A Theoretical Solution for Consolidation Rates of Stone Column‐Reinforced Foundations Accounting for Smear and Well Resistance Effects

Theoretical and experimental studies have proven that stone columns can be used for accelerating the consolidation rate of soft soil by providing a drainage path and reducing stresses in the soil. In constructing stone columns in fine‐grained soils, however, soil zones at the interface between the columns and their surrounding soil can become smeared and the fine‐grained soil particles can also be mixed into aggregates in the columns. The smear and well resistance due to aggregates contaminated with the fine‐grained soil particles reduce the effectiveness of stone columns in dissipating excess pore water pressures. A theoretical solution is developed in this article for computing the consolidation rates of stone column reinforced foundations accounting for smear and well resistance effects. In the derivations, stone columns and soft soil are both considered deforming one‐dimensionally and the stone columns having a higher drained elastic modulus than the surrounding soft soil. A modified coefficient of consolidation is introduced to account for the effect of the stone column‐soil modular ratio or stress concentration ratio. A parametric study investigates the influences of six important factors on the rate of consolidation. These influence factors include the diameter ratio of the influence zone to the stone column, the permeability of the stone column, the stress concentration ratio, the size of the smeared zone, the permeability of the smeared zone, and the thickness of the soft soil. To assist geotechnical engineers in utilizing the new solution for the design of stone column reinforced foundations, an illustrative design example is presented at the end of this article.     

Generalized Model for Geosynthetic-Reinforced Granular Fill-Soft Soil with Stone Columns

This paper pertains to the development of a mechanical model to predict the behavior of a geosynthetic-reinforced granular fill over soft soil improved with stone columns. The saturated soft soil has been idealized by Kelvin–Voight model to represent its consolidation behavior. The stone columns are idealized by stiffer springs. Pasternak shear layer and rough elastic membrane represent the granular fill and geosynthetic reinforcement layer, respectively. The nonlinear behavior of the granular fill and the soft soil is considered. Effect of consolidation of the soft soil due to inclusion of the stone columns has also been included in the model. Plane strain conditions are considered for the loading and reinforced foundation soil system. An iterative finite difference scheme is applied for obtaining the solution, and results are presented in nondimensional form. Comparison between the results from the present study and the analytical solution using theory of elasticity shows reasonable agreement. The advantage of using geosynthetic reinforcement is highlighted. Results indicate that inclusion of the geosynthetic layer effectively reduces the settlement. Nonlinearity in the behavior of the soft soil and the granular fill is reduced due to the use of geosynthetic reinforcement layer.     

Consolidation of a Finite Transversely Isotropic Soil Layer on a Rough Impervious Base

A semianalytical solution to axisymmetric consolidation of a transversely isotropic soil layer resting on a rough impervious base and subjected to a uniform circular pressure at the ground surface is presented. The analysis uses Biot’s fully coupled consolidation theory for a transversely isotropic soil. The general solutions for the governing consolidation equations are derived by applying the Hankel and Laplace transform techniques. These general solutions are then used to solve the corresponding boundary value problem for the consolidation of a transversely isotropic soil layer. Once solutions in the transformed domain have been found, the actual solutions in the physical domain for displacements and stress components of the solid matrix, pore-water pressure and fluid discharge can finally be obtained by direct numerical inversions of the integral transforms. The accuracy of the present numerical solutions is confirmed by comparison with an existing exact solution for an isotropic and saturated soil that is a special case of the more general problem addressed. Further, some numerical results are presented to show the influence of the nature of material anisotropy, the surface drainage condition, and the layer thickness on the consolidation settlement and the pore pressure dissipation.     

Rapid Construction and Settlement Behavior of Embankment Systems on Soft Foundation Soils

The I-15 Reconstruction Project in Salt Lake City, Utah required rapid embankment construction in an urban environment atop soft lacustrine soils. These soils are compressible, have low shear strength, and require significant time to complete primary consolidation settlement. Because of this, innovative embankment systems and foundation treatments were employed to complete construction within the approved budget and demanding schedule constraints. This paper evaluates and compares the construction time, cost, and performance of three embankment/foundation systems used on this project: (1) one-stage mechanically stabilized earth (MSE) wall supported by lime cement columns; (2) expanded polystyrene (geofoam) embankment with tilt-up panel fascia walls; and (3) two-stage MSE wall with prefabricated vertical drain installation and surcharging. Of the technologies evaluated, the geofoam embankment had the best performance based on settlement and rapid construction time considerations, but is more costly to construct than a two-stage MSE wall with PV drain foundation treatment. The one-stage MSE wall with lime cement treated soil was the most costly, and did not perform as well as expected; thus, it had only limited use on the project.     

Simplified Method for Consolidation Rate of Stone Column Reinforced Foundations

Field observations and numerical studies demonstrated that stone columns could accelerate the rate of consolidation of soft clays. A simplified method for computing the rate of consolidation is presented in this paper by assuming that stone columns; (1) are free draining; (2) have higher drained elastic modulus than soft clay; and (3) are deformed 1D. The formats of the final solutions in vertical and radial flows are similar to those of the Terzaghi 1D solution and the Barron solution for drain wells in fine-grained soils, respectively. Modified coefficients of consolidation are introduced to account for effects of the stone column-soil modular ratio. The new solutions demonstrate stress transfer from the soil to stone columns and dissipation of excess pore water pressures due to drainage and vertical stress reduction during the consolidation. Comparisons between the results from this simplified method and the numerical study by Balaam and Booker in 1981 exhibit reasonable agreement, when the stress concentration ratio is in the practical range (2–6). The discrepancies in the results from these two methods are discussed. This paper also includes design charts and a design example.     

软土地基处理方案的技术经济分析

结合井睦高速公路工程项目中的山涧淤泥质土-砂混合软土地基,通过采用松木桩、CFG桩和水泥深层搅拌桩对该软基进行处理,并对这3种软基处理方案进行技术比较与经济分析,以便得到最优处理方案.研究表明:松木桩的造价在3种处理方式中是最高的,而且松木桩受处理深度和承载力的限制,它不是最佳的选择方案;在相同的条件下,CFG桩法处理地基的造价是水泥深层搅拌法处理地基的1.4倍左右,造价较为昂贵;最终确定选用水泥深层搅拌桩法加固该工程的淤泥质土-砂混合软土地基.     

深层搅拌桩在软基加固中的应用

深层搅拌法是以水泥作固化剂，通过专用的深层搅拌机械，在地基深部就地将土和水泥搅拌而成加固体，应用该法处理地基可增加地基承载力，减小沉降。文章结合工程实例，简述了深层搅拌法处理软土地基的设计步骤。     

Responses of Excess Pore Water Pressure in Soft Marine Clay around a Soil–Cement Column

The soil ground treated by deep cement mixing (DCM) in the field normally consists of cement–soil mixed columns and untreated soils. Although many attempts have been made, research on the consolidation behavior of the treated soil ground has been limited. To better understand the consolidation process of the DCM treated ground, in this study, an axisymmetric physical model test with full instrumentation was carried out. The physical model ground consisted of a central cement–soil column and surrounding soft soil. Excess pore water pressures in the soil and vertical pressures carried by the DCM column and the untreated soil were recorded throughout the test. Responses of excess pore pressure under loading and unloading stages are highlighted. Based on the data analysis, it is revealed that the improved ground consolidates faster than the pure soil ground. The major reason is considered to be that the DCM column reduces the vertical stress increment in the soil and results in a lower value of excess pore pressure. The decrease of excess pore water pressure in the middle of soil seems to be controlled by the reduction of total stress in the soil. Besides, a delayed pore water pressure increase was observed in the early period of the loading stage. During the unloading stages, the stress on the DCM column was found to be reduced by approximately the same magnitude as the decrease in the vertical pressure on the model ground. In addition, a small residual pore pressure in the soil was also found at the end of the unloading stages.     

Consolidation of Soft Clay Foundations Reinforced by Stone Columns under Time-Dependent Loadings

This paper presents an analytical solution for the consolidation of soft soil foundations reinforced by stone columns under time-dependent loadings. The differential equations of the foundations reinforced by stone columns are obtained including smear and well resistance under arbitrary applied loadings. The closed-form solutions of pore pressure and the overall average degree of consolidations are obtained for some common types of loadings, such as step loading, ramp loading, and cyclic trapezoidal loading. By solving the equations using a semianalytical method, the comparisons agree very well with the existing analytical solutions, which verify the correctness and accuracy of the proposed methods. Using the solutions obtained, some selected charts are presented and the relevant consolidation behavior is investigated and discussed.     

Deep Mixing Induced Property Changes in Surrounding Sensitive Marine Clays

This paper presents a field study of installation effects of deep mixed columns on properties of the sensitive Ariake marine clay. Cone penetration tests were performed in the field to evaluate the change in the strength of the surrounding clay with time. Soil samples were taken before and after column installation to evaluate variations of physical, mechanical, and chemical properties of the surrounding clay. Test results indicated that the water content of the surrounding clay decreased while the concentration of cations increased as sampling locations approached the columns. Shear strength of the surrounding clay decreased during the installation but recovered after a short period of curing. Shear strength continued to increase with time over a period of 70?days. Based on the regression results, the surrounding soil after the installation of the columns took approximately 10?days to recover to the strength value before installation. On average, the shear strength of the surrounding clay increased over the original strength by approximately 23% after 40?days and 50% after 70?days, respectively. Discussion is presented on strength changes and key influence factors including soil disturbance and fracturing, thixotropy, consolidation, and diffusion of cations from deep mixed columns to the surrounding clay.     

Transient Flow into a Partially Penetrating Well during the Constant-Head Test in Unconfined Aquifers

This study derives a semianalytical solution for drawdown distribution during a constant-head test at a partially penetrating well in an unconfined aquifer. The constant-head condition is used to describe the boundary along the screen. In addition, a free-surface condition is used to delineate the upper boundary of the unconfined aquifer. The Laplace-domain solution is then derived using separation of variables and Laplace transform. This solution can be used to identify the aquifer parameters from the data of the constant-head test when integrated with an optimization scheme or to investigate the effects of vertical flow caused by the partially penetrating well and free-surface boundary in an unconfined aquifer.     

Performance Monitoring of a Rammed Aggregate Pier Foundation Supporting a Mechanically Stabilized Earth Wall

The use of rammed aggregate pier (RAP) foundations for support of retaining walls and earth fill embankments has increased in recent years to become a geotechnical solution for rapid construction of earth structures in soft ground conditions. A nominal 6-m mechanically stabilized earth wall was constructed over piers installed in relatively compressible soil to investigate the performance of RAP foundation elements in terms of stress-deformation and settlement behaviors for such applications. Geotechnical instrumentation consisting of total earth pressure cells, settlement plates, and vibrating wire piezometers was installed within the pier elements and at the foundation surface for short- and long-term monitoring of pier response. Monitoring data indicate: (1) mobilization and concentration of vertical stress on pier elements and matrix soil; and (2) load transfer response for the boundary condition associated with support of geogrid-reinforced earth fill. The practical implications of the experimental research findings are briefly discussed.     

Coupled Mechanical and Hydraulic Modeling of Geosynthetic-Reinforced Column-Supported Embankments

Geosynthetic-reinforced column-supported (GRCS) embankments have increasingly been used in the recent years for accelerated construction. Numerical analyses have been conducted to improve understanding and knowledge of this complicated embankment system. However, most studies so far have been focused on its short-term or long-term behavior by assuming an undrained or drained condition, which does not consider water flow in saturated soft soil (i.e., consolidation). As a result, very limited attention has been paid to a settlement-time relationship especially postconstruction settlement, which is critical to performance of pavements on embankments or connection between approach embankments and bridge abutments. To investigate the time-dependent behavior, coupled two-dimensional mechanical and hydraulic numerical modeling was conducted in this study to analyze a well-instrumented geotextile-reinforced deep mixed column-supported embankment in Hertsby, Finland. In the mechanical modeling, soils and DM columns were modeled as elastic-plastic materials and a geotextile layer was modeled using cable elements. In the hydraulic modeling, water flow was modeled to simulate generation and dissipation of excess pore water pressures during and after the construction of the embankment. The numerical results with or without modeling water flow were compared with the field data. In addition, parametric studies were conducted to further examine the effects of geosynthetic stiffness, column modulus, and average staged construction rate on the postconstruction settlement and the tension in the geosynthetic reinforcement.     

Plane Strain and Axially Symmetric Consolidation in Unsaturated Soils

A method is presented for the analysis of coupled consolidation in unsaturated soils due to loading under conditions of plane strain as well as axial symmetry. The method is based on the transformation of the governing differential equations by the Fourier transform, when the soil system is deformed under plane strain conditions, or Hankel transform for problems exhibiting axial symmetry. The effect of such transformations is to simplify considerably the solution from a computational point of view. In addition, using these transformations the same differential equations can be used to analyze consolidation under both the above conditions. Results are presented to point out some aspects of the consolidation in unsaturated soils generated by the application of strip as well as circular loads.     

Use of Deep Cement Mixing to Reduce Settlements at Bridge Approaches

Differential settlements between a bridge and the backfill behind the abutment have been a major problem in the construction of expressway embankments over a soft clay foundation. Deep cement mixing (DCM) columns with varying lengths were used to reduce such differential settlements along the Fu-Xia Expressway, Fujian Province, China. The performance and the feasibility of the DCM method were investigated in a trial embankment constructed prior to the actual construction. This paper presents the results of the instrumentation including total settlements, multipoint settlements, soil pressures on both the DCM columns and soil surface, pore-water pressures, and lateral movements obtained from in situ monitoring. The strength of the soil-cement from laboratory mix tests and from in situ quality control tests on DCM columns is presented in the paper. Study results indicate that DCM columns with varying lengths were a simple and effective method to reduce the total and differential settlements from a soft clay foundation at a bridge approach.     

Consolidation of Clays for Variable Permeability and Compressibility

The theory of consolidation has its origin in the effective stress concept developed by Terzaghi, which was derived based on several assumptions to arrive at a simplified theory. Considering the limitations involved in Terzaghi’s theory, various attempts are being made by researchers to idealize the problem to represent various field situations. This paper presents a more generalized theory for vertical consolidation of a compressible medium of finite thickness, subjected to suddenly applied loading, assuming small strain and no creep. The theory assumes small deformations, and thus the settlement is governed by vertical strains generated by an increment of loading, neglecting the effect of self-weight of the soil. The analytical solution presented here takes into account e–log?K and e–log?σ′ linear responses under instantaneous loading. With Cc as the slope of the e–log?σ′ line and M the slope of the e–log?K line, a parameter Cc/M is identified which is found to govern the rate of consolidation. In this paper, an analytical closed form solution is obtained for vertical consolidation considering the variation in the compressibility and permeability.     

Finite-Element Parametric Study of the Consolidation Behavior of a Trial Embankment on Soft Clay

The paper presents a case study for numerical analysis of the consolidation behavior of an instrumented trial embankment constructed on a soft soil foundation. Details are given to the geological profile, field instrumentation, laboratory test results, and determination of soil parameters for numerical modeling. Embankment settlement is estimated based on one-dimensional consolidation analysis and nonlinear finite-element analysis following Biot’s consolidation theory. Finite-element results are calibrated against the measured field data for a period of more than 3?years. Development and dissipation of excess pore pressure, long-term settlement, and horizontal displacement are predicted and discussed in light of sensitivity of embankment performance to some critical factors through a parametric study.     

Staged Construction and Settlement of a Dam Founded on Soft Clay

Alibey Dam is located near Istanbul in Turkey on the Alibey Stream, 4.5?km north of its point of confluence with Golden Horn, an ancient submerged river mouth. It was constructed as an earthfill dam over 30-m-thick soft valley sediments. Before the construction of the dam, field and laboratory tests were performed to determine the geotechnical characteristics of the foundation soils. During the construction and many years after the construction of the earthfill embankments, including the cofferdams and the intermediate fills, the response of the foundation soils was monitored by extensive field instrumentation generating a unique long-term (over 25 years) database. With proper instrumentation and careful monitoring of the collected data, field construction rates could be adjusted and the earth dam was safely constructed on the thick soft deposits. Approaches to settlement prediction were evaluated in a historical context, starting with the simplified one-dimensional approach available at the time of construction to more sophisticated analyses including the employment of modern numerical methods, in terms of the recorded data. Standard subsurface exploration and field testing supplemented with conventional laboratory testing provided the relevant material parameters that were used in the finite element method. The only exception to this was the overall hydraulic conductivity of the deposit, which controlled the rate of consolidation. Early field observations were used to assign the appropriate hydraulic conductivity. An elastoplastic soil model in a coupled analysis of consolidation was employed in the analysis that yielded realistic predictions of field behavior in response to the complex construction history. The accurate prediction and monitoring of the behavior of soft and thick soil layers subjected to staged construction, as in the case of Alibey Dam, is very important for planning of the construction as well as the expected behavior after construction.     

Dynamic Stiffness of Foundations on Inhomogeneous Soils for a Realistic Prediction of Vertical Building Resonance

The aim of this contribution is a practice-oriented prediction of environmental building vibrations. A Green’s functions method for layered soils is used to build the dynamic stiffness matrix of the soil area that is covered by the foundation. A simple building model is proposed by adding a building mass to the dynamic stiffness of the soil. The vertical soil-building transfer functions with building-soil resonances are calculated and compared with a number of measurements of technically induced vibrations of residential buildings. In a parametrical study, realistic foundation geometries are modeled and the influence of incompressible soil, deep stiff soil layering, soft top layers, and increasing soil stiffness with depth is analyzed. All these special soil models reduce the resonant frequency compared to a standard homogeneous soil. A physically motivated model of a naturally sedimented soil has a stiffness increasing with the square root of the depth and yields a foundation stiffness that decreases with foundation area considerably stronger than the relatively insensitive homogeneous soil. This soil model is suited for the Berlin measuring sites and reproduces satisfactorily the experimental results.     

Plate Load Tests on Cemented Soil Layers Overlaying Weaker Soil

This paper addresses the interpretation of plate load tests bearing on double-layered systems formed by an artificially cemented compacted top soil layer (three different top layers have been studied) overlaying a compressible residual soil stratum. Applied pressure-settlement behavior is observed for tests carried out using circular steel plates ranging from 0.30 to 0.60 m diameter on top of 0.15 to 0.60-m-thick artificially cemented layers. The paper also stresses the need to express test results in terms of normalized pressure and settlement—i.e., as pressure normalized by pressure at 3% settlement (p/p3%) versus settlement-to-diameter (δ/D) ratio. In the range of H/D (where H = thickness of the treated layer and D = diameter of the foundation) studied, up to 2.0, the final failure modes observed in the field tests always involved punching through the top layer. In addition, the progressive failure processes in the compacted top layer always initiated by tensile fissures in the bottom of the layer. However, depending on the H/D ratio, the tensile cracking started in different positions. The footing bearing capacity analytical solution for layered cohesive-frictional soils appears to be quite adequate up to a H/D value of about 1.0. Finally, for a given project, combining Vésic’s solution with results from one plate-loading test, it is possible (knowing of the demonstrated normalization of p/p3%-δ/D, where the pressure-relative settlement curves for different H/D ratios produce a single curve for all values of H/D) to estimate the pressure-settlement curves for footings of different sizes on different thicknesses of a cemented upper layer.