The hybrid of floating stone column by numerical and physical evaluation

Rapid population growth amplifying demand for accommodation and infrastructure has resulted in soft ground being increasingly used in construction. Problems related to soft ground can be remedied by adopting a ground improvement technique. The stone column is one of the most effective and feasible techniques for soft clay soil improvement. Stone columns increase bearing capacity and reduce the settlement of soil. However, soft ground of more than 40 meters depth makes stone column treatment costlier. The design of floating stone columns within soft ground is sometimes needs to adopt. However, this method is not popular compared to the end bearing stone columns due to low mobilised shear resistance and resulted in higher occurrence of punching failure. This research is aimed for addressing the shortcoming floating stone columns with proposing the hybrid dimension floating stone columns. The hybrid stone column size able to increase the mobilised shear resistance, decrease punching failure, and reduce the volume of aggregates. In the present work, finite element analysis was performed using the program PLAXIS 2D. An elastic-perfectly plastic constitutive soil model relation based on the Mohr-Coulomb criterion was utilized to predict the behaviour of soft clay strengthen by stone column. Response Surface Methodology (RSM) was used to optimize the hybrid stone column size with the Design-Expert 6.0.4 software. The laboratory physical model tests were performed based on the sizes of optimum hybrid stone column size proposed by RSM. The results revealed that the optimal parameter of the uniform diameter of 44 mm with a length of 100 mm increases its load bearing capacity of 3260.7 N and the lowest settlement was recorded at a diameter of 24.2 mm with a length of 400 mm to achieve 25.8 mm of settlement. Moreover, the hybrid column size i.e. the first stone column diameter of 43 mm and second diameter of 21.2 mm with the same lengths of 200 mm each diameter able to achieve load-bearing capacity of 3350.9 N and settlement of 24.5 mm. Thus, by comparing with the uniform diameter stone column of 44 mm and length of 400 mm, the hybrid column able to increase the load bearing capacity by 3% and decrease the settlement by 5%. In addition, a good agreement was obtained between the numerical and physical models with variation 25%. In addition, the hybrid stone column size is able to reduce the volume of aggregates up to 40%

Geosynthetic-encased stone columns: analytical calculation model

This paper presents a newly developed design method for non-encased and encased stone columns. The developed analytical closed-form solution is based on previous solutions, initially developed for non-encased columns and for non-dilating rigid-plastic column material. In the present method, the initial stresses in the soil/column are taken into account, with the column considered as an elasto-plastic material with constant dilatancy, the soil as an elastic material and the geosynthetic encasement as a linear-elastic material. To check the validity of the assumptions and the ability of the method to give reasonable predictions of settlements, stresses and encasement forces, comparative elasto-plastic finite element analyses have been performed. The agreement between the two methods is very good, which was the reason that the new method was used to generate a parametric study in order to investigate various parameters, such as soil/column parameters, replacement ratio, load level and geosynthetic encasement stiffness on the behaviour of the improved ground. The results of this study show the influence of key parameters and provide a basis for the rational predictions of settlement response for various encasement stiffnesses, column arrangements and load levels. The practical use of the method is illustrated through the design chart, which enables preliminary selection of column spacing and encasement stiffness to achieve the desired settlement reduction for the selected set of the soil/column parameters. (C) 2010 Elsevier Ltd. All rights reserved

A behavior of reinforced vibrocompacted stone column in peat

In the literature, several methods of ground improvement have been presented including compacted stone columns. The bearing capacity of the granular column is governed mainly by the lateral confining pressure mobilized in the soft soil to restrain or prevent bulging of the granular column. Therefore, the technique becomes unfeasible in peat that does not provide sufficient lateral confinement. This condition can be overcome by encasing the stone column with geogrid. This paper investigates the performance of the geogrid encased vibrocompacted stone column in peat. This study was carried out using PLAXIS software equipped with unit cell concept. The peat was modelled using soft soil model and the stone column using Mohr-Coulomb soil model, respectively. The geogrid was modelled using the geogrid option and could take only tensile force. The results indicate that the geogrid encased stone column can take much higher load in comparison to ordinary stone columns as the stiffness of the column increases. Meanwhile, the length of encasement also varied and it was observed that it was very effective up to about two times the diameter of the column. It also increased the column stiffness, and therefore led to a significant strain reduction. It was also observed that the columns at a spacing of three times the diameter are very effective. The results presented here can be used by the geotechnical engineers to design the geogrid reinforced stone column based on the strength of the soil, diameter of the column, spacing of the columns and stiffness of the geogrid

Effect of vibro stone column installation on the performance of reinforced soil

Empirical design methods for stone column foundations are often on single stone columns or as a homogeneous medium of soil/column. These methods underestimate the capacity of the composite system because they do not take into account the increased confining stress acting on the stone column or the increased stiffness of the soil. This study used Plaxis 2D to study the effect of the installation method on the confining pressure and soil stiffness around a single column by assuming the installation of the column could be modelled as an expanding cavity followed by consolidation of the surrounding soil. The mean stress and stiffness generated during installation between two, adjacent columns was used in Plaxis 3D to compare the settlement of circular foundations on estuarine deposits reinforced by stone columns at a site in Santa Barbara, California. Good agreement was found between the predicted and actual settlement of the trial foundations on three column arrangements. The predictions gave a better estimate of the settlement compared to those using a unit cell or homogeneous medium showing that improvements to the soil should be taken into account when assessing stone column performance

A homogenization approach for assessing the yield strength properties of stone column reinforced soils

International audienceThe macroscopic strength properties of a purely cohesive soil reinforced by a periodic distribution of "stone columns" made of a highly frictional granular material are investigated in a rigorous way on the basis of the yield design homogenization approach. Starting from a first crude lower bound approximation to the macroscopic strength criterion of the stone column reinforced soil, a much more accurate failure surface is then drawn in the space of stresses as a result of a series of numerical elastoplastic simulations performed on the reinforced soil unit cell subject to radial strain controlled loading paths. The anisotropic characteristics of the so obtained original criterion are then highlighted by means of its representation in the Mohr plane attached to any oriented facet. The paper concludes with a first illustrative implementation of the method on the derivation of an upper bound estimate for the ultimate bearing capacity of a stone column reinforced foundation

The White Stone Band of the Kimmeridge Clay Formation, an integrated high resolution approach to understanding environmental change

The Kimmeridge Clay is a Jurassic mudrock succession that shows Milankovitch Band climatic cyclicity. A key issue is to determine how the subtle changes that define this cyclicity result from climatic change. Using material from the Natural Environment Research Council Rapid Global Geological Events (RGGE) Kimmeridge Drilling Project boreholes, the White Stone Band was investigated at the lamination scale using backscattered electron imagery and quantitative palynofacies. Fabric analysis shows the lamination to represent successive deposition of coccolith-rich and organic-matter-rich layers. Individual laminae contain unsorted palynological debris with a consistent ratio of marine and terrestrial components. Such mixed organic matter input is interpreted as the result of storm transport. Linking water column processes to laminae deposition suggests seasonal input with a coccolith bloom followed by a more diverse assemblage including dinoflagellates and photosynthetic chlorobiacean bacteria. As the photic zone extended into the euxinic water column organic matter export to the sea bed underwent minimal cycling through oxidation and subsequently became preserved through sulphurization with greatly increased sequestration of carbon. This was significantlyincreased by late season storm-driven mixing of euxinic water into the photic zone. Increased frequency ofstorm systems would therefore dilute the coccolith input to give an oil shale. Hence climatically induced changes in storm frequency would progressively vary the organic content of the sediment and generate the climate cycle signal. Keywords: Milankovitch theory, Kimmeridge Clay, organic matter, high-resolution methods, climate change

Numerical analyses of stone column installation in Bothkennar clay

The paper presents the results of numerical simulations studying the installation effects of stone columns in a natural soft clay. Stone column installation is modelled as an undrained expansion of a cylindrical cavity, using the finite element code PLAXIS that allows for large displacements. The properties of the soft clay correspond to Bothkennar clay, a soft Carse clay from Scotland (UK). The complexity of this material is simulated via two advanced recently developed constitutive formulations able to account for the soil structure, namely S-CLAY1 and S-CLAY1S. Modified Cam Clay model is also used for comparison purposes. The paper shows the new stress field and state parameters after column installation and the subsequent consolidation process. This sets the basis for including installation effects in studying the settlement reduction caused by stone columns

Tamm-Horsfall protein in recurrent calcium kidney stone formers with positive family history: abnormalities in urinary excretion, molecular structure and function

Tamm-Horsfall protein (THP) powerfully inhibits calcium oxalate crystal aggregation, but structurally abnormal THPs from recurrent calcium stone formers may promote crystal aggregation. Therefore, increased urinary excretion of abnormal THP might be of relevance in nephrolithiasis. We studied 44 recurrent idiopathic calcium stone formers with a positive family history of stone disease (RCSFfam) and 34 age- and sex-matched healthy controls (C). Twenty-four-hour urinary THP excretion was measured by enzyme linked immunosorbent assay. Structural properties of individually purified THPs were obtained from analysis of elution patterns from a Sepharose 4B column. Sialic acid (SA) contents of native whole 24-h urines, crude salt precipitates of native urines and individually purified THPs were measured. THP function was studied by measuring inhibition of CaOx crystal aggregation in vitro (pH 5.7, 200mM sodium chloride). Twenty-four-hour urine excretion of THP was higher in RCSFfam (44.0±4.0mg/day) than in C (30.9±2.2mg/day, P=0.015). Upon salt precipitation and lyophilization, elution from a Sepharose 4B column revealed one major peak (peak A, cross-reacting with polyclonal anti-THP antibody) and a second minor peak (peak B, not cross-reacting). THPs from RCSFfam eluted later than those from C (P=0.021), and maximum width of THP peaks was higher in RCSFfam than in C (P=0.024). SA content was higher in specimens from RCSFfam than from C, in native 24-h urines (207.5±20.4mg vs. 135.2±16.1mg, P=0.013) as well as in crude salt precipitates of 24-h urines (10.4±0.5mg vs. 7.4±0.9mg, P=0.002) and in purified THPs (75.3±9.3μg/mg vs. 48.8±9.8μg/mg THP, P=0.043). Finally, inhibition of calcium oxalate monohydrate crystal aggregation by 40mg/L of THP was lower in RCSFfam (6.1±5.5%, range −62.0 to +84.2%) than in C (24.9±6.0%, range −39.8 to +82.7%), P=0.022, and only 25 out of 44 (57%) THPs from RCSFfam were inhibitory (positive inhibition value) vs. 25 out of 34 (74%) THPs from C, P<0.05. In conclusion, severely recurrent calcium stone formers with a positive family history excrete more THP than healthy controls, and their THP molecules elute later from an analytical column and contain more SA. Such increasingly aggregated THP molecules predispose to exaggerated calcium oxalate crystal aggregation, an important prerequisite for urinary stone formatio

Approximations of the macroscopic strength criterion of reinforced soils, with application to structural stability analyses

International audienceThe macroscopic strength properties of a stone column reinforced soil are investigated numerically. Using the kinematic approach of the yield design theory applied to the reinforced soil's unit cell, a numerical upper bound estimate of its strength domain is provided. Since this domain cannot be used directly for a structural stability analysis, an approximation method is performed, using a sum of ellipsoidal sets. The result is a rigorous upper bound estimate for the macroscopic strength criterion depending on few parameters. The relative error of this method is quantified not exceeding a few percents. Then, this approximation is used in order to treat the problem of an embankment resting upon a stone column reinforced soil. Again, performing the kinematic approach on this structure, a rigorous upper bound estimate of the ultimate stability factor is obtained numerically. The gain in terms of ultimate capacity improvement is observed, as compared to the non reinforced configuration. This result is compared to a simplified analysis, based on a rule of mixture formula, where strong disparities are highlighted