1209 MADRONA AVE 2016-10-05 - Laserfiche WebLink

Geotechnical Engineering Evaluation NGA File No. 870113 Packer Property March 11,2013 Retaining Walls and Steep Slope Evaluation Page 4 Everett,Washington We measured slope inclinations below the lower western tier of the walls in the recent landslide area to be approximately 20 to 60 degrees (36 to 173 percent), as shown on Cross-section A-A" in Figure 3. The landslide exposed undocumented fill directly below the wall closest to the slide scarp with dense sand and silt soils within the lower portions of the slope. Debris from the recent landslide was deposited at the toe of the slope near the railroad tracks. The remaining portions of the steep slope are vegetated with young to mature trees and dense underbrush. We did not observe significant indications of recent instability within the steep slopes outside of the landslide area. However, we did observe groundwater emitting from the face of the slope near the walls along the western property line. We were informed that this groundwater is generally observed year round within this area. There are five tiers of concrete block retaining walls that were constructed along the steep ravine slope to the northwest of the existing residence. The blocks used for the construction consisted of Diamond Blocks. The walls range from approximately 3.0 to 6.0 feet tall. We understand that the lower two wails were founded on reinforced concrete foundations with 6-inch diameter, 4.0-foot deep concrete piers that were placed at approximately 3.0 to 4.0 feet on-center. Robert Packer informed us that the upper three walls were founded on the native soils. We understand that an 8-to 10-inch crushed rock drainage layer along with a 4-inch perforated corrugated drain was placed behind each of the walls. These wall drains along with drains associated with the residence were tightlined through a series of pipes to daylight on the steep slope below the lower walls. A gutter was also formed into the foundation of the lower western wall with a catch basin. During our site visit,we observed groundwater emitting from the slope along the western property line that was captured and routed to this catch basin. These drains from the wall and catch basin then descend down the slope and outfall near the toe of the steep slope along the railroad tracks. During our site visit on February 1, 2013, we heard water flowing through these pipes and observed water discharging from the pipes near the toe of the slope. The remaining portions of the wail were backfilled with on-site soils. Subsurface Conditions Geology: The geologic units for this area are shown on the Geologic Map of the Everett 7.5-Minute Quadrangle, Snohomish County Washington, by James P. Minard(U.S.G.S., 1985). The site is mapped as Qtb (Transitional Beds). The Transitional Beds unit is described as a deposit of clay, silt,and very fine sand. Our explorations encountered silty sand and silt generally consistent with the description of Transitional Beds. NELSON GEOTECHNICAL ASSOCIATES, INC.