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The AAPG/Datapages Combined Publications Database

Utah Geological Association

Abstract


Environmental and Engineering Geology of the Wasatch Front Region, 1995
Pages 349-358

Construction on Soft Clay Using Wick Drains and Preload - A Case History

Russell L. Owens

Abstract

This paper presents a case history for foundation improvement and placement of two steel, above-ground tanks for waste-water storage in the Salt Lake City area. The site vicinity is surrounded by ponds or reservoirs and is underlain by approximately 40 feet of soft, compressible clay. The proposed location for the tanks had previously been used as an aeration pond for waste-water treatment. Site development required: (1) dividing and draining the aeration pond, (2) determining adequate setback distance of the tanks from adjacent dikes, and (3) improving the foundation to minimize tank settlement. In order to meet regulatory time constraints, a condensed construction schedule was required which limited options for foundation improvement. Final design included using granular structural fill as a preload and wick drains to enhance drainage and consolidation.

The aeration pond, measuring 300 feet long by 250 feet wide by 10 feet deep, was divided into two sections by an earthen divider dike. The northern section of the pond was drained and the bottom sediment was removed. Two feet of sand was placed in the bottom of the drained section of the pond and wick drains were installed on 10-foot centers to a depth of 40 feet. Twelve feet of granular structural fill was placed over the wick drains to achieve the preload and desired grade for tank construction. Stability analyses were performed to determine appropriate tank set-back distances from nearby dikes. Areal settlement resulting from fill placement was predicted and the time for consolidation due to fill placement was assessed. Additional settlement due to tank loads was also evaluated.

Settlement monitors were installed beneath and extending up through the fill to document actual versus predicted settlement. Following construction, tank settlement was also monitored.

Final tank placement included a 40-foot tank setback from adjacent dikes. No signs of dike instability were noted resulting from site improvement or subsequent construction. Total recorded settlement resulting from fill and tank loads was approximately 21 inches. Predicted settlement ranged from 26 to 29 inches. Tiltage of tanks towards each other was minimal.


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