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In the fall of 1999, Doug McClelland, USFS Northern Region Geotechnical Engineer, implemented an independent peer review of the Aquoneering and Womack proposal. Four independent experts in geotechnical and landslide technology were hired. They included Landslide Technology (Portland, OR), Klohn-Crippen (Richmond, B.C., CN), G.N. Richardson & Associates (Raleigh, NC), and Don Hyndman (Professor of Geology, University of Montana). In these independent reviews, significant problems associated with the surface water diversion and collection system and sediment dam in the Aquoneering/Womack proposal were identified. The major problems included: 1) inaccurate hydrologic estimates for the drainage area, which led to an overdesigned, impractical water diversion and collection system; 2) the sediment dam location at the toe of an active slide mass which could saturate the toe of the slide and potentially increase the risk for unstable conditions; 3) costly and difficult maintenance of the sediment dam; and 4) difficult maintenance of the lateral piping system because of potential ground movement and the high probability of debris plugging the pipe.However, some success was accomplished by the installation of Womack's experimental drainage systems, which were installed in fall 1999. Two prototype elements of their proposed drainage system were installed including 1) a subsurface geocomposite (Eljen) drain above the headscarp of the slide and 2) a geotextile-reinforced embankment and drainage sump. In addition, three surface-water diversion ditches, constructed of geotextile liner and vertical log posts, were installed at the request of the Forest Service to divert surface-water laterally off the slide through the pressure ridge on the west edge of the landslide. During his May 2000 field review, Doug McClelland noted that the experimental drainage systems installed in the fall of 1999 appeared to have some beneficial effect, and removing the surface water with the three diversions through the lateral pressure ridge reduced potential sediment generation by significantly drying out the surface of the landslide.

Rodney Prellwitz, P.E., former USFS geotechnical research engineer, was hired by the Forest Service to serve as a technical advisor for the installation of the monitoring wells. Also, he was to assist with the development and implementation of a monitoring program to better understand the ground water and surface water characteristics affecting the stability of the slide. The monitoring program included installation of state-of-the-art instrumentation to continuously record ground water levels. Monitoring results will be used to determine the effectiveness of mitigation measures to control ground water and surface runoff. For example, following the installation of ground water observation wells, it was noted that the Eljen drain located above the headscarp appeared to have some drawdown effect on ground water levels, observed during the installation of 11 ground water-monitoring wells in July 2000 (Installation Report, Prellwitz, November 2000).

Ground water observation wells were installed in July 2000 using a tripod and motorized cathead to advance the holes by standard penetration test (ASTM D1586) techniques. The drilling equipment was sling-loaded to the top of the slide by helicopter. Holes were cased with 1.5-inch-sch 40-PVC pipe, which was slotted in the anticipated aquifer zone. Continuous monitoring of ground water levels in 10 of the observation wells was initiated in October 2000 and will continue for the duration of the project. Standard penetration tests were conducted from the ground surface to the bottom of each well. Soil samples throughout the entire well depth were collected and analyzed. Depths to ground water also were recorded for each well. This subsurface information helped to more accurately define characteristics of the slidemass failure in the stability analysis. Using this analysis, the critical depth of ground water (at which failure of the slidemass is expected to begin) was determined.

Prellwitz was recruited to advise in the development of alternatives and, eventually, the final drainage system design. He completed a new, more thorough slope stability analysis. The new analysis, based on additional investigation and data from drilling the ground water monitoring wells, reflected a more accurate evaluation of the slide mass geotechnical properties, such as soil shear-strength parameters, density, depth to failure surface, and so on (Stability Analysis Report, Prellwitz, January 2001). He describes the failure mode as primarily translational. The mode is characterized by the downslope displacement of slide mass material moving on a surface that is generally parallel to the general ground surface with little rotational movement. Ground water characteristics of the landslide include both confined and unconfined aquifers.

The resistivity subsurface profiles and hydrographs from the first year of ground water monitoring indicate that more than multiple aquifers are feeding ground water into the slidemass at different locations. The Aquoneering and Womack plan addressed only one unconfined aquifer at the main headscarp. This aquifer would have been drained and collected in a closed piping system running from the top of the slide to the sediment dam near the toe of the slide. Prellwitz recommended installing additional subsurface drains throughout the upper two-thirds of the slide to pull the ground water levels down below the critical level. This would control ground water levels not only at the top of the slide, but also in other locations not necessarily recharged completely by the aquifer located at the top of the slide.

The settlement agreement requires the removal of surface water from the landslide and abatement of heavy sedimentation from McClain Creek. The drainage plan (Drainage Analysis Report, Prellwitz, January 2001) included lined ditches at several locations across the width of the landslide to direct the surface water and sediment outside of the west pressure ridge to the forest floor. A lined ditch was designed over the top of a subsurface drain to minimize the amount of excavation and ground disturbance.

During the environmental assessment process, concerns arose about the potential risk of erosion caused by diverting flows to the forest floor west of the slide. McClelland addressed these concerns in several internal memorandums, including a December 12, 2000, memorandum to Betsy Ballard, Project NEPA (National Environmental Policy Act) Coordinator. He pointed out that the forest floor was already saturated, and mass stability was unlikely to be decreased with additional water because the phreatic surface would not be significantly increased. McClelland also pointed out that the forest floor was very porous and adsorbed water readily, and he hypothesized that the forest floor remained essentially the same from the end of the wet Pleistocene, when the original landslide occurred, creating the large lateral pressure ridges. Thus, the existing forest floor adjacent to the landslide had probably been exposed to numerous 100-year flood events, and many 1,000-year flood events, that generated flows far in excess of the 50-year design criteria for the McClain landslide stabilization project.

Based on initial observations of the experimental drainage systems and recommendations from Doug McClelland and Rodney Prellwitz, the interdisciplinary team selected the proposed alternative that included the installation of subsurface drains to lower ground water levels in conjunction with surface water drainage systems to control runoff and erosion.

The landslide is located in steep, unstable terrain. Because approximately two thirds of the landslide is located above the upper road, the majority of the slide is inaccessible by road. The portion of the slide located between the upper and lower roads (see figure 1) is extremely steep and inaccessible by conventional excavating equipment. After considering the terrain and potential for encountering high ground water, the team decided to eliminate conventional excavating equipment, and instead, use a spider hoe excavator with an experienced operator. All Terrain Excavating from Polson, MT, was selected because of the company's qualifications and past experience working in steep, rough terrain and difficult conditions, including wet and boggy terrain.

Description of Drainage Systems

The project included the installation of ground water drainage systems, surface water drainage systems, and monitoring stations. Geocomposite drains were selected as the subsurface drains because they are more lightweight and more easily transported by helicopter than conventional trench drains (constructed of graded aggregate and perforated pipe wrapped in geotextile). The relative characteristics of geocomposite drains were discussed by McKean and Inouye (Field Evaluation of the Long-Term Performance of Geocomposite Drains, December 2000). In 1999, Womack installed 100 feet of experimental Eljen drain on this landslide. Based on the continuous satisfactory performance, Eljen drains were selected for the geocomposite subsurface drains. On the McClain Creek landslide project, a total of 2,190 feet of Eljen drains were installed at 14 locations. Drain lengths varied from 50 to 260 feet with a typical trench depth of 6 feet (figure 2). In addition, about 2,000 feet of lined ditches were constructed at 9 locations to divert surface water off the landslide. The Eljen drain panels used were 4 feet high by 10 feet wide. They were folded and transported, two panels per package (about 2.5 feet by 1 foot by 4 feet), and weigh about 30 pounds. The heat-sensitive panels had to be stored in a shady location until installation.

Figure 2. Installation of Eljen
drains in depositional zone.

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