Standard Penetration Test: A Comparative Analysis Between Auto Trip & Manual Rig Systems

Abstract

The Standard Penetration Test (SPT) is one of the widely used tests for subsurface exploration in Bangladesh. Generally two types of SPT rig systems are used, namely manually operated and auto trip systems. Most of the SPT is performed by a manually operated SPT rig system where keeping the standard free-falling height of the SPT hammer is quite difficult. In addition to that, nonstandard equipment and operational procedure, unskilled rig operators, and absence of regulatory bodies, the quality of the SPT data is unreliable in Bangladesh. In this study four manually operated and four auto trip SPT rig systems were examined at eight different sites. Then both systems were compared with respect to the ASTM standard in terms of hammer weight, dimensions of the split-spoon sampler, free-falling height of hammer, and energy efficiency. It was found that an auto trip hammer weight is 3.31 lb and a manually operated hammer is 7.4 lb less than the standard weight while the remaining six hammers weigh within the standard range. The length of the split barrel of all sites were found within the standard range. The inner diameter of a split barrel was found 5.27 mm less and outer diameter of three split barrels were found 1.62 mm, 1.82 mm, and 1.45 mm larger than the standard diameter. The thickness of a driving shoe was found 1 mm less and three driving shoes were found 0.49 mm, 0.62 mm, and 1.29 mm greater than the standard thickness respectively. The inner and outer diameter of five and six driving shoes were found greater than the standard diameter. The inner and outer surface of the split spoon samplers were found rusty in different digress. In case of manually operated rig system total 1252 hammer blows were recorded where only 468 blows were released from the standard free-falling height of 30 inch, which is 37.4% of total blows and 784 blows were released from a height either greater or lower than the standard free-falling height of 30 inch, which is 62.6% of total blows. In case of auto trip rig systems total 643 hammer blows were recorded and the standard free-falling height of 30 inches were maintained. It was found that the hammer energy transferred to the anvil for manually operated and auto trip rig systems varies between 99.4% to 99.5% and 89.4% to 113.7% respectively. The nonstandard practice is higher in case of manually operated rig systems as compared to the auto trip systems. Nonstandard practices are responsible for over/under estimation of soil’s strength. Due to the unreliability of the SPT data, the projects are often over designed, hence uneconomic.