Effects of drop energy on SPT value of soil in Bangladesh context

Abstract

Standard Penetration Test (SPT) is the most popular subsurface investigation test for foundation design in Bangladesh as well as in the whole world. Unfortunately the SPT values are not unquestionable in Bangladesh because of non-standard parameters that are being in practice here. There are numerous factors considered by ASTM that effect the value of SPT. Some of these factors include the drill stem length and cross section the type of anvil, the blow rate, the technique of operation, the alignment and type of hammer, the use of liners or borehole fluid etc. Of all of the documented variables the hammer type is perhaps the most significant parameter due to its variability in energy delivered to the drill rods. Researchers have shown that energy transfer efficiency can be 30% to 90% depending on the type of hammer used. In Bangladesh, majority of the SPT is done manually. A hammer is attached with a rope. The workers pull the rope and lift the hammer by 30 inch or so and release it. The hammer hits the drilling rod and the attached split spoon sampler is penetrated. Human error is involved in this system. Moreover, number of turns in the pulley varies from site to site. Thus SPT remains in a very nonstandard form in Bangladesh. No previous study is reported to find the efficiency of energy transfer of these systems that are being used in Bangladesh. In very few cases use of auto trip hammer has been reported. In this study SPT Analyzer was used with PDA Machine to observe the efficiency of the hammer system. Two feet long NW-1 drill rod section, hand-held unit and the software for evaluating the data were used. Foil strain gauges were glued directly onto the rod to measure strain, which was converted to force by using the cross sectional area and modules of elasticity of the rod. The acceleration measured by the accelerometer was integrated to obtain velocity. The instrumented rod was affixed to the top of the drill string just below the anvil during testing. Standard Penetration Tests were conducted in three selected locations of Dhaka city, using both by Manual and Auto trip hammer. The sites were Rajarbag Police Line, Dhaka, Sarkari Shishu Paribar, Tejgaon and Business studies faculty, Dhaka University. The test data showed that the energy transfer ratio (ETR) in automatic hammer was around 40-70% that might be considered within the range of standard 60% efficiency. The ETR in manual hammers were found to be around 33-66% and the range was found inclusive of standard 60% efficiency. In case of auto trip hammer, ETR was found inversely proportional to SPT which was indicated by ASTM D4633-05.In case of manual hammer ETR was found directly proportional to SPT. As expected, better consistency of ETR was observed for auto trip hammer as compared to manual hammers.