Comparison of Ultimate Pile Capacity Based on Theoretical and Pile Load Test Methods
Abstract
The estimation of axial load carrying capacity of bored piles is a complicated problem because it depends upon number of factors which may include boring method, concrete quality, concreting method, experience of the staff and the ground conditions. Validation of pile design through testing of piles either test piles or working piles is considered an essential part of deep foundation design. This is recognized as being the most reliable means of dealing with the uncertainties that may rise during design and construction phase of pile foundations.
In this paper, different theoretical methods have been used for the evaluation of pile capacities and then their comparison with the pile load test evaluations have been made for the project of Rathoa Haryam Bridge being constructed in Mirpur Azad Jammu & Kashmir at Mangla reservoir. The subsoil at the bridge site consists mostly of lean clay with ground water table at a shallowest depth of 0.3 m below NSL during the period of field investigations. Six pile load tests have been conducted on test piles with length in the range of 35 m and 45 m and with diameter of 1000 mm.
Based on pile capacity analysis, it has been revealed that the theoretical method for pile capacity estimation by NAVFAC DM 7.02 gives 20% to 40% lesser pile capacity relative to the pile capacity evaluated from pile load test data. The SPT method proposed by Decourt gives higher pile capacity relative to the pile capacity derived from pile load test data and therefore, it is not recommended to be used for cohesive soils. The CPT based methods suggested by Schmertmann and Philipponnat give close results to each other but with a difference of 10% to 20% relative to the pile load test interpretation and may be considered reliable methods for cohesive soils. Pile capacities from the pile load test interpretation by Limit Value method and 90% Hansen method match closely whereas the 6 mm net settlement method gives slightly lower estimate of pile capacity as compared to the other load test methods. The best approach for pile capacity evaluation from pile load test data is to use the average value of pile capacity of all the load test interpretation methods.
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