A Study of Parameters Influencing Efficiency of Micropile Groups

Micropiles support loads individually, as group or as a network. In cases of groups and networks, the micropiles and the surrounding soil will form a composite block to resist the applied loads. This may lead to a group capacity or network capacity that is different from the total capacity of individual piles consisting of the group or network. The group effect of piles in granular soil has been studied by many researchers, the results of which so far have been inconsistent. Therefore, it is difficult in practice to compare the performance of micropile groups from one test program with other, without considering all the factors together. This work consists of a model experimental study of group behaviour in sand. In this paper three important parameters, namely length-to-diameter ratio (L/D), spacing of piles and relative density of sand were considered. For this purpose 12 micropile groups with each group arranged in a square grid of four piles, with centre-to-centre spacing of 2D, 4D, and 6D and with L/D ratio of 20, 35, 50, and 65 were installed in sand bed having relative density of 50%. The group effect of ultimate total resistance on contractive (loose) sand was studied by installing the same number of micropile groups in sand having relative density of 30%. Similarly the group effect on dilative (dense) sand was studied by installing another twelve groups in sand having a relative density of 80%. The model micropile groups were subjected to axial compressive loading. The group efficiency came out to be high at 4D spacing for the entire different L/D ratio for the loose and the medium dense sand. Positive group effect was observed in most cases but in case of micropile group in 80% relative density, mostly negative group effect was observed. Most importantly the group efficiency was found to be a function of L/D ratio, spacing of the micropile groups and relative density of sand.