO classification for FHWA gravel surfacing, the soils are likely to be a A-2-6 to A-2-4 material. The reference indicates a A-2-4 and A-2-6 modulus values range from 21,500 to 37,500 psi, but typical values range from 26,000 to 32,000. Based on these values, we assumed a gravel surfacing elastic modulus of 30,000 psi for design.

E.4.d. Existing Gravel Surfacing Crust. As previously indicated, to evaluate existing gravel surfacing, we generally broke the project up into two segments. Segment 1 extends from the beginning of the project to the beginning of the Claggett Hill reconstruction project completed in 2006. Existing gravel surfacing generally ranged from 0 to 4 1/2 inches thick with an average thickness of 2 inches. As can be seen in Table 1 previously presented, two larger areas between about MP 30.25 and 31.5, and MP 34.5 and 37.0, have little to no gravel surfacing present.

The Claggett Hill project, originally constructed in 2006, included 9 inches of MDT Crushed Top Surfacing, Type B, Grade 3. The borings generally indicated gravel surfacing thicknesses ranging from about 5 to 9 inches with an average existing thickness of about 6 1/2 inches. Considering these values and the length of time they have been in place, it appears that gravel loss due to traffic, maintenance, and environmental factors since installation has been about 0.2 inches per year.

As previously discussed, the existing gravel surfacing outside of the Claggett Hill area is very compacted, most likely due to repeated traffic and limited maintenance, creating a very hard, concrete like crust. In our opinion, this harder crust can act like a slip-plane and reduces the bond between new gravel surfacing and the existing gravel surfacing/subgrade. Therefore, it is our opinion to provide better long-term performance, it is critical the top 2 to 4 inches of the material be loosened prior to placing new gravel surfacing. This loosening is needed to break this crust up and provide a better interface for bonding of the new gravel surfacing. This can be accomplished several ways, but we anticipate blading using a bulldozer to windrow the material and respread it is likely the most cost-effective method. Although reclaimers are available within the state, we anticipate the cost to pulverize the existing gravel surfacing crust will likely be higher when compared to conventional earth moving equipment.

Throughout Claggett Hill, the surficial crust was generally not observed, therefore, additional grading is not warranted in our opinion.

Project 22-4208G Page 19

E.4.e. Gravel Surfacing Thickness. Based on our analysis, we estimate about two-thirds of the alignment from the beginning of the project to Claggett Hill generally has a minimum of 2 inches of A-2 gravel surfacing material in-place over the primarily clay subgrade. The remaining one-third has little to no existing surfacing. Assuming no existing gravel surfacing is present, the analysis indicates a minimum gravel surfacing thickness of 7 3/4 inches is required. This thickness is then adjusted for loss, as described below.

The Claggett Hill surfacing was constructed in 2006 with 9 inches of MDT Crushed Top Surfacing material. The borings indicated an average remaining thickness of about 6 1/2 inches of surfacing is present, resulting in a total gravel loss of about 2 1/2 inches, or about 0.2 inches per year. Assuming a 10-year design life with similar maintenance and traffic patterns for the entire roadway, we recommend assuming a total gravel loss of 2 inches for the project. Step 9 of the analysis method therefore indicated an additional 1 inch of surfacing is recommended for a total thickness of 8 3/4 inches.

We also evaluated the thickness based on MDT’s Pavement Design Manual. The analysis also indicates the minimum required gravel thickness is 8 3/4 inches for traffic levels less than 10 ESAL’s per day placed directly on the clay subgrade.

It is our understanding FHWA recommended providing a uniform thickness of new crushed top surfacing of 9 inches for the entire project limits. This will exceed the minimum required gravel surfacing thickness indicated by the analysis.

E.5. Unstable Areas and Digouts

E.5.a. General. The risk of encountering unstable subgrades was evaluated at each of the borings based on conditions during drilling and were indicated in Table 1.

While the risk was generally considered low, the borings were completed in the fall months when subgrades are generally drier and stronger. The risk of encountering/creating unstable subgrades is directly associated with the construction schedule, methods, and repetitive loads due to heavy rubber-tired construction equipment. Also, the clayey subgrade soils are considered highly moisture sensitive, and more unstable conditions will be encountered in the spring after spring/thaw and due to rainstorms and poor runoff. Late-summer and early-fall are normally much drier and hotter, and fewer unstable conditions will likely occur.

E.5.b. Identification of Unstable Areas. Once the existing gravel surfacing is near or at final grade, we recommend proof-rolling with a loaded tandem-wheel dump truck or water truck. If the material deflects 1/4-inch or more, then the underlying subgrade is considered unstable, and a subexcavation and placement of a stabilized gravel surfacing section is recommended. Even though we have anticipated stabilized pavement extents below, the best method is to have the Project Manager determine the extents

Project 22-4208G Page 20 during construction by proof-rolling observations. We recommend repairs of unstable areas generally follow Section 303 of the FP-14 specifications.

In our opinion, consideration can be given to discussing the solution with the contractor as well as leaving the area(s) alone for several days. If given favorable weather and not subjected to construction equipment, some clays can become stable, i.e., clays become stronger as pore pressures dissipate.

E.5.c. Stabilized Surfacing Section. Where unstable subgrades are identified, the extent should be subexcavated and minimum of 12 inches below proposed subgrade elevation with smooth-bladed excavation equipment. The subexcavated material should be replaced with imported, relatively impermeable, lean clay or clayey sand, having AASHTO classification of A-2-6 or A-2-7, to avoid creating a bathtub and provide a more uniform subgrade.

It is our opinion granular backfill including Select Borrow per Section 704.07 or Select Granular Backfill per Section 704.08 can also be considered provided the bottom of the subexcavation can be extended to the road edges and graded to drain (daylighted) to avoid creating a bathtub in the roadway.

All backfill materials should be moisture conditioned, placed, and compacted in accordance with Section 204 of the Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects, FP-14.

E.5.d. Extents. Based on the borings and field observations, we anticipate the following extents in Table 8 will have a moderate high risk of being unstable during construction and will require subexcavation and placement of the stabilized surfacing section. This is especially true if the work occurs during the wetter spring/early summer months.

Table 8. Anticipated Extents for Unstable Subgrades

Extent

(MP)

Description Approximate Length (ft)

Lane Width

28.62 Soft Subgrade, rutting, potholes, washboard 100 Full width

29.87 Soft Subgrade 100 Full width

30.25 Soft Subgrade, rutting, potholes, washboard 30 Full width

31.19 Soft Subgrade 200 Full width

31.70 Soft Subgrade, rutting 350 Full width

32.19 Soft Subgrade 500 Full width

32.88 Soft Subgrade 200 Full width

33.83 Soft Subgrade 100 Full width

34.44 Soft Subgrade 200 Full width

34.75 Soft Subgrade 300 Full width

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Extent

(MP)

Description Approximate Length (ft)

Lane Width

35.08 Soft Subgrade 100 Full width

35.19 Soft Subgrade 100 Full width

35.30 Soft Subgrade 200 Full width

35.35 Soft Subgrade, rutting 60 1/2 width, NB

35.70 Soft Subgrade 100 Full width

35.91 Soft Subgrade 50 Full width

36.20 Soft Subgrade 200 Full width

36.57 Soft Subgrade 100 Full width

36.76 Soft Subgrade 200 Full width

37.71 Soft Subgrade 200 Full width

38.27 Soft Subgrade 200 Full width

38.45 Soft Subgrade 500 Full width

38.66 Soft Subgrade, rutting 150 Full width

39.14 Soft Subgrade 300 Full width

39.40 Soft Subgrade 100 Full width

40.48 Soft Subgrade 100 Full width

Subtotal (ft) 4,740

Subtotal (mi) 0.90 NB-Northbound Lane

Apparent unstable areas were evaluated based on the observations in the field in the fall of 2021 and 2022, therefore, should be considered approximate. Additionally, if construction is delayed until the late summer/early fall months (July-October), the amount of stabilization will be reduced.

We wish to point out that the recommended and best method to determine the extents for subexcavation and placement of the stabilized surfacing section is by proof rolling during construction. These extents can vary, especially when considering the weather prior to and during construction, frequency and size of construction equipment, drainage during construction, and many other conditions dependent on nature and construction methods.

E.6. Chemical Subgrade Treatment Alternatives The primary benefits of chemical stabilization of the subgrade, particularly for this project, would theoretically reduce the required gravel surfacing thickness and improve long-term performance of the roadway. Unfortunately, to our knowledge, chemically stabilizing subgrade soils has not been performed

Project 22-4208G Page 22 on large-scale projects in Montana. Additionally, very few small-scale projects have been successfully completed using these methods, with none by local contracting forces.

Additionally, stabilization practices for highway projects in our surrounding states of North Dakota, South Dakota, Wyoming, Idaho, and even the Saskatchewan province in Canada are either not used or not very well established. Therefore, we assume that very few contractors in Montana or nearby states have much, if any, experience with large-scale chemical stabilization of subgrade soils. There are projects where pulverized base mixture has been stabilized with cement, or full depth reclamation. We have also discussed chemical subgrade stabilization with MDT's geotechnical personnel on past projects. They have indicated since these methods are unprecedented, caution should be exercised with regard to the amount of additional supervision and testing resources that will be required during design and construction.

Considering the lack of experience of contractors and design personnel in Montana, the lack of sulfate testing of subgrade soils to evaluate potential adverse effects, and this being a secondary highway with a limited and constrained budget, it is our opinion that the risk of unanticipated problems during construction due to a lack of experience is relatively high and there is the potential for premature failure and/or major change orders. Additionally, several local gravel sources are available near the project, and chemically treating the subgrade is likely cost prohibitive when considering the local contracting forces and gravel availability.

E.7. High Performance Geosynthetics Incorporating high performance geosynthetics such as biaxial, triaxial, or Mirafi woven fabrics was also considered to reduce surfacing thickness and improve stability. Based on the observed conditions, soil borings, and our analysis, incorporating a high performance geosynthetic would reduce the gravel thickness to 6 inches. Thinner sections are not recommended. We also performed a simple cost analysis comparing current geosynthetics pricing and the anticipated gravel savings. The results indicate the high performance geosynthetics will add significant project costs.