AN IMPROVED EFFICIENY OF DROP WEIGHT SYSTEM IN HIGH STRAIN DYNAMIC LOAD TEST

High strain dynamic load test (HSDLT) is commonly used in testing for displacement piles. Recently, it has also increasingly used as an economy method to evaluate the capacity and integrity of cast-in-situ foundation system. To get a reliable pile analysis, a good quality stress-wave signal in HSDLT is utmost important. Secondly, an adequate stresses mobilization along the pile shaft and the toe is also very critical for the capacity determination. A proper selection of drop weight system for HSDLT on drilled shaft is one of the key components so as a uniform and sufficiently large impulse can be impacted on the pile head to sufficiently mobilize the pile-soil interaction behaviour. The typical conventional drop weight system in HSDLT engages a free-fall ram with a rigid guide frame. However, several studies reported a large variation of energy transfer efficiency (ETR) from the free-fall ram onto the drilled shaft. The ETR can be ranged from 3%-98% due to varies reasons. An uneven ram impact could be one of the possible reasons that causes a low ETR even with the best practice to align the verticality of the rigid guide frame with the pile head. Paikowsky (2004) conducted a series of numerical simulation to study the effect of a tilted ram impact, at very small angle of 1o, onto the pile capacity determination. The study suggested that the induced stress-wave can only become uniform after a distance of 2 to 3 times of pile diameter (D) below the pile top. This paper presents a new drop weight system for HSDLT that was innovatively designed with a hydraulic-lifted modular ram built on four independent automatic self-adjusted outrigger system. It enables the almost perfect vertical alignment of the ram to the axial direction of the pile head. Based on the compiled case histories, the conventional drop weight system registers an averaged ETR of 39% associated with a standard deviation of 19.9%. As a comparison, this new drop weight system shows a remarkable y improved ETR of 56% associated with standard deviation of 8.2% after minimizing the effect of uneven ram impact