Reducing Static Settlement Potential Using Prefabricated Wick Drains – A Case History

Soft fine-grained soils subjected to heavy loads, such as those imposed by rigid structures or fill, have a high potential to consolidate over time. Depending upon subsurface conditions, soil consolidation can take many years to occur. This consolidation results in settlement, and subsequent differential settlement of the ground surface, which can have damaging consequences to structures. To reduce the potential for damage to structures due to consolidation settlement, various solutions have been devised by geoprofessionals. Such solutions include pile foundations, rigid mat/raft foundations, chemical stabilization, and over-excavation/replacement. Another method is the use of prefabricated wick drains in combination with a temporary surcharge load. Wick drains installed vertically in a grid pattern across a project site create small voids within the soil matrix. These voids reduce the drainage distance, allowing excess pore water to dissipate. Decreasing the drainage distance significantly reduces the time needed for consolidation to occur. Wick drains and a temporary surcharge were used prior to construction of a building pad for a large retail structure in San Luis Obispo, California. To construct the building pad, 8.5 feet of engineered fill was placed over a field of prefabricated wick drains. Four additional feet of fill was used as a surcharge load during a period of 69 days. Throughout the approximate 4 month monitoring period during building pad construction, an average settlement of 3.43 inches was measured; with over 6 inches in some locations. The use of prefabricated wick drains and a surcharge load reduced the settlement period from several years to about four months, allowing construction to occur and the retail store to open on schedule