On February 28, 2001, Northwest Washington experienced a powerful 6.2 earthquake centered in the lower Puget Sound basin. This caused damage to a large area of Western Washington. Included in this damage was an earthen berm located at the Indian Ridge Correctional Facility outside of Arlington, WA. The berm, which measured ~50′ x 35′, revealed settlement of up to 4″ on the downsloping face of the berm. The Washington State Department of Natural Resources hired Merit Engineering to evaluate the berm. Included in this evaluation was a visual site reconnaissance, a topographic survey of the berm and adjacent areas, and an investigation of the subsurface soil and groundwater conditions. With all of the information gathered, a conclusion could be made as to the cause of the failure, the extent of the failure, and possible remedial techniques that could be applied to the berm.
The berm was constructed across a small stream valley. The unnamed stream is about 2′ wide and averages 4″ deep. No salmon have been seen using the stream, however, residents of Indian Ridge use the stream for a volunteer hatchery release. A culvert is used to dissect the berm and allow the stream to remain free-flowing through the berm.
Three test borings were drilled in the vicinity of the berm to identify the subsurface conditions. The results of these borings confirmed the cause of the slope failure. The borings revealed that the berm was filled with unsuitable fill material. Inconsistent types of fill were used throughout the structure of the earthen berm. Soft sand and clay, silt and gravel were used in different depths throughout the berm. No part of the berm was constructed with engineered or structured fill. The result was a soft, loose, moist berm that failed upon stress due to the earthquake.
Merit Engineering concluded that the berm was failing and that the fill was unsuitable for use, thus should be removed and the berm be rebuilt. A plan was devised by Merit Engineering that would ensure a safe, long-lasting, secure, earth berm. The plan included specifications for removal of the existing berm and for the construction of the new reinforced berm. Structural fill, geotextile materials (Geogrid), excavation and backfill specifications, culvert replacement, stream simulation, erosion control, and compaction criteria were all incorporated within the new plan. Culvert replacement included the new culvert size and material used, as well as recommending that its installation be done according to WSDOT Standard Plan No. B-11 and WSDOT Standard Specifications 7-08. The grade of the culvert and the stream simulation gravel used inside of the culvert were specified in the plan as well.