540 ALVERSON BLVD 2017-06-20 - Laserfiche WebLink

amec19 landslide scarp. According to the geologic map, soils at the site consist of Advance Outwash (Qva) overlain by Glacial Till (Qvt). Boring B-1 encountered about 14. 5 feet of medium dense, brown-gray silty fine sand with trace iron-oxide staining and a 4.5 foot layer of very stiff gray silt interpreted to be Glacial Till. Below the silt, a 10-foot-thick layer of very dense, gray, silty fine to medium sand was observed and also interpreted to be Glacial Till. Below the Glacial Till, a very dense, brown-gray fine to coarse sand with varying amounts of silt was encountered and interpreted to be Advance Outwash. Perched groundwater was encountered in boring B-1 at a depth of 37.5 feet at the time of drilling (January 2008). Our visual observations of the slide scarp confirmed the subsurface descriptions above. Specifically, we noticed the upper 2 feet consisted of fill, topsoil and bark mulch placed over black plastic tarp. Below the tarp, was loose to medium dense, light brown silty sand, interpreted to be weathered glacial till. Approximately 10 feet vertically down the slope, we observed a tan silt layer at least 5 feet thick. Slight seepage made the surface of the silt wet at the scarp. Slide debris covered the surface below this level, but hand digging encountered dense, gray to light brown, silty sand. The enclosed Geologic Cross-Section A-A' (Figure 2) illustrates our stratigraphic interpretations at the landslide. CONCLUSIONS AND RECOMMENDATIONS In order to stabilize the slope and restore it to near its previous condition, we recommend constructing a reinforced soil slope (RSS) with the proprietary flexible geocell facing known as Geoweb. The Geoweb facing will provide stability and allow vegetation to grow on the surface. Based on topographic information and embedment requirements, the slope height is expected to be about 15 feet. The paragraphs below present our design and construction recommendations and the enclosed Typical Geoweb Reinforced Soil Slope Detail (Figure 3) illustrates our design recommendations. Slope Analysis: We evaluated global stability of a representative cross section using the computer program SLOPE/W. Initially, in order to calibrate the model, we estimated the topography of the slope prior to the landslide, and then adjusted the soil strength properties and groundwater levels until a slip surface shaped like the actual landslide had a factor of safety near 1.0. Then we used the same soil strength properties to model the slope with the Geoweb reinforced slope repair. The groundwater level was lowered due to the drainage blanket that is part of the designed repair. The designed Geoweb reinforced slope has a calculated slope stability of 1.6, which is better than typical design values of 1.25 to 1.5. Slope stability figures displaying the results are attached with the calculations. RSS Components: We recommend that stacked perforated Geoweb cells (GW30V) be used as facing for the slope. Each cell is 6 inches high with an expanded width of 11 to 14 inches and expanded AMEC Project No.3-917-17245-A 3 W1_Projects\17000s\17245 Alverson-Maul sby Bluff Property LLC\17245-A\Geotech Report\Finley Geotech Report 130722.docx