Surcharge Embankment on Marine Clayey Silt Case Study and Lessons Learned

This case study summarizes an extensive surcharge monitoring program for a project in Juneau, Alaska. The project required working in the inter-tidal zone to facilitate the expansion of the existing uplands area for future development. The geotechnical exploration encountered a deposit of soft, low plasticity clayey silt of variable thickness over a sloping bedrock profile. The preliminary stability analysis performed as part of the geotechnical report determined that the low undrained shear strength was inadequate to support the total new embankment height. The geotechnical recommendations consisted of a temporary surcharge embankment constructed in stages to induce consolidation and increase the strength of the clayey silt prior to construction of the permanent embankment. Due to the thickness of the clayey silt layer, prefabricated vertical drains (PVDs) were installed across the site to expedite consolidation and strength gain, as well as to minimize and dissipate excess pore water pressures. An extensive instrumentation monitoring program was implemented which included inclinometers, piezometers, and settlement plates to monitor and evaluate horizontal and vertical deformations of the embankment and development of excess pore pressure during and after surcharge placement. The monitoring program was developed to ensure that excess pore pressure was minimized in the clayey silt. The results from the monitoring determined that consideration of construction sequencing and fill placement, coupled with drainage from the PVD would have yielded an improved design over the “wish-in-place” alternative. This paper summarizes the lessons learned from geotechnical site conditions, embankment stability analyses, PVD design, instrumentation installation and data reduction, prediction and observation methods for settlement, and general comments regarding the staged construction procedure.