Assessment of methods for construction of an equivalent top loading curve from O-cell test data

This paper presents a systematic review of existing methods (original, modified, and load-transfer methods) for constructing an equivalent top loading (ETL) curve using the results of O-cell tests for drilled shaft foundations. The authors performed parametric studies using load-transfer analyses to investigate the effects of the slenderness ratio, foundation stiffness, and the stiffness of the surrounding geomaterials on the elastic shortening of the foundation, which is one of the key components when constructing an ETL curve. Results from the parametric study showed that the foundation compressibility increased with an increasing slenderness ratio, increasing stiffness of the surrounding medium, and decreasing stiffness of the foundation material. It was further shown that when O-cell tests are performed on drilled shafts with very high slenderness ratios or drilled shafts with moderate slenderness ratios, but installed in a very stiff medium, the elastic shortening must be taken into consideration when constructing ETL curves. Full-scale load test data from projects with both the conventional top-down and O-cell load tests performed at the same site were collected and used to assess the validity of the existing ETL methods through three case studies. Analyses of the case studies suggested that the differences among the three existing ETL methods were not significant in terms of ultimate capacity. However, in terms of head settlement, the original ETL method yielded a significantly stiffer load-settlement response than a conventional top-down load test. In contrast, ETL curves constructed by the load-transfer method or the modified method were both practically accurate enough to estimate the head settlement under the service load.