Analysis of Soft Ground Reinforced by Columnar Inclusions

Abstract

A general method of analysis to solve an important class of problems encountered in the field of geotechnical engineering is developed in this dissertation. The analysis is formulated from the fundamental equation of equilibrium. It leads to develop a simple integro-differential equation to characterize the overall behaviour of the system. The behaviour of rigid columns (e.g. concrete/timber piles, lime/cement columns), deformable columns (e.g. stone columns/granular piles, sand compaction piles), pile groups, pile-raft foundations are the type of situations that can be analyzed by the proposed method. A numerical scheme using finite difference method is proposed to solve the governing equation with the aid of the. Relevant boundary conditions. A systematic process of trial is proposed to identify the possible slip and its magnitude developed at the column-soil interface. The numerical scheme can be used for end bearing and floating columns and for the situation involving stratified layers.

The behaviour of soft ground reinforced by a group of columnar inclusions such as stone columns/granular piles, sand compaction piles and lime or cement columns, are predicted using the proposed method of analysis. The reinforced ground is covered by a layer of granular fill. The response of reinforced ground ranges from flexible to rigid loading conditions depending on the magnitude of thickness and deformation modulus of overlaying granular fill. The compacted granular fill over the column reinforced ground is very effective in reducing both the overall and the differential settlements of the composite ground. The compressibility of S the granular fill has an appreciable influence on the settlement response of the composite ground as long as the modulus of granular fill is less than approximately fifty times that of the soft ground.

The depth of slip zone at column-soil interface increases with the decreasing value of limiting shear stress. It also increases with the increase of degree of penetration of column. Slip situation predicts higher depth of neutral plane than its no slip counterpart and it increases with the decreasing value of limiting shear stress. In case of end hearing column, a good portion of column sustains little or no interface shear stress at all but for floating column, the whole length of column is subjected to shear stress either positive or negative. The stress in column increases with depth and attains it high value at the bottom for end hearing columns. But in case of floating column, it increases up to the depth of neutral plane beyond which decreases up to the bottom of column. In both the cases no slip situation predicts higher depth of neutral plane than that of possible slip. The differential settlement of the composite ground is noticeable for the case of uniform flexible loading acting over the entire area. The overall and the differential settlements are more for slip analysis than that of no slip case. As the differential settlement does not reduce in case of floating column, end bearing column is more effective due to giving less overall settlement. The influence of soil stratification is evident in the predictions. This is, of course, as expected; the present analysis quantifies it. The spacing, length to diameter ratio, the degree of penetration of columns, the relative stiffness of column and soil, and the angle of friction between column and soil have a significant influence on the mobilization of shear stress, variation of normal stresses in column and soil and the settlement of the treated ground. But the Poisson's ratio of soil has little influence on them. A simple uncoupled consolidation model is proposed to determine the time-dependent response soft ground reinforced by columnar inclusions. This method is simple compared to Biot's coupled consolidation theory. The radial inhomogeneity of the soil properties such as deformation modulus and shear modulus and also soil stratification can be handled easily by the proposed method. From predicted results, it is realized that to evaluate the subsequent response of reinforced ground, the value of degree of consolidation predicted at every nodal points should be used rather than using the average value. The mobilization of shear stress at column-soil interface, the stress in column and soil and the settlement profile with time can be evaluated easily and reasonably accurately by the proposed method.

The proposed foundation model has been compared with the existing approaches and verified by the finite element analysis as well as experimental results both in the laboratory and in the field. The existing approaches can be used for rigid loading condition but their application is restricted for flexible loading. In both cases flexible and rigid loading, the proposed model offers better solution as it can be used by taking account the role of overlaying granular fill, soil stratification, slip and no slip situations, can be used for both end bearing and floating columns and also for time-dependent analysis. The comparison of results obtained from finite element method and those of by the proposed model, indicates that the proposed model can be used with a reasonable degree of accuracy to depict the settlement behaviour of end bearing or floating columns reinforced ground subjected to either flexible or rigid loading. The prediction obtained from the proposed model show also good agreement with test results both in laboratory and field.