ida-bonner841-1_Final Geotechnical Recommendations.pdf

Memorandum

Western Federal Lands Highway Division 610 E. Fifth Street

Vancouver, WA 98661-3801

In Reply Refer To: HFL-17

TO: Brandon Stokes, PE Project Manager

FROM: Diann Engquist, PE Geotechnical Engineer

DATE: October 10, 2022

SUBJECT: Geotechnical Memorandum GM23-22 Final Geotechnical Recommendations Trestle Creek Road, MP 100 to 103.7

ID FLAP BONNER 841(1)

Bonner County, Idaho

INTRODUCTION

This memorandum provides the final geotechnical recommendations for the Trestle Creek Road, Milepost (MP) 100 to 103.7 project, WFLHD Project No. ID FLAP BONNER 841(1). This project is located along Trestle Creek Road on Forest Service (FS) Road 275 in the Kaniksu National Forest (part of the Idaho Panhandle National Forests) in Bonner County, Idaho. Figure B-1, located in Attachment B, shows a project location map. Project photographs are contained in Attachment A. The project stationing referenced in this memorandum is from the 70 Percent Plan Set, dated April 21, 2022.

PROJECT DESCRIPTION

Trestle Creek Road provides primary access for private landowners and recreational and commercial visitors from Idaho’s State Highway (SH) SH-200 to the Trestle Creek Recreational Area and more than 88,000 acres of Forest Service lands. This access to the National Forest sees year-round recreational and commercial use.

Trestle Creek Road is a gravel road with a width varying between 14 and 27 feet with a few turnouts. The scope of this project is to reconstruct and pave the existing gravel road to match the current width and, in locations where topography allows, the roadway will be widened to provide vehicle turnouts. The project starts at Station 10+00, Milepost (MP) 100, intersecting with SH-200 at MP 42.07, and continuing in a northeast direction to the east end of the snowmobile

Trestle Creek Road MP 100 to 103.7

GM23-22 Final Geotechnical Recommendations Page 2 October 2022 parking area located at Station 210+00. Work for the proposed project includes asphalt-concrete pavement structure, subgrade stabilization, turnout construction, streambank stabilization, and safety improvements.

GENERAL SITE CONDITIONS

Topography

The project is located within a river valley with a topographic relief of about 3100 feet. The road generally follows Trestle Creek starting at an elevation of about 2090 feet at the beginning of the project. At about Station 134, the roadway grade increases as it moves away from Trestle Creek.

The elevation at the end of the project is about 2900 feet.

Climate

Table 1 shows Sandpoint Experiment Station, Idaho Period of Record: 1981 to 2010, located at GPS coordinates 48° 17’N, 116° 34’W at an elevation of 2100 feet. According to the Idaho Association of Building Officials, the frost depth in Sandpoint, Idaho, is 24 inches.

Table 1: Climate Summary from WRCC Station 108137, Sandpoint, Idaho.

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual

Average Max.

Temperature (°F)

34.4 39.4 47.6 56.8 65.9 72.9 81.3 81.3 71.1 56.7 41.1 33.8 57.0

Average Min.

Temperature (°F)

22.4 24.1 28.8 34.3 40.9 46.7 50.1 48.6 41.4 33.6 27.8 22.6 35.2

Average Total Precipitation (in.)

4.13 3.23 2.94 2.29 2.74 2.65 1.48 1.12 1.66 2.79 4.88 4.70 34.61

Geology

The Prichard Formation consists of Middle Proterozoic-age (2.5 billion years ago to 541 million years ago (mya)) rocks, contained in the Proterozoic Belt -Purcell Supergroup, are the oldest and most abundant rock within the general project area consisting of siltite, argillite, and quartzite rock. The Prichard Formation contains abundant sulfides. Prichard Formation (Ypm) is described as a structureless massive unit of siltite and quartzite rock. The Prichard Formation is also subdivided into subgroups including member a and b (Ypab) and is described as even parallel laminated rock while member c (Ypc) exhibits uneven laminated characteristics. Intrusive rocks found in localized pockets include Middle Proterozoic-age mafic (dark colored) intrusive rocks, undivided (Ymi) and (light colored) Quartz diorite (Yqd) as well as Tertiary-Eocene-age (66 to 2.6 mya) porphyritic dacite dikes, which are also light in color.

GM23-22 Final Geotechnical Recommendations Page 3 October 2022

Quaternary aged (approximately 2.6 mya to present) deposits include Pleistocene-aged (approximately 2.6 mya to 11,700 years ago) glacial (Qgu) and glacial till (Qgt) deposits and Holocene-aged (11,700 years ago to present) alluvial fan deposits. Glacial deposits consist of unstratified to poorly bedded, unsorted (well-graded) to moderately sorted (poorly-graded) gravel, sand, and silt while glacial till deposits consist of clayey pebble, cobble, and local rock sourced boulders. Alluvial fan deposits consist of well-sorted (poorly graded) gravel. An excerpt of Geologic Map of the Trout Peak Quadrangle showing the mapped geologic units in the project vicinity is provided in Figures B-3 and B-4.

SITE RECONNAISSANCE OBSERVATIONS

General

Site observations discussed in this section have been developed based on the conditions present during the geotechnical site visits performed on May 19 and 20, 2021, and October 13 and 14, 2021.

Partially exposed, cobble-sized rocks within the driving surface were observed throughout the project. It must be noted that the exposed cobble-sized rock surfaces are just the visual size of the exposure. The actual size of the rocks will vary, ranging from cobble size to potentially large boulders. Boulder-sized rocks up to 4 feet in diameter were observed in the cut and fill slopes along Trestle Creek Road. There is a high probability that cobbles, and boulder-sized rocks are located directly under the existing driving surface throughout the project.

Stations 10+00 to 52+00

Photographs A-1 and A-2 show general conditions between these station limits. Starting at the beginning of the project at Station 10+00 to about Station 52+00, Trestle Creek Road is located within a nearly flat, alluvial fan (transverse with the road alignment) and a roadway profile averaging about 3 percent. Sub-angular to subrounded cobbles are prevalent on the ground surface. The roadway widths generally range between 24 to 27 feet, with clean ditch lines on both sides of the road. No standing or flowing water was observed in the ditch lines.

Stations 52+00 to 95+00

Photographs A-3 to A-10 show general conditions between these station limits. Between Stations 52+00 and 95+00, the width of the valley floor narrows; the roadway is immediately adjacent to Trestle Creek for about half of this section. The roadway profile exhibits a relatively flat grade, nearly matching the creek profile, and the roadway width varies between 16 and 22 feet. Cut slope ratios range between 1V:2H and 1V:1H. Shallower cuts are vegetated and appear to consist of colluvium material. In comparison, steeper cuts are sparsely vegetated and appear to consist of completely weathered rock composed of grayish brown, fine silty sand. Natural slopes directly above the cut slopes have measured slope ratios of 1V:4H to 1V:2H. Embankment slopes range between 1V:2H and 1V:1H and undisturbed slopes below the road range from 1V:6H to1V:4H toward the creek. Generally, the ditch line is shallow and narrow (less than 1-foot deep and about 12 to 15 inches wide as measured from the top of the ditch) and filled with

GM23-22 Final Geotechnical Recommendations Page 4 October 2022 vegetation and roadway aggregate. Where the road is directly adjacent to Trestle Creek, it appears the embankment has been armored in the case of Photograph A-3 and is built upon boulder rich alluvium, as shown in Photograph A-7 in Attachment A.

The distance the boulders extend below the existing road surface is unknown. Occasional, large trees are also growing within the embankment between the road and creek and appear to be protecting/buttressing the roadway embankment; Photographs A-5, A-6, A-7. The ditch exhibits standing water through most of this section.

Stations 95+00 to 132+00

Photographs A-11 to A-16 show general conditions between these station limits. The valley floor generally widens between Stations 95+00 and 132+00, allowing the location of the road not to be influenced directly by the creek, except between Stations 130+00 and 132+00. Road width ranges between 15 and 20 feet with a relatively flat profile. Cut and fill slopes range between 1V:3H and 1V:2H, and the ditch line is more defined compared to the previous section. Between Stations 130+00 and 132+00, boulders were observed within the roadway embankment. A private cistern is located in Trestle Creek at approximately Station 131+00, Photograph A-15.

Water is collected in the cistern and pumped by the residence across the road. The water supply pipe is assumed to cross under the road perpendicularly, and the depth is unknown. The ditch is generally dry through this section with occasional lengths of road with saturated soils or standing water.

Stations 132+00 to 210+00

Photographs A-17 to A-42 show general conditions between these station limits. Starting at Station 132+00 until the end of the project at Station 210+00, the roadway widths range between 14 and 20 feet, and the roadway profile ranges from 3 to 10 percent. Cut slopes vary between 1V:2H to 1V:1.5H, natural slopes vary between 1V:3H to 1V:1.5H and fill slopes range between 1V:3H to 3V:1H. Old timber crib walls or logs spanning between tree trunks were observed retaining steep fill slopes, Photographs A-25, A-27, A-28, A-30, A-35, A-36, A-38, A-39, and A-40.

Generally, the effectiveness of the ditch line through this section is poor because it is poorly defined. The ditch is often clogged with vegetation growth obstructing positive, efficient shedding of water from the roadway prism, like the condition shown in Photograph A-23.

In addition, there is a higher rate of observed partially exposed rocks along the roadway surface between Stations146+20 to 147+50 and 180+00 to 181+50, Photographs A-18, A-21, A-23, A-24, and A-29. The ditch line was observed to have more saturated soils or standing water than dry conditions.

Groundwater Seeps and Unstable Slopes

Numerous seeps within the roadway cut slopes were observed during the field visits between Stations 60 and 195. Surface water originating from seeps follows the ditch line unless the ditch line is impacted by vegetation or debris. When the ditch line is obstructed, the surface water flows down the road, or across it, following the path of least resistance. Other occurrences

GM23-22 Final Geotechnical Recommendations Page 5 October 2022 observed the surface water disappearing into the subsurface within the ditch lines. Some of the primary seeps that were noted in the cut slopes are as follows:

• Stations 93 to 96,

• Stations 157 to 159,

• Stations 166 to 172,

• Stations 176 to 177, and

• Stations 184 to185.

In addition to some of the primary seeps observed and noted above, three sites were noted for their wide (25 to 50 feet) “U” shaped channels within the natural slopes on the inboard side of the road. These three sites listed below exhibit slightly depressed to generally flat topography with saturated soils and standing water between the road and the toe of the slope, Photographs A-8, A-21, and A-34. The distance from the edge of the road to the toe of the natural slope above the road ranges from approximately 25 to 65 feet and is generally shallower, between 1V:3H to 1V:2H, than adjacent slopes. These sites also contain localized hydrophilic (wet area) vegetation species. These sites are located between the following stations:

• Stations 84+00 to 85+00,

• Stations 146+20 to 147+50, and

• Stations 180+00 to 181+50.

The road embankment from Station 84+00 to 85+00 is adjacent to Trestle Creek with slope ratios between 1V:1.25H and 2V:1H. At this site, Trestle Creek is eroding and undercutting the embankment. A riprap revetment is planned for this area and the design will be provided by the Hydraulics Team under separate cover.

The embankment slope ratio from Station 146+20 to 147+50 is about 1V:2H and about 4 to 8 feet high. About 20 feet from the toe of the embankment is a slightly concave area measuring about 100 feet long and 75 feet wide that is void of large trees, Photograph A-19. The roadway gravel surface in this area exhibits more exposed rocks than the other sections. It is suspected that this area has a high groundwater table based on these observations.

Between stations 180+00 and 181+50, the area below the road is littered with fallen trees and standing trees that either has broken branches or are leaning, Photographs A-36 and A-37. The embankment slope exhibits a steep, stair-stepped slope ratio measured between 1.25V:1H and 1V:1H, approximately 20 feet high, and is generally void of trees. Below the stair-stepped portion of the slope, it flattens to a 1V:2H slope until it reaches Trestle Creek. This area is vegetated with trees that appear tilted slightly back towards the road. The area adjacent to the inboard edge of the road has a sag pond, where the water overflows into the ditch towards a 12-inch corrugated metal culvert (CMP). The natural slope above the sag pond exhibits a series of benches extending up about 20 feet (vertical) at an approximate overall slope ratio of about 1V:3H. It is interpreted that a more extensive pre-historic landslide feature extends above the existing road and lateral extents at the roadway extend beyond a 150-foot width.

GM23-22 Final Geotechnical Recommendations Page 6 October 2022

Based on our surficial assessment, the slope stability of the first two sites appears adequate. The area below the road between Stations 180+00 and 181+50 appears to be a relatively recent landslide and it is unknown if the site is still active or not. No tension cracks within the existing gravel road were observed during our site visits. The top of the more recently active landslide headscarp is about 13 feet down slope of the road, measured from the centerline. As mentioned above, saturated soils and standing water was noted in this section of the road. Characterizing and mitigating this site is beyond the scope of this project but improving drainage and out-falling the drainage along this section of road outside the landslide limits will likely help improve slope stability. The risk of not characterizing and possible mitigating this slope failure is it may continue to retrogress toward the roadway over time. No subsurface investigation or slope stability analyses was conducted as part of this geotechnical assessment.

SUBSURFACE INVESTIGATION AND PROFILE

Drilling

The subsurface investigation for this project consisted of nine subgrade borings and one deeper geotechnical boring. Figure B-2 shows the general locations of the borings, and Table 2 below shows the boring identification, purpose, approximate station and offset from road centerline, and GPS coordinates. Subgrade borings are generally located every ½-mile along the existing alignment, alternating in the center of each lane, and are denoted with the prefix SG-. Subgrade borings were advanced with a 12-inch outside diameter (O.D.) solid stem auger and a 14-inch slightly tapered one-dimensional auger bit from the center towards the outside to a depth of 4.5 feet. However, within the first subgrade boring, approximately 3 inches of one end of the auger head fractured, resulting in the remaining subgrade borings being drilled with an 11-inch auger head. One deeper geotechnical boring, identified as BH21-01, was advanced with an 8.25-inch O.D. hollow stem auger until refusal and was then advanced using an HQ3 (3.8-inch O.D.)

wireline coring system between 8 and 18 feet. This boring was included in the drill plan as a potential cut slope rockery site on the left-hand side of the road for a slight alignment shift into the cut.

Standard Penetration Tests (SPTs) were performed with a 140-pound automatic hammer dropped 30 inches to drive a standard, 2-inch O.D., split barrel sampler into the ground to recover disturbed soil samples. In boring, BH21-01 SPTs were performed at intervals ranging from 2 to 3 feet. Rock core samples were collected between the depths of 8 and 13 feet. The core barrel became blocked during drilling, so the innertube of the HQ3 coring system was removed at a depth of 18 feet, and SPTs were performed at 6-foot intervals to the bottom of this boring.

SPTs were performed in subgrade boring SG21-07, after auger refusal at a depth of 4 feet, and in SG21-09 at a depth of 2.5 feet. Bulk samples of subsurface materials were obtained in all the subgrade borings from each soil layer encountered.

Drilling operations and field inspection for the subsurface investigation were contracted with Vine Laboratories, Inc., located in Commerce City, Colorado. The borings were completed using a CME 55 truck-mounted drill rig.

GM23-22 Final Geotechnical Recommendations Page 7 October 2022

Project boring logs are provided for both the subgrade borings and BH21-01 in Attachment C.

Attachment D presents the rock coring recovery photographs from boring BH21-01.

Table 2: Subsurface boring location summary.

Boring ID Purpose Station

Offset from Existing CL

(ft.)

Coordinates Approx.

Latitude (Deg.)

Approx.

Longitude

(Deg.)

BH21-01 Potential Cut Side Wall 84+44 5 LT 48.292928 -116.323575

SG21-01 Subgrade Characterization 14+50 6 RT 48.283954 -116.348147

SG21-02 Subgrade Characterization 40+44 5 LT 48.287231 116.339124

SG21-03 Subgrade Characterization 67+92 4 RT 48.290987 -116.329543

SG21-04 Subgrade Characterization 91+42 4 LT 48.293527 -116.32111

SG21-05 Subgrade Characterization 115+88 4 RT 48.296867 -116.312749

SG21-06 Subgrade Characterization 146+24 6 RT 48.300527 -116.301626

SG21-07 Subgrade Characterization 157+72 5 LT 48.301924 -116.297735

SG21-08 Subgrade Characterization 181+96 8 RT 48.302664 -116.28797

SG21-09 Subgrade Characterization 203+72 6 LT 48.30337 -116.279631

Laboratory Testing

Representative soil samples collected during the subsurface investigation were submitted to the WFLHD Materials Laboratory for soil index testing. Laboratory testing was conducted in accordance with appropriate AASHTO specifications to obtain engineering properties necessary for design. The results of the soil index tests were used to classify the samples according to the Unified Soil Classification System (USCS). The USCS classification of individual laboratory tested soil samples is reported on the boring logs. A list of the laboratory tests performed are shown in Table 3 below, and a summary of the laboratory test results are shown in Table B-1a and B-1b in Attachment B. Laboratory test reports are contained in Attachment E.

GM23-22 Final Geotechnical Recommendations Page 8 October 2022

Table 3: Laboratory Test Descriptions Laboratory Test Testing Standard

Sieve Analysis AASHTO T 11/T 27 Particle Size Analysis AASHTO T 88 Liquid Limit of Soils AASHTO T 89 Plastic Limit and Plasticity Index of Soils AASHTO T 90 Apparent Specific Gravity of Soils AASHTO T 100 Soil Classification of Soils AASHTO M 145 R-Value at 300 PSI Exudation AASTHO T 190 Natural Moisture Content of Soils AASTHO T 265 Organic Percent by Ignition AASTHO T 267 Minimum Laboratory Soil Resistivity AASHTO T 288 pH of Soil for Use in Corrosion Testing AASTHO T 289

Subsurface Profile

Boring BH21-01 was advanced to a depth of 30.6 feet. The top 6 inches of the boring encountered poorly graded sand with silt and gravel which is interpreted as road base. Under lying the road base from 6 inches to 6 feet is medium dense, poorly graded gravel with silt, sand, and cobbles. Between the depths of 6 and 7 feet, stiff, sandy lean clay was encountered. Below the lean clay, between the depths of 7 and 30.6 feet, the boring encountered very dense, poorly graded gravel with silt and sand with cobbles and a 1.7-foot boulder between 10.5 and 12.2 feet and possible boulders between 13.1 and 30.0 feet. The formation is interpreted as alluvial stream deposit in this boring.

The subgrade borings encountered 3.5 to 6 inches of either silty sand with gravel (SM), silty gravel with sand (GM), or poorly-graded gravel with silt and sand (GP-GM) that is interpreted to be road base. Underlying the road base, the subgrade borings encountered either well-graded gravel with silt and sand (GW-GM), well-graded sand with silt and gravel (SW-SM), silty sand with gravel (SM), or silty gravel with sand (GM) that is interpreted as alluvium in subgrade borings SG21-01 through SG21-05, colluvium in subgrade borings SG21-07 and SG21-09 and fill in subgrade borings SG21-06 and SG21-08.

The observed volume of cobbles was estimated by the Contractor’s drill inspector, and it ranged between 15 and 45 percent; however, most of the cobbles were removed from the sample that was shipped to WFLHD for our visual confirmation and testing.

Utility (phone) cables were encountered in borings SG21-04 and SG21-09 at about 3.5 below ground surface.

Groundwater

Groundwater was measured in boring BH21-01 while drilling at a depth of 7.5 feet. Groundwater was not observed in any of the subgrade borings.

GM23-22 Final Geotechnical Recommendations Page 9 October 2022

DISCUSSION AND GEOTECHNICAL RECOMMENDATIONS

Frost Susceptible Soils

Frost action can cause differential heaving, surface roughness, and pavement cracking. Also, frost can cause a bearing capacity reduction during thaw periods. Three conditions must be present to cause frost action in the subgrade soils:

• Source of water,

• Subfreezing temperatures in the soil; and,

• Frost-susceptible soils.

Groundwater was not encountered in the subgrade borings; however, water in the ditch line was observed disappearing into the subsurface as noted in the site conditions section under groundwater and seeps discussion. It is assumed that seeps and resulting ditch flowing water is migrating through the roadbed in many sections.

From Table 1, the mean low air temperatures are historically below 32 degrees between November and March.

All the soils tested contain non-plastic, inorganic silt, except for the top 6 inches in SG21-04, which contains 10.8 percent low plasticity silt. Inorganic silt exhibits relatively low shear strength, high capillarity and frost susceptibility, and relatively low permeability. Inorganic silt is difficult to compact more than a couple of points above or below optimum moisture content. The percentage of material passing the #200 (0.075-millimeter (mm)) sieve ranges from 8.9 to 44.1 percent. To further classify the material passing the #200 sieve, silt is defined as material retained on the 0.005 mm sieve, and clay fraction is material passing the 0.005 mm sieve. The percentage of silt in the samples ranges from 8.9 to 41.7 percent, and of the samples where material passing the 0.005 mm was determined, the percentage of clay fraction ranges between

1.6 and 3.05 percent.

According to the Federal Highway Administration, Soils and Foundation Manual (Samtani, 2006), ice lenses form in mixed-grain soils when grains less than 0.02 mm constitute at least 3 percent of the total aggregate. Five of the eleven soil samples met this criterion, with the percent passing a 0.02 mm sieve ranging from 5.3 to 10.7 percent. The Army Corp of Engineers Cold Regions Research and Engineering Laboratory (CRRL), Frost Susceptibility of Soil (1981) publication also suggests a rating system for frost-susceptible soils. This rating system classifies soils based on the type of soil and percent finer than 0.02 mm and is presented in Table B-2, located in Attachment B. This table also discusses material defined as very fine silty sand. We understand and interpret that material passing the #100 ((0.15 mm) sieve and retained on the #200 sieve is very fine sand.

In addition to this information, it has been suggested that there is a correlation to frost susceptible soils with soil characteristics passing the #200 sieve and exhibiting a low Plasticity Index (Edgar, 2014). In this case, very fine sand is a contributing factor to frost-susceptible soils. These correlations are shown in Figure B-5, Heave Related to Percent Fines in Non-cohesive Soils, and

GM23-22 Final Geotechnical Recommendations Page 10 October 2022

B-6, Heave Related to the Plasticity Index, PI. Figure B-5 in the Attachments shows material percent finer than the #200 sieve and the relative vertical expansion in millimeters. In contrast, Figure F-6 shows that material with a low Plasticity Index has an increased vertical expansion tendency.

As stated above, the percentage of material passing the #200 sieve ranges from 8.9 to 44.1 percent, and all the material, except the sample from SG21-04, between 0 to 6 inches from the ground surface, is classified as non-plastic. Based on Figure B-5, the possible range of the non-plastic material may heave between 10 and greater than 30 mm, extending the curve to the limits of the figure.

As demonstrated above, the data suggests that there is a risk that frost heaves may occur, resulting in damage to the pavement structure, and the costs to treat the entire project length will exceed the budget, so a pragmatic, phased approach is recommended in the preceding subsection based on our field reconnaissance and subsurface geotechnical investigation observations.

Subgrade and Subgrade Stabilization

Due to the exposed cobble to boulder-sized rocks near the road's surface, it is recommended to build the structural pavement section on top of, or near the top of, the existing road grade to avoid excavating into the potentially problematic existing roadbed materials.

Details for the following subgrade stabilization recommendations are provided in Figure B-6.

Subgrade Stabilization Type 1

Overall, the existing roadway surface appeared to be in good condition. Rutting and potholes were not observed during the site visits. R-Value test results ranged from 71 to 79, indicating good subgrade support for the pavement structure. However, due to the percentage of material passing the #200 sieve that is silt and the high volume of water noted along some areas of the road, a Class 1, Type A (woven) separation and stabilization geotextile meeting FP-14, subsection 714.01 should be utilized on top of the subgrade prior to placing the structural pavement section. The geotextile should be installed full-width of the roadway at the following locations in accordance with FP 14, subsection 207.04:

Priority Sections:

• Stations 84+00 to 85+00,

• Stations 146+20 to 147+50, and

• Stations 179+50 to 182+00.

Secondary Sections:

• Stations 93 to 96,

• Stations 157 to 159,

• Stations 166 to 172, and

GM23-22 Final Geotechnical Recommendations Page 11 October 2022

• Stations 176 to 177.

Turnouts

New turnouts are planned in locations requiring excavation or embankment construction outside the existing roadway.

Subgrade Stabilization, Type 2

Locations where a turnout is planned requiring excavation outside the existing roadway or where embankment construction heights are 2 feet or less, excavate to a depth 2 feet below the top of the finished subgrade. Slope the bottom of the excavation 2 percent away from the road and daylight it. Place a layer of FP-14, subsection 714.01, Class 1, Type A, (woven), separation and stabilization geotextile on top of the subexcavation and backfill with select borrow meeting FP 14, subsection 704.07. Place the second layer of geotextile between the top of the new subgrade and the bottom of the pavement structural section as shown in Figure B-6. Extend the second layer of geotextile 5 feet beyond the back of the subexcavation (at top of subgrade) towards the centerline.

Subgrade Stabilization, Type 3

Turnout locations, where the fill height is greater than 2 feet at its highest point, slope the bottom of the excavation 2 percent away from the road and daylight it. Place a layer of FP-14 subsection 714.01, Class 1, Type A, (non-woven), separation and stabilization geotextile along the excavation limits before placing the first layer of FP14, subsection 714.03, biaxial stabilization geogrid subgrade along the bottom of the excavation. Backfill with select borrow, meeting FP-14, subsection 704.07 prior to placing additional horizontal layers of biaxial geogrid at a spacing of 1-foot to the top of subgrade. Extend it from the face of the embankment to the back of the excavation. For the final layer of biaxial geogrid at the top of the new subgrade and the bottom of the pavement structural section, place it over the backfilled select borrow immediately ahead of the aggregate base placement as shown in Figure B-6. Extend this final layer a minimum of 5 feet beyond the back of the excavation (at top of subgrade) towards the centerline.

Slope Instabilities

We understand this project does not include roadway widening or cutting slopes. Steep embankment slopes and non-engineered retaining structures such as logs placed parallel to the centerline against two trees were observed along much of the length of the project and was specifically observed between Stations 134 and 206.

It is also noted that the roadway width narrows in areas where the topography exhibits steep embankment slopes between Stations 134 and 206. These steep embankment slopes and retaining structures have a high risk of reduced lateral support for the pavement structure if the steep slopes creep or once the old logs and wood loses its strength and/or rots. Gravel driving surfaces can conceal evidence of slope instability, so some embankment instability may be recognized once the new roadway is paved. However, without evidence of slope instability, GM23-22 Final Geotechnical Recommendations Page 12 October 2022 shoulder stabilization is not prescribed at this time, but placing the pavement the pavement structure so the top of the pavement hinge point is a minimum of 2 feet left of the subgrade daylight point is recommended.

It is unknown if the active slope failure between Stations 180+00 and 181+50 will continue to retrogress toward the roadway. Consideration of a maintenance level deep patch through this road section may extend the life of the road if erosion and landsliding retrogression continues. At this time, it is recommended that Bonner County Road Maintenance Department inspect this location monthly during the wetter periods of time for tension cracks after the project is completed for early intervention of possible retrogression of this recently active landslide.

Borrow Material

Most of the excavated material from the top 6 inches of the existing surfacing is expected to meet FP-14, Subsection 704.06 specifications for unclassified borrow. Surfacing material near the ditch line and localized zones throughout the project are expected to contain materials that will not meet the specifications for unclassified borrow. Material from the subgrade stabilization excavation areas is not likely to encounter a large quantity of unclassified borrow due to the in-situ fine grain and organic soils.

Pavement Recommendations

Pavement design recommendations are summarized in the Pavement Structure Recommendations, Trestle Creek Road memorandum from Corbin Williams, WFLHD Assistant Materials Engineer, to the Cross-Functional Team, dated March 25, 2022.

Ditch Lines

Ditch lines should be cleaned out and reconditioned, but cuts into the existing slopes should be avoided. Undercutting the existing cut slopes or oversteepening them may result in slope creep and/or shallow slope failures. This may require the road width to be slightly reduced to accommodate properly functioning ditch lines.

REFERENCES

AASTHO (2021) Standard Specifications for Classification of Soils and Soil-Aggregate Mixtures for Highway, AASHTO M 145, Construction Purposes, American Association of State Highway and Transportation Officials, Washington D.C., accessed January 24, 2022.

ASTM (2017) Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System (USCS)), Designation: D2487-17, West Conshohocken, Pennsylvania, accessed January 24, 2022.

Chamberlain, E.W., (1981) Frost Susceptibility of Soil, Review of Index Tests, CRREL Monograph 81-2, Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire, U.S. Army Corps of Engineers, Prepared for the Federal Aviation Administration and Federal Highway Administration, Washington D.C.

GM23-22 Final Geotechnical Recommendations Page 13 October 2022

Edgar, T.V., (2014) Instrumentation and Analysis of Frost Heave Mitigation on WY-70, Encampment, WY, University of Wyoming, for Wyoming Department of Transportation and Federal Highway Administration, Cheyenne, Wyoming.

Idaho Association of Building Officials, Design Load Guidelines, Frost Depth, Sandpoint, Idaho, https://idabo.org/Design, accessed February 25, 2022.

Lewis, R.S., Breckenridge, R.M., McFaddan, M.D., Burmester, R.F. (2006) Geologic Map of Trout Peak Quadrangle, Bonner County, Idaho, DWM-58, Idaho Geologic Survey, University of Idaho, Moscow, Idaho, Scale 1:24,000, https://www.idahogeology.org/pub/Digital_Data/Digital_Web_Maps/TroutPeak_DWM-58-m.pdf, accessed July 2021.

NCEI, Climate Data Online, Daily Precipitation Summary, Sandpoint Experimental Station, ID US, Weather Station No. USC00108137, National Center for Environmental Information (NCEI), National Oceanic and Atmospheric Administration (NOAA), U.S. Department of Commerce, Washington D.C., https://www.ncdc.noaa.gov/cdo-web/search, accessed Feb 25, 2022.

Samtani, N.C., Nowatzki, E.A. (2006) Soils and Foundations Reference Manual -Volume 1, FHWA-NHI-06-088, Federal Highway Administration, U.S Department of Transportation, Washington, D.C.

Stokes, Brandon (February 23,2021). Trestle Creek Road, MP 100 to 103.7. Project Plan, Western Federal Lands Highway Division, Vancouver, WA.

Western Regional Climate Center (WRCC), Period of Record Monthly Climate Summary, WRCC Station 108137, Sandpoint, Idaho, https://wrcc.dri.edu/cgi-bin/cliMAIN.pl?id8134, accessed 1/23/2022.

CLOSING

Please contact Diann Engquist at (360) 619-7609 or Diann.Engquist@dot.gov with any questions regarding this memorandum.

INITIALS

CC: Geotechnical File

Included: Attachment A – Project Photographs

Attachment B – Figures and Tables Attachment C – Boring Logs Attachment D – Rock Core Photographs Attachment E – Laboratory Test Result Reports https://idabo.org/Design https://www.idahogeology.org/pub/Digital_Data/Digital_Web_Maps/TroutPeak_DWM-58-m.pdf https://www.ncdc.noaa.gov/cdo-web/search https://wrcc.dri.edu/cgi-bin/cliMAIN.pl?id8134 mailto:Diann.Engquist@dot.gov

ATTACHMENT A

Project Photographs

Photograph A-1: (2021) Station 30+00, ahead right.

Photograph A-2: (2021) Station 45+20, ahead.

Photograph A-3: (2021) Station 75, ahead.

Photograph A-4: (2021) Station 75, back and left.

f ?:t·~t-~ t-· \~~~:~~i ~·-;~r-:J.>··:-: ~ .. -l: ~~;--"· ~-~¥;"':

Photograph A-5: (2021) Station 80+00, ahead right.

Photograph A-6: (2021) Station 81+00, ahead.

Exposed cobble or boulder

Photograph A-7: (2021) Station 82+50, back.

Photograph A-8: (2021) Station 84+50, left.

Photograph A-9: (2021) Station 94, left.

Photograph A-10: (2021) Station 94, right.

Photograph A-11: (2021) Station 96, back.

Photograph A-12: (2021) Station 96, left.

Photograph A-13: (2021) Station 107, ahead.

Photograph A-14: (2021) Station 110, ahead.

Photograph A-15: (2021) Station 131+50, back.

Photograph A-16: (2021) Station 131+00, back.

Cistern

Photograph A-17: (2021) Station 146, back.

Photograph A-18: (2021) Station 146+50, right.

Photograph A-19: (2021) Station 147+00, right.

Photograph A-20: (2021) Station 147+50, ahead.

:i.• ,'

·_.._ "1

Photograph A-21: (2021) Station 147+50, back. Arrows in upper middle of photograph show locations of wet, depressed areas.

Photograph A-22: (2021) Station 148+50, right.

Exposed cobble-sized rocks

Photograph A-23: (2021) Station 158, back.

Exposed cobble-sized rocks

Photograph A-24: (2021) Station 158.

Photograph A-25: Station 160, ahead.

Photograph A-26: Station 160, left.

Photograph A-27: (2021) Station 169, ahead.

Photograph A-28: Station 167, back.

Timber Crib Wall

-r;~ ... ~ -- :·

Photograph A-29: (2021) Station 167.

Photograph A-30: (2021) Station 170+40, back.

Photograph A-31: (2021) Station 170+40, back.

Photograph A-32: (2021) Station 176+40, back.

Photograph A-33: (2021) Station 179+00, Ahead.

Photograph A-34: (2021) Station 180+00, ahead.

_ ,r

•• • t._l.;, Photograph A-35: (2021) Station 180+00, right.

Photograph A-36: (2021) Station 180+25, back.

Photograph A-37: Station 180+50, right.

Photograph A-38: Station 183+60, ahead.

Photograph A-39: Station 183+60, back.

Photograph A-40: Station 185+10, back.

Photograph A-41: Station 192+50, back.

Photograph A-42: Station 207, back.

-·· ... 1- "'• ~

'· · :~\-i·. -~; . .

ATTACHMENT B

Figures and Tables

610 East Fifth Street

Vancouver, WA 98661

Project No. TITLE FigureID BONNER 841(1) Scale

TRESTLE CREEK ROAD

PROJECT LOCATION MAP

B-1 N.T.S.

File Name:

Date:

June 3, 2021

N

Project

Vancouver, WA 98661

Project No. TITLE FigureID BONNER 841(1) Scale

TRESTLE CREEK ROAD

BORING LOCATION MAP B-2

N.T.S.

File Name:

Date:

June 3, 2021

N (l'\ USDOT, Federal Highway Admtnistrahon

I.CJ Office of Federal Lends Highwoy

Vancouver, WA 98661

Project No. TITLE FigureID BONNER 841(1) Scale EXCERPT OF GEOLOGIC MAP OF THE

TROUT PEAK QUADRANGLE, BONNER

COUNTY, IDAHO

B-3File Name:

Date:

January 2022

Qad

I Oad Tres, le Creek Road

I Beginning of proje

~ tate Route 20 i?

I e;;;;;;;;;;i:=:::::i;;;;;;;;;;;;r:==i,;;;;;;;;;;;;i==::i;;;;;;;;;;;;;;;i::==:,;;;;;;;;;~=::i;;;;;;;;;;;;;;;;;;;;,;;;;;;;;;,;;;;;;;;;,;;;;;;;;;,;;;;;;;;;,;;;;;;;;;,;;;;;;;;;,;;;;;;;;;,;;;;;;;;;,;;;;;;;;;,;;;;;;;;;;;;;;;;;;;,;;;;;;;i M ILE

0.5 0 s:::s=::s::====::i;;;;;;;;;;;;;;;;;;,~====::::i,;;;;;;;;;;;;;,;;;;;;;;;,,c:::====,;;;;;;;;;;;;;;;;;;;;;;..i===::::::i FEET

0 1000 2000 JOOO 4000 5000 6000 7000

(}'\ USDOT Federal Highway Adm1n1strat1on 1.C;J Office of Federal Lends Highway

17' 30"

S348

Ypd

Vancouver, WA 98661

Project No. TITLE FigureID BONNER 841(1) Scale EXCERPT OF GEOLOGIC MAP OF THE

TROUT PEAK QUADRANGLE, BONNER

COUNTY, IDAHO

B-4File Name:

Date:

January 2022

SYMBOLS

Contact: line showing the boundary between one map uni t and another; dashed where approximate . The location accuracy of contact is 80 feet or more on the ground.

1'1'-1'!!,l+H++++..+++-~ H igh-ang le fault: ba ll and bar ind icates downthrown side of

EB

19 f a norma l fau lt; dashed where approx imately located;

dotted where concea led .

Str ike and dip of bedding.

Str ike of vert ica l bedd ing.

Str ike and dip of bedding, ball indicates bedding known to be upright.

Horizontal bedd ing.

Str ike and dip of overturned bedding.

Variab le strike and d ip of bedding .

Approximate strike and dip of bedding.

-...__ Str ike and dip of fol iation .

/f so Str ike and dip of my lonitic folia tion.

~5

~50

J'25 'No

~ 43

Str ike and dip of cleavage.

Vertica l cleavage.

Bearing and plunge of sma l I fold ax is.

Bearing and plunge of recumbent sma ll fo ld.

Bearing and plunge of crenulation li neat ion.

Bearing and plunge of asymmetrica l sma l I fold showing counterc lockwise rotation viewed down plunge.

Q uartz vein: Arrow indicates dip.

~~---;:~~~-- Fold ax is.

An ticline.

D irect ion of ice flow through gap.

Crest of mora ine or debris flow levee.

~ Alluvial fan deposits (Holocene)

0 0

, • ~ g ·, Till deposits, undivided (Pleistocene) ou 0 oo;o 0 c

~ Glacial deposits, undivided (Pleistocene)

~ Porphyritic dacite dikes (Tertiary-Eocene)

~ Quartz diorite (Middle Proterozoic)

~ Mafic intrusive rocks, undivided (Middle Proterozoic)

~ Prichard Formation, member c (Middle Proterozoic)-

I Ypab I Prichard Formation, members a and b (Middle Proterozoic)

I Ypm I Prichard Formation, massive unit (Middle Proterozoic)

(}'\ USDOT Federal Highway Adm1n1strat1on 1.C;J Office of Federal Lends Highway

Table B-1a: Laboratory Test Results Summary

BORING NO.

FIELD

SAMPLE

NO.

SAMPLE

DEPTH

PARTICLE SIZE PASSING AASHTO FRACTIONS

101.6 mm

76.2 mm 50 mm 37.5 mm 25.0 mm

19.0 mm

12.5 mm

9.5 mm

4.75 mm

2.00 mm

0.425 mm

0.15 mm

0.075 mm

0.02 mm

0.01 mm

0.005 mm GRAVEL COARSE

SAND

FINE

SAND

SILT

CLAY 4" 3" 2" 1 ½" 1" ¾" ½" 3/8" #4 #10 #40 #100 #200 20µm 10µm 5µm

ft. % % % % % % % % % % % % % % % % % % % %

SG21-01 G-2 0.5 - 4.5 100 96 87 80 69 62 54 49 38 29 18 12 8.9 - - - 67 11 9.1 8.9

SG21-02 G-1 0.0 - 0.5 - 100 99 98 97 97 91 83 62 46 31 21 15.1 - - - 54 15 15.9 15.1

SG21-02 G-2 0.5 - 4.5 100 97 90 85 81 79 74 70 60 49 25 14 9.8 - - - 48 24 15.2 9.8

SG21-04 G-1 0.0 - 0.5 - 100 100 98 94 90 80 71 50 32 18 13 10.8 - - - 68 14 7.2 10.8

SG21-04 G-2 0.5 - 3.0 - 100 100 98 95.4 93.1 89.5 85.8 75.6 66.5 46.7 - 26.8 10.14 6.19 3.05 33.5 19.8 19.9 26.8

SG21-04 G-3 3.0 - 4.5 - 100 100 100 100 99.1 98.1 97.6 95.5 91.1 74.2 - 44.1 10.7 4.85 2.41 8.9 16.9 30.1 44.1

SG21-06 G-1 0.0 - 0.3 - 100 99 95 93 89 79 73 58 45 32 23 17.4 - - - 55 13 14.6 17.4

SG21-06 G-2 0.3 - 4.5 - 100 84.7 75.2 62.6 56.9 49.5 46.1 40.6 36 27.6 - 15 5.3 3.12 1.8 64 8.4 12.6 15

SG21-07 G-2 0.5 - 4.0 - 100 97.8 96.9 94.7 92.2 81.6 74.7 56.8 41.8 29 - 15.4 6.06 3.58 1.6 58.2 12.8 13.6 15.4

SG21-09 G-1 0.0 - 0.5 - 100 100 99 95 91 81 72 49 33 19 14 11.1 - - - 67 14 7.9 11.1

SG21-09 G-2 0.5 - 4.5 100 98.5 95.3 90.2 81.7 74.8 63.6 57.3 45.7 36.7 28.9 - 17.4 7.84 4.54 2.46 61.8 7.8 11.5 17.4

Table B-1b: Laboratory Test Results Summary Continued

BORING

NO.

FIELD

SAMPLE

NO.

SAMPLE

DEPTH LL PL PI

NATURAL

MOISTURE

, wn

APPARENT

SPECIFIC

GRAVITY,

Gs

AASHTO

CLASSIF-

ICATION

UNIFIED SOIL CLASSIFICATION SYSTEM

(USCS)

R-

VALUE

BY

EXUD.

R-VALUE

DENSITY

R-VALUE

MOISTURE ORGANICS pH RESISTIVITY

ft. - - - % - - lb/ft3 % % - (ohm-cm)

SG21-01 G-2 0.5 - 4.5 NP NP NP 1.59 - A-1-a(0) GW-GM; Well-graded GRAVEL with silt and sand - - - - - -

SG21-02 G-1 0.0 - 0.5 NP NP NP - - A-1-b(0) SM; Silty SAND with gravel - - - - - -

SG21-02 G-2 0.5 - 4.5 NP NP NP - - A-1-a(0) SW-SM; Well-graded SAND with silt and gravel 79 123.8 8.1 - - -

SG21-04 G-1 0.0 - 0.5 19 16 3 - - A-1-a(0) GP-GM; Poorly-graded GRAVEL with silt and sand - - - - - -

SG21-04 G-2 0.5 - 3.0 NP NP NP 11.75 2.683 A-2-4(0) SM; Silty SAND with gravel 71 126 8.5 - 6.2 6600

SG21-04 G-3 3.0 - 4.5 NP NP NP - 2.659 A-4(0) SM; Silty SAND - - - 1.6 - -

SG21-06 G-1 0.0 - 0.3 NP NP NP - - A-1-b(0) GM; Silty GRAVEL with sand - - - - - -

SG21-06 G-2 0.3 - 4.5 NP NP NP 7.29 2.716 A-1-a(0) GM; Silty GRAVEL with sand - - - - 5.6 6600

SG21-07 G-2 0.5 - 4.0 NP NP NP 5.48 2.727 A-1-b(0) GM; Silty GRAVEL with sand - - - - - -

SG21-09 G-1 0.0 - 0.5 NP NP NP 3.31 - A-1-a(0) GP-GM; Poorly-graded GRAVEL with silt and sand - - - - - -

SG21-09 G-2 0.5 - 4.5 NP NP NP - 2.729 A-1-b(0) GM; Silty GRAVEL with sand 73 129.3 8.8 - 6.9 4400

Table B-2: U.S. Army Corps of Engineers Frost Design Classification System

Reference: Chamberlain (1981) Frost Susceptibility of Soil, Review of Index Tests.

Frost

Frost susceptibility* group

NFS**

Possiblyt

Very low to high

Medium to high

Negligible to high

Medium to high Low to high

. Very low to very high

None

F1

F2·

F3

Low to very high F4 Very low to high

Low to very high Very low to very high

Kind of soil

(a} Gravels {b} Sands

(a} Gravels {b} Sands

Gravels

{a} Gravels

(b} Sands

(a} Gravels

{bl Sands, except very fine silty sands

(c) Clays, Pl > 12

(a) All silts (b} Very fine si lty sands (c} clays, Pl < 12 (d} Varved clays and other fine grained, banded sediments

• Based on laboratory frost heave tests.

Amount finer than 0.02 mm

Typical soil type under Unified Soil

(% by weight) Classification Systemt

0..:1.s 0-3

1.5-3 3-10

3-10

10-20

10- 15

>20 >15

>15

GW, GP

SW, SP

GW, GP

SW, SP

CW, GP, CW-CM,

GP-GM

CM, CM-GC, GW-GM,

GP-CM

SW, SP, SM,

SW-SM, SP-SM

CM,GC

SM, SC

CL, CH

ML, MH

SM

CL, CL-ML

CL and ML; CL, ML, and SM; CL, CH, and ML; CL, CH, ML, and SM tG = gravel, S = sand, M = silt, C = clay, W = well-graded, P = poorly graded, H = high plasticity, L = low plasticity.

••Non-frost-susceptible.

*Requires laboratory frost heave test to determine frost susceptibility.

Figure B-5: Heave related to percent fines in non-cohesive soils.

Reference: Edgar, T.V. (2014) Instrumentation and Analysis for Frost Heave Mitigation on Highway WY-70.

.--. 30 ~ --(J.)

(1)

I -c.n tL 20

0 10 20 30

Percent Passing No. 200 Sieve

Existing ground surface

Bottom of pavement structure

Separation-stabilization geotextile, class 1, type A (woven)

Asphalt concrete

Base course

Asphalt concrete

Base course

Bottom pavement structure

Asphalt concrete

LJ �2%slope 5' (min.)

NPS PMIS No. xxxxxx NPS Drw No. xxxxxx

PROJECT

ID FLAP BONNER 841(1)

Separation-stabilization geotextile, class 1, type A (woven)

2' (max.)

1V:4H Select borrow

Separation-stabilization geotextile, class 1, type A (woven)

SHEET

NUMBER

SUBGRADE STABILIZATION, TYPE 1

84+00 to 85+00

Base course

Asphalt concrete

Base course

146+20 to 147+50 179+50 to 182+00

Separation geotextile class 1, type A (non-woven)

Bottom pavement structure 5' (min.)

2% slope Select borrow

SUBGRADE STABILIZATION, TYPE 2

(TURNOUT)

Edge of existing road

2% slope

SUBGRADE STABILIZATION, TYPE 3

(TURNOUT)

Fills Greater than 2 Feet

Cut or Fill 2 Feet or Less

Stabilization geogrid, biaxial

NO SCALE

Fill greater than 2'

FIGURE B-6

SUBGRADE

STABILIZATION DETAILS

ATTACHMENT C

Boring Logs

H ol lo w

S te m A ug er

H Q

17-15-10 (15" = 83%)

22-10-9 (2" = 11%)

1-2-9 (11" = 61%)

13-43-28 (6" = 33%)

Rec = 71%

RQD = 54%

Rec = 56%

RQD = 0%

S-1

S-2

S-3

S-4

R-1

R-2

Auger refusal at 8 feet.

Poorly-graded SAND with silt and gravel, medium dense to dense, brown to gray, moist to wet from precipitation, fine to coarse, subangular to rounded, heavily compacted or binder added. Road Base.

0.5 ft Poorly-graded GRAVEL with silt, sand, and cobbles, medium dense, brown, moist, fine to coarse, subangular to rounded, cobbles inferred from drill action. Alluvium.

6 ft Sandy lean CLAY, stiff, brown to gray, moist to wet, fine, rounded sand, trace gravel. Alluvium.

7 ft Poorly-graded GRAVEL with silt, sand, and cobbles, very dense, brown, moist, fine to coarse, subangular to rounded, cobbles and possible boulders inferred from drill action. Alluvium.

10.5 ft BOULDER approximately 1.7 feet in diameter, one discontinuity at 45 degree from vertical with calcite infilling, slightly weathered to fresh, moderately strong to strong rock. Alluvium.

12.2 ft Poorly-graded GRAVEL with silt, sand, very dense, brown, moist, fine to coarse, subangular to rounded, cobbles inferred from core recovery. Alluvium.

12.7 ft COBBLE, same as above boulder, approximately 4 inches in diameter.

13.1 ft Poorly-graded GRAVEL with silt, sand, and cobbles, very dense, brown, moist, fine to coarse, subangular to rounded, cobbles and possible boulders inferred from drill

Station and Offset: 84+44 5 ft Lt.

Groundwater Depth:

While Drilling: 7.5 ft At Completion: --- After Drilling: ---

Notes:

Latitude: 48.292928° Longitude: -116.323575° Datum: WGS84

Weather: 30's-40's, Cloudy with Rain

Date Started: 10/14/21 Date Completed: 10/14/21 Driller/Company: Bill/Vine Laboratories, Inc.

Hammer Type: 140 lbs Automatic

Drill: CME 55 Truck-Mount Hammer Efficiency: 80 %

Logger/Company:Robbie Deister/Shannon & Wilson

BH21-01

Sheet: 1 of 2

Project Location: Bonner County, Idaho

20 40 60 80

PL LLWC

N VALUE

20 40 60 80

Project Name: Trestle Creek Road

D ep th ft)

D ril lin g

M et ho d

Field Blow Count (Recovery)

Core Rec., RQD, and Frac. Freq.

No. Test Results

T yp e

SAMPLE

MATERIAL DESCRIPTION

Boring Log

U. S. DEPARTMENT OF TRANSPORTATION

FEDERAL HIGHWAY ADMINISTRATION

FEDERAL LANDS HIGHWAY DIVISION

F H

W A

B O

R

IN

G L

O G

F

H W

A _D

A T

A T

E M

P

LA

T E

_2

3.

G

D T

/1 6/

:5

C :\P

W -W

O R

K \D

\1

T R

E S

T

LE

C R

E E

K

1) .G

P J

20 40 60 80

Recovery

RQD

G ra ph ic

L og

I

I I I I I _______________________ J

Fl

LJ

H Q

A dv an ce d w ith ou t i nn er b ar re l

Rec = 46%

RQD = 0%

23-35-50/5" (8" = 47%)

25-31-50/1" (6" = 46%)

39-50/1" (6" = 86%)

R-3

S-5

S-6

S-7 action. Alluvium.

Poorly-graded GRAVEL with silt, sand, and cobbles, very dense, brown, moist, fine to coarse, subangular to rounded, cobbles and possible boulders inferred from drill action. Alluvium. (continued)

30.6 ft Bottom of borehole at 30.6 ft.

BH21-01

Sheet: 2 of 2

Project Location: Bonner County, Idaho

20 40 60 80

PL LLWC

N VALUE

20 40 60 80

Project Name: Trestle Creek Road

D ep th ft)

D ril lin g

M et ho d

Field Blow Count (Recovery)

Core Rec., RQD, and Frac. Freq.

No. Test Results

T yp e

SAMPLE

MATERIAL DESCRIPTION

Boring Log

U. S. DEPARTMENT OF TRANSPORTATION

FEDERAL HIGHWAY ADMINISTRATION

FEDERAL LANDS HIGHWAY DIVISION

F H

W A

B O

R

IN

G L

O G

F

H W

A _D

A T

A T

E M

P

LA

T E

_2

3.

G

D T

/1 6/

:5

C :\P

W -W

O R

K \D

\1

T R

E S

T

LE

C R

E E

K

1) .G

P J

20 40 60 80

Recovery

RQD

G ra ph ic

L og

~"'~ 'rJ~ ~..;,_.V

I • I m Gill .

0 -

-0 - )0 ---o< ----0 - )0 ---o< ----o I -- )0 ---o< ---o I -- )0 - ---o< ---o I - 0 - )0 ---o< ---o I -- )0 ----o< ---o I -- )0 ---o< ---o I -- )0 ---o< ---o I -- )0 ---o< ---o I -- )0 ---o< ---o I -- )0 ---o< ---o I - 0 - )0 ----o< ---o I -- )0 ---o< ---o I -- )0 ---o< ---o I -- )0 ---o< ---o I -- )0 ---o< ---o I -- )0 ---o< ---o I -- )0 --

-o< - ~I 0

S id -S te m

A ug er

G-1

G-2

Poorly-graded SAND with silt and gravel, medium dense to dense inferred, brown to gray, moist to wet from precipitation, fine to coarse, subangular to rounded, heavily compacted or binder added. Road Base.

0.5 ft Well-graded GRAVEL with silt and sand (GW-GM), with cobbles up to 12-inches in diameter, very dense inferred, brown, moist, fine to coarse, subangular to rounded, cobbles up to 25% estimated from observed volume. Alluvium.

4.5 ft Bottom of borehole at 4.5 ft.

Station and Offset: 14+50 6 ft Rt.

Groundwater Depth:

While Drilling: --- At Completion: --- Groundwater not observed during drilling.

After Drilling: ---

Notes:

Latitude: 48.283954° Longitude: -116.348147° Datum: WGS84

Weather: 30's-40's, Cloudy with Rain

Date Started: 10/13/21 Date Completed: 10/13/21 Driller/Company: Bill/Vine Laboratories, Inc.

Hammer Type: 140 lbs Automatic

Drill: CME 55 Truck-Mount Hammer Efficiency: 80 %

Logger/Company:Robbie Deister/Shannon & Wilson

SG21-01

Sheet: 1 of 1

Project Location: Bonner County, Idaho

20 40 60 80

PL LLWC

N VALUE

20 40 60 80

Project Name: Trestle Creek Road

D ep th ft)

D ril lin g

M et ho d

Field Blow Count (Recovery)No. Test Results

T yp e

SAMPLE

MATERIAL DESCRIPTION

Boring Log

U. S. DEPARTMENT OF TRANSPORTATION

FEDERAL HIGHWAY ADMINISTRATION

FEDERAL LANDS HIGHWAY DIVISION

F H

W A

B O

R

IN

G L

O G

F

H W

A _D

A T

A T

E M

P

LA

T E

_2

3.

G

D T

/1 6/

:5

C :\P

W -W

O R

K \D

\1

T R

E S

T

LE

C R

E E

K

1) .G

P J

G ra ph ic

L og

~"'~ 'rJ~ ~..;,_.V

I • I

\ r o I )0 p<

Do p<

Do p<

Do ~ o< o I )0 o<

Do p<

Do id -S te m

A ug er

G-1

G-2

Silty SAND with gravel (SM), medium dense to dense inferred, brown to gray, moist to wet from precipitation, fine to coarse, subangular to rounded,…