Landslide Evaluation by GeoEngineers/PUD

On January 15, 2014, Craig Erdman of GeoEngineers met with Mark Flury from Public Utility District No. 1 of Snohomish County on-site at his request to discuss overall conditions and initial impressions regarding the recent landslide that has impacted the hairpin curve on the private road that serves as access to PUD owned real property and overhead power facilities. Representatives of the State Emergency Management Office, and Steven Slaughter from the Washington State Department of Natural Resources were present as well as representatives of Mount Index Riversites Community Club (MIRCC) (Mark Bollman and Lynne Kelly).

MIRCC claims ownership and maintenance responsibility for the private road.

The site is in an area where geologic units consist of granodiorite bedrock overlain by glaciolacustrine

deposits. The glaciolacustrine deposits consist of silt and clay with variable sand content and occasional

cobble to boulder-sized rock. Shannon & Wilson (1982, as referenced in EES, 2009) mapped landslide

features along the hillside where the recent landslide complex is located. They mapped landslide features

across most of the adjacent hillsides as well, as shown in attached Figure 1. 

SITE CONDITIONS

We commenced our site reconnaissance along the toe of the recent landslide area along Mt. Index River

Road, including the area visited by Jim Miller of GeoEngineers on December 20, 2013. The two landslides

have merged and expanded toward the south, such that a clear delineation is not evident between the two

landslide masses. The contractor working for MIRCC has opened two full lanes along most of the access road

along the toe of the landslide complex. It is our understanding that the contractor wanted to continue digging

into the landslide mass beyond (uphill and east of) the road prism, but it is our opinion that such action is illadvised,

as discussed further below. Steven Slaughter concurs with this opinion.

We observed bedrock consisting of granodiorite along a driveway that extends north-northwesterly from the

hairpin curve. We understand from the MIRCC contractor that they encountered bedrock in the road bed on

the inboard side, slightly north of the bedrock exposed in the driveway.

We traversed north along the toe of the recent slide area and observed the portion of the landslide mass that

moved in mid-December 2013. The contractor has most of the width of road cleared of landslide debris in

this area. It is our opinion that it would be good to remove the felled timber from the slide mass, provided

that MIRCC can do so without disturbing the landslide. Steven Slaughter concurs.

Around the bend in the road to the east, and outside of the recent (December – January) landslide area, we

observed a ditch on the inboard side of the road. We also saw the inlet of a 12-inch-diameter corrugated

metal pipe (CMP). We understand that the CMP is buried along the inboard edge of the road, extending about

150 to 160 feet to the south of the inlet. The CMP is believed to be buried under landslide deposits to the south. The drainage ditch is ponding water. To the east along the roadway, we observed a concrete culvert

perched above the ditch line. The culvert does not allow water to flow between the ditch segments until the

water is about 6 inches or so deep.

We then traversed south and up a foot path over the perched concrete culvert and east of the recent

landslide complex. The ground surface outside of the recent slide area is hummocky, sparsely vegetated with

trees and salmon berries, and has an arcuate head at the crest of the slope. This area was mapped as a

landslide in 1982 by Shannon & Wilson, but we did not observe signs of recent ground movements in this

area at this time. An exposure of bare soil along the crest of the slope consisted of glacial deposits (silty

gravel with sand, cobbles and boulders). The recent landslide complex is to the west, as indicated on the

attached site plan (Figure 1).

We continued our traverse west and along the crest of the ridge above the recent landslide complex.

Steven Slaughter informed me that there is a clearcut on the south side of the ridge. The crown of an earlier

mapped landslide area is located at the western crest of the ridgetop. The ridgetop and a zone approximately

150 feet wide (slope distance) sub-parallel to the crest of the west-facing slope is vegetated with western red

cedar, hemlock and Douglas fir and a ground cover of ferns. The earlier landslide area that is vegetated with

sparse to moderately dense conifers appears to be dormant at this time, since we observed no signs of

recent ground displacement such as open cracks or bulging slopes. However, most of the trees in the earlier

landslide area exhibit swept, pistol-butted or corkscrewed trunks, which are likely a result of past slope

movements as the trees were growing. An approximately 50-foot-wide bench is present about 35 feet (slope

distance) below the earlier landslide crest. The bench is relatively continuous for a distance of over 400 feet

sub-parallel to the crest of the slope (i.e. in a north-south direction). This bench was formed by earlier

landslide movements. The recent landslide complex is located about 150 feet (slope distance) west and

northwest of the crest of the earlier landslide area.

We traversed down the earlier landslide crest and bench, descending to the top of the recent landslide scarp.

There are a series of steps in the recent slide scarp ranging from 10 to 20 feet in vertical relief. Soil exposed

in the face of the scarps for the recent landslide complex consists of 1- to 2-foot-thick beds of fine sandy silt

to silty fine sand that grade upwards to clay. Thinner beds 1 to 3 inches thick are also exposed that consist of

silt with a variable sand content and interbedded clay layers. The thinner layers are laminated. The sandier

zones were wet and contained water seepage. Downslope of the earlier landslide bench and upslope of the

northern portion of the recent slide complex, tree cover consists of alders that are estimated to be 6 to

12 inches diameter at breast height. Within the area of recent landsliding, most of the trees have toppled or

are leaning.

The recent landslide mass extends southward of the hairpin turn and just south of the northernmost cabin on

Falls Place. The toe of the recent slide appears to be located about 40 or 50 feet east of the small cabin.

The recent landslide complex is about 400 to 500 feet wide from the southwestern lateral margin upslope of

Falls Place to the toe of the east-northeast lateral margin. We observed at least three linear drainages about

4 to 8 feet wide with steep, convergent headwalls that descend into the body of the recent landslide complex.

We observed hydrophyllic vegetation such as devil’s club and seeps of water within the linear drainages.

One of the linear drainages is located along the approximate southern limit of the recent landslide complex.

Flows of turbid water were observed within about 20 to 25 feet of the cabin at the toe of the slope. A cedar Page 3

GeoEngineers, Inc.

File No. 0482-040-01

tree at the toe of the slope appeared to be in place, and did not appear to be displaced or tilting from recent

slope movements.

CONCLUSIONS AND RECOMMENDATIONS

The recent landslide complex consists of reactivated landslides within glaciolacustrine deposits. It is our

opinion that these are relatively slow moving landslides, but precipitation likely accelerates ground

movements and helps some of the landslide debris to become more fluid. The base of the landslide near the

road appears to be on or near the contact with granodiorite bedrock.

The recent landslide complex is located about 600 feet from the closest boring that was completed for the

Sunset Fish Passage & Energy Project, with that work completed more than three months prior to the

landslide in December 2013. None of our geotechnical borings are located directly uphill of the recent slide

mass; therefore, water used during drilling and testing could not reach the recent landslide complex.

Because the closest boring is relatively distant from the recent slide area, and because all of our geotechnical

borings were completed almost entirely in competent rock, it is our opinion that the exploratory drilling

activities did not trigger the recent landslide movements that have impacted the Mt. Index River Road.

Based on our reconnaissance, it is our opinion that continued excavation of landslide debris from the toe of

the landslide adjacent to the roadway is inadvisable. It is further our opinion that continued excavation could

result in renewed ground movements, expansion of the landslide area, and increased short-term and

long-term costs for mitigation.

The main landslide mass affecting the roadway will be difficult and costly to stabilize. The length of landslide

mitigation measures could be 300 feet or more to reduce the risk of renewed landslide movement onto the

road. We anticipate that landslide mitigation construction costs could be several hundred thousand dollars,

depending on type, length, height and other factors. A conceptual discussion for one landslide mitigation

option, which extends from the hairpin turn to the northern edge of the recent landslide complex, is described in a subsequent section titled “Landslide Stability Improvement.”

Relocating the hairpin curve to the inside of the curve may be the best option for maintaining short-term

roadway access, as this will provide greater separation from the toe of the recent landslide.

Property ownership issues in this vicinity would need to be resolved.

Residence at Risk

There is one cabin on the lot identified as Assessor’s Parcel Number 00525800213700 (located on

Falls Place) that is within about 25 feet of the toe of the slope and within 40 to 50 feet of recent landslide

movements at the time of our January 15 site visit. This structure and its occupants are at risk if additional

landslide movements occur. It would be advisable to limit occupancy to day time hours only, when slope

movements can be readily observed.

Landslide Stability Improvement

Two episodes of landslide movement have resulted in slide debris extending over the MIRCC community

access road. The base of sliding appears to be at or above (higher than and east of) the road.

Nevertheless, slide debris has moved downhill to cover the roadway, thus limiting PUD and other vehicular

access and incurring significant costs for cleanup, soil removal and tree hazard mitigation.

Landslide movements are initiated when gravitational forces within a mass of soil or rock exceed the

available resisting forces (soil strength and friction along the base of the slide surface). Downhill movement

of a slide mass tends to reduce the overall angle of a slope and thus reduce gravitational driving forces.

However, excavation at the base of a slide mass (such as removing landslide debris from the roadway)

reduces resisting forces and can result in renewed slide movements. Excavation of slide debris from the

roadway serves a community need in re-opening roadway access. However, the process of excavation at the

base of the slide mass also increases the risk of additional landslide movement.

Landslide stability could be improved by: (1) reducing gravitational driving forces by removing soil from the

crest of the landslide mass, (2) adding weight to the base of the slide area, (3) improving soil strength at the

base of the slide surface, and (4) improving surface and subsurface drainage of the slide area. Excavation at

the crest of the landslide is likely infeasible. However, the other three stability improvement methods could

potentially be implemented through construction of a toe berm fill (also known as a toe buttress) adjacent to

the east (uphill) side of the road.

Conceptually, a toe buttress would be constructed in short segments by removing slide debris at the south

end of the slide mass (just south of the hairpin curve that was recently buried by slide debris) for a width of

about 20 feet (measured uphill to downhill), a length of about 50 feet (measured parallel to the road) and

extending to about 3 feet below adjacent roadway grade. The excavated area would be replaced with quarry spall rock in a fill that extends perhaps 10 feet above road grade and to the uphill limits of the excavation.

Once this initial section is excavated and backfilled, the process would be repeated by excavating the

adjacent zone immediately north of the backfilled area. This step-wise construction would continue around

the corner where the initial slide occurred in December, for a minimum total constructed length of about

300 feet of toe buttress. The quarry spall rock would add additional weight and soil strength to the toe of the

slide mass, thus improving landslide resisting forces. In addition, the quarry spalls would serve as a

subsurface drain to improve soil drainage within the slide mass.

If a slope repair method is pursued, it is important that the repair be analyzed technically and designed by a

qualified geotechnical engineer and permitted by the local permitting agency. In addition, it is also important

that construction activities be implemented under the full-time observation of the engineer or their designee.

In the short-term, until a formal design can be developed, it is our opinion that a single vehicle travel lane

could be maintained through the affected slide area.

Drainage

There is stormwater flowing into a 12-inch-diameter CMP at the eastern margin of the active landslide

complex. Based on site conditions, it appears that reduction of flow into this CMP can be managed by

removing the concrete culvert to the east and grading the ditch to flow eastward. Water flowing down the road surface is perhaps best managed by constructing a French drain system near

the down-gradient terminus of the 12-inch CMP (north of the hairpin where the road bends eastward) and

tight-lining the outlet to the toe of the embankment. The outlet of the drain should be protected by an energy

dissipation system, such as a properly designed and constructed quarry spall blanket and/or a perforated teestructure.

This drain could be tied into the 12-inch-diameter CMP on the inboard side of the road, which

might require minimal excavation of landslide debris.

The runoff along the remaining segment of road along the toe of the landslide and around the hairpin curve

could be improved by removal of all loose, fine-grained sediment from the inboard ditch line and directing

runoff into an area where it can dissipate instead of flowing directly into the Skykomish River.

Erosion and Sedimentation Control

It is our opinion that the MIRCC should install and maintain adequate temporary erosion and sedimentation

control (TESC) measures. The roadway should be cleaned of muddy debris, since erosion of this material is

delivering sediment and muddy water directly into the Skykomish River. Turbid runoff should be directed to a

temporary sediment pond or to an area where the turbid waters can be filtered by vegetation and sediment

can settle out.

Best management practices (BMPs) and a general description of a TESC Plan may be found in the

Washington State Department of Ecology Stormwater Management Manual for Western Washington

(http://www.ecy.wa.gov/programs/wq/stormwater/manual.html). Snohomish County is the local permitting agency that regulates drainage control, critical area alterations, and

land disturbing activities (clearing and grading}. Snohomish County Planning and Development Services

should be consulted for permitting requirements prior to any land disturbing activity, critical area alteration, or

drainage improvements.

LIMITATIONS

We have prepared this report for Public Utility District No. 1 of Snohomish County and Hatch Associates, Inc.

to help with an evaluation of landslides that have impacted access to the PUD’s real property, electrical

facilities and portions of its proposed Sunset Fish Passage & Energy Project. This document is intended to

provide a preliminary evaluation of stability of the affected area, to provide our professional opinions

regarding the potential for future property damage related to earth movements, drainage or erosion and some

potential options for improvements. Qualified engineering and construction practices can help mitigate the

risks inherent in construction on slopes with landslide hazards, although those risks cannot be eliminated

completely. 

Is this what you call "respectful?"

Bright ideas are a dime-a-dozen and of course we have more than our share of arm-chair experts.

I don't see this as "Respectully submitted" at all. Insulting our intelligence is no way to garner support or create a respectful forum for the members. You need to word your posts more carefully. e.g., "We appreciate all of the suggestions given to us by our members." No agenda. No personal prejudices. You're shooting yourself and the board in the foot with such statements.

~ Daniel Lahey