Comparative Study on the Use of Reinforced Concrete Piles and Rock Columns in Weak Foundation Strata in Zambia

Abstract

All structures require transfer of loads to the competent strata. Where less competent soil strata is encountered, the use of either pile foundations or ground improvement methods is adopted. In Zambia, reinforced concrete pile foundations are the commonly used deep foundations since, to transmit loads to stronger strata by utilizing foundation soil skin friction, end bearing or both, while also guarding against differential settlement. Unfortunately, this has proved to be an expensive construction method and it requires the use of materials processed in high energy consuming industries. Stone columns, among many techniques, are one of the most commonly used soil improvement techniques. Although this ground improvement method is considered to be relatively cheaper than reinforced concrete piles, as it only requires the use of coarse aggregates that are widely available in Zambia, it is yet to be employed in the Zambian construction industry. As such this research aimed at comparing the use of the two construction techniques by using Finite Element Method (FEM) Analysis Software Plaxis V20 (2D and 3D) under similar loading conditions and soil properties (Lusaka West, Zambia). The resulting models and simulations were used for comparison of the various parameters which included axial loading (and lateral bulging for stone columns) for skin friction characteristics of the models. In the case of stone columns, the effects of internal angle of friction, spacing and surcharge settlement behaviour were taken into account to come up with the best performing stone column model. For calibration and validation purposes, a number of physical experiments were modelled and simulated so as to produce models that matched their predecessors as much as possible, then the soil properties were adjusted to match those obtained from a local site in Lusaka West, Zambia. Based on the results, concrete pile foundations were found to endure a larger loading before attaining the ultimate failure as compared to stone columns, but the serviceability loading state of stone columns was quite similar to that of the concrete pile foundation. Hence for structures requiring relatively low serviceability loading state on relatively large ground surface area, stone columns proved to be a reasonable construction alternative for pile foundations.