Surcharge Fill and Vertical Drain System Improves Soft Clay Site

The Intermountain Power Project in Delta, Utah required construction of a Railcar Service Center near Provo, Utah. Preliminary investigations related to the design of the proposed facility revealed the presence of soft clays at the site adjacent to the foothills of the Wasatch Mountains. Detailed geotechnical laboratory and field investigations were performed using relatively undisturbed California samples, thin-walled Shelby tube samples, and cone penetration test soundings. The detailed investigations revealed the presence of a significant layer of deep, soft varved clay. This soft clay layer originated from alluvial deposits from the old Lake Bonneville. Because of the need to develop the site to substantially higher final grade elevation, which in turn necessitated the placement of supplemental fill material, the project team decided to use a preload/surcharge fill (comprised of soil to be used in the ultimate site development) and prefabricated vertical drain system designed to mitigate long term settlements for the facility. The impetus for adopting this design approach was to use shallow foundation systems for the project, as opposed to a more expensive foundation system consisting of driven pile foundations. The designers used two basic approaches to the ultimate development of the site as impacted by the underlying soft, compressible varved clay. 1. Laboratory triaxial strength testing and cone penetration test (CPT) sounding were used to develop a site-specific shear strength profile. 2. One-dimensional consolidation testing, including time rate analyses, was used to define a past pre-consolidation confining pressures and the anticipated virgin compression parameters. The overall preload system design accounted for the rate of fill installation, global stability of the fill/natural soil profile, and anticipated internal drainage conditions of the varved clay. Design predictions related to magnitude, rate and duration of the preload settlement are compared with actual settlement measurements of the fill and deeper, intermediate strata within the varved clay. The magnitude of settlement related to the surcharge fill approached 7 inches. An ancillary and coincident activity related to the preload activities included the performance and interpretation of a pile load test installed into the varved clay profile. The strength profile developed for project design is also used to analyze the load transfer mechanisms during the pile load test, which also provided an understanding of the ultimate pile capacity as defined by the load test