ANALYTICAL METHOD FOR CHECKING STABILITY IN DEEP SOIL PILE BLOCK UNDER STATIC AND EARTHQUAKE LOADING IN A SOIL STRATA

Abstract

Geological stratification of Indian subcontinent is composed of formation of Rocky strata in the central, western and southern part of the sub-continent and a vast area beyond this upto Himalayan foot hills in the north and eastern part of the country including part of Bangladesh up to Burmese Plate on the east is deep sedimentary strata of varying depth up to 20km till it meets the top of the Indian Plate. Especially Gangetic plane of eastern India and Bangladesh is having a soft clayey strata upto 50m from ground level which is inadequately strong to bear heavy structural load more than 50metric ton/m2 coming from Nuclear Plant Buildings and Multistoried residential structures. With the use of technology for precast and cast in situ Reinforced Concrete Piles in this region of the subcontinent, the problem of low bearing capacity of founding strata was solved majorly. But designers were looking for a technology improving the strength of the soil instead of transferring the load over RCC Piles keeping the adjoining soil strata unchanged. There came this technique of deep soil-pile which is basically auguring maximum 1.0 m dia soil column up to a depth of 25-30m and mixing with cement slurry simultaneously so that a soil-cement piles of length 25-30 m with a minimum compressive strength of 2Mpa is developed in this engineered cement-soil pile (C-S Piles). A pattern of C-S pile-group is developed in both horizontal axis keeping a center to center distance of 75 cm between 2 adjacent piles center which overlaps each other by 25 cm and behaves like bonded pile block. Outer dimension of the pile block containing 36 overlapped C-S piles becomes 4.694mx 4.694m. Therefore, to make this geometry little more rhombic the engineered C-S pile block was optimized to a plan dimension of 5mx5m and the merged rhombic pile block was conceived to be a rhombus of dimension 5mx5mx25m(Fig-1&2). The other elements of the geometric model are the boundary soil which is original which encircles the C-S Pile block and transfer lateral soil pressure on the C-S Pile block in a hydraulic pressure mode. The model is loaded with a Live Load of 60 Metric Ton/m2 as a static Load. Additionally, an earthquake load in terms of acceleration time -history is also applied at the bottom of the boundary soil using program Ansys 2021vers R1. The size of the boundary soil 50mx50mx35m was made to see that the earthquake wave doesn’t travel back from the boundary. In this paper, the author has tried to investigate how the Deep Soil pile block is transferring the imposed load to a deeper subsoil layer while interacting with the boundary soil medium which is present all around the pile block with a gradual increase of compressive strength of the C-S Pile block from 2Mpa to 15Mpa. Further this approach was modified by inserting a RCC foundation plate of plan dimension 1mx1mx0.25m over which the imposed load was placed instead placing directly over the pile block. The RCC foundation plate was planned to have a high stiffness of grade M25(cube strength 25Mpa). Pile block 5mx5mx25m is embedded in boundary soil of 50mx50mx35m.