Hammer engery calibration of standard penetration test

Abstract

The Standard Penetration Test (SPT) is the most widely held in-situ test used for subsurface exploration and investigation. There are number of factors that affect the efficiency of the SPT system. The most dominant of them all is hammer efficiency. This research aims at finding the actual amount of energy transferred by the hammer with respect to the standardized 60 percent.For the purpose of reducing the major variationsof the N value due to large differencesin the values of energy transferred, it is suggested that N value should be changed to specific energy level by the use of certain correction factors. As per this study, these correction factors are different for the type of equipment used.Keeping in view all the previous work done, a system named as Standard Penetration Test Energy Measurement and Analysis Tool (SEMAT) has been developed which givesthe energy ratio of the actual energy transferred to the theoretical energy using accelerometers (for velocity) and strain transducers (for force)which are mounted on indigenously developed instrumented SPT rod. Actual energy transferred is calculated by the SEMATusing force velocity methodwhereastheoretical potential energy (mgh) is calculated manuallyand inserted to the software.Study incorporated field tests behind Civil Engineering wing MCE on different borehole depths. Equipment used in the test was the property of MCE Geotechnical laboratory. Major causes of loss in hammer energy in manual method are sliding of hammer and tilting of SPT rods. With each blow of hammer strike data is acquired from the accelerometers and strain transducers which are connected to compact Data Acquisition device which convert analog signal into digital data and gives the output on tough book in the form of acceleration, strain, velocity and force waveforms.A good data shows proportionality of force and velocity waveforms. Results were compared with the results of Pile Driving Analyzer (PDA) on the same site and the energy ratio of both the tests was almost similar to each other. The average energy ratio of MCE Geotechnical laboratory equipment came around 34 percent against the normalized 60 percent. Which means that it is around 57 percent out of 100 percent. Which means that blow count of every SPT test conducted on above mentioned equipment must be multiplied by 0.57 to get actual count of SPT N value. Which lead us to assume a FOS of around 4 for geotechnical designs based on SPT N value. During the conduct of tests, it was also observed that energy ratio increases with increase in depth up to 10 meters. iiSEMAT is recommended to be used on variety of SPT equipment being in service in Pakistan. For sensitive building designs Automatic SPT method should be used. In future, calibration should be carried out using foiled strain gauges instead of strain transducers and results be compared. Furthermore for depths greater than 5 meter some instrumented assembly could be developed that could be placed just above the sampler so as to get the actual energy transferred in the sampler instead of SPT rod.