Stone column technique is mostly used to reinforce soft cohesive soils. Settlements are decreased under foundations and bearing capacity is increased. This study initially focuses on a comprehensive review of literature about stone column reinforced soils. Afterwards, numerical modeling of stone column reinforced soft clays is done. Three different cases are chosen on different foundation soils mainly soft clays. Parametric studies are done to determine influence of parameters on settlement reduction ratio under three different foundation conditions. Analyses are converted to two dimensional conditions and this conversion is also compared within the scope of this study. Settlement reduction ratio response to variation in parameters revealed similar results under three different foundation conditions.

ABSTRACT A LABORATORY MODEL STUDY ON SETTLEMENT REDUCTION Effect OF STONE COLUMNS IN SOFT CLAY S&uuml / nnetcioglu, Mehmet Emrah M.Sc., Department of Civil Engineering Supervisor: Prof. Dr. Mehmet Ufuk Ergun August 2012, 177 pages An experimental study was conducted in order to examine settlement reduction ratios of footing supported by both floating and end bearing type of stone columns. For the floating types, tests were done with varying column lengths of one and two widths of footing (L=B,2B). Tests were conducted in 200 mm* 200 mm* 200 mm cubic loading tanks. The reinforcement effect was achieved by the installation of four stone columns with 20 mm diameter under 70 mm* 70mm model footing. Parameters such as area replacement ratio (a_s), loading plate dimensions, consolidation and vertical pressures applied, and the relative density (D_R) of the granular column were kept constant, the column length (L) was set as the only variable in the experimental tests conducted. In the tests, footing settlements together with subsurface settlements at depths equal to footing width (B) and two times the footing width (2B) were measured by specially designed telltales. The settlement reduction ratios both at surface and subsurface were evaluated in order to determine the effect of column length on settlement improvement. It has been found out that as the column length increases the settlement reduction ratios decrease for all depth intervals. However, there exists a threshold column length (L=2B), beyond which the composite ground demonstrates little settlement improvement.

This study discusses the results of numerical modeling aspect of aggregate pier foundations (aggregate piers) in soft, compressible soils. FLAC 2D (Fast Lagrangian Analysis of Continua), a finite difference code is utilized in the analyses. Use of axisymmetry enabled to visualize a three dimensional model throughout this research. The primary objective of this research is to make comparisons for stress concentration ratio &lsquo / n&rsquo / , and settlement reduction ratio b for given variables consisting of length, diameter, elastic modulus of the aggregate piers, and foundation pressures. Analyses have been carried out with 1, 1.5, 2, and 3 m long piers with diameters of 60 cm, and 80 cm, placed under a circular footing in 1.30 m diameter. Two values for elastic modulus of the piers have been used to reflect the effect of pier stiffness on settlement behavior. Analysis and design methodology have been carried out in three stages. The first stage consists of modeling the matrix soil with an elastic constitutive model and exerting foundation pressures to first check the accuracy of the mesh by comparing the effective vertical stress and settlement values by analytical methods. Once satisfactory results are achieved, modeling of a rigid foundation is carried out. Consequently, aggregate piers are modeled and loaded. For foundation pressures, a range of values consisting of 25, 50, 75, and 100 kPa have been chosen to see the behavior of piers under variable foundation pressures. There are solid outcomes of this study. It concludes by stating that the settlement behavior of piers having L/d ratios greater than 3.75, are alike. Thus, there is almost no additional settlement improvement achieved with piers longer than 3 m with 60 cm pier diameter.

Despite the developments in the last decades, field performance information for short aggregate pier improved ground is needed for future design and to develop a better understanding of the performance of the short (floating) aggregate piers. A full-scale field study was performed to investigate the floating aggregate pier behavior in a soft clayey soil. Site investigations included five boreholes and sampling, four CPT soundings, and SPT and laboratory testing. The soil profile consisted of 8m thick compressible clay overlying weathered rock. Four large plate load test stations were prepared. A rigid steel footing having plan dimensions of 3.0m by 3.5m were used for loading. Four 65cm diameter reaction piles and steel cross beams were used to load the soil in each station. First test comprised of loading the untreated soil up to 250 kPa with increments, and monitoring the surface settlements. Moreover, distribution of settlements with depth is recorded by means of deep settlement gages installed prior to loading. Other three tests were conducted on clay soil improved by rammed aggregate piers. In each station, seven stone columns were installed, having a diameter of 65cm, area ratio of 0.25, placed in a triangular pattern with a center to center spacing of 1.25m. The length of the columns were 3m, 5m in the two station resembling floating columns, and 8m in the last station to simulate end bearing columns to observe the level of the improvement in the floating columns. Field instrumentations included surface and deep settlement gages, and load cell placed on a aggregate pier to determine distribution of the applied vertical stress between the column and the natural soil , thus to find magnitude of the stress concentration factor, n , in end bearing and floating aggregate piers. It has been found that, the presence of floating aggregate piers reduce settlements, revealing that major improvement in the settlements takes place at relatively short column lengths. It has been also found that the stress concentration factor is not constant, but varies depending on the magnitude of the applied stress. The magnitude of stress concentration factor varies over a range from 2.1 to 5.6 showing a decreasing trend with increasing vertical stress.

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Deep Mixing involves the introduction of cementitious or specially formulated solutions directly into the ground through the use of purpose built blending injection augers. The system is mainly designed to increase strength and reduce compressibility of treated soil. In the first stage of the research effective mixture ratios and mixture types of stabilizing agents were investigated for soft clays (CL form Eymir lake and kaolinite) by means of unconfined compression (UC) tests on stabilized soils. The unconfined compressive strength (UCS) values were obtained for 7,28,90 and 365 days of curing time. The ratio of elastic modulus at 50% failure load (E50) to (UCS) of the stabilizing agents were also investigated. In the second part of the research programme, deep mixed model columns with the three column materials and four different column spacings are formed within the large scale consolidation tanks, and the consolidation characteristics of deep mixed improved clay were investigated. Based on the results of large scale consolidation tests on deep mixed columnar improved soft clay, compressibility characteristics of improved soft clay were determined in relation to spacing of columns namely, effective replacement ratio and binder content. The cement content (also UCS) of the column material was found to be the most important parameter for the improvement effects of DMM applications. Validity of the relations for the estimation of bulk compression modulus of soilcrete were discussed. The use of constrained modulus of the soil and the column material were found to be effective in predicting the compression modulus of the soilcrete. Settlement reduction factor versus replacement ratio and cement content relations were determined which may be used for preliminary design works. The stresses on the soil and the columns were backcalculated from the settlement values. The stress ratios were obtained.