Short-term and long-term behavior of geosynthetic-reinforced stone columns

Stone columns are often used to improve the load-carrying characteristics of weak soils. In very soft soils, however, the bearing capacity of stone columns may not significantly improve the load-carrying characteristics due to the very low confinement of the surrounding soil. In such cases, encased stone columns (ESCs) or horizontally reinforced stone columns (HRSCs) may be used. Although ESCs have been studied extensively, few studies have been done on HRSCs. In addition, very limited studies are available on ESCs and HRSCs under the same conditions. Moreover, no studies have been carried out to compare the long-term and short-term behavior of HRSCs with that of ESCs. In this research, therefore, numerical analyses are performed on various types of reinforced end-bearing stone columns to compare their behavior under both long-term and short-term conditions under various loading conditions. The Advanced Modified Cam-clay model for clay and the Hardening Soil model for stone column materials are used. The results show that with proper reinforcing stone columns, in addition to a considerable reduction in settlement, the consolidation time can be greatly decreased and most of the settlement will occur during the loading period. Also, the consolidation settlement rate may be increased by using a smaller column diameter and a larger area replacement ratio for the unit cell, stiffer geosynthetic reinforcements, and greater values for the internal friction angle of the stone column materials.     

Deformation and consolidation around encased stone columns

A new analytical solution is presented to study soft soil improvement by means of encased stone columns to reduce both settlement and consolidation time. The proposed solution aims to be a simple and useful tool for design. Only a unit cell, i.e. an end-bearing column and its surrounding soil, is modelled in axial symmetry under a rigid and uniform load. The soft soil is treated as an elastic material and the column as an elastic-plastic material using the Mohr-Coulomb yield criterion and a non-associated flow rule, with a constant dilatancy angle. An elasto-plastic behaviour is also considered for the encasement by means of a limit tensile strength. The solution is presented in a closed form and is directly usable in a spreadsheet. Parametric studies of the settlement reduction, stress concentration and consolidation time show the efficiency of column encasement, which is mainly ruled by the encasement stiffness compared to that of the soil. Column encasement is equally useful for common area replacement ratios but columns of smaller diameters are better confined. Furthermore, the applied load should be limited to prevent the encasement from reaching its tensile strength limit. A simplified formulation of the solution is developed assuming drained condition. The results are in agreement with numerical analyses.     

饱和盐渍土碎石排水桩加强夯试验研究

在青海察尔汗盐湖地区的饱和盐渍土上进行碎石排水桩加强夯试验研究。对不同的碎石排水桩参数、不同的强夯能量进行对比试验。通过静载荷试验、孔隙水压力测试、变形测试、静力触探试验、重型圆锥动力触探试验、波速测试等大量的原位测试试验,明确了饱和盐渍土碎石排水桩加强夯处理的碎石桩优化设计参数、强夯单点夯击能量的选择、强夯处理效果、强夯的影响深度等。给出了强夯处理后地基承载力、压缩模量等土性指标的确定方法。     

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.     

Groups of encased stone columns: Influence of column length and arrangement

This paper presents a set of systematic 2D and 3D finite element analyses that study the performance of groups of encased stone columns beneath a rigid footing. Those numerical analyses show that, if the area replacement ratio, i.e. area of the columns over area of the footing, and the ratio of encasement stiffness to column diameter are kept constant, the column arrangement (both number of columns and column position) has a small influence on the settlement reduction achieved with the treatment. For high encasement stiffnesses, placing the column near the footing edges may be slightly more beneficial reducing the settlement; on the contrary, the maximum hoop force at the encasement is notably higher. Based on the minor influence of column arrangement, this paper proposes a new simplified approach to study groups of encased stone columns, which involves converting all the columns of the group beneath the footing in just one central column with an equivalent area and encasement stiffness. This simplified model is used to conclude that, for settlement reduction and fully encased columns in a homogeneous soil, there is a column critical length of around two or three times the footing width. The critical length of the encasement for partially encased columns is slightly lower than that of the fully encased columns.     

Analyse numérique et expérimentale du comportement des ouvrages fondés sur un sol mou renforcé par des colonnes ballastées flottantes

This paper work is in the frame of the study about the reduction of settlements in the case of floating columns. The prediction of a settlement remains a prickly problem to which geotechnical engineer is confronted. Experience has shown that structural pathologies are caused by the effects of excessive differential settlements before reaching soil bearing capacity. To palliate this problems in the case of soft and compressible soils, we advocate, as an alternative solution, piles or the reinforcement of the soils with stone columns. This strengthening method, introduced in Algeria in 1999, is economical. However, it does not inihilate settlements but it speeds up the time factor. Numerical modelings by finite element method (FEM) have been carried out through interest into all mechanical and geometrical aspects that are inherent in the meshing of floating stone columns. An analytical formulation of a correction coefficent (μβ) and an abacus of the settlement reduction factor (β) has been suggested. On the other way, the monitoring instrumentation of experimental site since 2008 to 2017 at the level of the harbour lowland of Bejaia has been carried out. It concerns important storage structures fonded on a very compressible ground reinforced by floating stone columns. The records of the settlements at the scale of the structures, according to variable loads recorded in real time, have revealed excessive settlements under the effects of variable stresses from 1 to 3 bars. An exhaustive geological, geotechnical and geomechanical analysis has been carried out for the characterization of the grounds, and numerical modelling by finite element method (FEM) has been carried out. The experimental and numerical investigation of the concerned structures at issue allowed us to confront the results with regard to settlement prediction of an environment reinforced with floating stone columns. The result is a perfect matching in relation to the suggested formula.

     

A method for predicting consolidation settlements of floating column improved clayey subsoil

A method of predicting the consolidation settlement-time curve of floating soil-cement column on improved soft clayey subsoil has been proposed. The degree of the consolidation (U(t)) of the system is calculated by the double soil-layer consolidation theory, and the methods for evaluating the equivalent hydraulic conductivity (k) and the coefficient of volume compressibility (m _v) of the part of the column improved layer have been proposed. The effectiveness of the method was verified by comparing predictions with the results of finite element analysis (FEA) using a unit cell model. The consolidation settlement (s(t)) can be calculated by the method of treating a part of the column improved layer as an unimproved layer and using the corresponding average U(t) value. By comparing the predicted results with the measurements of laboratory model tests and three case histories in Fukuoka, Japan, the effectiveness of the proposed methods has been verified. It is suggested that the method can be used for designing the soft clayey subsoil improvement using floating soil-cement columns.     

双层散体材料桩复合地基固结解析理论

研究了考虑应力集中效应的双层散体材料桩复合地基固结问题 ,在得到此类双层地基系统正交公式的基础上 ,给出了解析解答 ,并通过编程计算 ,分析了此类地基固结基本性状。研究表明 ,根据本文提出的计算模型 ,桩径比和桩体刚度对地基的固结速率有较大影响 ;分别按孔压和沉降定义的总平均固结度有一定程度差别 ;采用竖井固结理论计算得到的平均固结度较本文理论的计算结果偏小。     

Comparative Study on the Behavior of Encased Stone Column and Conventional Stone Column

Stone columns, one of the most commonly used soil improvement techniques, have been utilized worldwide to increase bearing capacity and reduce total and differential settlements of structures constructed on soft clay. Stone columns also act as vertical drains, thus speeding up the process of consolidation. However, the settlement of stabilised bed is not reduced in many situations for want of adequate lateral restraint. Encasing the stone column with a geogrid enhances the bearing capacity and reduces the settlement drastically without compromising its effect as a drain, unlike a pile. The behavior of the encased stone column stabilized bed is experimentally investigated and analysed numerically. In the numerical analysis, material behaviour is simulated using Soft Soil, Mohr Coulomb and Geogrid models for clay, stone material and encasement respectively and is validated with experimental results. The parametric study carried out on varying the L/D ratio (L = length of the column; D = diameter of the column) of column, stiffness of geogrid and angle of internal friction of stone material gives a better understanding of the physical performance of the encased stone column stabilized clay bed.     

A method for predicting consolidation settlements of floating column improved clayey subsoil

A method of predicting the consolidation settlement-time curve of floating soil-cement column on improved soft clayey subsoil has been proposed. The degree of the consolidation (U(t)) of the system is calculated by the double soil-layer consolidation theory, and the methods for evaluating the equivalent hydraulic conductivity (k) and the coefficient of volume compressibility (m_v) of the part of the column improved layer have been proposed. The effectiveness of the method was verified by comparing predictions with the results of finite element analysis (FEA) using a unit cell model. The consolidation settlement (s(t)) can be calculated by the method of treating a part of the column improved layer as an unimproved layer and using the corresponding average U(t) value. By comparing the predicted results with the measurements of laboratory model tests and three case histories in Fukuoka, Japan, the effectiveness of the proposed methods has been verified. It is suggested that the method can be used for designing the soft clayey subsoil improvement using floating soil-cement columns.     

Long-term consolidation behavior interpreted with isotache concept for worldwide clays

The consolidation characteristics interpreted with the isotache concept have been studied by many researchers, including the authors. The aim of most of these studies has been to calculate secondary consolidation with high accuracy in order to evaluate the long-term settlement of large-scale structures. In a previous study, the long-term consolidation characteristics of Osaka Bay clay, collected from the construction site of the Kansai International Airport, were examined, and a simplified method based on the isotache concept, using a compression curve and the relationship between the consolidation yield stress (preconsolidation pressure) and the strain rate, was proposed. The former and the latter were obtained from constant rate of strain consolidation (CRS) tests and long-term consolidation (LT) tests, respectively. The latter is expressed by an equation with three isotache parameters. In addition, it is noteworthy that the isotache parameters can be commonly determined for the Osaka Bay clays retrieved from various depths up to 300 m below the seabed. In the present study, the proposed method was applied to worldwide clays with various characteristics using the common values for the isotache parameters determined for the Osaka Bay clays. It was found that the long-term consolidation behavior of those worldwide clays can be well characterized by the proposed method, along with the common values for the isotache parameters.     

Consolidation of partially saturated ground improved by impervious column inclusion: Governing equations and semi-analytical solutions

This study focuses on the consolidation behavior and mathematical interpretation of partially-saturated ground improved by impervious column inclusion. The constitutive relations for soil skeleton, pore air and pore water for partially saturated soils are proposed in the context of partially-saturated ground improved by impervious column inclusion. Settlement equation and dissipation equations of excess pore air/water pressures for a partially saturated improved ground are then derived. The semi-analytical solutions for ground settlement and pore pressure dissipation are then obtained through the Laplace transform and validated by the existing solutions for two special cases in the literature and the numerical results obtained from the finite difference method. A series of parametric studies is finally conducted to investigate the influence of some key factors on consolidation of partially saturated ground improved by impervious column inclusion. Based on the parametric study, it can be found that a higher value of the area replacement ratio or modulus of the pile results in a longer dissipation time of excess pore air pressure (PAP), a shorter dissipation time of excess pore water pressure (PWP), and a lower normalized settlement.     

强夯置换碎石桩复合地基承载力的试验研究

 为了解决强夯时产生的超孔隙水压力问题, 提出了加固饱和粉土、粉质粘土地基的强夯碎石桩法。工程试验研究表明, 该法克服了一般强夯法对这类场地效果不理想的缺点, 加固后的复合地基不仅碎石桩本身桩体长、影响深、强度高, 而且, 桩间土的承载力得到很大的提高。复合地基比天然地基的承载力提高幅度在一倍以上。工程应用结果表明, 该方法是可行的。     

饱和盐渍土地基处理孔隙水压力实测分析

通过对青海察尔汗盐湖地区的饱和盐渍土天然地层强夯和碎石排水桩加强夯进行孔隙水压力测试,重点探讨了碎石排水桩加强夯处理盐渍土地基孔隙水压力变化特性、强夯时孔隙水压力峰值与土层表面接触应力峰值的关系、不同夯击遍数下的孔隙水压力、排水桩的作用以及强夯夯击间歇时间等问题,对碎石排水桩加强夯处理饱和盐渍土地基技术的研究与工程实践有一定的指导意义。     

碎石桩复合地基承载力和沉降若干问题

本文首先简要地讨论了碎石桩复合地基可能的破坏模式，随后系统地分析了三十几组复合地基的实测数据，探讨了这种加固处理技术可能达到的效果。通过对桩土应力比、面积置换率以及天然地基压缩模量对复合地基承载力的提高和沉降折减系数的影响的讨论，指出了这些参数在设计取值时所需注意的问题。     

碎石桩加固油罐地基试验研究

油罐地基的载荷板试验、标准贯入试验、静力触探试验和充水预压试验等研究表明,碎石桩加固油罐地基能增强地基中饱和粘质粉土抗地震液化的能力;提高软土地基承载力;大大缩短油罐沉降时间和减小差异沉降量;在一定程度上能抑制起始超孔隙水压力的产生。     

地下室上浮事故实例分析与处理

在地下水丰富地区,施工期间地下室上浮事故较为多发.以某地下室上浮事故为例,详细分析上浮事故的发生、发展及后续的增补抗拔桩、结构修复过程.对过程中综合应用的锚杆静压桩加固、托梁换柱结构修复技术进行重难点剖析,同时观测全过程建筑沉降状况,可为类似项目提供借鉴.     

Design chart for the modified hyperbolic method

The modified hyperbolic method is used in practice to predict the ultimate primary consolidation settlement of compressible ground. This paper presents a new relationship between the average degree of consolidation for combined vertical and horizontal consolidation U_vh and the non-dimensional vertical time factor T_v, with a new parameter, ν_hv, introduced. ν_hv is defined as the ratio of the time factor in the horizontal direction to that in the vertical direction. This relationship is then adopted to calculate the slope of the linear segment of the theoretical hyperbolic plot (T_v/U_vh vs T_v), λ, which is a key factor in the modified hyperbolic observational method presented by Tan (1995). A design chart for λ as a function of ν_hv is proposed. Using this design chart, the determination for λ can be simplified from a procedure involving three parameters to only one, ν_hv. A new procedure for the use of the modified hyperbolic method is proposed. The new procedure is verified using a well-documented case history.     

Proposal of a Simple Method for Assessing the Susceptibility of Naturally Deposited Clay Grounds to Large Long-Term Settlement due to Embankment Loading

A simple method that utilizes the results of laboratory tests has been proposed for determining the susceptibility of soft clay grounds to large residual consolidation settlement due to embankment loading. It was found that there is a possibility of large long-term settlement if the sensitivity and compression index ratios of the clay material that constitutes the ground are equal to or more than 8.0 and 1.5, respectively. The compression index ratio is defined in this paper as the ratio (Cc/Ccr) of the steepest gradient of the compression curve of an undisturbed sample to that of the remolded sample. Through the SYS Cam-clay model, an elasto-plastic constitutive model that describes the actions of the soil skeleton structure, it was found that clays with large sensitivity and compression index ratios are characterized by initially highly structured soils and that decay/upgradation of the structure can easily occur due to plastic deformation. In addition, by following Schmertmann's graphic method for in-situ compression curve (1953), this paper proposes a method of deducing the in-situ initial conditions from the results of laboratory consolidation tests on undisturbed samples. These investigations revealed not only that large delayed settlement is facilitated in clays, which have higher degrees of structure and faster rates of structural decay, but also that the Δe method and other simple methods of predicting settlement may underestimate the amount of settlement.     

Performance enhancement of encased stone column with conductive natural geotextile under k0 stress condition

In this study, the performance of the stone column encased with conductive jute geotextile in improving the characteristics of soft clay under k₀ stress condition was evaluated using a custom-designed large-scale consolidation test setup. The electrokinetic-encased stone column (e-ESC) was designed as a cathode, while mild steel rods were chosen as anodes, and by applying a voltage gradient of 0.1 V/mm between the electrodes, electrokinetic processes were initiated. The efficacy of using the e-ESC in improving the performance of soft clay was assessed by comparing the results with an ordinary stone column (OSC) and encased stone column (ESC) reinforced soft clay. The results from the study reveal that the inclusion of stiffer material like OSC and ESC in soft clay has significantly increased undrained shear strength and modulus of subgrade reaction of the composite ground with a reduction in settlement and compression index. Due to the application of the voltage gradient between the electrodes, the rate of consolidation settlement of composite ground has significantly increased when compared to the OSC/ESC case. Further, it was confirmed from the chemical and mineralogical analysis that the pH, chemical composition, mineral phases, and microfabric of soft clay were altered with the voltage gradient.