GR 05-21 Final WFLHD Geotechnical Engineering Report (without GR 18-21 addendum).pdf

REPORT COVER PAGE

Final WFLHD Geotechnical Engineering Report No. 05-21

Charles M Russell Refuge Repairs Fergus, Petroleum and Phillips Counties, MT

May 26, 2021 Terracon Project No. C4205037

Prepared for:

DJ&A PC

Missoula, MT

Prepared by:

Terracon Consultants, Inc.

Great Falls, Montana

Terracon Consultants, Inc. 1392 13 t h Ave SW Great Fal ls, MT 59404 P (406) 453 5400 F (406) 761 6655 terracon.com

REPORT COVER LETTER TO SIGN

May 26, 2021

DJ&A PC

3203 S Russell Street Missoula, MT 59801

Attn: Mr. Paul Druyvestein, P.E.

P: (406) 721-4320 ext. 105 E: pauld@djanda.com

Re: Final WFLHD Geotechnical Engineering Report No. 05-21 Report Charles M Russell Refuge Repairs Charles M Russell Refuge Fergus, Petroleum and Phillips Counties, MT Contract No. DTFH7015D00003, Task Order No. 69056720F000090 Terracon Project No. C4205037

Dear Mr. Druyvestein:

We have completed the Final WFLHD Geotechnical Engineering Report No. 05-21 services for the project referenced above. This study was performed in general accordance with the Statement of Work (SOW) dated June 11, 2020 and subsequent signed Professional Services Agreement dated October 23, 2020. This report presents the findings of the subsurface exploration, including laboratory testing completed by Western Federal Lands Highway Division finalized on March 3, 2021, and provides geotechnical design recommendations for the proposed project.

We appreciate the opportunity to be of service to you on this project. If you have any questions concerning this report or if we may be of further service, please contact us.

Sincerely, Terracon Consultants, Inc.

Matthew D. Hoffmann, P.E. Brian J. Williams, P.E., P.G.

Office Manager Senior Geotechnical Engineer

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REPORT TOPICS

INTRODUCTION

SITE CONDITIONS

PROJECT DESCRIPTION

GEOTECHNICAL CHARACTERIZATION

GEOTECHNICAL OVERVIEW

EARTHWORK

SITE 1 – SIPARYANN CREEK CROSSING

SITE 2 – ROCK CREEK CROSSING

SITE 3 – SEVENMILE CROSSING

SITE 4 – NICHOLS CROSSING

SITE 6 – SAND CREEK CROSSING

SEISMIC CONSIDERATIONS

GENERAL COMMENTS

Note: This report was originally delivered in a web-based format. Orange Bold text in the report indicates a referenced section heading. The PDF version also includes hyperlinks which direct the reader to that section and clicking on the GeoReport logo will bring you back to this page. For more interactive features, please view your project online at client.terracon.com.

ATTACHMENTS

EXPLORATION AND TESTING PROCEDURES

PHOTOGRAPHY LOG

SITE LOCATION AND EXPLORATION PLANS

EXPLORATION RESULTS

SUPPORTING INFORMATION

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

http://client.terracon.com/

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INTRODUC TION

Final WFLHD Geotechnical Engineering Report No. 05-21 Report

Charles M Russell Refuge Repairs Charles M Russell Refuge

Fergus, Petroleum and Phillips Counties, MT Terracon Project No. C4205037

May 26, 2021

INTRODUCTION

This report presents the results of our subsurface exploration and geotechnical engineering services performed for the proposed Charles M Russell Refuge Repairs project to include design modifications at five sites throughout the Charles M Russell Refuge in Fergus, Petroleum and Phillips Counties, MT. The purpose of these services is to provide information and geotechnical engineering recommendations relative to:

■ Subsurface soil and rock conditions ■ Foundation design and construction

■ Groundwater conditions ■ Lateral earth pressures

■ Site preparation and earthwork

■ Excavation considerations

■ Seismic site classification per AASHTO LRFD Bridge Design Specifications, 8th Edition

The geotechnical engineering Scope of Services for this project, based on the Statement of Work (SOW) dated June 11, 2020 along with the approved Final Exploration Plan submittal dated August 28, 2020, included the advancement of a total of 15 test borings for the five sites explored.

The five sites included within our geotechnical scope and evaluated within this report are as follows:

■ Site 1 – Siparyann Creek ■ Site 2 – Rock Creek

■ Site 3 – Sevenmile ■ Site 4 - Nichols

■ Site 6 – Sand Creek

Thirteen of the borings, conducted at Sites 1, 2, 3, and 4, were extended to 26.5 feet below existing grade. Based on the 30% preliminary design, these sites included primarily surface improvements such as site grade raising, riprap erosion control installations, construction of concrete ford, and culvert replacements. For Site 6, Sand Creek Crossing, two borings were extended to 43.0 and 54.0 feet below existing grade as the site improvement will include the installation of a precast concrete bridge adjacent a former timber bridge structure to replace the current concrete ford used for crossing at this location.

Final WFLHD Geotechnical Engineering Report No. 05-21 Report Charles M Russell Refuge Repairs ■ Fergus, Petroleum and Phillips Counties, MT May 26, 2021 ■ Terracon Project No. C4205037

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Maps showing the sites and boring locations are shown in the Site Location and Exploration Plan sections, respectively. The results of the laboratory testing performed on soil samples obtained from the site during the field exploration are included on the boring logs and as separate graphs in the Exploration Results section.

SITE CONDITIONS

The following description of site conditions has been replicated from the Statement of Work (SOW) documents provided by DJ&A, along with additional description derived from our site visits in association with the project scoping phase and preliminary geotechnical field explorations, and our review of publicly available geologic and topographic maps.

Item Description

Parcel Information

The proposed project improvements are located at several locations both north and south of the Missouri River in the CMR Refuge.

Sites 1 through 4 are located north of the river on Route 201, which is a maintained jeep trail, beyond the end of Route 101, a maintained gravel surfaced road. Site 1 is approximately 7.3 miles east of Highway 191, Site 2 is approximately 11.0 miles east of Highway 191, Site 3 is approximately

22.6 miles east of Highway 191 and Site 4 is approximately 26.1 miles east of Highway 191.

Site 6 is located south of the Missouri River on Route 210, a maintained jeep trail east of Highway 191. Site 6 is located approximately 12.5 miles east of Highway 191 on Route 210.

See Site Location maps

Existing Improvements

The improvements are all located along maintained jeep trails within the CMR Refuge. Existing site features primarily include culverts, concrete fords, HDPE piping, and existing timber bridge structures that have been buried by more recent alluvial sediment deposits (Site 3) or decommissioned (Site 6).

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Item Description

Existing Topography

Each of the sites is located within alluvial fans deposited by the upslope drainage systems as they feed the Missouri River. The topography at each site has limited variation along the alignment, with the general slope tending from the upslope areas of the drainage features toward the Missouri River.

That is, north of the Missouri River, the general topography slopes south across the roadway alignments toward the river. South of the Missouri River, the general topography slopes north across the roadway alignment toward the river.

The general site Elevation above Mean Sea Level (MSL), as obtained from GoogleEarth Pro for each site is listed below:

■ Site 1 – Siparyann Creek Crossing, 2269 to 2272 feet

■ Site 2 – Rock Creek Crossing, 2273 to 2287 feet

■ Site 3 – Sevenmile Crossing, 2259 to 2262 feet

■ Site 4 – Nichols Crossing, 2259 to 2260 feet

■ Site 6 – Sand Creek Crossing, 2265 to 2270 feet

Geology

The project scope includes several locations both north and south of the Missouri River in the CMR Refuge. The general geologic setting of the CMR Refuge is dominated by overburden sands and moderate to high plasticity clay soils, which are deposits left on the floor of Glacial Lake Musselshell during the Bull Lake Ice Age (70,000 to 130,000 years ago). These glacially derived lakebed soils are overlying bedrock of Upper Cretaceous Age, predominantly Bear Paw Shale and Judith River Formation Sandstones. As the sites approach the Missouri River, the general subsurface conditions consist of alluvial deposits, alluvial fans deposited from drainages as they feed the river and flood bank deposits of sand, silt, clay and intermittent gravel. The alluvial deposits near the Missouri River have not been documented, but are anticipated to extend over 100 feet as the Missouri River Channel has greatly incised the bedrock in the area. Outcrops of Judith River Formation sandstone, a more resistant formation, are noted along Route 101 north of the River. Many of the sites align within the Bear Paw Shale Formation that form prominent sag/slough features in which past landslide movements have resulted in the current appearance consistent with the Missouri River Breaks area. Numerous recent to active landslide features with head scarps ranging from a couple feet to over 60 feet were observed throughout the general area of observation, not necessarily impacting the project sites but indicative of the tenuous slope conditions in the geologic setting.

We also collected photographs at the time of our field exploration program. Representative photos are provided in our Photography Log.

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PROJECT DESCRIPTION

Item Description

Information Provided

We were provided with several documents for our review and development of the geotechnical portions of this project throughout the course of this project by DJ&A including the following:

o Statement of Work (SOW) dated June 11, 2020 o Appendix “B” Preliminary Design dated October 31, 2018 o CMR Refuge Modified Design Summary dated February 2, 2019

Project Description

The overall objective of the project is to improve long-term performance of the maintained routes (Route 201 and Route 210) alignments through extreme runoff events through implementation of improved drainage features, grading, and installation of ford/bridge structures to allow for continued passage of vehicle traffic during runoff events. Description of individual design elements included in the preliminary design for each Site will be discussed separately in the report.

GEOTECHNICAL CHARACTERIZATION

We have developed a general characterization of the subsurface conditions based upon our review of the subsurface exploration, laboratory data, geologic setting, and our understanding of the project. This characterization forms the basis of our geotechnical calculations and evaluation of site preparation and foundation options. For the purposes of this report, the individual project sites will be broken out to provide brief descriptions of the subsurface conditions encountered along with the impacts to the proposed design in order to develop appropriate recommendations for the final design, economic considerations, life expectancy, and constructability. Conditions encountered at each exploration point are indicated on the individual logs. The individual logs can be found in the Exploration Results section of this report. The methods and procedures utilized for the exploration are outlined in the Exploration and Testing Procedures section of this report.

The borings were observed while drilling and immediately after completion for the presence and level of groundwater. The following table provides a summary of borings in which groundwater was observed during our exploration efforts.

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Site Boring ID Depth to Ground Water (ft)1, 2

Site 1 – Siparyann B-01 None Encountered

Site 1 – Siparyann B-02 15.5 (11/7/2020)

Site 1 – Siparyann B-03 18.0 (11/7/2020)

Site 2 – Rock Creek B-04 None Encountered

Site 2 – Rock Creek B-05 26.0 (11/7/2020)

Site 2 – Rock Creek B-06a None Encountered

Site 3 – Sevenmile B-07 None Encountered

Site 3 – Sevenmile B-08 None Encountered

Site 3 – Sevenmile B-09 14.5 (11/4/2020)

Site 3 – Sevenmile B-10 24.0 (11/4/2020)

Site 4 – Nichols B-11 24.0 (11/3/2020)

Site 4 – Nichols B-12 17.5 (11/3/2020)

Site 4 – Nichols B-13 12.5 (11/3/2020)

Site 6 – Sand Creek B-14 48.5 (11/5/2020)

Site 6 – Sand Creek B-15 12.5 (11/6/2020)

1. Below existing ground surface

2. Groundwater level measurements taken during drilling

The field investigation does not fully reflect seasonal or long-term groundwater conditions which will be influenced by precipitation, hydrologic impacts originating off-site, and other factors beyond the scope of this investigation. Therefore, groundwater levels during construction, or at other times in the life of the project, may vary from the conditions indicated on the Logs.

GEOTECHNICAL OVERVIEW

A primary concern throughout the CMR Refuge is the impact that standing water and runoff from the adjacent tributary watershed areas has on the ability to utilize the roadways within the CMR Refuge due to continued washouts and overtopping of the alignment. The predominant subgrade throughout the area consists of glacially derived, moisture-sensitive, moderate to high plasticity fat (lakebed) clay soils that become very unstable precipitation events. Any planned improvements to the areas need to consider the fat clay subgrade to be extremely sensitive to moisture and weak, such that typical construction traffic operating over prepared subgrades can create localized instabilities, especially after precipitation events. The establishment of effective surface drainage should be completed early in the construction sequence for all improvements planned, and maintained throughout and after construction to avoid potential issues associated with the moisture-sensitive clay soils. If possible, the grading should be performed during the

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Also, of consideration is the continued deposition of sediment from the drainages feeding the Missouri River at each of the sites. Planned improvements to the sites discussed has the intent of providing for improved performance of the roadway alignment, increased capacity and functionality of culverts, or fords, to allow for more water and sediment to cross the roadway while limiting the impacts on the travel path. With these improvements, it will be important to ensure that continued maintenance of the upslope drainage ditches is programmed so that the functionality of drainage improvements is maintained throughout the design life.

The General Comments section provides an understanding of the report limitations.

EARTHWORK

Earthwork is anticipated to include clearing and grubbing, subgrade preparation, subexcavations and replacements, as well as embankment fill placement. The Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects FP-14 applicable sections for earthwork will pertain for the project. Specific geotechnical considerations for pavement subgrade and structure site preparation are discussed in the corresponding sections of the following report..

Site Preparation

Prior to placing fill materials, the existing vegetation and root mat should be removed. Complete stripping of the topsoil should be performed in all areas that are to receive embankment fill.

Preparation of the subgrade should be consistent with Section 204.06(b), scarification of 6 inches below subgrade and compact according to Section 204.11 of the FP-14 throughout, unless noted otherwise. The following table lists the recommended subgrade preparation methods based on stationing.

Fill Material Types

Soil Type FP-14 Section Placement Locations

Structural Backfill 704.04

Fill placed below concrete ford construction to provide suitable uniformity and modular value for ford concrete placement (Sites 1 and 2) For abutment backfill at precast concrete bridge structure (Site 6)

Select Borrow (Pit Run) 704.07

Grade raising fill sections for embankment construction to provide additional freeboard (Sites 1 and 3)

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Soil Type FP-14 Section Placement Locations

Gabion Basket Fill 705.01 For gabion basket fill material used as overflow structure (Site 3)

Riprap (Class 2 & 4)

705.02 For armoring of culvert inlet and outlet, at concrete ford outfalls, and armoring of abutments for precast bridge structure (Sites 1, 2, 3, 4, and 6)

Native Clays, Gravels, and Sands 204.06

Native soils encountered may be suitable for embankment widening outside of the 1H:1V core with finished slopes at 2H:1V or flatter to match existing grade where moisture conditioning and compaction can be achieved. The predominant native soils encountered within the upper reaches of the subsurface profile consist of high plasticity, moisture-sensitive lakebed clays that may be difficult to place and compact within requirements. Native lean clays and granular materials appear suitable for reuse in embankment widening and are likely to require less effort to moisture condition and place within requirements.

Earthwork Construction Considerations

Shallow excavations for the proposed improvements are anticipated to be accomplished with conventional construction equipment. Where excavations are to be conducted within the predominantly clay soils, which are sensitive to construction disturbance, the use of equipment with a smooth bucket should be employed to limit such disturbance and provide for a “neat cut” excavation. Upon completion of filling and grading, care should be taken to maintain the subgrade water content prior to construction of foundation elements (bridge abutment, concrete ford, gabion basket materials). Construction traffic over the completed subgrades should be avoided. The construction sites should also be graded to prevent ponding of surface water on the prepared subgrades or in excavations. Water collecting over or adjacent to construction areas should be removed. If the subgrade freezes, desiccates, saturates, or is disturbed, the affected material should be removed, or the materials should be scarified, moisture conditioned, and recompacted prior to further construction.

As a minimum, excavations should be performed in accordance with OSHA 29 CFR, Part 1926, Subpart P, “Excavations” and its appendices, and in accordance with any applicable local, and/or state regulations.

Construction site safety is the sole responsibility of the contractor who controls the means, methods, and sequencing of construction operations. Under no circumstances shall the

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SITE 1 – SIPARYANN CREEK CROSSING

The field exploration at the Siparyann Creek Crossing included the advancement of three borings along the 400 feet of planned grade raising from Sta. 19+00 to 23+00. Boring B-01 was located approximately at Sta. 19+70, 12 feet right of centerline on the southern end of the planned embankment raising. The segment of the site represented by Boring B-01 is approximately 3 to 4 feet lower in elevation than the segment moving upstation. Boring B-02 was located at approximately Sta. 21+50, 6 feet right of centerline near the middle of the 400 foot grade raise section. Boring B-03 was located approximately at Sta. 22+75, 4 feet right of centerline near the north end of the planned site grade raising zone. Each boring was advanced to a depth of 26.5 feet below existing grade. Boring logs conducted for the Siparyann Creek Crossing site, Borings B-01 through B-03, are attached to this report along with a site plan and exploration plan showing the layout of the borings.

The planned repairs for Siparyann Creek Crossing include removal and replacement of four 36 inch diameter existing culverts with 48 inch culverts, the addition of a 48 inch culvert near Sta.

20+25, raise grade approximately 0-3 feet along 400 feet of the section and installation of a concrete ford from Sta. 19+50 to 20+50 (this portion of the site is well suited for a concrete ford as it is positioned naturally lower than the remaining alignment through this site). The planned repairs are focused on increasing freeboard and cover over the new, larger culverts which will be able to pass more water during runoff events. The ability to pass water through the roadway section within the larger culverts and over the concrete ford, will lead to improved subgrade and roadway surfacing performance due to less potential for standing water and increased stability of the roadway section elevated away from the flat alluvial fan.

Subsurface Conditions

The site is located along the mainline alignment of Route 201 in an alluvial fan deposit create by Siparyann Creek and its associated drainage. The subsurface conditions generally consist of a nominal roadway embankment fill layer (including some oversized cobbles/boulders placed to bridge soft areas of yielding subgrade) varying from 3.5 to 7.0 feet thick overlying native alluvial deposits of moderate to high plasticity clays. The overlying fill layer appears to have been constructed with a subgrade elevation near 2264 to 2266 feet. The alluvial clay deposit extends from the overlying clayey gravel with sand fill material to the termination of exploratory borings at all locations. See the attached Boring Logs for more detailed description of the subsurface conditions.

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The embankment fill material generally has a relative density of medium dense with Standard Penetration Test (SPT) recorded N-values ranging between 11 to 23 blows per foot (bpf). The natural moisture content of the clayey gravel with sand material was found to be within the range of 6.3 to 12.5 percent.

The alluvial clay generally has a soft to stiff consistency throughout the deposit, with SPT recorded N-values between 4 and 24 bpf, varying with the amount of sand and gravel within the deposit and was generally falling within the medium stiff range of blow counts. The natural moisture content of the clay material was found to be within the range of 13.5 to 33.8 percent. Liquid Limit values of 65 and 64 with Plasticity Index values of 44 and 44, respectively were determined for select samples of clay and gravely clay soils tested at this site. Particle-size analyses conducted on select samples of clay and gravelly clay determined approximately 60 to 91 percent of the matrix consisted of material passing the number 200 sieve. Consolidation testing of a clay sample from B-01 at 10.0 feet exhibited moderately high compressibility with initial consolidation of approximately 8.3 percent strain at an overburden pressure of 8,000 pounds per square foot (psf).

An additional consolidation test of a gravelly fat clay with sand sample from B-03 at 7.5 feet exhibited moderate swell potential when inundated, interpolated to be approximately 2.5 percent strain at 500 psf. Based on the testing results and manner of deposition, the clays at the site are normally consolidated such that the stress history of the soils is limited to the overburden pressure exerted on the material to date.

Groundwater was not observed in Boring B-01 at the time of our exploration, but was encountered at 15.5 feet and 18.0 feet within Borings B-02 and B-03, respectively. The field investigation does not fully reflect seasonal or long-term groundwater conditions which will be influenced by precipitation, runoff characteristics of the site, future development impacts, hydrologic impacts originating off-site, and other factors beyond the scope of this investigation or evident at the time the borings were performed. Therefore, groundwater levels during construction or at other times in the life of the structures may vary from the conditions indicated on the Logs.

Geotechnical Consideration and Recommendations

The primary geotechnical concern at the Siparyann Creek Crossing location is the ability to allow for water passage from upslope to the downslope side of the roadway to limit the potential for washouts and overtopping of the alignment. The installation of new culverts and a concrete ford will accommodate additional water passage at this location. The compressible nature of the native soils that will provide support of the embankment fill and concrete ford should be considered in the design of these elements. Although the embankment height will not result in substantial increased pressure, a three foot high new embankment structure can be anticipated to add approximately 350 to 400 psf of stress to the subgrade. With the added stress, calculated centerline of alignment settlements (assuming an embankment geometry of 3 feet of fill with 2H:1V slopes and fill material density on the order of 135 pounds per cubic foot), using the consolidation data obtained from Boring B-01 at 10 feet, results in approximately 1.25 to 2.0

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Culvert installations elevations and connections should consider the potential for movement magnitudes as discussed above. Culverts are currently designed to be riprap lined at the inlet and outlet for each which is appropriate based on evidence of past undermining or piping within existing roadway prism where culverts are currently located. Use of proper keying of riprap and bedding materials prior to riprap placement should be conducted in accordance with FP-14.

The concrete ford should be designed based on an expected modulus of subgrade reaction for the predominantly fat clay soils of 75 pounds per cubic inch (pci). Preparation of the subgrade to receive embankment fill or concrete ford planking should be conducted in accordance with the Earthwork section to provide sufficient subgrade uniformity. Also of note, the upper portion of the existing roadway consists of clayey gravel with sand material, with varying amounts of fine within the matrix. The installation of a non-woven separation geotextile to limit migration of fines from the subgrade into the embankment fill materials may provide for additional basal stability of the embankment and increase long-term performance of the fill zone.

Regarding the need to provide sufficient bearing resistance while limiting potential for settlement of the concrete ford structure, we recommend that foundations for the ford (utilized to key into the subgrade and provide protection from erosional forces) be designed to bear on properly prepared clayey gravel with sand soils encountered near Sta. 19+70 (Boring B-01). Our analysis has considered foundation preparation of the subgrade encounter to include recompaction of the bearing surface to a minimum of 98 percent of the maximum laboratory dry density value obtained by AASHTO T99 prior to placement of foundation concrete to recover disturbance caused by excavation activities.

We have estimated nominal bearing resistance for spread footings by evaluating soil strength parameters from the explorations we performed. The bearing soils are presented in the tables as the “Soil Type” and the Soil Layer per the subsurface conditions described in this section. These design sections are intended for use in design of new foundations. It should be noted that the bearing resistances presented in the following tables are nominal resistances and should be factored (reduced) by the resistance factors of the following:

AASHTO LRFD Resistance Factors (ϕ)

Limit State Bearing (ϕbc) Shear Resistance to Sliding (ϕτ)

Passive Pressure Resistance to Sliding (ϕep)

Strength 0.45 0.80 0.50

Service 1.0 N/A N/A

Extreme Event 0.9 0.9 0.9

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Based on review of the preliminary design detail for the concrete ford, we understand that it will function as an open-bottom system that is to bear on exterior strip footings at a depth of approximately 3 feet below grade, and preparation of recommendations following is based on this finding. The following table provides unfactored soil parameters for design of new spread footings at the Rock Creek Crossing concrete ford location:

Foundation Elevation (ft bgs)1

Soil Type

Total (Effective)

Unit Weight, γ

(pcf)2

Friction Angle, ϕ

(deg)

Cohesion, c (psf)

Nominal Bearing

Resistance, Qnom

(ksf)3

Nominal Sliding

Coefficient tan ϕ

3.0 Clayey Gravel

with Sand (GC)

(57.6)

24 400 11.5 0.45

1. Foundation elevation based on estimated minimum of 3 feet of footing embedment.

2. Groundwater estimated to be approximately 3 feet above base of crossing in runoff (short-term) condition; therefore, both moist and effective unit weights are presented for footing influence zone.

3. Nominal bearing resistance calculated on an assumed footing width of 1.0 feet and length of 200 feet to support concrete ford within the native clay deposit.

SITE 2 – ROCK CREEK CROSSING

The field exploration at the Rock Creek Crossing included the advancement of three borings along the planned repair area from Sta. 38+00 to 43+00. Boring B-04 was located approximately at Sta. 38+30, on centerline on the southern end of improvement area where an existing water control structure is to be removed, along with a 36 inch culvert, and installation of a 48 inch culvert is planned. Boring B-05 was located at approximately Sta. 40+00, 6 feet right of centerline near the southwest end of the existing concrete ford schedule to be replaced. Boring B-06a was located approximately at Sta. 42+50, 7 feet right of centerline near the northeast end of the existing concrete ford schedule to be replaced. Each boring was advanced to a depth of 26.5 feet below existing grade. Boring logs conducted for the Rock Creek Crossing site, Borings B-04 through B-06a, are attached to this report along with a site plan and exploration plan showing the layout of the borings.

The planned repairs for Rock Creek Crossing include removal and replacement of three 36 inch diameter existing culverts with 48 inch coated culverts, removal and replacement of dual 24 inch HDPE culverts below the existing concrete ford with dual 48 inch by 33 inch arched culverts with concrete headwalls, and replacement of the existing concrete ford with new 8 inch thick concrete ford (200 feet long by 14 feet wide). The new concrete ford structure is to be keyed into the subgrade and riprap placed at the outlet/downstream end to provide erosion control. The planned

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Subsurface Conditions

The site is located along the mainline alignment of Route 201 in an alluvial fan deposit create by Rock Creek and its associated drainage. The subsurface conditions generally consist of a nominal roadway embankment fill layer (clayey gravel with sand grading to gravelly lean clay) south/west of the concrete ford, encountered within Boring B-04 and B-05, varying from 4.5 to 7.0 feet thick overlying native alluvial deposits of moderate plasticity clays. The alluvial lean clay deposit extends from the overlying fill material south/west of the existing concrete ford, and at existing grade north/east of the existing ford, to the termination of exploratory borings at all locations. See the attached Boring Logs for more detailed description of the subsurface conditions.

The embankment fill material generally has a relative density/consistency of loose to medium dense/medium stiff to stiff with Standard Penetration Test (SPT) recorded N-values ranging between 7 to 11 blows per foot (bpf). The natural moisture content of the fill material was found to be within the range of 5.4 to 8.4 percent (where granular) and 13.0 to 17.5 percent (where cohesive).

The alluvial clay generally has a soft to stiff consistency throughout the deposit, with SPT recorded N-values between 4 and 13 bpf, generally decreasing with depth. The natural moisture content of the clay material was found to be within the range of 13.6 to 33.0 percent. Liquid Limit and Plasticity Index values of 47 and 31, respectively were determined for select samples of clay tested at this site. Particle-size analyses conducted on a select sample of clay determined approximately 69 percent of the matrix consisted of fine-grained material. Consolidation testing of a clay sample from B-04 at 10.0 feet exhibited moderate compressibility with initial consolidation of approximately 2.2 percent strain at an overburden pressure of 4,000 psf, or approximately 0.5 percent strain per 1 ksf loading. Based on the testing results and manner of deposition, the clays at the site are normally consolidated such that the stress history of the soils is limited to the overburden pressure exerted on the material to date.

Groundwater was not observed in Borings B-04 or B-06a at the time of our exploration, but was encountered at 26.0 feet within Boring B-05 within a thin lens of gravel and sand. The field investigation does not fully reflect seasonal or long-term groundwater conditions which will be influenced by precipitation, runoff characteristics of the site, future development impacts, hydrologic impacts originating off-site, and other factors beyond the scope of this investigation or evident at the time the borings were performed. Therefore, groundwater levels during construction or at other times in the life of the structures may vary from the conditions indicated on the Logs.

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Geotechnical Consideration and Recommendations

The primary geotechnical concern at the Rock Creek Crossing location is the ability to allow for water passage from upslope to the downslope side of the roadway to limit the potential for washouts and overtopping of the alignment. The installation of new culverts and a replacement concrete ford that will be sloped and embedded into the subgrade will accommodate additional water passage at this location. The most likely issue associated with the site will be proper preparation, embedment (keying), and armoring of the replacement concrete ford to eliminate the risk of undermining on the outlet side which has historically led to the existing concrete ford blowing out and needing to continually be repaired. The concrete structure will be relatively light weight, accounting for approximately 100 psf dead load. With the added stress of the concrete ford which will encapsulate structural backfill on the order of 3 to 4 feet thick, calculated centerline of alignment settlements (assuming an embankment geometry of 4 feet of fill with 2H:1V slopes and fill material density on the order of 135 pounds per cubic foot), using the consolidation data obtained from Boring B-05 at 10 feet, results in approximately 0.75 to 1.25 inches of settlement under primary consolidation. The edge of embankment settlement is expected to be on the order of 1/3 of the centerline settlement, or between 0.25 and 0.50 inches. Culvert installations elevations and connections should consider the potential for movement magnitudes as discussed above. Culverts are currently designed to be riprap lined at the inlet and outlet for each which is appropriate based on evidence of past undermining or piping within existing roadway prism where culverts are currently located. Use of proper keying of riprap and bedding materials prior to riprap placement should be conducted in accordance with FP-14.

The concrete ford should be designed based on an expected modulus of subgrade reaction for the predominantly fat clay soils of 75 pounds per cubic inch (pci). Preparation of the subgrade to receive embankment fill or concrete ford planking should be conducted in accordance with the Earthwork section to provide sufficient subgrade uniformity. Also of note, the upper portion of the existing roadway consists of some clayey gravel with sand transitioning to sandy lean clay material, with varying amounts of fine within the matrix. The installation of a non-woven separation geotextile to limit migration of fines from the subgrade into the embankment fill materials may provide for additional basal stability of the embankment and increase long-term performance of the fill zone.

Regarding the need to provide sufficient bearing resistance while limiting potential for settlement of the concrete ford structure, we recommend that foundations for the ford (utilized to key into the subgrade and provide protection from erosional forces) be designed to bear on properly prepared native gravelly/sandy lean clay soils. Our analysis has considered foundation preparation of the subgrade encounter to include recompaction of the bearing surface to a minimum of 98 percent of the maximum laboratory dry density value obtained by AASHTO T99 prior to placement of foundation concrete to recover disturbance caused by excavation activities.

May 26, 2021 ■ Terracon Project No. C4205037

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We have estimated nominal bearing resistance for spread footings by evaluating soil strength parameters from the explorations we performed. The bearing soils are presented in the tables as the “Soil Type” and the Soil Layer per the subsurface conditions described in this section. These design sections are intended for use in design of new foundations. It should be noted that the bearing resistances presented in the following tables are nominal resistances and should be factored (reduced) by the resistance factors of the following:

AASHTO LRFD Resistance Factors (ϕ)

Limit State Bearing (ϕbc) Shear Resistance to Sliding (ϕτ)

Passive Pressure Resistance to Sliding (ϕep)

Strength 0.45 0.80 0.50

Service 1.0 N/A N/A

Extreme Event 0.9 0.9 0.9

Based on review of the preliminary design detail for the concrete ford, we understand that it will function as an open-bottom system that is to bear on exterior strip footings at a depth of approximately 3 feet below grade, and preparation of recommendations following is based on this finding. The following table provides unfactored soil parameters for design of new spread footings at the Rock Creek Crossing concrete ford location:

Foundation Elevation (ft bgs)1

Soil Type

Total (Effective)

Unit Weight, γ

(pcf)2

Friction Angle, ϕ

(deg)

Cohesion, c (psf)

Nominal Bearing

Resistance, Qnom

(ksf)3

Nominal Sliding

Coefficient tan ϕ

3.0 Gravelly/Sandy

Lean Clay (CL)

(42.6)

10 1,000 9 0.18

1. Foundation elevation based on estimated minimum of 3 feet of footing embedment.

2. Groundwater estimated to be approximately 3 feet above base of crossing in runoff (short-term) condition; therefore, both moist and effective unit weights are presented for footing influence zone.

3. Nominal bearing resistance calculated on an assumed footing width of 1.0 feet and length of 200 feet to support concrete ford within the native clay deposit.

SITE 3 – SEVENMILE CROSSING

The field exploration at the Sevenmile Crossing included the advancement of four borings along the 1,400 feet of planned grade raising from Sta. 56+00 to 70+00. Boring B-07 was located approximately at Sta. 58+50, on centerline on the western end of the planned embankment raising near existing 48 and 60 inch culverts to be removed and replaced. Boring B-08 was located at approximately Sta. 60+80, 6 feet right of centerline and Boring B-09 was located approximately

May 26, 2021 ■ Terracon Project No. C4205037

Responsive ■ Resourceful ■ Reliable 15 at Sta. 64+00, 4 feet right of centerline. These two borings represent the middle portion of the 1,400 foot grade raise section. Boring B-10 was located approximately at Sta. 67+00, on centerline on the eastern end of the planned embankment raising. Each boring was advanced to a depth of 26.5 feet below existing grade. Boring logs conducted for the Sevenmile Crossing site, Borings B-07 through B-10, are attached to this report along with a site plan and exploration plan showing the layout of the borings.

The planned repairs for Sevenmile Crossing include removal and replacement of existing 48 and 60 inch culverts near Sta. 59+00 with a single 81 inch by 59 inch pipe arch, raising grade approximately 0-2 feet along 1,400 feet of the section, installation of a roadside ditch on the north side of the alignment, and installation of a Gabion Basket Overflow Structure from Sta. 60+50 to 61+50 where overtopping historically occurs. It is also reported that an old wooden bridge toward the east end of this Site has been filled with sediment, as the drainage creates alluvial deposition of sediments over approximately ¼ mile of the roadway. This gives an indication of the amount and widespread impacts of the sedimentation issue at the site.

The planned repairs are at Sevenmile Creek are focused on increasing freeboard and cover over proposed new, larger culverts, as well as adding surface drainage features including inboard ditching to improve flow of runoff toward culverts. These improvements are designed to allow more water to pass to the downslope side of the road to during runoff events. The ability to pass water through the roadway section within the larger culverts and over the overflow structure (as needed), will lead to improved subgrade and roadway surfacing performance due to less potential for standing water and increased stability of the roadway section elevated away from the flat alluvial fan.

Subsurface Conditions

The site is located along the mainline alignment of Route 201 in an alluvial fan deposit create by Sevenmile Creek and its associated drainage. The subsurface conditions generally consist of a more recent deposits of alluvial moderate to high plasticity clays overlying older deposits of alluvial clayey gravel with sand at relatively shallow depths along the east portion of the site, and shale bedrock at shallow depths on the west portion of the site. See the attached Boring Logs for more detailed description of the subsurface conditions.

The alluvial clay varies from soft to hard consistency throughout the deposit, with SPT recorded N-values between 2 and 40 bpf, varying randomly and was generally within the medium stiff range of blow counts. The natural moisture content of the clay material was found to be within the range of 16.1 to 44.0 percent, varying randomly. Liquid Limit values of 61, 65, and 75 with Plasticity Index values of 40, 38, and 51, respectively were determined for select samples of fat clay with sand, fat clay and sandy fat clay tested at this site. Particle-size analyses conducted on select samples of clay determined approximately 69 to 92 percent of the matrix consisted of material passing the number 200 sieve. Consolidation testing of select clay samples from Borings B-07, May 26, 2021 ■ Terracon Project No. C4205037

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B-08, and B-10 at 7.5 to 15.0 feet below existing grade generally exhibited moderate compressibility, in the range of 0.75 to 1.0 percent strain per ksf loading. Limited swell potential was exhibited at 500 psf confining pressures, with swell of 1.0 to 1.8 percent recorded. Based on the testing results and manner of deposition, the clays at the site are normally consolidated such that the stress history of the soils is limited to the overburden pressure exerted on the material to date. Unconfined compressive strength testing of a sample of clay from Boring B-08 indicated an unconfined compressive strength of 4,600 psf.

The underlying older alluvial deposit of clayey gravel with sand was generally medium dense with SPT recorded N-values between 11 and 16 bpf. The natural moisture content of the clayey gravel with sand material was found to be within the range of 13.1 to 27.7 percent.

The shale bedrock of the Bear Paw Formation, was encountered in the western most boring at the site, Boring B-07, adjacent the western slope confining the edge of the alluvial fan at the location. The shale was generally medium hard to hard rock, with SPT recorded N-value between 30 and greater than 100 blows per foot. The rock was moderately fractured, highly weathered in the upper reaches and became more massive and competent with depth. Laminated bedding and very close fracture spacing were noted. Natural moisture content of the shale was between

16.4 and 24.4 percent.

Groundwater was not observed in Borings B-07 and B-08 at the time of our exploration, but was encountered at 14.5 feet and 24.0 feet within Borings B-09 and B-10, respectively. The field investigation does not fully reflect seasonal or long-term groundwater conditions which will be influenced by precipitation, runoff characteristics of the site, future development impacts, hydrologic impacts originating off-site, and other factors beyond the scope of this investigation or evident at the time the borings were performed. Therefore, groundwater levels during construction or at other times in the life of the structures may vary from the conditions indicated on the Logs.

Geotechnical Consideration and Recommendations

The primary geotechnical concern at the Sevenmile Crossing location is the ability to allow for water passage from upslope to the downslope side of the roadway to limit the potential for washouts and overtopping of the alignment. The compressible nature of the native soils that will provide support of the embankment fill and Gabion Basket Overflow structure should be considered in the design of these elements. Although the embankment height will not result in substantial increased pressure, a two foot high new embankment structure can be anticipated to add approximately 250 to 300 psf of stress to the subgrade. With the added stress, calculated centerline of alignment settlements (assuming an embankment geometry of 2 feet of fill with 2H:1V slopes and fill material density on the order of 135 pounds per cubic foot), using the consolidation data obtained from Borings B-07 and B-08 at 7.5 feet below existing grade, results in approximately 0.5 to 1.5 inches of settlement under primary consolidation. The edge of embankment settlement is expected to be on the order of 1/3 of the centerline settlement, or

May 26, 2021 ■ Terracon Project No. C4205037

Responsive ■ Resourceful ■ Reliable 17 between 0.25 and 0.50 inches. Arch pipe installation elevation and connections should consider the potential for movement magnitudes as discussed above. The planned arch pipe should be designed to be riprap lined at the inlet and outlet based on evidence of past undermining or piping within existing roadway prism where culverts are currently located. Use of proper keying of riprap and bedding materials prior to riprap placement should be conducted in accordance with FP-14.

Ditching currently planned for the north side of the roadway will improve water flow toward the planned arch pipe and reduce the potential for standing water/saturated subgrade along the embankment section.

The installation of a non-woven separation geotextile to limit migration of fines from the subgrade into the embankment fill materials may provide for additional basal stability of the embankment and increase long-term performance of the fill zone.

Regarding the need to provide sufficient bearing resistance for Gabion Basket Overflow Structure while limiting potential for settlement of the structure, we recommend that subgrade preparation for the gabion baskets structure be conducted as follows prior to placement of leveling course.

Our analysis has considered foundation preparation of the subgrade encountered to include recompaction of the bearing surface to a minimum of 98 percent of the maximum laboratory dry density value obtained by AASHTO T99 prior to placement of Gabion Basket Leveling course to recover disturbance caused by excavation activities. The Gabion Basket should be embedded a minimum of 1B (where B is the width of the selected basket) below finished grade, below this embedment depth a nominal 12 inches of level course material should be placed to provide uniformity of bearing for the basket system. Gabion baskets should be designed to include a PVC coating of the wire mesh system for added corrosion protection based on historical data for the project vicinity.

SITE 4 – NICHOLS CROSSING

The field exploration at the Nichols Crossing included the advancement of three borings along the planned repair areas from Sta. 74+00 to 80+00. Boring B-11 was located approximately at Sta. 74+50, on centerline on the western end of the repair area near the location of a culvert that has previously blown out and is to be replaced with an 60 inch by 46 inch pipe arch culvert. Boring B-12 was located at approximately Sta. 76+30, 2 feet right of centerline and Boring B-13 was located approximately at Sta. 76+90, 6 feet right of centerline. These two borings represent the west and east ends of the existing timber bridge located at the crossing. Each boring was advanced to a depth of 26.5 feet below existing grade. Boring logs conducted for the Nichols Crossing site, Borings B-11 through B-13, are attached to this report along with a site plan and exploration plan showing the layout of the borings.

The planned repairs for Nichols Crossing include installation of a new 60 inch by 46 inch pipe arch culvert and installation of a gabion baskets keyed into creek bank filled with riprap upstream and downstream of the bridge location.