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.