GM 32 20 WA FS ERFO 2018(1)-23(1) Okanogan-Wenatchee 2018 Repairs Phase 1.pdf

158377846

M E M O R A N D U M Western Federal Lands Highway Division

610 E. Fifth Street Vancouver, WA 98661

DATE: September 24, 2020

TO: Craig Sanders Project Manager

FROM: Robert Kraig Geotechnical Engineer

SUBJECT: Geotechnical Memo 32-20

Geotechnical Recommendations Okanogan-Wenatchee 2018 Repairs, Phase 1 Project

WA FS ERFO 2018(1)-23(1)

Okanogan-Wenatchee National Forest, Washington

INTRODUCTION

This memorandum presents geotechnical recommendations for three (3) sites in the Okanogan-Wenatchee National Forest in Washington State.

PROJECT DESCRIPTION

In April through June 2018, rapid snowmelt and flooding caused damage to roads within the Okanogan-Wenatchee National Forest that provide recreational and administrative access, and act to serve fire suppression needs. The Emergency Relief of Federally Owned Roads (ERFO) program will be used to restore these damaged roads to their original condition.

The overall project will repair 12 damage sites located within the Forest from near Yakima to near the Canadian border, with geotechnical design assistance not required for all but three of the damage sites (listed in bold type below). The larger project work includes repair of eroded ditches, roadbeds and slopes, subsided roadways and repair and replacement of small culverts.

• FR 1504 MP 0.89

• FR 3525-300 MP 1.15

• FR 3700-300 MP 1.5

• FR 4930 MP 3.2

• FR 5008-100 MP 2.3

• FR 5008-200 MP 2.4

• FR 5010 MP 9.3

• FR 5160-250 MP 3.2

• FR 5215-400 MP 3.8

• FR 6200 MP 16.6

• FR 6200 MP 17.4

• FR 6210 MP 4.65

The geotechnical recommendations contained within this memorandum are provided for sites FR 1504 MP 0.89, FR 5215-400 MP 3.8, and FR 6210 MP 4.65 for which special rock embankments (SREs) and mechanically stabilized earth (MSE) walls are proposed. Figure 1 shows the relative locations of the three sites requiring geotechnical design assistance.

Figures 2a through 2c show the vicinities of the individual sites.

Figure 1: Site Locations

As can be seen in Figure 1 the three sites are in central to north-central Washington. Straight line distance between 5215-400 MP 3.8 to 1504 MP 0.89 is approximately 130 miles (as measured by Google Earth). The sites are located from top to bottom in Yakima, Okanogan, and Chelan Counties, respectively, on the eastern slopes of the Cascade Range. FR 5215- 400 is located approximately 17 miles driving distance from Winthrop (not visible in the figure), FR 6210-MP 4.65 is located about 57 miles driving distance from Wenatchee, and FR 1504 MP 0.89 is located approximately 37 miles driving distance from Yakima.

Figure 2a: Site Vicinity--FR 1504 MP 0.89

Figure 2b: Site Vicinity--FR 6210 MP 4.65

Figure 2c: Site Vicinity--FR 5215 MP 3.8

GEOLOGY

FR 1504 MP 0.89

This site is located within the Wapshilla Ridge lava flow (Map Unit Tgw) and is located at an elevation that is above a mapped talus deposit (Map Unit Qta) that appears to be located downslope to the west. An excerpt from the geologic map is provided as Figure 3a.

Mapped Unit Tgw is part of the Middle to Upper Miocene Stratified Volcanic Deposits and one of the lower two flows of the Grande Ronde Basalt Formation (15.6 to about 16 million years ago (Ma) or middle Miocene). The Grande Ronde Basalt Formation consists of six individual basalt lava flows that are dark gray to grayish black to black with some flows consisting of multiple injections of lava. All flows contain fine-grained plagioclase (23.9 percent - 26.1 percent) and clinopyroxene (15.6 percent – 26.3 percent) in a very fine-grained black groundmass of tightly interwoven crystallites of plagioclase, clinopyroxene, and granular ilmenite-magnetite. Tiers of some flows contain fine-grained phenocrysts of plagioclase (1.0 percent - 4.0 percent) and clinopyroxene (0.4 percent - 3.0 percent). All flows are compositionally basaltic andesite rather than truly basalt. The thickness of the Tgw unit in this area is reported to be 0 to 148 feet, and averages about 98 feet.

Mapped Unit Qta is described as rock that is generally monolithic, brown to gray to white, depending on color of composing rock. Clasts are angular and pebble to boulder size, some larger than 3 feet in diameter. It is reported to lack fine-grained matrix and can be unsorted or poorly sorted, but is generally coarser downslope. Most surface blocks are unstable. The deposits have restricted extent, at base of a cliffs or steep, rocky slopes and is commonly partly overgrown. Thickness ranges from 3 to 100 feet but are thicker at the base.

Figure 3a: Excerpt from Geologic Map of the Meeks Table and Western Two-Thirds of the Nile

7.5-Minute Quadrangles

FR 5215-400 MP 3.8 (Heifer Road) According to the geologic map, the site is in the Virginian Ridge Formation of Barksdale and Winthrop Sandstone of Barksdale, as undivided (Map Unit Kvwu), but is shown to be adjacent to Older Alluvium to the east (Map Unit Qoa). Examination of the topographic mapping (not presented in this memorandum) shows the Kvwu unit is located on the hillside above the Qoa unit but may be at the upper limits of Map Unit Qoa as described below, which could mean that the soils at the site may be those of the Older Alluvium Qoa unit. An excerpt from the geologic map appears as Figure 3b.

The Kvwu mapped unit (Early and Late Cretaceous, 145 and 67 Ma) consists of interbedded black siltstone, mudstone, chert-pebble conglomerate, and arkosic sandstone. It is a steeply dipping section of marine lithic sandstone, siltstone, and chert-pebble conglomerate that crops out in Wolf Creek and on neighboring Virginian Ridge, about five miles west of Winthrop.

The Qoa mapped unit (Holocene) is comprised of stratified deposits of sand, silt, and gravel that once filled stream valleys to depths at least as great as 330 feet. These deposits consist of almost totally reworked Pleistocene glacial drift. Modern streams have dissected the older alluvium leaving patchy, abandoned high stream terraces, or remnants along the sides of valleys.

Figure 3b: Excerpt from Geology of Part of the Mazama Quadrangle

FR 6210 MP 4.65 (Also known locally as Chikamin Ridge Road)

According to the geologic mapping, this site is in the Chumstick Formation (Map Unit Tc) (Middle Eocene, 40 Ma) and sits adjacent to the Entiat Pluton to the east (Map Unit Kcet) (Late Cretaceous, 67 Ma), which is part of the Chelan Complex of Hopson and Mattinson (1971). An excerpt from the geologic map is provided as Figure 3c.

Map Unit Tc is described as sandstone, shale, and conglomerate—white to buff-gray, medium- to coarse-grained, micaceous, feldspathic to lithofeldspathic sandstone.

Composition averages 35 to 40 percent quartz, 45 percent to 50 percent feldspar, and 10 percent to 15 percent lithic clasts of which 90 percent are feldspar, and 10 to 15 percent are lithic clasts of which 90 percent are volcanic rock. The unit also includes lesser amounts of interbedded pebbly sandstone, conglomerate, and minor shale. Sedimentary structures include graded, cross bedded and channeled sandstone, and imbricated pebbles in conglomerate.

Map Unit Kcet is described as hornblende-biotite and biotite-hornblende tonalite and tonalite gneiss. The unit ranges from a uniform tonalite to quartz diorite and is a rare granodiorite with pronounced lineation and rare mafic-rich layering to strongly schistosed flaser gneiss, the latter located mostly along the western margin. Massive tonalite is mostly medium-grained hypidiomorphic to xenomorphic granular with crystalloblastic aggregates of hornblende and biotite.

Qoa unit

Figure 3c: Excerpt from Geologic Map of the Chelan 30-Minute by 60-Minute Quadrangle

SUBSURFACE EXPLORATIONS

Subsurface explorations were outside the scope of this ERFO project and not conducted.

Samples were not obtained and no laboratory testing was performed.

SITE CONDITIONS

Climate data and site descriptions are discussed in this section. Climate data was obtained from the National Oceanic and Atmospheric Administration (NOAA) National Centers for Environmental Information.

Climate Data Figure 4 shows the locations of three selected weather reporting stations near the repair sites.

Tables 1 through 3 contain a summary of the most recent available climate observations for those stations showing 1) average maximum and minimum temperatures 2) average monthly precipitation and 3) average monthly snowfall data for the observation period from 1981 to 2010 (NOAA National Centers for Environmental Information, 2020). The tables contain the observations for the Ellensburg, Holden Village, and Mazama weather reporting stations.

Note that this information is not a summary of all possible observations at those reporting stations. The tables are provided to give prospective bidders from outside the geographical area a relative idea of the local weather conditions. It is not intended, nor should it be used to predict how much precipitation to expect during construction, but is instead intended to show a monthly average based on 30 years of statistical data. In addition to the selected weather stations, other weather reporting stations were found to be located near the FR 6210 MP 4.65 and FR 1504 MP 0.89 sites, and in some cases may have been located closer than the selected stations; however, those stations were found to contain incomplete data and thus were deemed less useful.

Figure 4: Weather Reporting Stations Relative to Repair Sites

The Ellensburg, Washington (WA) Weather Station is located approximately 28 miles to the east-northeast of the FR 1504 MP 0.89 site. The reporting station is positioned at elevation 1480 feet and is about 1790 feet higher elevation from the FR1504 MP 0.89 site, which is at approximate elevation 3270 feet. Period of record 1981 to 2010 climate averages are shown in Table 1.

Station: Ellensburg, WS US USC00452505

Ja n

Fe b

M ar

A pr ay

Ju

Ju l ug

Se p

O ct

N ov

D ec nn ua l

Avg. Max. Temp. (F) 35.3 42.0 53.1 60.6 68.7 75.5 84.0 83.8 75.6 61.8 45.1 33.0 59.9

Avg. Min. Temp. (F) 21.0 23.6 29.2 34.4 42.3 48.8 53.8 52.3 43.4 33.5 26.9 19.5 35.7

Avg. Total Precip. (in.) 1.19 0.82 0.68 0.61 0.65 0.59 0.33 0.24 0.45 0.60 1.24 1.56 8.96

Avg. Total Snowfall (in.) 6.9 2.6 0.7 0.0 0.0 0.0 0.0 0.0 0.0 0.0 3.4 7.5 21.1

Note: 1) Period of Record: 1981 to 2010

Table 1: Ellensburg, WA Reporting Station - Summary of Monthly Normals 1981 – 2010

The Holden Village, WA Weather Station is located approximately 16 miles to the north of the FR 6210 MP 4.65 site. The station is reported at elevation 3218 feet and the site is about 352 feet higher at approximate elevation 3570 feet. Period of record 1981 to 2010 climate averages appear in Table 2.

Station: Holden Village, WS US USC00453730

Fe b

Ju l ua l

Avg. Max. Temp. (F) 30.5 35.9 43.6 51.8 61.2 68.3 76.9 77.2 68.6 53.4 36.3 28.1 52.6

Avg. Min. Temp. (F) 16.1 16.5 22.3 28.1 34.6 40.8 44.4 44.0 37.5 30.1 23.1 14.6 29.3

Avg. Total Precip. (in.) 6.65 4.05 3.25 1.61 1.45 1.29 0.78 0.96 1.47 4.12 6.81 6.98 39.42

Avg. Total Snowfall (in.) 64.0 41.8 26.8 5.5 0.5 0.0 0.0 0.0 0.0 4.5 41.7 77.9 262.7

Note: 1) Period of Record: 1981 to 2010

Table 2: Holden Village, WA Reporting Station - Summary of Monthly Normals 1981 - 2010

The Mazama, WA Weather Station is located approximately 6.6 miles to the west of the FR 5215-400 MP 3.8 damage site. The station sits at elevation 2141 feet with the damage site being about 2141 feet higher at approximate elevation 4060 feet. Period of record 1981 to 2010 climate averages are presented in Table 3.

Station: Mazama, WS US USC00455133

Fe b

Ju l ua l

Avg. Max. Temp. (F) 29.8 36.7 46.6 57.8 66.9 74.1 82.7 82.8 73.3 56.3 37.5 27.0 56

Avg. Min. Temp. (F) 16.2 19.1 25.8 31.8 39.9 47.1 52.5 52.0 43.1 32.9 24.8 14.7 33.3

Avg. Total Precip. (in.) 3.68 2.35 1.69 1.03 1.06 1.03 0.79 0.58 0.66 1.82 3.90 3.82 22.41

Avg. Total Snowfall (in.) 32.9 17.6 7.6 0.3 0.0 0.0 0.0 0.0 0.0 2.2 16.1 37.8 114.5

Note: 1) Period of Record: 1981 to 2010

Table 3: Mazama, WA Reporting Station - Summary of Monthly Normals 1981 - 2010

Table 4 shows a summary comparison of the annual averages based on the summary of monthly normals from the three selected weather reporting stations.

Site Avg. Annual

Max. Temp. (F) Avg. Annual

Min. Temp. (F)

Avg. Annual Total Precip.

(in.)

Avg. Annual Total Snowfall

(in.)

Ellensburg, WA

(FR 1504 MP 0.89)

59.9 35.7 8.96 21.1

Holden Village, WA

(FR 6210 MP 4.65)

52.6 29.3 39.42 262.7

Mazama, WA

(FR 5215-400 MP 3.8)

56 33.3 22.41 114.5

Table 4: Summary Comparison of Annual Averages

Note that the average annual maximum and minimum temperatures are somewhat higher at the Ellensburg reporting station near FR 1504 MP 0.89 and the average annual total precipitation and average annual total snowfall are significantly lower when compared to the observations at the Mazama and Holden Village reporting stations. This is likely due to the Ellensburg station being at a lower elevation and situated further to the east and off the leeward slopes of the Cascade Range, where warmer and dryer conditions would be expected. It is likely that the reported temperature and precipitation observations at the Mazama and Holden Village reporting stations are more representative of the typical conditions at the FR 1504 MP 0.89 damage site.

Site Descriptions & Conditions The WFLHD Geotechnical Section did not have the opportunity to observe the sites directly due to COVID-19 travel restrictions, and unfortunately independent assessment by us was not possible. Because of this, and because subsurface investigations were not performed, the descriptions and “observations” described below were obtained from the Damage Site Reports (DSRs) that were prepared in late-October to early November 2018. Additional information was obtained from customized soil reports obtained from the USDA Natural Resources Conservation Service (NRCS) and the cross sections developed for the project.

The custom soil reports address local surface soils typically no deeper than 5 feet below the ground surface. The cross sections show that the upper slopes adjacent to the road have inclinations that range from approximately 1:4 (V:H) to 1:1.

Annual Average Daily Traffic (AADT) is reported to be less than 50 for all sites. The WFLHD performed the survey for the FR 1504 MP 0.89 site and the Forest Service performed surveys for the FR 6210 MP 4.65 and FR 5215-400 MP 3.8 sites.

Photographs appearing in the DSRs for the three damage sites are attached to this memorandum. Based only on the photographs, in general, the exposed soils appear to be granular in nature but as such might contain a significant fine-grain portion (silt and/or clay).

The failure sites appear to be located in tight, narrow road corridors that are bounded by upper hillside slopes and downhill slopes of unknown length that could be moderately steep to steep. Road width in the vicinity of the sites is estimated to be approximately 14 to 16 feet.

All roads appear to be gravel-surfaced. Slopes appear to be lightly to moderately vegetated.

Road fill soils may have been used through some areas of the corridor. The individual sites are discussed below.

FR 1504 MP 0.89

This site is in Yakima County at approximate elevation 3270 feet. See Photos 1 through 4 in the attachment. According to the DSR, damage occurred here when water saturated the fill slope and the road slumped approximately 6 to 10 feet for about 92 feet along the travel way. The failure reached across the travel way to the inside shoulder as seen in Photos 1 and 2.

The Custom Soil Resource Report shows the site is in Map Soil Unit 90, Rock outcrop and Rubble land and is adjacent to a Map Soil Unit 19, Bograp ashy fine sandy loam.

Pertinent information for both units are provided below. A map excerpt from the resource report is provided as Figure 5.

Mapped Soil Unit 90: Rock outcrop and Rubble land:

Rock outcrop:

• Rock outcrops – 50 percent

• Rubble land – 45 percent

• Minor components – 5 percent

• Typical profile – 0 to 60 inches unweathered bedrock

Rubble land:

• Typical profile: 0 to 60 inches fragmental material

Mapped Soil Unit 19: Bograp ashy fine sandy loam:

10 to 35 percent slopes Parent material: colluvium and residuum from basalt mixed with volcanic ash.

Typical profile: 0 to 9 inches of ashy fine sandy loam, 9 to 14 inches fine sandy loam, 14 to 47 inches loam, and 47 to 60 inches silty clay loam Natural drainage class: well drained Depth to water table: > 80 inches.

Figure 5: A map excerpt from the Custom Soil Resource Report for Wenatchee

National Forest, Naches Area, Washington, Parts of Kittitas and Yakima Counties

According to the descriptions above and based on the DSR photographs it is possible that the site is within the limits of Soil Unit 19 ashy fine sandy loam soils since rock outcrops are not readily visible in the photographs and the soils appearing in the photographs appear to be more typical of the sandy loam soils described above. (It should be noted that in the soil surveys, loam is defined as soil material that is 7 to 27 percent clay particles, 28 to 50 percent silt particles, and less than 52 percent sand particles.) The cross sections developed for the site show the upper slope to be standing at or near 1:1, thus indicating substantial internal strength.

FR 6210 MP 4.65 (Also known locally as Chikamin Ridge Road)

This site is in Chelan County at approximate elevation 3570 feet. See Photos 5 through 7 in the attachment. According to the DSR, damage occurred when water overtopped the ditch and flowed over the road resulting in saturation and erosion of the fill slope. Length of damage along the road was reported at approximately 35 feet and extended into the travel way approximately 4 feet. There is a culvert located adjacent to the damage area. A map excerpt from the report is provided as Figure 6.

The Custom Soil Resource Report shows the site is in Map Unit 74, Choralmont cindery sandy loam, till substratum. Pertinent information about this unit is presented below. It was noted that the cross sections developed for the site show the upper slope to be standing at or near 1:1, thus indicating significant internal strength.

Mapped Soil Unit 74: Choralmont cindery sandy loam, till substratum:

30 to 60 percent slopes Choralmont and similar soils: 75 percent Parent material consists of volcanic ash and pumice over till and in a few places residuum.

Typical profile: 0 to 9 inches of paragravelly ashy sandy loam, 9 to 17 inches very paragravelly ashy sandy loam, 17 to 33 inches very paragravelly ashy loamy sand, 33 to 48 inches cinders, 48 to 54 inches ashy loamy sand, 54 to 60 inches ashy fine sandy loam.

Natural drainage class: well drained Depth to water table: > 80 inches.

Figure 6: A map excerpt from the Custom Soil Resource Report for Cashmere

Mountain Area, Washington, Parts of Chelan and Okanogan Counties

FR 5215-400 MP 3.8 (Heifer Road)

This site is in Okanogan County at approximate elevation 4060 feet. See Photos 8 through 10 in the attachment. According to the DSR the damage at this site was caused when the fill slope became saturated followed by slumping of the road for approximately 47 feet length. Most of damage occurred across a 10-foot-wide turnout and reached into the travel way approximately 3 feet as seen in Photos 9 and 10. The uphill slope was noted to be wet when the site was inspected during preparation of the DSR in early November 2018 as shown in Photo 9. Possible outcroppings of bedrock are pointed out in Photo 10. There is an existing corrugated metal pipe (CMP) which is anticipated to be removed as part of the proposed project. Cross sections developed for this site show that the upper slope is standing at or near 1:1.4 (V:H).

The Custom Soil Resource Report shows the site lies in Map Unit 279, Nicmar-Santop complex. Pertinent unit information appears below. A map excerpt from the resource report is provided as Figure 7.

Mapped Soil Unit 279: Nicmar-Santop complex, 35 to 65 percent slopes:

Nicmar and similar soils: 60 percent

• Parent material: mixed volcanic ash 10 to 25 inches over glacial till from volcanic and sedimentary rock

• 0 to 16 inches: gravelly ashy loam

• 16 to 33 inches: very cobbly clay loam

• 33 to 60 inches: very gravelly sandy clay loam

• Natural drainage class: well drained

• Depth to water table: > 80 inches.

Santop and similar soils: 25 percent

• Parent material: mixed volcanic ash 10 to 18 inches thick over colluvium and residuum from volcanic and sedimentary rock

• 0 to 5 inches: gravelly ashy sandy loam

• 5 to 15 inches: very gravelly ashy sandy loam

• 15 to 34 inches: extremely stony sandy loam

• 34 to 38 inches: unweathered bedrock

• Natural drainage class: well drained

• Depth to water table: > 80 inches.

Minor components: 15 percent

Figure 7: A map excerpt from the Custom Soil Resource Report for Okanogan National

Forest Area, Washington

PROPOSED DESIGN & ANALYSIS

As discussed earlier, the sites are located along narrow corridors with existing slopes that are observed to be relatively steep. Repair considerations for mitigating the damage sites included reconstructing the damaged slopes using imported fill (unreinforced), and constructing reinforced soil slopes (RSS), special rock embankments (SRE) or mechanically stabilized earth walls (MSE or MSE walls).

Reconstructing unreinforced slopes with imported fill was considered but eliminated from further consideration based on the need for this type of repair to have a finished face of 1:1.75 (V:H), or flatter, to achieve a “stable” slope face, but also realizing that unreinforced slopes with finished faces flatter than 1:1.75 would be preferable by being more resistant to erosion and increasing the ability to vegetate the slope. This option would inherently require deeper fills with the toe extending at a greater horizontal distance than other options being considered. The expense of importing larger quantiles of fill was also considered.

A RSS is steepened slope that is constructed by benching into the existing slope face and constructing a compacted fill embankment that includes horizontally-placed layers of reinforcement at specified vertical spacings to enhance the stability of the soil structure.

Although RSSs can be constructed with a steeper finished face (typically up to 70 degrees from horizontal), they oftentimes require significant fill volume and finished height. In addition, the larger the structure becomes, typically longer reinforcements are required. In tight corridors, such as are present on the project, the longer reinforcement lengths would necessitate the back of the temporary excavation to extend past the inside shoulder or ditch line, thus undermining the uphill slope and creating a safety hazard. When this is the case, use of temporary shoring or other approved methods would be needed to stabilize the back cut; however, shoring typically adds additional cost and complexity to an otherwise straightforward repair. Another consideration in using the RSS option is how to protect and stabilize the face, particularly with steeper structures, which can also increase complexity and cost. Considering these issues, using an RSS was eliminated from further consideration for this project.

A SRE is constructed with angular rock that is machine-placed and can be constructed with a finished face approaching 1:1.25 (V:H); however, if reinforcements are incorporated in the design then these structures can be constructed with a finished face up to 1:1, thus resulting in reduced structure height and a closer toe position compared to unreinforced slopes and some RSSs. Because it is constructed out of angular rock fill, the face is inherently resistant to erosion and comparatively stable.

A MSE wall is a retaining wall that is a composite structure consisting of alternating layers of compacted backfill and soil reinforcement elements that are typically fixed to wall facing, or in some cases may serve to comprise the facing by wrapping at the face. A MSE wall can be constructed with a finished face as steep as vertical (90-degrees from horizontal), depending on the type of facing used. The steep face allows for reduced structure height, less backfill as compared to the other options, and a toe that is closer in plan (horizontally) to the top of the outside edge of the wall.

After considering the options, the SRE and MSE wall options were considered suitable for use on the project and were selected by the Design Team for use in mitigating the damage sites.

The proposed MSE wall and SREs were analyzed using the software program ReSSA 3.0.

ReSSA 3.0 is a program developed by Adama Engineering that models rotational and translational stability of slopes. The analyses were performed for SRE structures by modelling the slopes at FR 1504 MP 0.89 and FR 5215-400 MP 3.8 using a rock fill with welded wire reinforcement (WWR) spaced at 2-foot vertical spacings. A gabion-faced MSE wall was modelled with rock-filled wire gabion baskets which are attached to layers of welded wire reinforcement embedded into the backfill behind the gabion baskets. Refer to the plan set and special contract requirements for the required reinforcement type, reinforcement spacing, and configuration for the SRE and gabion-faced MSE structures and the gabion basket and rock fill requirements.

As stated earlier, no subsurface investigation or laboratory testing was performed and we did not have the opportunity to observe the sites directly. Allowing that, the soil parameters used in the models were conservatively assumed based on examination of the site photographs from the DSRs, the geometry of the existing slopes as made known by the site surveys, and the information obtained from the soil resource reports. Soil parameters used in the models are shown in Table 5. A surcharge of 250 psf was applied at the top of the embankments to simulate a traffic surcharge on the road.

Site Repair Type 1, 2

Backfill In-Situ Soils

Unit Weight, pcf (γ)

Internal Friction Angle, degrees (φ)

Cohesion, psf (c)

Unit Weight, pcf (γ)

Internal Friction Angle, degrees (φ)

Cohesion, psf (c)

FR 1504

MP 0.89

SRE 125 43 0 116 28 30

FR 6210

MP 4.65

MSE 125 34 0 115 33 60

FR 5215-400

MP 3.8

SRE 125 43 0 120 32 40

1. SRE signifies Special Rock Embankment

2. MSE signifies Gabion-Faced Mechanically Stabilized Earth Wall

Table 5: Soil Parameters Used in Analysis

The safety factors resulting from the analyses are shown in Table 6. The analyses resulted in several of the safety factors being lower than we would typically target. Typically, safety factors at 1.35 or above are sought, but exceptions are possible. In this case, we found that higher safety factors were elusive given the site geometries and the conservative assumptions used in the analyses. It is possible that the assumed soil properties could have benefited from site-specific information, such as the type that would result from subsurface investigations and laboratory testing. This, in turn might have permitted more comprehensive analyses that would have resulted in potentially higher safety factors. Nevertheless, given the level of scope for many of our ERFO projects, the results are considered to fall within an acceptable risk level with the lowest safety factor of 1.26 at FR 6210 MP 4.65 and at FR 5215-400 MP 3.8.

Site

Repair Type 1, 2 Failure Modes & Safety Factors

Rotational Direct Sliding

Three-Part Wedge

FR 1504

MP 0.89

SRE

1.31 1.39 1.43

FR 6210

MP 4.65

MSE

1.32 1.26 1.28

FR 5215-400

MP 3.8

SRE

1.35 1.33 1.26

1. SRE signifies Special Rock Embankment

2. MSE signifies Gabion-Faced Mechanically Stabilized Earth Wall

Table 6: Analyses Safety Factor Results

DESIGN RECOMMENDATIONS

Two repair types are proposed for the project based on the site conditions and as selected by the Design Team. Special rock embankment (SRE) structures are proposed for the FR 1504 MP 0.89 and FR5215-400 MP 3.8 sites, and a gabion-faced mechanically stabilized earth (MSE) wall is proposed for the FR 6210 MP 4.65 damage site. Figure 5 shows a conceptual illustration of a SRE intended for the FR MP 0.89 and FR5215-400 MP 3.8 sites and Figure 6 shows a conceptual illustration of a gabion-faced MSE wall for the FR 6210 MP 4.65 site.

The SREs for the project are constructed by benching into the existing slope and reconstructing with the placement of angular rock meeting the requirements of 705.04 (FP 14). The rock material is mechanically placed according to 252.04 and have horizontal layers of reinforcement spaced at 2 feet (maximum) vertical spacing. We anticipate that the SREs will have a finished face that ranges from 1:1 to 1.25 and that welded wire reinforcement will be used.

The gabion-faced MSE wall for the project is constructed by benching into the existing slope and placing select granular backfill meeting the requirements of 704.08 that is backfilled and compacted according to 209.09 and 209.10, and utilizing gabion baskets and gabion rock fill as facing elements that meet the requirements of 253 and 705.01, respectively.

Figure 5: Conceptual Illustration of SRE (FR MP 0.89 and FR5215-400 MP 3.8)

Figure 6: Conceptual Illustration of Gabion-Faced MSE Wall (FR 6210 MP 4.65)

CONSTRUCTION CONSIDERATIONS

It is possible that the onsite soils are granular in nature but may contain a significant silt/clay portion. It is expected that the contractor will provide construction materials that meet the requirements of the contract specifications and that those materials will be obtained from a private or commercial source. If onsite materials are to be considered for use on the project, it will be the contractor’s responsibility to determine whether the onsite soils meet the contract specifications.

Based on the study discussed within this memorandum, the Contractor should be aware that there is a possibility that bedrock of unknown strength might be encountered at the FR 5215- 400 MP 3.8 site, although bedrock outcrops are not apparent at the failure surface as shown in the photos. If competent bedrock should be encountered, the reinforcement lengths and associated excavation may be reduced with the approval of the CO and the project geotechnical engineer. Any consideration for reduction of reinforcement lengths will be evaluated in the field by the project geotechnical engineer for the potential to result in adverse design impacts prior to a change in the design and implementation in the field.

REFERENCES

Hammond, Paul E., May 2009, Geologic Map of the Meeks Table and Western Two-Thirds of the Nile 7.5-Minute Quadrangles, Yakima County, Washington, Washington Division of Geology and Earth Resources, Geologic Map GM-74.

Tabor, R.W., Frizzell, V.A., Whetten, J.T., Waitt, R.B., Swanson, D.A, Byerly, G.R., Booth, D.B., Hetherington, M.J., and Zartman, R.E., 1987, Geologic Map of the Chelan 30-Minute by 60- Minute Quadrangle, Washington, Department of the Interior, U.S. Geological Survey, Miscellaneous Investigations Series, Map I-1661.

Todd, Victoria R., Geology of Part of the Mazama Quadrangle, Okanogan County, Washington, U.S. Department of the Interior, U.S. Geological Survey, Open-File Report 95-523 (Preliminary), 1995.

NOAA National Centers for Environmental Information (2020), Summary of Monthly Normals, Ellensburg, WA US USC00452505, Holden Village, WA US USC00453730, and Mazama, WA US USC00455133, https://www.ncdc.noaa.gov/cdo-web/datasets#NORMAL_MLY, accessed June 24, 2020.

USDA Natural Resources Conservation Service, Custom Soil Resource Report for 1) Cashmere Mountain Area, Washington, Parts of Chelan and Okanogan Counties, 6210 MP 4.65, 2) Okanogan National Forest Area, Washington, 5215-400 MP 3.8, and 3) Wenatchee National Forest, Naches Area, Washington, Part of Kittitas and Yakima Counties, 1504 MP 0.89, https://websoilsurvey.nrcs.usda.gov/app/, each Accessed June 24, 2020.

https://websoilsurvey.nrcs.usda.gov/app/

CLOSING

Please contact Robert Kraig at (360) 619-7699 or robert.kraig@dot.gov with any questions regarding this memorandum.

RK

CC: Craig Sanders, Project Manager

Mark Martini, Highway Design Engineer Matt Dillin, Hydraulics Engineer Benn Oltmann, Bridge/Structures Engineer Holly Weiss-Racine, Environmental Engineer Larry Gobble, Geomatics Team Megan Chatfield, Materials Engineer

Geotechnical File

Attachments: Photos mailto:robert.kraig@dot.gov

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Robert.Kraig Text Box

Photos Attachment

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Photo 1: FR 1504 MP 8.9 - Looking back. Note turnout at left in foreground. (11/1/2018)

Photo 2: FR 1504 MP 8.9 - Looking ahead. Slump is 6’ -10’ deep. Note turnout. (11/1/2018)

Turnout

Turnout

Photo 3: FR 1504 MP 8.9 - Looking from turnout toward slump & cutslope. (11/1/2018)

Photo 4: FR 1504 MP 8.9 - Looking from slump area up toward cutslope. (11/1/2018)

Photo 5: 6210 MP 4.65 – Looking back at top of failure area (11/8/2018)

Photo 6: 6210 MP 4.65 – Looking back at failure area (11/8/2018)

Photo 7: 6210 MP 4.65 – Looking ahead at top of failure area (11/8/2018)

Photo 8: FR 5215-400 MP 3.8 – Looking back at the failure area. (10/31/2018)

Photo 9: FR 5215-400 MP 3.8 – Looking back at the failure area. Note wet slope area. (10/31/2018)

Photo 10: FR 5215-400 MP 3.8 - Looking ahead at the failure area. (10/31/2018)

Wet slope area

Possible bedrock outcrops?