An Alternative Approach To Assessing Post Construction Settlement With Incomplete Preload Settlement Data

Soft clays are commonly encountered in construction projects undertaken in coastal alluvial environments and can undergo large settlements when subjected to vertical stresses larger than previously experienced. Building on soft clay sites typically requires geotechnical assessment and treatments to manage the risk associated with excessive total and differential settlement resulting from primary consolidation and creep mechanisms. Preloading or surcharging of soft clays is a common treatment used to induce the majority of primary consolidation settlement prior to construction, and to reduce the magnitude and variability in long-term settlement post-construction. Review of monitoring data captured within the preload or surcharge period is normally necessary to assist in the prediction of post-construction settlement over the design life of the development. The monitoring data is typically used to identify when the surcharge can be removed, allowing a prediction of the long-term settlement based on key parameters including the achieved over- consolidation ratio (OCR). This paper presents an alternate approach to assessing post-construction settlement when settlement monitoring data is unavailable or incomplete. The method relies on in-situ Cone Penetration Test (CPTu) data to estimate the pre-consolidation pressure of the soft clay deposit under the surcharge load and a calculation to consider the resulting OCR after surcharge removal. The resulting values of OCR are used to assess reduced creep settlement rates based on published information to predict the post-construction settlement. The method is demonstrated with data from a site in south-east Queensland.