GR10-12_Middle_Fork_Snoqualmie_Final_Geotech_Report.pdf

[Type text] Page 0

Geotechnical Report No. 10-12

Final Geotechnical Report

Middle Fork Snoqualmie River Road

WA PFH 29-1(1)

Mt. Baker-Snoqualmie National Forest King County, Washington

January 2013

Intentionally blank.

GR10‐12 Page i

Table of Contents

FIGURE 1. PROJECT LOCATION MAP ................................................................................................................................. iii FIGURE 2. AREA GEOLOGIC MAPPING .............................................................................................................................. iv

1. INTRODUCTION

1.01. EXECUTIVE SUMMARY

1.02. PROJECT LOCATION

1.03. PROJECT SCOPE

1.04. SITE GEOLOGY

1.05. GENERAL SITE CONDITIONS

1.06. OTHER GEOTECHNICAL INFORMATION

1.07. CLIMATE

2. SUBSURFACE INVESTIGATIONS

2.01. 2003 NTL GEOTECHNICAL BORING INVESTIGATION

2.03. 2011 WFLHD GEOTECHNICAL BORING INVESTIGATION

2.03(a) Investigation Procedures 2.03(b) Vane Shear Tests 2.03(c) Slope Inclinometer and Vibrating Wire Piezometer Instrumentation 2.03(d) Groundwater Observations

2.04. LABORATORY TESTING PROGRAM

2.05. ROCK CUT INVESTIGATIONS

3. ANALYSIS AND RECOMMENDATIONS

3.01. ALIGNMENT AND GRADE DISCUSSION

3.02. SURFACING SECTION AND SUBGRADE

3.03. SUBGRADE STABILIZATION

3.04. EARTHWORK RECOMMENDATIONS

3.04(a) Soil Cut Slope Recommendations 3.04(b) Rock Cut Slope Recommendations 3.04(c) Suitability of Excavation Material for Embankment 3.04(d) Ribbability of Bedrock 3.04(e) Shrink‐Swell Factors

3.05. DEEP PATCH

3.06. ROCK BUTTRESS

3.07. ROCK FILL (SPECIAL ROCK EMBANKMENT)

3.08. UNDERDRAINS

3.09. RETAINING WALLS

3.10. MP 10 LANDSLIDES

3.10(a) MP 10 Landslide “A” (south of outcrop) 3.10(b) MP 10 Landslide “B” (east of outcrop) 3.10(c) MP 10 Landslide Mitigation Recommendations

4. MATERIAL SOURCES

5. USE OF THIS REPORT

6. REFERENCES

APPENDICES A – G

GR10‐12 Page ii

Appendices

APPENDIX A

Project Photos, pages A‐1 to A‐10

APPENDIX B

Boring Location Plans, pages B.1 to B.18

APPENDIX C

Unified Soil Classification System, page C‐1

Soil Descriptive Terminology, page C‐2

Rock Descriptive Terminology, page C‐3

Boring Logs & Core Photographs – ordered by stationing:

o WFLHD 2011 Boring Logs, pages C‐4 to C‐97 o NTL 2004 Boring Logs, pages C‐98 to C‐118

APPENDIX D

Hand‐Held Vane Shear Test Results on Shelby Tube Samples, page D‐1

In Situ Test Results:

o Wildcat Penetrometer Test Results, pages D‐2 to D‐6 o Dynamic Cone Penetrometer Test Results, pages D‐7 to D‐13

APPENDIX E

Laboratory Test Reports – 2011 WFLHD Investigation, pages E‐1 to E‐75

Laboratory Test Reports – 2004 NTL Investigation, pages E‐76 to E‐91

Laboratory Test Summary – 1992 USFS Investigation, page E‐92

APPENDIX F

Slope Inclinometer And Vibrating Wire Piezometer Data Reports: B‐57, B‐58, B‐59, B‐64, B‐65, pages F‐1 to F‐15

APPENDIX G

2011 Rock cut investigations report by Golder Associates, pages G‐1 to G‐28

Page iii

Figure 1. Project Location Map, Middle Fork Snoqualmie River Road, WA PFH 29-1(1).

North Bend

I‐90 exit 34 Begin Project

171+65.00

End Project

685+50.00

MP 10 Landslides

540+00 to 565+00

Bridge Replacement

588+50

Bridge Replacement

471+85

New MSE Wall

281+90 to 285+10

Slump Area

435+00 to 445+00

Page iv

Figure 2. Area Geologic Mapping

[top half from Geologic Map of the Skykomish River 30x60 Minute Quadrangle (Tabor et al, 1993), bottom half from Geologic Map of the Snoqualmie Pass 30x60 Minute Quadrangle (Tabor, et al, 2000)].

Legend of Geologic Symbols:

Qa - Alluvium Qb - Bog deposits Tsgs - Tonalite and granodiorite

Qvgl - Glaciolacustrine deposits TKwa - Argillite and graywacke Tsm - Mafic diorite and gabbro

Qag - Alpine-glacier deposits TKwv - Metavolcanic rocks gb - Gabbro

Qvr - Recessional outwash deposits Tsg - Granodiorite and granite Tv - Volcanic rocks

Qpf - Glacial and nonglacial deposits Tst - Tonalite and granodiorite

Begin Project 171+65.00

End Project 685+50.00

MP 10 Landslides

GR10‐12 Page 1

1. INTRODUCTION

1.01. Executive Summary

This geotechnical report presents subsurface investigation results and geotechnical recommendations for

9.7 miles of the Middle Fork Snoqualmie River Road project near North Bend, WA. This report is the culmination of several geotechnical investigations, analyses, and reports; and presents final geotechnical analysis and recommendations with consideration of project scope, available funding, and environmental impacts. Recommendations contained herein supersede all previous geotechnical recommendations unless specifically noted otherwise.

The project includes reconstructing the existing gravel road into an asphaltic concrete paved road with drainage improvements, soil and rock cuts and embankments, one retaining wall, and two new bridges.

The existing road has been in use dating back to the early 1900s when its primary use was mining and timber extraction from the Middle Fork and its tributary valleys. The existing road generally follows the same alignment of a railroad that was constructed in the 1920s. Remnants of railroad trestles are still present in some of the creek beds along the alignment.

In general, the existing road consists of 1.5 to 5 feet of gravel overlying saturated lake deposited silt and clay. The subgrade soils are extremely sensitive to disturbance and will not support conventional construction equipment. Therefore the construction will generally consist of a raised grade above the existing alignment to prevent equipment from coming in contact with the silt and clay. Subexcavation and replacement will be used to provide a stable construction platform in areas where the road grade will not be raised.

Two significant landslides exist near milepost 10 (project Stationing 540+00 to 565+00). These landslides are acres in size with shear zones extending 50 feet or more below the ground surface.

Mitigation of these landslides is outside the scope and budget of this project.

Several smaller scale landslides, referred to as slumps, are also present at several locations along the project. These areas will be mitigated with roadway realignment and/or deep patches.

Note that the stationing references in this report refer to the Plan-in-Hand (70%) Plan Set dated August 2012. Subsequent changes to the stationing are not anticipated.

1.02. Project Location

This project is located in King County east of North Bend, WA, north of I-90 near the west end of Snoqualmie Pass. The beginning of the project is about 3 miles northeast of exit number 34 off of I-90.

Figure 1, at the beginning of this report, shows the project location and highlights some of the significant geotechnical aspects of the project that are discussed within this report.

The Middle Fork Snoqualmie River Road is also known as National Forest Road 56 and Lake Dorothy Road.

GR10‐12 Page 2

The project begins at the easterly intersection of SE Middle Fork Road and SE Lake Dorothy Road.

These two roads run parallel to each other for about 1 ¼ miles and are therefore referred to as the Couplet. The project begins at Station 171+65.00 (elevation 786.55 feet) and extends along the Middle Fork Snoqualmie River Road northeasterly 9.7 miles to Station 685+50 (elevation 1072 feet) just ahead of the Taylor River bridge.

1.03. Project Scope

Work will include excavation and embankment, rock blasting, improving roadway structural stability (subgrade stabilization and outboard lane stabilization/deep patch), drainage (cross-grade, ditches, and culverts), roadway safety (signing, striping, and guardrail), one mechanically stabilized earth retaining wall, replacement of two bridges, and asphaltic concrete paving.

1.04. Site Geology

The geology of the Middle Fork Snoqualmie River Valley is dominated by glacial and alluvial processes.

During the last ice age alpine glaciers advanced down the valley from the surrounding mountains. The advance and retreat of the glaciers deposited glacial till and glacial outwash, respectively. Massive continental ice that flowed down from Canada into filling the Puget Lowland flowed up the Snoqualmie River Valley following I-90 just past North Bend. I-90 crosses the hills east of North Bend, which were formed as a terminal moraine of the continental ice up the I-90 valley. Glacial ice dams impounded glacial lakes that resulted in thick accumulations of fine-grained glacial lake deposits (glaciolacustrine) in the valley bottom.

Bedrock exposures along the valley walls are mapped as gabbro, granodiorite and granite, and argillite and greywacke, and the soils in the valley floor are mapped as alluvium, recessional outwash, and glaciolacustrine deposits with mantles of alpine glacier deposits up some of the tributary valleys (Tabor et al, 1993, and Tabor et al, 2000). Figure 2 at the beginning of this report is a geologic map of the valley combined from the two quadrangle maps that cover the area.

The subsurface conditions along the alignment consist of three generalized materials:

1) coarse-grained silty sandy gravelly cobbly glacial till, glacial outwash, and alluvium,

2) fine-grained glacial lake silt and clay, and

3) bedrock.

Fill that was encountered in borings was generally silty sandy gravelly cobbly material.

1.05. General Site Conditions

As the name implies, the road closely follows the alignment of the Middle Fork Snoqualmie River along the valley floor. Since the road was at one time used as a railroad alignment there are several long through-cut tangent sections. Conversely, due to the road being pinched between the river and the adjacent mountains there are several sections in curvy rolling terrain. The Google Earth imagery in

GR10‐12 Page 3

Figure 3 shows how the road traverses the relatively flat valley bottom with very steep mountainous terrain surrounding both sides of the valley.

Figure 3. Google Earth Imagery of Middle Fork Snoqualmie Valley (road alignment in yellow).

The project begins at about elevation 786 and travels uphill to about elevation 1072 at the end of the project.

1.06. Other Geotechnical Information

WFLHD Geotechnical Memorandum GM20-12, prepared by WFLHD Geotechnical Engineer, Brian Collins, provides geotechnical investigation results, analysis, and final foundation recommendations for replacement of two bridges at Station 471+85 and 588+00. GM20-12 is complementary to this report.

Rock cut investigations were completed by Golder Associates, Inc. (Golder) on behalf of WFLHD and included detailed geologic mapping at three sites and five seismic refraction surveys at proposed rock cut locations. Golder’s recommendations are summarized in WFLHD Geotechnical Memorandum GM29-11. The memorandum is contained in Appendix G.

Preliminary design phase geotechnical investigations along the Middle Fork Snoqualmie River Road were completed on behalf of WFLHD by Cornforth Consultants, Inc. (Cornforth), and NTL Engineering

GR10‐12 Page 4 and Geoscience, Inc. (NTL). Cornforth completed one boring near Station 288+80 in 2001. That boring encountered 10 feet of sandy gravel overlying argillite bedrock. NTL completed 19 borings within the project limits in 2003. NTL’s findings are summarized in WFLHD Geotechnical Report GR23-04, dated February 28, 2005. GR23-04 provides baseline geotechnical information for preliminary design and therefore the recommendations appearing in that report are superseded by this report.

The Mt. Baker-Snoqualmie National Forest provided a copy of a slope stability investigation report dated March 1992 for one of the large landslides near milepost 10. Note that the report title reads “Slump at Mile Point 8.79”, but the location was verified as current milepost 10 based on maps and descriptions in the report. This report provides geotechnical investigation results and repair alternatives for the landslide. The investigation included three geotechnical borings, one test pit, and multiple soil index and direct shear tests. The results of the USFS laboratory testing program are summarized in Appendix E.

The recommended alternative was relocation of the road; however, the extent of the relocation is unknown because the report does not include any plans showing the limits of relocation. It also appears that some lightweight fill (sawdust/hog fuel) was incorporated into some of the fills and riprap slope protection and riprap barbs were installed along the river bank. It is also unknown if the landslide movement halted after implementation. At this time the landslide is still active so the recommendations in the report are no longer considered relevant.

Kevin Kelsey, King County Senior Engineer, provided three reports completed by King County for locations within the project limits. The referenced stationing is the current WFLHD project stationing.

The first report completed by King County, dated January 26, 2000, provides geotechnical parameters for the bridge over the Middle Fork Snoqualmie River at Station 326+05 to 328+95. Two borings were drilled, one at each end of the bridge. The borings encountered layers of sand, gravel, and clay.

The second report completed by King County, dated August 15, 2000, provides alternatives and recommendations for mitigation of road embankment scour that occurred near Station 289+00 +/-. The rock slope recommendation was carried forward at this location and appears to also have been used to mitigate road embankment scour at several other locations between Station 229+00 to 235+00 and 274+00 to 290+00.

The third report completed by King County, dated March 8, 2006, provides recommendations for mitigation of a localized landslide between Station 333+00 and 335+00. Two borings were drilled at the landslide and they encountered layers of sand, gravel, and lean clay. The recommended mitigation was to excavate the slide mass and reconstruct a new embankment using select borrow or gravel borrow.

Where conflicts exist, recommendations in this report supersede all previous geotechnical recommendations.

GR10‐12 Page 5

1.07. Climate

The local climate is typical of the mountainous valleys in the northern Cascades – long wet winters with mild summers. The closest Western Regional Climate Center (WRCC) monitoring station (Station 451233 at elevation 1560 feet) is about 8 miles southwest of the beginning of project. Data from the station is summarized in Tables 1. US Forest Service staff report that November 9 is the typical date at which the weather becomes very wet and the rivers and creeks rise quickly.

Table 1. Climate Summary from WRCC Station 451233, Cedar Lake, Washington.

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

Average Max.

Temperature (F)

40 43.8 47.6 53.7 60.8 65.5 72 71.9 66.4 57.4 46.8 41 55.6

Average Min.

Temperature (F)

30.1 31.5 33 36.2 41.4 46.2 49.9 50.4 46.9 41.4 35.7 31.7 39.5

Average Total Precipitation (in.)

13.52 10.06 10.36 8.01 6.52 5.16 2.22 2.68 5.2 9.45 13.87 14.07 101.13

Average Total SnowFall (in.)

18.9 14 12.3 4.3 0.4 0 0 0 0 0.4 4.7 13.8 68.8

Average Snow Depth (in.)

4 4 2 0 0 0 0 0 0 0 1 2 1

NOTES:

Data from Western Regional Climate Center website, www.wrcc.dri.edu, Cedar Lake, Washington (451233). Period of record monthly climate summary from 1/1/1900 to 9/30/2012; Percent of possible observations for period of record: Max. Temp.:

99.3% Min. Temp.: 99.3% Precipitation: 99.7% Snowfall: 99.4% Snow Depth: 97.4%.

It should be noted that the weather station at the Seattle Tacoma International Airport (Station 457473), located 25 miles west of the Cedar Lake Station, has average monthly temperatures that are within a few degrees; however, the average annual precipitation at the Airport is only 38 inches compared to 101 inches at the Cedar Lake Station. Thus the project location is much wetter mountainous terrain compared to the nearby Puget Sound lowlands.

Short-duration high-intensity rainfall events frequently occur in the Middle Fork Snoqualmie River Valley and often result in rapidly rising water levels in the river and tributary creeks. Some of these events resulted in road damage that was repaired by WFLHD under the Emergency Relief for Federally Owned (ERFO) Road Program. The most recent ERFO event occurred in November 2008 and resulted in damage to 9 sites along the Middle Fork Road. Damage at all of the sites was generally attributed to erosion caused by the high water events. Those sites were repaired in summer 2009.

GR10‐12 Page 6

2. SUBSURFACE INVESTIGATIONS

Subsurface investigations completed by WFLHD during development of the Middle Fork Snoqualmie River Road project included geotechnical borings with hollow-stem auger and coring, installation of slope inclinometer casing and vibrating wire piezometers in several borings at the MP 10 landslides, geologic structural mapping, and geophysical surveys.

2.01. 2003 NTL Geotechnical Boring Investigation

In 2003 NTL completed 19 geotechnical borings on behalf of WFLHD within the project limits as part of the preliminary engineering phase – Borings DH-5 through DH-13 and DH-14 through DH-24. Borings DH-1 through DH-4 were drilled outside the current project limits and DH-14 was not drilled.

The 2003 investigation procedures are described in WFLHD Geotechnical Report GR23-04, prepared by NTL. The subsurface investigation locations are summarized in the Boring Location Plans contained in Appendix B. NTL’s boring logs are contained in Appendix C.

2.03. 2011 WFLHD Geotechnical Boring Investigation

In 2011, WFLHD contracted with Haztech Drilling of Meridian, Idaho, and completed an additional 76 geotechnical borings along the project alignment. The subsurface investigation locations are summarized in the Boring Location Plans contained in Appendix B.

2.03(a) Investigation Procedures The borings were drilled through the existing road and shoulders. A track-mounted CME-850 drill was used to drill borings. 8-inch outside-diameter hollow-stem augers were used to advance the borings and HQ3 coring was used to advance the holes into bedrock.

Standard Penetration Tests (SPTs) were completed at 2.5 to 5 foot intervals. The SPT consists of using a 140 pound hammer with a 30 inch drop to drive a 2 inch outside diameter split-barrel sampler 18 inches below the bottom of the bore hole and recording the number of blows for each 6 inch increment of penetration. The last two numbers (for the final 12 inches of penetration) are added together to determine the N-value, also known as the blow count.

A 2.5 inch outside diameter split-barrel sampler (modified California sampler) was alternated with the 2 inch (SPT) sampler in most of the borings. 3-inch diameter Shelby tubes were pushed in into soft clay layers when encountered.

Bedrock was encountered in some of the borings. HQ3 wire-line coring methods were used to advance some of the borings into bedrock. Photographs of all core boxes were taken and are shown on the respective boring log.

The materials and groundwater conditions encountered in the borings were logged in the field by Kent Nelson, WFLHD Geotechnical Engineer. Soils were visually classified in the field according to the

GR10‐12 Page 7

Unified Soil Classification System. The soil samples and boring logs were reviewed by Brian Collins, WFLHD Geotechnical Engineer, and selected samples were submitted to the WFLHD Materials Laboratory for index testing. Boring descriptive terminology used for soil and rock, boring logs, and core photographs are contained in Appendix C.

Boring locations were determined by measuring offsets from known features on the plans set with a fiberglass tape or measuring wheel. The locations listed on the boring logs and shown on the Boring Location Plans should be considered the approximate location where the boring was drilled. Minor discrepancies may exist between the boring logs and Boring Location Plans due to subsequent changes in the alignment. Bore hole elevations were determined by WFLHD Design based on the existing ground elevation at that location from WFLHD survey data.

2.03(b) Vane Shear Tests Vane shear testing was completed on selected Shelby tube samples using a Geotest E-286 Inspection Vane Set. The vane shear tests were completed following the manufacturer’s recommendations and in general conformance with AASHTO T223. The hand-held vane shear test results are contained in Appendix D.

2.03(c) Slope Inclinometer and Vibrating Wire Piezometer Instrumentation Instrumentation consisting of slope inclinometer casing and vibrating wire piezometers was installed at 5 locations (Borings B-57, B-58, B-59, B-64, and B-65) within the MP 10 landslides to monitor groundwater levels and track slope movement. The vibrating wire piezometers were attached to the slope inclinometer casing and grouted in the boreholes.

Data loggers allowed continuous monitoring of groundwater levels between June 2011 and August 2012.

The data logger installed in boring B-58 was defective so water levels were only recorded between April and August 2012. Readings were taken in the slope indicator casing on the following dates: 7/28/11, 11/3/11, 3/26/12, 4/2/12, and 8/16/12.

The slope inclinometer casing used on this project consists of 2.75 inch diameter blue plastic pipe with two orthogonal sets of precision grooves on the inside of the casing. The casing extends down the bore hole past the zone of expected movement. A slope inclinometer probe is a portable device used to measure ground movement. The probe is 30 inches long and it has two sets of wheels spaced at 24 inches that fit into the precision grooves on the inside of the casing. The probe contains two force-balanced servo-accelerometers that measure the tilt of the probe in two orthogonal planes. Readings are taken from the bottom of the casing up at two-foot intervals in each set of grooves. The initial survey establishes the shape of the casing. Subsequent surveys measure changes in the casing profile caused by ground movement. Displacement profiles are used to determine the magnitude, depth, direction, and rate of ground movement. The probe has an accuracy of ±0.01 inch per reading and ±0.3 inch per 50 readings (Slope Indicator Digital Inclinometer Probe datasheet, http://www.slopeindicator.com/pdf/digitilt-vertical-inclinometer-probe-datasheet.pdf, viewed 5/29/2012).

Cumulative and incremental displacement data reports for the slope inclinometer readings and a chart of the groundwater levels from the vibrating wire piezometers are contained in Appendix F. Interpretation of the instrumentation data and discussion of the MP 10 landslides is presented in Section 3.10.

GR10‐12 Page 8

2.03(d) Groundwater Observations Groundwater levels observed in the boreholes are shown on the logs of borings contained in Appendix C.

Groundwater was observed in some borings at depths ranging from 0 to 92 feet below the ground surface during the months of May and June 2011. Typically the groundwater was encountered at depths less than 10 feet below the ground surface. Groundwater levels are anticipated to be even closer to the ground surface during the wet winter months between November and March. Five vibrating wire piezometers with dataloggers were installed at the MP 10 landslide in borings between 546+00 and 560+00.

Groundwater levels were recorded at depths between 4 and 19 feet below the ground surface. The groundwater levels in the piezometers fluctuated as much as 5 feet during the one year observation period and rises of 1 to 3 feet were typically recorded following heavy precipitation events (see data reports in Appendix F).

The observed water levels and/or conditions indicated on the boring logs are as recorded at the time of the exploration. The water levels and/or conditions may vary considerably, with time, according to the climate, rainfall, and other factors and are otherwise dependent on the duration of and methods used in the exploration program. Groundwater levels fluctuate, and it is interpreted that the groundwater level in areas close to the river will rise and fall with the level of the river. Groundwater levels anywhere along the project corridor are likely to rise following periods of heavy precipitation.

2.04. Laboratory Testing Program

Selected split barrel samples obtained from the 2003 and 2011 borings were submitted to the WFLHD Materials Laboratory for testing. NTL retained the Shelby tube samples from the 2003 investigation for direct shear and index testing. The laboratory test reports are attached in Appendix E.

The following is a list of laboratory tests that were completed and the protocols that were followed:

Natural Moisture Content – AASHTO T265 (sample dried at 140° F) Particle Size Analysis – AASHTO T88 Liquid Limit, Plastic Limit, and Plasticity Index – AASHTO T89/T90 pH of Soil – AASHTO T289 Apparent Specific Gravity – AASHTO T100 Minimum Resistivity – AASHTO T288 Moisture-Density Relationship – AASHTO T99 (Standard Proctor) R-Value (at 300 psi exudation pressure) – AASHTO T190 Direct Shear – AASHTO T236 Vane Shear – AASHTO T223

The results of the index tests were used to classify the samples according the Unified Soil Classification System (USCS) and the American Association of State Highways and Transportation Officials (AASHTO) Classification System. The soil classifications are presented on the laboratory test reports.

The 1992 Mt. Baker-Snoqualmie National Forest Geotechnical Report (see description in Section 1.06) for the MP 10 landslide contains a substantial number of laboratory index and direct shear test results on the glaciolacustrine silts and clay encountered at the site. A summary of those laboratory test results is also contained in Appendix E.

GR10‐12 Page 9

2.05. Rock Cut Investigations.

Rock cut investigations were completed by Golder Associates, Inc. (Golder) on behalf of WFLHD and included detailed geologic mapping at 3 sites and 5 seismic refraction surveys. The purpose of Golder’s investigation was to develop rock cut recommendations and assess the rippability at proposed rock cut locations. The locations are summarized below:

Geologic Mapping Locations Seismic Refraction Survey Locations

• 230+00 to 235+00 RT (Area 1) • 224+90 to 229+50 RT (SL-1)

• 279+50 to 290+00 RT (Area 2) • 272+50 to 274+80 RT (SL-2)

• 391+00 to 395+00 LT (Area 3) • 285+05 to 289+65 RT (SL-3)

• 364+00 to 364+35 LT (SL-4B)

• 364+40 to 368+90 LT (SL-4A)

Golder’s recommendations are presented in WFLHD Geotechnical Memorandum GM29-11. The memorandum is contained in Appendix G. Golder’s rock cut recommendations were incorporated into the cut slope recommendations in Section 3.04(a), except that the ditch design recommendations have been modified based on the revised cut slope heights.

GR10‐12 Page 10

3. ANALYSIS AND RECOMMENDATIONS

3.01. Alignment and Grade Discussion

The existing road surfacing section is supported on a fairly thick layer of fill generally measuring 1 to 5 feet thick consisting of sandy gravelly cobbly material. Beneath this fill saturated clay and silt deposits were encountered in many of the borings. These fine-grained soils are highly sensitive meaning they will lose significant strength when disturbed. These soils will not support construction earth moving equipment. Any excavation into the existing surfacing section is perceived to have a very high risk of exposing weak subgrade soils that will not support construction equipment without costly subexcavation and replacement. Therefore, it is recommended to maintain the existing alignment as much as possible and raise grade to eliminate excavation into the subgrade.

In areas where the alignment is over existing, the grade should be set so the new pavement section (asphaltic concrete and base course) is entirely above the existing surfacing section. In areas where the alignment is off of existing, the grade should be set so the surfacing section is at least 18 inches above the existing ground elevation. Areas where these requirements cannot be met have been reviewed and specific locations identified for subgrade stabilization.

Typically the groundwater was encountered in borings at depths less than 10 feet below the ground surface. It is likely that during most of the year much of the road subgrade is saturated, especially following periods of heavy precipitation. This was a major consideration in setting the new road grade above existing in order to keep the construction out of the subgrade soils as much as possible.

3.02. Surfacing Section and Subgrade

A summary of the surfacing section measurements and laboratory test results completed on the surfacing and subgrade materials is contained at the beginning of the 2003 NTL laboratory test data in Table 2.

The existing road surfacing section is supported on a fairly thick layer of fill generally measuring 1 to 5 feet thick consisting of sandy gravelly cobbly material. Beneath this fill saturated clay and silt deposits were encountered in many of the borings.

The liquidity index (LI) of fine-grained soil is an indicator of how the soils will behave upon disturbance.

It is the authors experience that LI values greater than 0.3 indicate the soils will not likely support conventional earth moving equipment. LI values greater than 1.0 indicate the material will behave like a fluid when disturbed. LI values in Table 2 range from 0.4 to 1.4 and average 1.1. Thus it is interpreted that these fine-grained soils are highly sensitive meaning they will lose significant strength when disturbed. These soils will not support construction earth moving equipment. Any excavation into the existing surfacing section is perceived to have a very high risk of exposing weak subgrade soils that will not support construction equipment without costly subexcavation and replacement.

Laboratory tests indicate R-values of the existing fill and coarse subgrade materials range from 54 to 76 and average 66. The underlying clay and silt subgrade material has R-values that range from 12 to 60 and average 39.

GR10‐12 Page 11

Table 2. Summary of Surfacing and Subgrade Measurements and Laboratory Test Results, 2003 NTL Subsurface Investigation (from Geotechnical Report GR23-04)

Station

THICKNESS (1) DEPTH (2)

Boring/ TP No.

Sample Depth

SUBGRADE MATERIAL CLASSIFICATION LABORATORY TEST RESULTS (3)

AC

AGG.

COURSE

FILL:

gravel/ sand/ cobble USCS

USCS

Description AASHTO In Situ

MC LL PL PI LI Gravel Sand P200 S.G.

R-value

@300 psi

R-value

MC @

R-value

Max.

Density, T-99 - C

Opt.

MC,

T-99 - C ft in in ft ft % % % % % % % % pcf % pcf %

179+87 3 6 4.2 DH-5 5.5-10.0 GP-GM PG GRAVEL w/silt & sand A-1-a 5.5 np np 64.3 24.7 11.0 2.711 54 122.8 5.3

196+30 S-1 cut GM silty GRAVEL w/sand A-1-b 17.8 np np 44.5 38.7 16.8 2.588 111.2 13.6

209+16 2 2.0 DH-6 0.2-5.0 GP-GM PG GRAVEL w/silt & sand A-1-a 3.1 np np 73.1 21.5 5.4 2.68 66 127.6 3.8

245+72 1 0.8 DH-7 0.8-5.0 SM silty SAND A-4 47.5 np np 7.1 53.7 39.2 2.65 76 92.1 25.3

252+00 S-2 cut SM silty SAND A-2-4 22.2 np np 0.6 76.3 23.1 2.725 101.5 19.5

262+20 1 0.9 DH-8 0.9-5.0 SW-SM WG SAND w/silt & gravel A-1-b 7.5 np np 32.7 62.2 5.1 2.852 67 155.9 8.2

278+50 1 1.4 DH-9 1.4-4.0 SM silty SAND w/gravel A-2-4 10.6 np np 29.1 43.8 27.1 2.759 72 119.2 10.2

306+30 1 1.0 DH-10 1.0-2.0 CL lean CLAY 24 306+30 1 1.0 DH-10 2.0-5.0 SM silty SAND A-1-b 21.1 np np 8.0 71.0 21.0 2.795 47 115.4 14.5

320+25 1 1.3 DH-11 1.3-5.0 ML sandy SILT w/gravel A-4 26.6 np np 15.8 27.1 57.1 2.587 60 107 15.6

349+80 1 2.2 DH-12 0.1-2.2 GW-GM FILL: WG GRAVEL w/silt & sand A-1-a 1.8 np np 68.0 25.8 6.2 2.734 67 130.3 5.6 349+80 1 2.2 DH-12 2.2-5.0 CL SUBGRADE: sandy lean CLAY A-7-6 42.6 43 20 23 0.98 7.3 34.2 58.5 2.788 21 101.6 23.9

378+75 1 0.7 DH-13 0.7-5.0 CL lean CLAY w/sand A-6 29.1 39 22 17 0.42 1.1 16.6 82.3 2.761 35 98.5 23.6

406+60 1 3.7 DH-15 5 CL lean CLAY 37

428+90 1 1.2 DH-16 1.2-5.0 ML SILT w/sand A-4 28.9 29 24 5 0.98 2.9 23.9 73.2 2.691 39 100.2 20.6

434+70 S-5 cut ML SILT A-7-6 48.3 41 27 14 1.52 0.1 8.6 91.3 2.818 94.8 25.8

439+70 S-7 cut ML sandy SILT A-4 25.1 np np 14.1 31.4 54.5 2.696

448+00 1 1.0 DH-17 1.0-5.0 CL lean CLAY A-6 48.9 38 19 19 1.57 1.8 10.4 87.8 2.733 12 99.7 24.6

472+32 1 1.9 DH-18 2 ML sandy SILT 46

486+10 S-8 cut ML SILT w/sand A-4 np np 0.1 29.6 70.3 2.749 85.8 29.5

486+60 1 0.6 DH-19 0.6-5.0 ML sandy SILT A-4 34.8 34 27 7 1.11 6.2 24.7 69.1 2.766 37 100 23.3

520+10 1 2.7 DH-20 2.7-5.0 SM silty SAND A-4 30.2 28 23 5 1.44 10.2 41.9 47.9 2.805 47 111.4 15.2

559+00 S-6 cut GM silty GRAVEL w/sand A-1-b 25.9 np np 59.7 27.7 12.6 2.578

560+50 6 0.5 DH-21 2.5 CH fat CLAY 40

588+35 1 0.5 DH-22 2.5 GP-GM PG GRAVEL w/silt & sand

608+70 1 1.7 DH-23 1.7-5.0 SM silty SAND w/gravel A-2-4 np np 25.8 47.8 26.4 2.71 57 111.8 14.7

645+90 1 2.0 DH-24 0.1-3.5 GM FILL: silty GRAVEL w/sand A-1-a np np 63.9 25.4 10.7 2.78 65 119.8 7.8

645+90 1 2.0 DH-24 3.5 GP-GM SUB: PG GRAVEL w/silt & sand 3

Notes:

(1) Left column indicates AC thickness (inches), right column indicates base thickness below AC or crushed gravel surfacing thickness.

(2) Depth to bottom of gravel/sand/cobble fill (including surfacing).

(3) MC = water content, LL = liquid limit, PL = plastic limit, PI = plasticity index, LI = liquidity index, P200 = percent of material by weight passing no. 200 sieve, S.G. = specific gravity.

GR10‐12 Page 12

The project does not include any significant changes in alignment. Generally, the subgrade conditions are very good considering that the new surfacing section will generally be supported above the existing gravel surfacing or on a stabilized subgrade section. The average R-value of the existing fill and coarse subgrade would correlate to a modulus on the order of 20,000 psi.

Pavement design recommendations are summarized in the Final Structural Pavement Design memorandum from Brad Neitzke, WFLHD Senior Materials Engineer, to the Cross-Functional Team, dated December 2, 2011.

3.03. Subgrade Stabilization

In areas where the alignment is over existing, the grade should be set so the new pavement section (asphaltic concrete and base course) is entirely above the existing surfacing section. In areas where the alignment is off of existing, the grade should be set so the surfacing section is at least 18 inches above the existing ground elevation. Areas where these requirements cannot be met have been reviewed and specific locations identified for subgrade stabilization. Three types of subgrade stabilization are recommended. Subgrade stabilization details as described below are provided in the project plan set.

Type 1 subgrade stabilization is recommended in areas where the subgrade is not anticipated to support construction equipment, areas where the grade could not be raised, and areas at MP 10 where the road is interpreted to cross the MP 10 landslide terrain. Type 1 subgrade stabilization consists of subexcavation to a depth 2 feet below finished subgrade, placing earthwork geotextile (type III-A) then biaxial geogrid along the bottom of the subexcavation (or existing ground in areas where the fill height exceeds 2 feet), and backfilling with select borrow. During backfill placement, a second layer of biaxial geogrid is placed 1 foot below subgrade elevation. Type 1 subgrade stabilization is full embankment width.

Type 1 subgrade stabilization is recommended at the following locations:

176+50 to 183+00 438+90 to 441+50 540+00 to 551+00 558+00 to 561+00 564+00 to 565+00

Type 2 subgrade stabilization is recommended in areas where the new alignment will be off of the existing road and the subgrade soils are not suitable to support construction earthmoving equipment.

Type 2 subgrade stabilization consists of subexcavating 2 feet below subgrade elevation for the section of road off the existing alignment, placing earthwork geotextile (type III-A) along the bottom of the subexcavation (or existing ground in areas where the fill height exceeds 2 feet), and backfilling with select borrow. A biaxial geogrid should be placed over the backfilled select borrow immediately ahead of the aggregate base placement to mitigate differential movement between the old and new fill. The geogrid should extend 3 feet past the subexcavation limits towards the existing road.

GR10‐12 Page 13

Type 2 subgrade stabilization is recommended at the following locations:

203+00 to 203+90 RT 449+00 to 459+00 RT

In several locations embankment widening will be constructed in areas where the subgrade at the toe of the slope is interpreted to be soft and weak and/or standing water was observed above the ground surface. Embankment toe stabilization, hereafter referred to as special rock embankment Type 1, is recommended in those locations. Special Rock Embankment Type 1 consists of subexcavating 2 feet below existing grade in the area below the widened embankment prism, placing earthwork geotextile (type IV-A, nonwoven), and backfilling to an elevation 2 feet above existing grade with mechanically placed special rock embankment material.

Special rock embankment type 1 is recommended at the following locations:

268+17 to 270+00 RT 394+00 to 395+80 RT 529+60 to 539+00 LT 530+00 to 539+00 RT 578+80 to 585+00 LT 578+80 to 585+00 RT

3.04. Earthwork Recommendations

There are not many significant cuts on this project since the new alignment generally follows the existing road. Most of the cut slopes are in areas where the road is perched on a narrow bench between the river and a steep hillside. Bedrock outcrops can be observed in many areas where the road abuts the river. Besides the areas that will encounter bedrock it is anticipated that the cuts will encounter silt, clay, and silty sandy gravelly soils. The estimated amount of roadway excavation on the project is about 60,000 cubic yards.

A significant amount of borrow will be needed to construct this project due to grade raises along most of the alignment. Estimated embankment quantities exceed 110,000 cubic yards. Most of the excavated material will be unsuitable; therefore, most of the embankment material will have to be imported to the project and the excavated material will be hauled to a waste area.

3.04(a) Soil Cut Slope Recommendations Table 3 provides station by station cut slope recommendations, including a description of the existing cut slope and the materials that are expected to be encountered in the cut. In addition to considering long-term geotechnical stability, cut slope recommendations were made also bearing in mind erosion potential, environmental impacts, constructability, and project costs.

As can be seen in Table 3 many of the existing soil cuts slopes are standing at inclinations between 1V:1.5H and 1:1. Considering that many of these cuts may date back to the original construction in the early 1900s, and since very few locations have any indication of cut slope instability, it is interpreted that the majority of the existing cuts have geotechnical stability factors of safety well in excess of 1.0. It is also interpreted that cut slope inclinations made at the same or flatter angles than existing will have satisfactory long-term factors of safety for this project.

GR10‐12 Page 14

WFLHD’s experience with other projects in similar geologic conditions within the Mt. Baker- Snoqualmie National Forest is that cut slope inclinations between 1:1 and 1V:1.5H provides satisfactory long-term stability.

The existing roadway has a mature tree canopy along much of its length (see project photos in Appendix A). It is desirable to minimize disturbance to the existing mature tree population and retain as much of the existing canopy as possible.

Table 3 includes soil cut slope recommendations for gradients between 1:1 and 1V:2H. WFLHD typically uses 1V:1.5H cuts in the Mt. Baker-Snoqualmie National Forest. Flatter slopes of 1V:2H are recommended in some locations to reduce post-cut erosion potential and thereby improve revegetation. Steeper soil cut slopes, up to 1:1, are recommended in some locations where flatter slope inclinations would result in excessively high cuts, which would not only disturb more existing vegetation, but would expose more soil and increase potential for post-construction erosion.

Rolled erosion control products (RECPs) are recommended at multiple locations to protect the exposed soil on the cut until vegetation can be re-established.

3.04(b) Rock Cut Slope Recommendations Rock cut recommendations are made in Table 3. New rock cuts will range from 25 to 65 feet in height. Most rock cuts will be 30 to 40 feet high. Several of the rock cuts are in locations where the proposed cut is on a narrow bench perched above the river. It will be critical that the contractor designs the blast and uses appropriate measures to mitigate the potential for rock entering the river.

Controlled blasting methods are recommended for all rock cuts exceeding 10 feet in height to minimize blast damage beyond the finished cut face and produce a uniform, stable finished cut.

The contract should include a sufficient quantity of controlled blast holes assuming 30 inch center-to-center spacing between holes along the cut line. The contract should also include a bid item for a Blasting Consultant to provide the expertise to optimize blast production and cut slope stability and to minimize impacts to the adjacent river.

Post-construction rockfall mitigation will consist of widened ditch areas along the base of rock cuts to provide rockfall catchment. Recommendations for cut slopes and ditch widths in areas where rock will be encountered are contained in Table 3. The recommendations for ditch widths are for the cut heights listed in the table and the cut heights shown in the referenced plan set. Increasing cut heights above those currently proposed will require additional analysis of rockfall catchment areas.

The ditch recommendations will provide approximately 80-90% catchment for the recommended cut slopes listed based on the design charts in the Rockfall Catchment Area Design Guide (Pierson et al, 2001). It is anticipated that the rock from these area can be used to partially fulfill the material needs on this project.

It should be anticipated that some spot doweling or rock bolting will be needed to stabilize isolated areas in rock cuts, especially for rock cuts in excess of 30 feet. It is recommended to include a quantity of 100 20-foot long rock dowels consisting of #8 grade 60 threaded or deformed rebar.

Rock dowels shall be installed according to Section 261 of the FP-03 Specifications.

GR10‐12 Page 15

Table 3. Cut slope recommendations (for cuts >5 feet high).

Location

Existing and Proposed Cut Slopes1

(V:H) Subsurface Conditions on Cut Side of Roadway2 Cut Slope Recommendations3

189+00 to

205+00 RT

Existing cut:

10‐15’ @ 1:2 to 1:1.5

Proposed cut:

10‐15’ @ 1:2 to 1:1.5

Silty gravel and sand (colluvium).

[Likely unsuitable].

1V:1.5H or flatter cut slope.

203+75 to 204+05

1V:1.3H cut slope with rock buttress at culvert inlet.

Granite Creek Road Approach @202+92= 10+25 to

10+40 RT

Existing cut:

10‐12’ @ 1:1 to 1:1.25

Proposed cut:

15‐17’ @ 1:1.5

Silty gravel and sand (colluvium).

[Likely unsuitable].

Use 1V:1.5H or flatter cut slope.

Cover slopes with RECP type 3.B.

216+00 to

219+00 RT

Existing cut:

8‐18’ @ 1:2 to 1:1

Proposed cut:

8‐25’ @ 1:2 to 1:1.5

Silty gravel and sand (colluvium).

[Likely unsuitable].

1V:1.5H or flatter cut slope.

Cover slopes with RECP type 3.B.

221+00 to

223+00 RT

Existing slopes:

25‐50’ @ 1:1.25 to 1:1

Proposed cut:

None, rock buttress stabilization of existing surficial sloughing

Silty gravel and sand (colluvium).

[Likely unsuitable].

1V:1.25H cut slope with rock buttress.

232+00 to

234+50 RT

Existing cut:

20‐35’ @ 2:1 to 4:1

Proposed cut:

25‐40’ @ 4:1

Argillite bedrock.

[Suitable].

Use 4V:1H cut slope.

Provide minimum 8’ ditch width.

Use 1V:4H ditch slope.

238+00 to

240+50 RT

Existing cut:

20‐26’ @ 4:1

Proposed cut:

25‐30’ @ 2:1 in rock 10’ @ 1:1 in soil

Argillite bedrock. Rock outcrop ends at 239+50.

[Suitable].

238+00 to 239+50

Use 2V:1H cut slope.

Provide minimum 8’ ditch width.

Use 1V:4H ditch slope.

239+50 to 240+50

Transition out of outcrop using 1:1 or flatter cut slope.

Cover slope with RECP type 4.

242+00 to 264+50

RT& LT

Existing cut:

5‐15’ @ 1:1 to 1:2

Proposed cut:

5‐15’ @ 1:1.5 to 1:2

Silty sand and gravel (outwash).

[Likely unsuitable].

Use 1V:1.5H or flatter cut slope.

Table 3 continued on next page.

GR10‐12 Page 16

Table 3, continued.

Existing and Proposed Cut Slopes1

(V:H) Subsurface Conditions on Cut Side of Roadway2 Cut Slope Recommendations3

272+00 to

274+60 RT

Existing cut:

5‐10’ @ 1:1 in soil 10‐20’ @ 2:1 in rock

Proposed cut:

11’ @ 1:1.5 in soil 16‐18’ @ 1:1 in soil 20‐40’ @ 2:1 in rock

Silty sand with gravel and cobbles (alluvial fan) from 272+00 to 272+60.

[Likely unsuitable].

Thin layer of colluvium over bedrock from pipe inlet at 272+60 to 273+10.

[Likely unsuitable].

Argillite bedrock from 273+10 to 274+25. Abrupt transition out of bedrock at Station 274+25.

[Suitable].

Silty sand and gravel (colluvium) from 274+25 to 274+60.

[Likely unsuitable].

272+00 to 272+60

Use 1V:1.5H or flatter cut slope.

Cover slope with RECP type 4.

272+60 to 273+10

Use 1:1 or flatter cut slope.

Cover slope with RECP type 4.

273+10 to 274+25

Use 2V:1H cut slope.

Provide minimum 7’ ditch width.

Use 1V:4H ditch slope.

274+25 to 274+60

Use 1:1 or flatter cut slope.

Cover slope with RECP type 4.

279+50 to

282+25 RT

Existing cut:

10‐30’ @ 2:1 to 4:1

Proposed cut:

45‐63’ @ 2:1

Argillite bedrock.

[Suitable].

Use 2V:1H cut slope.

Provide minimum 13’ ditch width.

Use 1V:4H ditch slope.

285+00 to

293+50 RT

Existing cut:

10‐20’ @ 2:1 to 4:1 in rock

Existing slope: 1:1 to 1:1.5 in soil

Proposed cut:

7‐41’ @ 2:1 in rock <10’ @ 1:1 in soil 10‐40’ @ 1:1.5 in soil

Argillite bedrock from 285+00 to drainage at 290+20.

[Suitable].

Silt, sand, and gravel (outwash) over bedrock from 290+20 to 292+00.

Overburden thickness increases from Station 290+20 ahead.

[Likely unsuitable].

285+00 to 290+10

Use 2V:1H cut slope.

Provide minimum 9’ ditch width from 285+00 to 285+78. (H<40’)

Provide minimum 8’ ditch width from 286+00 to 287+00. (H<30’)

Provide minimum 9’ ditch width from 287+50 to 288+50. (H<40’)

Provide minimum 8’ ditch width from 289+00 to 290+00. (H<30’)

Use 1V:4H ditch slope.

290+10 to 290+30

Use 1:1 cut slope at pipe inlet.

290+30 to 293+50

Use 1V:1.5H or flatter cut slope.

Steepen up to 1:1 as needed for heights up to 10 feet to minimize amount of cut.

Cover slope with RECP type 4.

GR10‐12 Page 17

Existing and Proposed Cut Slopes1

(V:H) Subsurface Conditions on Cut Side of Roadway2 Cut Slope Recommendations3

365+00 to

366+00 LT

Existing slope:

13’ @ 1:1.25

Proposed cut:

14’ @ 1:1.5

Bedrock outcrop left from 360+00 to 365+00. Bedrock outcrop disappears into slope abruptly at Station 365+00.

Silty sand and gravel (colluvium) from Station 365+00 to 366+00.

[Likely unsuitable].

Install curb and gutter to minimize cut.

Use 1:1 cut at 365+00 to keep cut height <5’, then transition to 1V:1.5H cut slope at 365+50.

392+00 to

395+00 LT

Existing cut:

35‐40’ @ 4:1

Proposed cut:

8’ @ 4:1

Argillite bedrock.

[Suitable].

Use 4V:1H cut slope.

Provide minimum 6’ ditch width.

Use 1V:4H or steeper ditch slope.

398+00 to

403+00 LT

Existing cut:

14‐24’ @ 2:1

Proposed cut:

15‐30’ @ 2:1

Thin layer of duff and colluvium (1‐2’ thick) over argillite bedrock.

[Suitable].

Use 2V:1H cut slope.

Provide minimum 8’ wide ditch. (H<30’)

Use 1V:4H or steeper ditch slope.

407+50 to 409+50

RT & LT

Existing cut:

8‐12’ @ 1:1.5

Proposed cut:

6‐10’ @ 1:2 to 1:1.5

Silt and clay (glaciolacustrine).

[Unsuitable].

Use 1V:1.5H or flatter cut slope.

417+00 LT

Existing cut:

16’ @ 1:1

Proposed cut:

19’ @ 1:1.5

Silt and clay (glaciolacustrine).

[Unsuitable].

Use 1V:1.5H or flatter cut slope.

Cover slope with RECP type 3.B.

419+50

RT & LT

Existing cut:

8‐12’ @ 1:1

Proposed cut:

26’ @ 1:1.5

Silt and clay (glaciolacustrine).

[Unsuitable].

Use 1V:1.5H or flatter cut slope.

Cover slope with RECP type 3.B.

434+50 to 436+50

RT & LT

Existing cut:

11‐13’ @ 1:1 to 1:2

Proposed cut:

10‐16’ @ 1:1.5 to 1:2

Silt and clay (glaciolacustrine).

[Unsuitable].

Use 1V:1.5H or flatter cut slope.

GR10‐12 Page 18

Existing and Proposed Cut Slopes1

(V:H) Subsurface Conditions on Cut Side of Roadway2 Cut Slope Recommendations3

439+00 to 441+50

RT & LT

Existing cut:

10‐14’ @ 1:1 to 1:1.25

Proposed cut:

8‐12’ @ 1:1.5 to 1:2

Silt and clay (glaciolacustrine).

[Unsuitable].

Use 1V:1.5H or flatter cut slope.

443+50 to 444+00

RT

Existing cut:

8’ @ 1:1

Proposed cut:

8‐10’ @ 1:1.5 to 1:2

Silt and clay (glaciolacustrine).

[Unsuitable].

Use 1V:1.5H or flatter cut slope.

556+00 to

542+55 LT

Existing cut:

20‐23’ @ 1:1 to 1:1.5

Proposed cut:

20‐24’ @ 1:1.5

Silt and clay (glaciolacustrine).

[Unsuitable].

Use 1V:1.5H or flatter cut slope.

Cover slope with RECP type 3.B.

556+00 to

557+00 LT

Existing cut:

None

Proposed cut:

6‐16’ @ 4:1 to 1:1.5

Granitic bedrock.

[Suitable].

Use 4V:1H or flatter cut slope.

Provide minimum 8’ ditch width.

Use 1V:4H or steeper ditch slope.

562+50 to

565+00 LT

Existing slope:

15‐30’ @ 1:1 to 1:1.5

Proposed cut:

20‐35’ @ 1:1.5

Sand, gravel, cobbles, and boulders (alluvial fan).

[Likely suitable, but may be over optimum moisture].

Use 1V:1.5H or flatter cut slope.

Cover slope with RECP type 3.B.

587+02 to 588+00

RT & LT

Existing cut:

8‐20’ @ 1:1 to 1:1.5

Proposed cut:

8‐20’ @ 1:1.5 to 1:2

Silt and clay (glaciolacustrine).

[Unsuitable].

Use 1V:1.5H or flatter cut slope.

Cover slope with RECP type 3.B.

588+92 to

593+00 LT

Existing cut:

6‐18’ @ 1:1 to 1:1.5

Proposed cut:

12‐16’ @ 1:1.5

Silt and clay (glaciolacustrine).

[Unsuitable].

Use 1V:1.5H or flatter cut slope.

Cover slope with RECP type 3.B.

611+00 to

611+50 LT

Existing cut:

8‐12’ @ 1:1 to 1:1.25

Proposed cut:

8‐12’ @ 1:1.5

Silt and clay (glaciolacustrine).

[Unsuitable].

Use 1V:1.5H or flatter cut slope.

NOTES:

1Slope heights measured vertical from ditchline to crest. Slope inclinations listed as Vertical:Horizontal (V:H).

2Subsurface conditions listed are those observed in existing cuts and interpreted from nearby subsurface investigation locations.

The actual conditions in each cut are likely to be variable and the description is intended to describe the predominant geology observed at each cut location. An assessment was made as to the materials suitability for embankment considering both the soil type and the in situ moisture relative to optimum moisture for compaction.

3Ditch width is defined as the distance from the slope break at the edge of the pavement to the ditch bottom elevation. The recommendations for ditch widths are for the cut heights listed or for the cut heights shown in the referenced plan set.