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Embankment preloading, in conjunction with prefabricated vertical (PV) drains, was used to accelerate primary consolidation and to eliminate most of the anticipated secondary settlements of the soft clay foundation due to loading from a proposed wastewater treatment plant (WWTP). Subsurface exploration conducted during preliminary design indicated the presence of up to 60-m thick deposit of soft to very soft clay underlying the site. Estimates of settlement due to filling of the existing lagoon and construction of the WWTP indicated that the soft clay deposits will undergo up to 150 cm of settlement. Approximately 80 cm of this settlement was estimated to occur in the first 10 years of the plant operation and an additional 60 cm was estimated to occur during the following 30 years. UP to 19 settlement plates were installed under the embankment fill to monitor the preload performance. The measured settlements were then used to analyze the behavior of the soft clay foundation, and to develop recommendations regarding the time for preload removal and subsequent construction of the proposed WWTP. This paper discusses the results of settlement analyses and highlights the procedure that was used to predict the magnitude and time-rate of settlements based on field measurements. I N T R O D U C T I O N The city of Chewelah is located approximately 75 km northwest of Spokane, Washington along U.S. Highway 395. As a part of its ongoing improvement, the City has proposed the construction of a new wastewater treatment plant (WWTP). The new facility will consist of biological secondary treatment system providing nitrification, a disinfection facility, sludge handling and treatment facilities, headworks, a laboratory building, and related electrical and instrumentation control equipments. The proposed WWTP will be constructed by filling an existing sewage lagoon (called Lagoon #3) by as much as 2 m of soil to bring the grade to the current top elevation (El. 500 m) of the perimeter dikes. The lagoon covers and area of roughly 90 m by 135 m (see Figure 1). From the as-built drawings provided by the City, it appears that Lagoon #3 was constructed by building the perimeter dikes 1.5 to 2.4 m above the original ground surface. tProject Geotech. Engr., CH2M Hill, Bellevue, WA; 2Sr. Geotech. Engr., CH2M Hill Inc., Corvallis, OR; 3Project Manager, CH2M Hill Inc., Spokane, WA; 4principal, Campbell Services Group, Spokane, WA; SSr. Geotech. Engr. and Staff Manager, CH2M Hill Inc., Bellevue, WA; 6Senior Geotech. Engr., CH2M Hill Inc., Bellevue, WA; 7President, Drainage and Ground Improvement Inc., Bridgeville, PA |
