Hyperbolic method for settlements in clays with vertical drains

Abstract

Estimation of the total primary settlement is critical in determining when the required compression has been achieved and a surcharge loading can be removed. Conventionally, the total primary settlement is predicted using laboratory-determined compressibility data. However, the rectangular hyperbolic-fitting method allows estimation of the total primary settlement during the process of consolidation. In this paper, the hyperbolic-fitting method is extended to the case of consolidation with combined vertical and radial flow, which occurs when vertical drains are used to speed consolidation. Solutions are presented for the slope of the first linear segments between the 50 and 90% settlement points of the theoretical hyperbolic plots (time/settlement versus time). The proposed hyperbolic-fitting method was tested using data from the case history of the Changi Airport field test in Singapore, where Geodrains and sand drains were used for soil improvement following land reclamation and prior to airfield pavement construction. The predictions of the proposed method agree well with the measured and estimated soil settlements from compressibility calculations. The method can complement existing methods to provide a practical means of monitoring the progress of consolidation in field applications using drains and surcharge, and to determine the appropriate time for surcharge removal, if required. Key words : consolidation, vertical drains, settlements, hyperbolic method.