Proceedings-DFI/EFFC 11th International Conference on Piling and Deep Foundations, 2014, Stockholm, Sweden, (DFI)
Comparison of α and β Methods with CPT and Pile Load Test Data: A Case Study with Attention on Soil Setup Effects
Kazem Fakharian and Mohammadreza Khanmohammadi, Amirkabir University of Technology, Iran
In driven piles, the gradual dissipation of excess pore water pressure and consequently the soil setup significantly contribute to increasing the pile capacity with time. The question is risen that how the scale differences affect the stress state variations around the pile shaft and at the pile tip? Would the CPT predictions better correlate with End-Of-Drive (EOD) or Beginning of Restrike (BOR)conditions? On the other hand, are the proposed α and β parameters for clayey soils mostly predicting the EOD or BOR situation? The main objective of this paper is to back calculate the α and β parameters from CPT profiles, pile Dynamic Load Tests (DLT) at the EOD and BOR as well as Static Load Tests (SLT) and evaluate how they compare with those suggested in literature. Four precast 400 mm square concrete piles driven in North Azadegan Oilfield in Khuzestan Province were selected for the comparisons. CPT test results were available from the extensive geotechnical investigation at the study site. DLT tests using PDA were carried out at EOD, 1 day and 14-day BOR on all the 4 piles. All the piles were SLT tested as well. The UniCone software was used to estimate the tip and shaft resistances using 4 methods including the Dutch, Eslami & Fellenius, LCPC and Schmertmann. The results indicate that the α-method based on the proposed values in literature has predicted a shaft capacity of 90% of the 14-day BOR. This is while the β-method has predicted a shaft capacity equivalent to 1-day BOR results only. Therefore, both methods have under-predicted the shaft capacities compared to real capacities measured after the significant soil setup in the study area. Two of the CPT methods have underestimated, one has overestimated and one has matched the estimated 60-day capacity.
|article #1898; publication #100 (IC-2014)|