The influence of creep on the settlement of foundations supported by stone columns
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Vibro-replacement stone columns are widely used in geotechnical practice to improve the bearing capacity and settlement characteristics of weak natural soils and man-made fills, accelerate consolidation in fine soils and reduce soil liquefaction potential. The vibro-replacement technique is becoming increasingly popular for the treatment of soft cohesive (and often organic) soil deposits in which creep settlements can make up a significant proportion of the total settlement. The majority of field trials, numerical studies, and laboratory tests carried out to date have focused on estimating the improvement to bearing capacity or the reduction in settlement (almost exclusively primary settlement), with very little consideration given to the potential of stone columns to arrest long-term creep settlements. This research addresses this gap in knowledge by using two-dimensional finite element analysis techniques to assess the effectiveness of using vibro-replacement stone columns to treat creep-prone soils. The majority of the numerical modelling work described in this thesis is based on axisymmetric unit cell models, implemented using PLAXIS 2D, using the Soft Soil Creep (SSC) model to represent the host soil. The SSC model is a three-dimensional isotropic elasto-viscoplastic model suitable for normally consolidated and lightly overconsolidated clays, silts, and peat. The granular column material has been modelled using the elasto-plastic Hardening Soil (HS) model. The soil profile adopted is that of the Bothkennar geotechnical test bed in Scotland, consisting of an overconsolidated crust overlying two layers of soft lightly overconsolidated Carse clay. Simplified single-layer profiles based on the Bothkennar parameters were used for preliminary numerical modelling purposes, before the full profile was considered. An examination of the evolution of settlement improvement factor (defined as the ratio of the settlement of untreated to treated ground) with time, using PLAXIS 2D, indicates that the inclusion of creep leads to a lower ¿total¿ settlement improvement factor than would be obtained for primary consolidation settlement alone. Parametric studies have indicated, as expected, that this effect is more pronounced in situations where creep settlements account for a greater proportion of the total settlement. The numerical output has also been used to derive separate ¿primary¿ and ¿creep¿ settlement improvement factors. The ¿primary¿ settlement improvement factors, which were found to be in relatively good agreement with a selection of pre-existing analytical formulations (which pertain to primary settlement only), tend to be much larger than the ¿creep¿ settlement improvement factors. Nevertheless, the latter factors are greater than unity, suggesting that columns help reduce creep settlements. Consideration of the variations of radial, vertical, and hoop stresses and strains with time and depth predicted by PLAXIS 2D has highlighted how creep influences the behaviour of the composite system, in particular the stress transfer process from soil to column. As the soil creeps, vertical stress is transferred from soil to column; the amount of stress transferred increases with depth. It is demonstrated in the thesis that the lower ¿total¿ settlement improvement factors (owing to the presence of creep) occur because the columns, which have already yielded, are forced to carry additional vertical stress, inducing additional shear-plane formation. In addition, both the radial and hoop stresses in the soil are lower in treated ground than in untreated ground; these radial and hoop stresses are lower in a soil that creeps. The radial stress reduction means that the lateral support imparted onto the column by the soil is lower; this will also contribute to lower ¿total¿ settlement improvement factors but is not as influential as the additional yielding caused by the vertical stress transfer process. The hoop stress reductions for the ¿with creep¿ case are caused by the additional plastic deformation but do not contribute to the lower ¿total¿ settlement improvement factors. The impact of creep on the stress transfer process for floating columns is similar to that for end-bearing columns; the soil is unloaded and the magnitude of this stress transfer (unloading) increases with depth. Given that this geotechnical problem is being investigated numerically for the first time, it is appropriate that the emphasis of the work is on obtaining practical estimates of the likely behaviour of stone columns in creep-prone soils rather than on the subtleties of complex higher-order models. Nevertheless, selected analyses, repeated using the advanced Creep-SCLAY1S model (which incorporates anisotropy, bonding, and destructuration and is not yet commercially available with PLAXIS), have yielded very similar findings to those obtained using the SSC model. However, settlement improvement factors (¿primary¿ and ¿total¿) are lower when destructuration is considered because column presence triggers bond degradation. The majority of existing analytical settlement design methods pertain to primary settlement only, and in the absence of further guidance, designers will tend to apply the same improvement factor to creep settlements as they have estimated for primary settlements. To overcome this, a simplified empirical design procedure that accounts for the influence of creep has been developed based on a parametric study carried out using the SSC model with a view to identifying appropriate variables which influence the aforementioned improvement factors. The parameters have been altered in a specific range so that they are representative of those typically encountered for soft (creep-prone) clays in practice. It is suggested that the method is used in conjunction with an existing primary settlement design method that captures all key features of primary settlement behaviour. Finally, a novel procedure which uses Cylindrical Cavity Expansion (CCE) theory in conjunction with the conventional 'wished-in-place' installation technique to account for column installation has also been used for a selection of analyses. A two-step process was implemented: (i) CCE theory has been used to work out post-installation lateral earth pressure coefficients caused by the lateral expansion of the vibrating poker when columns are installed in soft clay, and (ii) the new earth pressure coefficients have been incorporated in a standard axisymmetric unit cell model to establish their influence on settlement improvement factors for an infinite column grid. This two-step approach can be used as an improvement upon the conventional ¿wished-in-place¿ column installation technique, with larger settlement improvement factors predicted when installation (increased earth pressure coefficients) is taken into account. For stage (i), a comparison of the SSC model output with and without creep has been used to give an indication of the possible effect of column construction on lateral earth pressure coefficients surrounding columns in creep-prone soils. The stage (ii) output has indicated that the conclusions heretofore are unaffected if installation is or is not considered; incorporating creep leads to lower settlement improvement factors.
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