Hammer Energy Ratios of Different Spt Setups Using Indigenous Hammer Energy Measurement Apparatus
Standard Penetration Test (SPT) is carried using different components and lifting mechanisms, which causes variation in energy efficiency and the inverse relationship between energy transferred to drill rod and N value was found. It was later accepted that N values should be normalised to a standard energy ratio of 60 %. Energy measurement has been a part of SPT procedure in countries like US, UK, and Japan since the standard guidelines in these countries recognises the importance of knowledge of energy transfer efficiency of SPT setup to use N values for important projects. Indian SPT practices show that energy measurement during SPT is still very uncommon, as IS 2131 does not recommend the same. Donut hammer with manual lifting is most commonly used in India, which is reported to have the highest operational variability. ASTM D1586 suggests that hammer lifting systems with spooling winch operated manually have very low energy ratios (Er), this is also one of the most common hammer setups used in India. To account for variation in energies development of indigenous energy measurement apparatus – SPT HEMA (Standard Penetration Test Hammer Energy Measurement Apparatus) was initiated in 2013 and updated in 2016, but this setup was not convenient for energy measurement on actual field SPT tests. Hence, this study was carried with the aim of modifying the existing version of equipment to be used effectively in the field testing and taking field energy measurements on SPT setups used widely in India. Energy measurement was carried on sixteen SPT setups, tested on 30 boreholes with 144 SPT tests at various depths and soil types. The energy was measured at all depths on some boreholes, and for few boreholes, it was measured only on selected depths. These two measurement methodologies were intended to study the effect of different rod lengths and different setups on Er values. Three distinct types of hammer setups were observed during this study, of which HRL(Hydraulic drill rig) and RRL (Rotary drill rig) consist of manually operated spooling winch system as a hammer release mechanism, and other RRH (Rotary drill rig with rope and pulley) comprised of rope and pulley operated manually. Field SPT energy measurements were made by recording force and acceleration time histories of hammer impact using SPT HEMA instrumented rod and data logger. Acceleration time history was integrated to get velocity time history by writing Matlab code and further integral of Force (F) and velocity (V) time histories, i.e. FV result gave energy (EFV) of each blow. Velocity was subjected to a linear baseline correction to remove minor noise in acceleration data. This procedure of energy calculation is as per FV method of energy calculation suggested in ASTM D4633. The ratio of EFV and theoretical potential energy of the hammer is expressed in percentage and called Energy ratio (Er). Er values from field SPTs show that setups with spooling winch have lower mean Er, for HRL setups it is 26.68 % and for RRL setups it is 31.84 %. whereas RRH setups work at an Er of 46.58 %. For one test Er was measured using SPT HEMA and PDI SPT Analyzer connected together in one drill rod assembly, where the SPT setup was HRL setup. SPT Analyzer reported mean Er of 27 % which matches with the result of SPT HEMA which is developed indigenously at a comparatively lower cost. Details of hammer setups such as hammer and anvil dimensions and weights, hammer blow rate, combinations of rod lengths, soil type, N etc. were noted during field tests, and their effect on energy ratio was studied. Although FV method considers the integration of F and V over full record time, energy transfer pattern is influenced by soil resistance for short rod lengths. It was found that Er values do not show any correlation with rod length. Furthermore, the effect of soil resistance is negligible for rod lengths >15 m. Effect of hammer-anvil impedance ratio on Er was found to be matching with the theoretical trend. Hammer blow rates were calculated based on videos recorded during field tests. Blow rates from all the field tests were found to be in the range of 10 to 25 blow/min, which is lower than the range 30 to 40 blows/min, specified in IS 1893 for liquefaction assessment, and other international literature. Recording of hammer blow rates is not a part of field SPT procedure, but blow rates are an important factor to be known in liquefaction assessment. Few Laboratory tests were also performed where SPT like condition was recreated and sampler was inserted in a cylinder filled with soil and hammer dropped from a loading frame. Lab tests show that larger and heavier anvils reduce the Er, this is in agreement with previous findings and also in more recent specifications by ASTM D1586.
- Civil Engineering (CiE)