C4185017.Yale Kilgore Final Geotechnical Engineering Report_stamped.pdf

REPORT COVER PAGE

Geotechnical Engineering Report

GR 10-23

Yale-Kilgore Road – Idaho Route 6805 Fremont and Clark Counties, ID

DTFH7015D0003, Task Order No. 69056719F000067 – MOD P00003

May 25, 2021 Terracon Project No. C4185017

Prepared for:

DJ&A, P.C.

2000 Maple Street Missoula, MT 59808

Prepared by:

Terracon Consultants, Inc.

1392 13th Ave SW Great Falls, MT 59404

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REPORT TOPICS

INTRODUCTION

SITE CONDITIONS

PROJECT DESCRIPTION

GEOTECHNICAL CHARACTERIZATION

EARTHWORK

TAYLOR CREEK CULVERT

MILL CREEK TRAIL BRIDGE

SEISMIC CONSIDERATIONS

PAVEMENTS

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

Geotechnical Engineering Report Yale-Kilgore Road – Idaho Route 6805

Fremont and Clark Counties, ID DFTH7015D00003, Task Order No. 69056719F000067

Terracon Project No. C4185017 May 25, 2021

INTRODUCTION

This report presents the results of our subsurface exploration and geotechnical engineering services performed for the proposed Yale-Kilgore Road – Idaho Route 6805 improvement project located in Fremont and Clark Counties, ID. 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 ■ Seismic site classification per IBC

■ Site preparation and earthwork ■ Lateral earth pressures

■ Pavement design and construction

The geotechnical engineering Scope of Services for this project, based on the Statement of Work (SOW) dated March 4, 2019, and SOW for MOD P00003 dated July 21, 2020, included the advancement of 30 test borings for the two explorations conducted. The first exploration was conducted for preliminary subsurface characterization of the roadway alignment and included 23 subgrade borings made to depths between 5.0 and 7.0 feet below existing grade, along with a single geotechnical (deep) boring having a depth of 21.6 feet. The second exploration was conducted for the planned structures at Taylor and Mill Creeks with six borings conducted to depths ranging from approximately 21.5 to 41.0 feet below existing ground surface.

Maps showing the site and boring locations are presented 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/or 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 visit in association with the field explorations and our review of publicly available geologic and topographic maps.

Yale-Kilgore Road – Idaho Route 6805 ■ Fremont and Clark Counties, ID May 25, 2021 ■ Terracon Project No. C4185017

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Item Description

Parcel Information (provided in SOW documents)

The proposed project start is located just east of Kilgore in Clark County, Idaho with the alignment laying out generally east to the intersection with US 20 near Island Park, Idaho. Approximate Global Positioning System (GPS) coordinates for the beginning of the project are 44°27'19.24"N, 111°45'25.43"W and end of the project are 44°27'10.70"N, 111°21'39.94"W.

The proposed project comprises a 21.8-mile section of Yale-Kilgore Road. It starts near the intersection of Yale-Kilgore Road and A-2 Clark County Road and ends at the Yale-Kilgore Road and US 20.

See Site Location

Existing Improvements

The Clark County portion of the alignment consist of existing gravel surfaced roadway of width varying from 22 to 26 feet. This portion of the alignment is surrounded primarily by agricultural pasture land and State or Federally owned undeveloped lands.

The Fremont County portion of the alignment consists of primarily asphalt surfaced (bitumen surface treatment or asphalt concrete pavement) roadway of width varying between 24 and 28 feet. This portion of the alignment is boarded by varying private, State and Federally owned lands that are both developed, undeveloped and leased for logging/agricultural use.

Geology

The project alignment traverses the south edge of the Eastern Centennial Mountains on the border between Idaho and Montana. The project starts on Yale-Kilgore Road (Idaho Route 6805) near MP 9.0 located in Clark County, ID within the general Shotgun Valley area and ends near MP 30.8 at the intersection of Yale-Kilgore Road and US Highway 20 in Fremont County, ID. The geology of the area consists predominantly of mapped zones of alluvial sand, gravel, and silt deposits (Quaternary), some zones of glacial meltwater deposits, and colluvium through Clark County portion, and transitions to alluvial/colluvial materials mantling basalt flow and rhyolite of the Snake River Plain (Middle and Early Pleistocene) through the Fremont County section.

Photographs were taken at the time of our field exploration programs with representative project locations shown in the 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 A2 Highway Phase II and III Geotechnical Reports, prepared by American Geotechnics, provided on April 18, 2018 o Yale-Kilgore Road Wetlands Map, May 2018 o Yale-Kilgore Road Ownership Map, May 2018 o Yale-Kilgore Road Clark County Segment, MP 9.0 to 16.9

(Preliminary), May 2018 o Yale-Kilgore Road Fremont County Segment, MP 16.9 to 30.8

(Preliminary), May 2018 o Yale-Kilgore Road Reconstruction Aerial Overlay Set for Field

Visit, May 2018 o Statement of Work (SOW) dated March 4, 2019 o Preliminary plan set dated 25 September 2019 o SOW for MOD P00003 dated July 21, 2020 o Mill Creek Trail Bridge Preliminary Plan Set dated April 2020

Project Description (provide in SOW documents)

The proposed project for the Clark County Segment consists of excavation, grading, drainage, bridge structures, soft spot repair, aggregate base, culvert replacement, and asphalt surface course improvements of the Yale- Kilgore Road from MP 9.0 to MP 16.9 (7.9 miles). The intent of the project is to provide a consistent roadway width of 28 feet by slightly widening the existing travel way beyond the current 22-26 feet.

The Fremont County Segment consists of excavation, drainage, shoulder repair, pavement patching, existing pavement pulverizing or reclamation and asphalt surface course improvements of the Yale-Kilgore Road from MP 16.9 to MP30.8 (13.9 miles). The project will consist of constructing a 27 foot wide asphalt overlay on a pulverized base. From MP 26.25 to MP

30.55 (3.7 miles) a new separated ATV and snowmobile gravel trail will be constructed on the right side of the road. The trail design will provide for a 10-foot wide graveled surface travel way and 16 feet of clearing to accommodate a snow grooming machine.

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Item Description

Pavements

Planned use of asphalt paved surface for the entire roadway alignment, with new pavement to be placed on the Clark County Segment, and rehabilitation of the Fremont County Segment surfacing.

Design standards provided in SOW as follows:

Functional Classification: Major Collector Terrain: Rolling Design Vehicle: WB-40 Preliminary Design Speed: 45-55 mph Max. Superelevation: 6% Finished Roadway width: Travel Lane – 12’

Shoulders – 2-3’ Average Daily Traffic (ADT) ADT2015 = 650 (7% Trucks)

ADT2035 = 966 (12% Trucks)

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 sections 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 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.

Boring ID Depth to Ground Water (ft) Observation Note

B-02 5.6 6/27/2019, while drilling

B-19B 7.0

11.0

6/25/2019, while drilling

6/25/2019, following drilling (30 minutes after completion)

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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.

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 or pavement/subgrade 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

Select Borrow 704.07 Subexcavation replacement materials from subgrade to base course adjacent Mill Creek Trail Bridge abutments

Existing Granular Soils (unclassified borrow) 704.06

Sliver fill sections for embankment widening (where needed), embankment and subgrade replacement materials for Trail areas

Native Silts and Clays 204.06

Native silts and clays encountered may be suitable for embankment widening outside of the 1H:1V core with finished slopes at 3H:1V or flatter to match existing grade where moisture conditioning and compaction can be achieved

Aggregate Base Course, Grading D 703.5 Throughout project limits as part of the structural pavement section

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Earthwork Construction Considerations

Shallow excavations for the proposed structures and pavement sections are anticipated to be accomplished with conventional construction equipment. Upon completion of filling and grading, care should be taken to maintain the subgrade water content prior to construction of foundations or pavement sections. 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.

The groundwater table could affect overexcavation efforts, especially for over-excavation and replacement of lower strength soils at the Mill Creek Trail Bridge location and foundation construction at the Taylor Creek Culvert location. A temporary dewatering system consisting of sumps with pumps could be necessary to achieve the recommended depth of over-excavation or foundation bearing.

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 information provided herein be interpreted to mean Terracon is assuming responsibility for construction site safety, or the contractor's activities; such responsibility shall neither be implied nor inferred.

TAYLOR CREEK CULVERT

The field exploration at the Taylor Creek Culvert site included the advancement of two (2) borings for the structure, one near the south end and one near the north end of the planned culvert crossing to be located near MP 14.3. The structure borings were advanced to an approximate depth of 21.5 feet below existing grade. Boring logs for the structure borings conducted for the proposed culvert, Borings DH-01 and DH-02, are attached to this report along with a site plan and exploration plan showing the layout of the borings.

Subsurface Conditions

The culvert is located along the mainline alignment of Idaho Route 6805 in an alluvial channel create by Taylor Creek and its associated drainage. The subsurface conditions generally consist of a nominal roadway embankment fill layer overlying native alluvial deposits of moderate plasticity clays with varying amounts of sand and gravel (Quaternary). The alluvial clay deposit

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The near surface alluvial clay generally has a soft consistency within the upper 3 to 6 feet with Standard Penetration Test (SPT) recorded N-values of 4 blows per foot in both borings.

Consolidation testing of a clay sample from DH-01 at 5.0 feet exhibited extremely high compressibility with initial consolidation of approximately 5.5 percent strain at an overburden pressure of 4,000 psf. The sample was rebounded to a confining pressure of 1,000 psf and inundated, with no swell observed in the sample. The sample was then wetted and consolidated to 8,000 psf in which it exhibited approximately 9.8 percent strain. The results indicate the highly compressible nature of the overlying clay soils at the site. The underlying native sand and gravel deposits have more substantial shear strength and lower compressibility based on SPT recorded N-values ranging from 13 to greater than 50 blows per foot.

No groundwater was observed at the time of our exploration in any of the borings. 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.

Shallow Foundation Analysis and Recommendations

The primary geotechnical concern at the Taylor Creek location is to provide sufficient bearing resistance while limiting potential for settlement of the structure. The near-surface soft clays are extremely compressible and have the potential for adverse settlement of elements founded on shallow foundations bearing within the clay zone. With the presence of a more competent alluvial sand and gravel deposit at relatively shallow depths, we recommend that spread foundations for the culvert be designed to bear below the clay zone to bypass the weak, compressible clay soils and provide bearing within the medium dense to dense sand/gravel. 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:

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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 discussions with DJ&A regarding hydrological impacts and potential scour at the Taylor Creek Culvert location, we understand that the open-bottom culvert is to bear on spread footings at a depth of approximately 10 feet below grade which is below the potential scour depth at the site, 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 Taylor Creek Culvert 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

10.0

Clayey Gravel with Sand (GC) / Poorly-graded

Gravel with Sand

(GP)

(67.6)

36 - 12 0.73

1. Foundation elevation based on estimated minimum of 3 feet of footing embedment after assumed potential scour at culvert location.

2. Groundwater estimated to be approximately 3 feet above base of culvert; therefore, both moist and effective unit weights are presented for footing influence zone.

3. Nominal bearing resistance calculated on an assumed footing width of 2.5 feet and length of 25 feet to support culvert within the native gravel deposit.

Based on the planned approach of an open bottom (or three-sided) culvert to be designed at the Taylor Creek site, the structure will act as a retaining wall for the roadway embankment. The following table provides earth pressure coefficients for evaluation of the new foundations. Active earth pressure is commonly used for design of retaining walls and assumes wall movement. The “at-rest” condition assumes no wall movement and is commonly used for walls restrained at the top such as will be the case for the open bottom culvert location.

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Soil Above Bottom of Footing Earth Pressure Coefficients

Soil Type Total Unit Weight, γ 3

(pcf)

Friction Angle, ϕ

(deg) At-rest, Ko Active, Ka1 Passive, Kp2

Sandy Lean Clay (CL) 110 10 0.83 0.70 1.42

Sand (SP) 120 32 0.47 0.31 3.25

Gravel

(GP/GC) 130 36 0.41 0.26 3.85

1. For active earth pressure, wall must rotate about base, with top lateral movements 0.002 H to 0.004 H, where H is wall height.

2. For passive pressure presented to develop, wall must move (Δ) horizontally to mobilize resistance, generally greater than 1 percent of the buried height or Δ/H > 0.01

3. Buoyant (Effective) unit weights should be used to calculate equivalent fluid pressure below creek elevation.

To calculate Effective Unit Weights, reduce the Total Unit Weight by 62.4 pcf.

MILL CREEK TRAIL BRIDGE

The field exploration at the Mill Creek Trail Bridge site included the advancement of four (4) borings for the structure, one at each abutment and bent location, between approximate Sta.

13+24 and Sta. 15+37 along centerline of the proposed trail bridge alignment. The structure borings were advanced to depths ranging between 31.4 and 41.0 feet below existing grade.

Boring logs for the structure borings conducted for the proposed trail bridge, Borings DH-03 through DH-06, are attached to this report along with a site plan and exploration plan showing the layout of the borings.

Subsurface Conditions

The trail bridge is located south of Idaho Route 6805 in an alluvial channel created by Mill Creek and its associated drainage. The subsurface conditions generally consist of a nominal layer of organics and topsoil overlying native alluvial deposits of clayey sand with gravel and moderate to high plasticity clays with varying amounts of sand and gravel (Quaternary). The alluvial clay overburden overlies basalt flow of the Snake River Plain (Middle and Early Pleistocene) at depths varying between approximately 11 feet and 17 feet below existing ground surface. See the attached Boring Logs for more detailed description of the subsurface conditions.

The near-surface alluvial clayey sand with gravel (gradational to sandy lean clay) generally has a loose relative density (SPT N-values ranging between 6 and 12 blows per foot) within the upper 3 to 5 feet and transitions to a more general high plasticity clay that has a soft to very soft (SPT N-values between 1 and 7 blows per foot) consistency at depths of greater than 5 feet below existing grade. The clay soils exhibit weak strength characteristics with unconfined compressive strength results of 1,523 pounds per square foot (psf) and 2,449 psf for samples obtained at 6.1

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This strength and compressibility of the overburden is in contrast to the underlying basalt which is generally severely to moderately weathered and moderately hard near the contact zone. The basalt becomes more resistant, less weathered, and hard with depth. Recovery of basalt core obtained was between 63 and 100 percent, with the recovery generally increasing with depth.

Rock Quality Designation (RQD) values varied from 0 to 90 percent, varying randomly with depth.

Several rock cores were tested for unconfined compressive strength, with results tabulated below:

Boring ID Sample Depth (ft)

Unconfined Compressive Strength (psi)

DH-03 16.0 8,515

DH-03 22.0 13,944

DH-04 16.0 6,959

DH-04 25.0 5,025

DH-05 17.0 8,486

DH-05 23.0 12,569

DH-06 25.0 5,154

DH-06 30.4 4,011

No groundwater was observed at the time of our exploration in any of the borings. 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.

Deep Foundation Analysis and Recommendations

The primary geotechnical concern for the trail bridge is to provide sufficient support for the bridge structure with adequate axial and lateral resistance while limiting potential for settlement and lateral deflections. The near surface clays, while having a firm to stiff upper crust, become soft to very soft with depth providing potential for adverse settlement of abutments founded on shallow

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Our analysis included a review of the Mill Creek Trail Bridge 75% Design Drawings provided by DJ&A, in which the preliminarily proposed deep foundation system included driven piles (HP14x73 section). The use of driven piles is a generally standard solution for bridge foundations.

However, during preliminary analysis of the use of driven piles with the subsurface profile conditions encountered during our supplemental drilling at the bridge site, the depth of embedment to satisfy the Strength I requirement of 161 kips per pile (for a three pile bent) was on the order of 3 feet below basalt contact. Based on the required embedment depth and the observed rock qualities, recovery, and unconfined compressive strength, there is a low probability that the pile sections will be able to be advanced to the required embedment without pile damage and overstressing of the sections. Due to the high risk of pile damage or the refusal of piles at less embedment requiring modifications to pile capacity in the field, we recommended that a drilled-in, micropile system be utilized for support of the trail bridge. This form of deep foundation is recommended in lieu of a driven pile system due to installation characteristics that are more positive and better suited to the subsurface conditions.

The use of micropiles for support of the trail bridge within the shallow basalt flow layer has the advantage of achieving relatively large axial capacity with small-diameter drilling, using a system with an installation technique well-suited for the advancement into the basalt. That is, the micropile system of installation using an a cased hole to the bedrock (cored or advanced with downhole hammer equipment) can be accomplished based on past experience of the ability to core the basalt layer during field exploration activities. Also, the use of micropiles will allow for installation with smaller, track-mounted equipment that can easily access the abutment/bent locations. This equipment has tracks with generally low contact pressure, and marsh mats can further be used to reduce intensity.

Micropile Selection

There are a variety of micropile sizes, types, and installation methods which could be designed and employed for the project. The foundation design recommendations contained herein are based on a Type A, composite micropile consisting of a casing with a centralized reinforcing bar installed as defined by the Federal Highway Administration in the Micropile Design and Construction Reference Manual, FHWA NHI-05-039, December 2005. This method would include the following basic installation steps:

n Drillhole advanced with permanent casing to the basalt contact zone

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Responsive ■ Resourceful ■ Reliable 12 n Smaller diameter drillhole (5 inch) advanced from bottom of casing (top of basalt) to minimum embedment depth (7.5 feet) through the overburden casing n Reinforcing bar centralized for full depth of drillhole, beyond base of casing into basalt n Casing and open-hole (within basalt) to be tremie-filled with grout (neat cement)

For purposes of this report, the composite micropile has been considered to consist of the following basic elements as listed below and shown on the Micropile Conceptual Detail:

n Steel pipe casing: American Petroleum Institute (API) N-80 casing, 7-inch minimum outside diameter and 0.498-inch wall thickness, meeting ASTM A252, Grade 3 requirements with a minimum yield strength of 80,000 psi, inclusive of pipe joints.

n Steel reinforcing bars: No. 14, Grade 75 (ASTM A615) Williams All-thread bars with couplers of similar strength and centralizers at intervals not greater than 10 feet.

n Grout: Neat cement grout consisting of a pumpable mix batched with Type I-II cement at a water:cement ratio not exceeding 0.45 that will attain a minimum 3-day compressive strength of 2,000 psi and a 28-day compressive strength of at least 4,000 psi.

Micropile Axial Capacity

The primary factor in the geotechnical design of micropile is determination of an appropriate grout-to-ground bond strength αbond. This parameter depends on the method of grouting that is used, as described above. For our analysis we have utilized Table C10.9.3.5.2-1 Summary of Typical αbond Values (Grout-to-Ground Bond) for Preliminary Micropile Design (modified after Sabatini, et al., 2005) from the AASHTO LRFD Bridge Design Specifications, 8th Edition. For a Type A micropile in Basalt (fresh-moderate fracturing), the typical range of grout-to-ground bond Nominal Resistance is 28.8 to 87.7 kips per square foot (ksf), or 200 to 609 pounds per square inch (psi).

Based on the results of the bedrock core measured RQD, recovered and unconfined compressive strength testing and variable weathering, we have selected to utilize the low end of the Nominal Resistance and have performed our analyses assuming a grout-to-ground bond strength αbond of 200 psi.

The micropiles will develop their load bearing in bond strength and movement is expected to be less than 0.5-inch due primarily to elastic shortening of the pile steel. We have assumed that the upper 11 to 17 feet above basalt contact zone will remained cased, and any resistance contribution has been conservatively neglected in this cased zone in the loose sand/soft clay soils.

Therefore, we are assuming full load support within the basalt layer. We have utilized a resistance factor for compression resistance of a single micropile of 0.55 in accordance with Table 10.5.5.2.5-1 Resistance Factors for Geotechnical Resistance of Axially Loaded Micropiles, Side Resistance (Bond Resistance):Presumptive Values from the AASHTO LRFD Bridge Design Specifications, 8th Edition. The results of the analysis are tabulated below:

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Location

Assumed Depth To Base Of Casing

(ft) 1

Projection Above, Or

Bury Depth

(ft) 2

Minimum Embedment Into Basalt

(ft)

Approximate Total Micropile

Length (ft)

Factored Compressive Resistance

(kips) 3

Abutment 1 17.0 -3.0 7.5 21.5 155

Bent 2 14.5 2.0 7.5 24.0 155

Bent 3 11.0 1.5 7.5 20 155

Abutment 4 11.0 -3.0 7.5 15.5 155

1. Based on depth below existing grade

2. Based on 75% Design Elevation data for abutment/bent projection or bury, along with ground surface elevation at boring locations provided by DJ&A.

3. For individual micropile factored resistances and assumes that piles will be spaced a minimum of

8*D (D = diameter of bond zone, in this case 5-inch for a minimum spacing of 40 inches) apart and no more than 4 piles at each abutment/bent. Closer spacing or greater number of micropiles in groups will require a reduction in axial load capacity.

Micropile Lateral Capacity

The structure will be subject to moderate lateral loads, and the micropiles are capable of providing limited bending resistance via the small-diameter cased section of the pile. The lateral load analysis has been conducted at this point in project design assuming that micropiles will be separated by sufficient space in the pile caps to allow consideration as individual elements. That is, pile spacing should not be closer than 3 diameters between piles perpendicular to the load direction and not closer than 6 diameters in the direction of load. Since lateral load direction, particularly in the seismic case, is uncertain, these criteria would allow a nominal center-to-center spacing of 4-feet (provided by DJ&A) within a pile cap for consideration of piles as individual elements for lateral loading.

Lateral load analysis has been conducted using the program AllPile Version 7 to calculate groundline/top of pile deflection, moment, and shear for the pile section. Our analysis included two scenarios, the first for a fixed head abutment pile in which the pile/abutment connection is at ground line or embedded. The second scenario analyzed was for interior bents in which the pile head would be unsupported for approximately 2 feet prior to being affixed to the pile cap/bridge superstructure. The results of this analysis have been used to check combined bending and axial stress in the pile section. It has been assumed that acceptable groundline/top of pier deflections for a fixed-head pile should not exceed 0.5-inch for static conditions and 1.0-inch for the seismic case. For the seismic case, the lateral modulus of subgrade reaction has been reduced by 50%.

The nominal analysis parameters are tabulated as follows and can be used in computer programs such as AllPile or LPILE:

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Stratum Unit Weight (pcf) Friction Angle, ϕ

Cohesion, c (psf)

Subgrade Modulus, k

(pci) ε50

Stiff Upper Clay 120 0 750 150 0.0115

Soft Lower Clay 110 0 500 50 0.0165

Basalt1 145 5 20,000 1200 0.0005

1. Conservative parameters provided based on the variability of weathering of basalt at within contact zone

Analysis output for 155-kip micropiles in a profile of 11 to 17 feet of clay over basalt rock indicates that the lateral load per pile must be limited to about 7 kips in order to avoid overstressing the pile due to the combined effects of axial load and moment. This is on similar order as the resistance requirement of 5.8 kips per pile provided for the preliminary strength case by DJ&A for the total reaction at top of pile.

Additional interaction with our geotechnical engineer will be required during final design to ascertain axial and lateral capacities as compatible with the recommended micropile section.

Pending detailed structural analysis, it may be necessary to adjust the section, particularly for any unsupported length of pile sections. It may be required to increase casing diameter in such instances to account for lateral load considerations, especially for the seismic case.

Micropile Construction Considerations

Proper micropile installation is highly operator-sensitive and requires a significant dependence on quality workmanship and quality control monitoring. Thus, the successful micropile completion largely depends on the equipment and installation procedures along with strict quality control and engineering observation.

The micropiles will require pre-production verification load testing and in-production proof testing to reasonably ascertain that design axial resistance is being attained. It is recommended that one verification test be conducted at each abutment, and one proof test at each bent location. This testing is usually conducted as a tensile load test.

Abutment Considerations

The subsurface conditions at the bridge abutments consist of weak, compressible clay overlying bedrock at greater depth, as such it has been recommended that all abutment and bents be founded on micropiles to transfer support of the bridge structure below this clay zone and provide resistance in the underlying basalt. Therefore, the primary consideration for the abutment approaches will be to provide a compacted fill zone with limited differential performance relative

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Responsive ■ Resourceful ■ Reliable 15 to the abutment structures. It is recommended that a nominal 2 feet of subexcavation by “neat cut” to limit disturbance to the native clay soils to provide a subgrade cap for the approach section.

Following subexcavation, a separation/reinforcing geotextile should be provided for the subgrade prior to placement of select borrow; this preparation should be utilized within 50 feet of the abutments. The select borrow material should be compacted according to Section 204.11 of FP-

14. The subgrade cap has the intent of providing a more stable subgrade/subbase for placement of the trail surfacing section at the abutment approaches and to reduce the potential for abrupt differential performance between the abutment backfill zone and structure approach.

SEISMIC CONSIDERATIONS

According to USGS (United States Geologic Survey) 2014 National Seismic Hazards Maps on-line the closest deterministic fault to the project is located 6.1 miles away, the Centennial Fault north of the alignment on the border between Montana and Idaho. Based on information obtained from the deeper borings located at Taylor Creek and Mill Creek for the structure subsurface investigation, the Seismic Site Classification will vary from C (Mill Creek) to D (Taylor Creek), depending on the depth to bedrock within the given profile. Based on USGS information, the design seismic event can be taken as having a Maximum Moment Magnitude of 7.17g and a Peak Ground Acceleration of 0.275g for a 5 percent exceedance in 50 years (975 year event).

PAVEMENTS

The project is to consist of asphalt pavement section construction in Clark County, where existing gravel surfaced roadway exists, and rehabilitation/reconstruction within Fremont County, were the current surfacing varies from bitumen surface treatment (double shot, chip seal, etc.) to asphalt concrete pavement section. The existing pavement condition was evaluated during our initial field exploration efforts for use in determining appropriate methods of pavement rehabilitation and/or reconstruction. Following presentation of our preliminary recommendations within separate pavement memorandums, review comments/discussions were held with Western Federal Lands Highway Division (WFLHD) Materials Engineering Staff and design team members to finalize pavement recommendations. The outcome of these interactions is included in the recommendations presented below.

Subsurface Conditions

The field exploration included the advancement of 24 borings along the 21.8-mile segment of ID Route 6805. The exploration points included 23 subgrade borings advanced to depths ranging between 5.0 and 7.0 feet below existing grade and one geotechnical boring extended to a depth of 21.6 feet below existing grade (near Mill Creek within the existing roadway embankment). The 21.8-mile segment includes a 7.9-mile gravel surfaced road segment within Clark County and a 13.9-mile segment within Fremont County that is currently asphalt surfaced.

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The following table provides a summary of the subsurface conditions encountered along the alignment as related to existing surface, base course, subbase and subgrade materials.

Boring

ID

Existing AC Existing BC Existing Subbase Subgrade TypeInches Inches Inches

B-01 0 4 12 SM

B-02 0 7 23 CL-ML

B-03 0 4 14 CL-ML

B-04 0 4 25 CL-ML

B-05 0 4 12 SM

B-06 0 4 21 CL-ML

B-07 0 4 8 CL-ML

B-08 0 7 29 GC

B-09 0 4 20 CL-ML

B-10 3 5 28 CL-ML

B-11 0.875 5 21 CL-ML

B-12 1.625 4 12 GM

B-13 0.625 4 20 SC

B-14 1.5 6 22 CL

B-15 1.5 5 48 CL-ML

B-16 1.25 2.5 10 SM

B-17 0.75 0 13 GP-GM

B-18 1.125 2.25 11 SP-SM

B-19 2.875 7 4 GP

B-19B 3.5 7 19 SP-SM

B-20 3.75 7 7 SC

B-21 3.125 7 4 GM

B-22 5 12 31 Rhyolite

B-23 3.875 7 7 SC

For our analysis of the pavement section thickness design, we have utilized the average daily traffic (ADT) provided in the SOW to develop appropriate design ESALs for layered analysis in accordance with AASHTO Guide for Design of Pavements and Structures and the FLH PDDM.

The following items were utilized in our analysis:

■ Provided ADT of 650 (7% trucks) in 2015, our analysis included a traffic growth rate of 2% to obtain an ADT of 966 in 2035 (maintaining 7% trucks);

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■ Assumption made in developing ESAL for design life included that 50 percent of the trucks were loaded (i.e. an unloaded truck would enter the project limits and a loaded truck would exit the project limits, or vice versa, such as water tenders to fill cisterns, logging trucks, livestock/cattle trucks, etc.);

■ Directional Traffic Factor – 0.6

■ 75% Reliability

■ Standard deviation of 0.49 (flexible)

■ Initial Serviceability Index of 4.2 and Terminal Serviceability Index of 2.5

■ Based on these parameters the following table indicates 20-yr design ESAL’s for varying truck traffic conditions, assuming 2% annual growth:

TRUCK TRAFFIC, % 20-Year DESIGN ESAL’s 7 307,911 6 267,995 5 214,784

■ Resulting required Structural Number, for 20-yr Design Life using 7% truck traffic and 307,911 lifetime ESALs, based on WFLHD laboratory test results:

o Clark County Section; for design subgrade consisting of existing surfacing course of Clayey SAND with Gravel (SC) subgrade with R-value of 25, SN – 3.04 o Fremont County Section; for design subgrade consisting within the upper 2 feet of existing profile of Silty, Clayey Sand or Gravel (GC-GM/SC-SM) or Poorly Graded Sand with Silt (SP-SM) with minimum R-value of 69, SN – 1.67

■ Layer Coefficients used in analysis o New Hot Mix Asphalt (HMA) – 0.41 o New Crushed Base Course (CBC) – 0.14 o Stabilized Existing Base Course/Pulverized Asphalt – 0.10

Clark County is the first 7.9 miles of the road (MP 9.0 to 16.9 and Borings B-01 to B-09) with only a gravel surfaced road section with an average 4.5 inches of existing surfacing/base course (predominantly clayey sand with gravel) over approximately 18 inches of subbase material (predominantly clayey gravel with sand or clayey sand). Throughout this section, samples of the existing surfacing base course (clayey sand with gravel) and underlying controlling subbase course (silty clay with sand) were tested for index properties and subgrade support characteristics. It was found that the overlying surfacing base course was the controlling condition with an R-value at 300 psi exudation pressure of 25, while the underlying silty clay with sand subbase material had an R-value of 36 at exudation pressure of 300 psi. Based on the test results, the design recommendations provided have been developed using a design R-value of 25, which has been adjusted from a direct correlation of California Bearing Ratio (CBR) based on review of existing performance of the subgrade to a design CBR of approximately 3. This support value is lower than what was assumed in the initial preliminary finding’s memorandum, and the

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The remaining 13.9-mile section (MP 16.9 to 30.8 and Borings B-10 through B-23) is located in Fremont County. This portion of the existing road consists of poor-condition, existing asphalt surfacing ranging from 5/8” thick bituminous surface treatment (BST) to 5 inches of hot mix asphalt (HMA). It has been discussed for this section to consider Full Depth Reclamation (FDR), in which the existing pavement surface is pulverized in place, stabilizing/blending the pulverized asphalt surfacing with underlying crushed aggregate course, and then construction of a new section on top of this stabilized base. Laboratory testing was conducted on several samples throughout this section of aggregate base course and subbase materials to determine appropriate subgrade support values for analysis of the new section. Within the upper 2 feet of the existing profile (the anticipated zone of influence for subgrade support), the minimum R-value obtained was 69, which correlates to a CBR of approximately 32. This support value is higher than what was assumed in the preliminary findings and pavement recommendations memorandum which utilized a CBR of 20 for analysis. Therefore, our analysis has been conducted assuming an R-value of 69 for subgrade support below the FDR section for development of the section below.

Existing Surface Observed Distress Patterns

Photos of the existing gravel and asphalt surface distress patterns are attached to this report in our Site Photography Log. For the gravel surface section, routine maintenance of the roadway will address most distress patterns; however, at the time of our observations, some areas of minor pot-holing (generally less than 2 inches deep) were observed. Some minor washboard areas had developed near turns and private access roads, with very minimal observation of rutting of the gravel surfaced section.

The observed pavement distress varied from the bituminous surface treatment (BST) section between MP 16.9 to MP 25.5 to those observed on the existing hot mix asphalt (HMA) section between MP 25.5 to MP 30.8. At the transition from gravel surfaced roadway to BST surfacing (MP 16.9), heavy fatigue cracking, alligator cracking, rutting, potholing, and isolated complete breakups of the BST surface were evident. This section is unique in that traffic entering or exiting the BST surface is likely slowing or accelerating, causing differing dynamic stresses within the transition zone. Generally, the BST section exhibited limited to moderate transverse cracking, some minor flushing/bleeding of oil, fatigue cracking, and some rutting with the largest isolated area of this distress near approximate MP 21.3 over a 200-foot-long section. Primary distress patterns observed were moderate to severe, isolated pot-holes and patched pot-holes (generally on outboard wheel paths), along with limited to moderate rutting, edge cracking, and transverse cracking. Ride quality was moderate throughout the section from MP16.9 to MP 25.5.

It should be noted that, from approximately MP 25.5, at the intersection with Cherokee Road, to MP 30.8, the existing asphalt surfacing (HMA) has an asphalt thicknesses between 2.9 and 5.0

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Responsive ■ Resourceful ■ Reliable 19 inches and is very heavily distressed (heavy to moderate fatigue cracking, rutting, alligator cracking, block cracking, full-width transverse cracking, longitudinal wheel-path deflection cracking, etc.) in this area. Rutting and longitudinal deflection cracking within wheel paths was fairly pronounced throughout this section, with rutting depths randomly measured between 1 to 2-inches deep. As an example, observations near Boring B-19, at the access to a commercial trucking/pit operation, the pavement section exhibited extreme fatigue cracking and substantial rutting in both the east and west bound lanes. The severity and intensity of cracking increased throughout this section (MP25.5 to MP30.8), likely resulting from the increased traffic associated with the section of the corridor. The severity of the pavement distress, the pattern and distress type, and discussions with WFLHD Materials Engineering members have been taken into consideration during finalizing of the pavement recommendations provided. Of note, the severity and amount of rutting observed during our pavement evaluation indicates that due to the high variability of subgrade conditions and observed performance differences, stabilization of the FDR section should be included in the rehabilitation.

Recommendations of Pavement Section

The following recommendations have been based on the parameters indicated above, which were updated following receipt of laboratory test results from WFLHD, and our resultant pavement section thickness calculations.

■ Clark County (MP 9.0 to 16.9): provide a final section to include 4 inches of HMA over 10 inches of new crushed aggregate base course supported on existing surfacing course as a subbase material; this results in a provided Structural Number, SN of 3.04. This section will provide for a more durable final section, which will have sufficient structural carrying capacity for the existing weak subgrade conditions. Based on our understanding, we are providing the following table for consideration in approaches to varying the section thicknesses (both HMA and crushed base course) to meet project goals for consistent section thickness between Fremont County and Clark County as well as potential consideration of economic constraints.

SECTION LIFETIME ESALs

4” HMA

10” CBC 307,911

3.5” HMA

11” CBC 265,000

3.5” HMA

10” CBC 205,000

■ Fremont County (MP 16.9 to 30.8): provide a final section to include 3.5 inches of HMA placed on a minimum of 6 inches of FDR (pulverized and stabilized existing asphalt surfacing and existing base course), which results in a provided Structural Number of 2.24.

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Full Depth Reclamation…