Soft Clayey Soil Research Articles

A plane strain yield design approach to stability analysis of a column‐reinforced soil foundation under inclined loading

SummaryThe ultimate bearing capacity problem of column‐reinforced foundations under inclined loading is investigated within the framework of static and kinematic approaches of yield design theory. The configuration of a native soft clayey soil reinforced by either a group of purely cohesive columns (lime‐column technique) or a group of purely frictional columns (stone‐column technique) is analyzed under plane strain conditions. First, lower bound estimates are derived for the ultimate bearing capacity by considering statically admissible piecewise linear stress distributions that comply with the local strength conditions of the constitutive materials. The problem is then handled by means of the yield design kinematic approach of limit analysis through the implementation of several failure mechanisms, allowing the formulation of upper bound estimates for the ultimate bearing capacity. A series of finite element limit load solutions obtained from numerical elastoplastic simulations suggests that the predictions derived from the kinematic approach appear to be more accurate than the estimates obtained from the static approach. Comparison with available results obtained in the context of yield design homogenization demonstrates the accuracy of the proposed direct analysis, which may therefore be viewed as complementary approach to homogenization‐based approaches when a small number of columns is involved. Copyright © 2016 John Wiley & Sons, Ltd.

Performance of small group of geosynthetic-reinforced granular piles

ABSTRACTGranular piles are frequently used as a method of improving soft grounds as they provide increased bearing capacity and reduce foundation settlements. However, in very soft clayey soils, they may not derive their load-carrying capacity by low confining pressure provided by the surrounding soil. In such circumstances, granular piles may be reinforced with suitable geosynthetic to increase its load-carrying capacity and to reduce excessive bulging. In this study, the performance of small group of geosynthetic-reinforced granular piles (GRGPs) is examined in terms of load-carrying capacity, settlement, and modulus by laboratory model tests. The parameters investigated include modulus of reinforcement material, area replacement ratio (ARR) based on the column diameter and reinforcement length. The results indicated that increasing the modulus of the reinforcement and the ARR based on the column diameter enhances the overall performance of the GRGP group. It was also observed that reinforcement on top portion of the granular pile is sufficient to substantially increase the load-carrying capacity of granular pile group.

Experimental and Numerical Analysis of Geosynthetic-Reinforced Floating Granular Piles in Soft Clays

The installation of reinforced granular piles is a commonly adopted technique to improve load carrying capacity and reduce settlements in very soft clayey soils. This paper presents results of a series of laboratory model tests and numerical analysis carried on geosynthetic reinforced granular pile under short term loading. Unit cell concept has been adopted. Laboratory model tests were conducted on unreinforced, vertical encased, reinforced with horizontal strips and combined vertical-horizontal reinforced granular piles. The loading was applied either over the entire cylindrical tank area or only over the area of granular piles. The effects of various parameters such as reinforcement, encasement stiffness, shear strength of clay, length and diameter of granular piles have been studied. Experimental results in the form of vertical load intensity-settlement relationship have been compared with that obtained from PLAXIS 3D. The results of laboratory model tests indicated significant influence of reinforcement on the ultimate load intensity of granular piles and ultimate bearing capacity of treated ground. Lateral bulging in reinforced granular piles has also been controlled by incorporating geosynthetic materials.

Behavior of polymer columns in soft clayey soil: A preliminary study

Deep soil mixing with cement and cement-lime mixtures has been widely used for decades to improve the strength of soils. In this study, small-scale laboratory model tests of polymer columns in soft clayey soil were conducted to evaluate the feasibility of using various polymeric compounds as binders in deep soil mixing. Floating and end bearing polymer columns were used to examine the load-settlement relationship of improved soft clayey soils for various area replacement ratios. The results indicate that polymer columns show good promise for use in deep mixing applications.

Comparison of Effects of Using Geosynthetics and Lime Stabilization to Increase Bearing Capacity of Unpaved Road Subgrade

So many soil improvement methods have been developed in order to increase bearing capacity of superstructure of the road to be constructed on the soft clayey road base soils, decrease settlements, and increase other strength specifications (CBR,k,MRvalues, etc.). In this paper, lime stabilization of clayey road base soil with high water content and its improvement with geosynthetics (geocell + geotextile) reinforcement and comparisons of these two different improvement methods were made. For this purpose, plate loading experimental comparisons of clayey soil, which had high water content by 10% increasing the optimum water content, were made after it was improved with lime at the rates of 3, 6, and 12%, geotextile reinforcement, geocell reinforcement, geosynthetics reinforcement, and geosynthetics reinforcement + lime stabilization at various rates. It was understood that these improvement methods will not yield sufficient results on clayey soils with high water content on their own, and method of improvement with lime and then reinforcement with geosynthetics yields better results on these types of soils. Only one improvement state among ten different states examined in this study gave the sufficient results for the soil to be used for unpaved roads.

LABORATORY ASSESSMENT OF THE ELECTRO-OSMOTIC CONSOLIDATION TECHNIQUE FOR URMIA LAKE SEDIMENTS *

The electro-osmotic consolidation is a method to improve geotechnical properties of soft clayey soils. The feasibility of electro–osmotically induced consolidation can be examined through the soil coefficient of electro–osmotic permeability (ke) and the electro–osmotic coefficient of water transport (ki). Using an electro-osmotic testing cell, ke and ki parameters were measured in Urmia Lake west coast sediments. Three electric field intensities of 40, 60 and 80 V/m were used and salinity and pH change was monitored in cathode and anode reservoirs during the tests. ke and ki increased when soil void ratio increased. The results showed that increasing applied electric field intensity increased the rate of water pH and salinity change at cathode reservoir. The results for soil hydraulic conductivity (kh) and ke showed that kh is in the range of 10-10 m/s to 10-9 m/s for the range of void ratios between 0.5 to 0.8, respectively, and ke is in the range of 5.5×10-9 to 6.6×10-9 m2/sV for the range of void ratios between 0.72 to 0.83, respectively. The ratio of ke/kh controls electro–osmotically induced negative pore water pressure. The higher the ratio, the higher the pore water pressure difference and the faster the consolidation is. The range of ke/kh was between 5.2 to 9.4 m/V at the void ratios between 0.83 and 0.72, respectively. The ratio of ke/kh increased when void ratio decreased. The high range of ke/kh proved the effectiveness of electro-osmosis consolidation for the Urmia Lake sediments.

Assessment of concrete maturity and overturning mechanism of primary diaphragm walls in very soft clay during cold winter

Many underground excavation failures are caused by severe water inflow due to the piping through the opening in the diaphragm wall. The opening can be created by slurry pocket or air pocket during concrete placement by tremie method. This type of defect in diaphragm wall can be detected by sonic logging to show the condition of trench wall and by measuring actual concrete dosage during placing. The same detecting method can be used in two very rare case histories where just-completed primary panels in very soft and thick clay deposit were overturned from the adjacent tremie concreting secondary panel during cold winter. The abnormal increasing concrete dosage and the smooth trench wall profile of secondary panel from sonic logging identify the occurrence of this incident. The causes to trigger this incident include (1) panel size, (2) weather condition, (3) tremie concreting speed of secondary panel, (4) initial and final setting times of concrete in trench, (5) roughness of trench wall, etc. Since the temperature for the two case histories is very low during tremie concreting, the initial setting time of concrete in secondary panel, given by equivalent age maturity function, is much longer than the time to place the concrete in secondary panel. To clarify the failure mechanism, the force-equilibrium analysis of inclined primary panel is conducted taking into account the driving force induced by the concrete pressure in the unset secondary panel and the resistant force given by the adhesion between inclined primary panel and soft clayey soil. It can be concluded from this investigation that the concrete in the secondary panel due to lower concrete curing temperature still remains unset or in plastic form and this will generate much larger driving force to push the adjacent just-completed primary panel to tilt. The measures to prevent similar incidents from happening are also suggested in this paper.

Electrokinetics to Modify Strength Characteristics of Soft Clayey Soils: A Laboratory Based Investigation

The effects of Electrokinetic (EK) processing on compressive strength characteristics of two types of soils were investigated using laboratory experimental models. Soils were collected from soft alluvial soil deposit (Soil S1) and basaltic soil deposit (Soil S2) in central Victoria, Australia. A layer of soil was placed in glass tanks (90cm×18cm plan area) and compacted to a known density and water content typical of field conditions. Using electrodes inserted into the soil, a direct current was passed across the soil under various voltage gradients (0.5, 1.0, 2.0V/cm) for periods of 7, 14, 30 and 60 days. Throughout the experimental duration, test samples were supplied with a continuous flow of water via the hollow core anode from external reservoirs. Unconfined compression (UC) tests were conducted on original (untreated) soils and EK processed soils.From the test results it is noted that, soil compressive strength increases with the increasing processing time and increasing voltage gradients, at various rates. Under certain voltage gradients and processing times, around 175% and 200% strength increases are observed. In general, stress increases of at least 30% or more are reported for both soils under all test conditions.It is apparent that the variation in strength can be attributed to several complex and interrelated processes that become active under EK processing. These may include, (i) Electroosmotic advection - When a soil is subjected to EK processing with an open electrode configuration, the water content of the soil varies predominantly due to the electroosmotic advection while natural drying and evaporation could also add to the decrease in water content to some extent, depending on the time and environmental conditions such as temperature and humidity. The test results show that with the decrease in water content, there is a corresponding increase in the strength. (ii) Electromigration - The electromigration of charged ions and their interaction with clay minerals can also affect the soil strength due to the variations in the DDL (Diffused Double Layer) ionic concentration and subsequent modifications in the soil structure. (iii) Ionic Diffusion and Aging - After the complete termination of EK processes, the ionic concentrations still continued to modify at a slower rate. This is considered to be due to the ionic diffusion. In this phase too, cementation bonds may continue to develop that could contribute to the increase in soil strength. During this period, two other processes, i.e. natural drying of soil and aging may continue that can also affect the variation of soil strength.

Cooperation Between the N. M. Gersevanov Scientific-Research Institute of Foundations and Underground Structures and Geotechnical Engineers from the Chinese People's Republic

Scientific and methodical cooperation in the field of geotechnical construction between the Research Center ("Civil Engineering") and the N. M. Gersevanov Scientific-Research Institute of Foundations and Underground Structures (NIIOSP) was continued in 2014 at the Institute of Foundation Engineering of the CABR, and the Heilongjiang Cold Region Construction Science and Research Institute. A Chinese-Russian symposium was held to discuss results of scientific-research studies, and also exchange experience acquired on construction projects in the Chinese People's Republic under complex geologic engineering conditions. Russian and Chinese specialists attended the symposium (Fig. 1). In a paper submitted by Director of the NIIOSP Igor V. Kolybin Ph.D (Eng.) "Lessons learned from emergency situations during construction of pits under urban conditions," basic errors during design and construction are presented, and failures and emergency situations during installation of pits in Russia, and also abroad are analyzed. Factors resulting in emergency situations − design errors, lack of monitoring, redesign during construction, neglect of risks associated with rupture of water-carrying utilities, temperature and climatic effects, etc. were examined. Moreover, the critical role of geologicengineering surveys, selection of soil-performance models, and determination of the parameters in the mass is noted for assurance of construction safety. A paper delivered by Assistant Director of the College of Natural Resources and the Environment at the Northeastern University of China Professor Zhao Wen raised questions concerning protection of buildings and structures during sheet-pile opening of subway tunnels. In the paper "Construction experience on peaty soils and in alluvial areas," delivered by the Manager of the Laboratory "Beds and foundations on soft soils" (NIIOSP) Farshed F. Zekhniev Ph.D (Eng.), light is shed on problems associated with propagation of highly compressible soils and classification of their properties. Basic principles and peculiarities of the design of beds for structures on organo-mineral and organic soils were examined in conformity with building Code 22.13330.2011 "Beds of buildings and foundations." The construction site of the Logistical Center in Riga on soft silty soils for which a plan for bed preparation with a temporary surcharging fill with use of vertical strip drains is cited as an example. A system is presented for monitoring deformations of the soil mass during its consolidation. Verification of computed (predicted) data indicated good convergence with settlement measurements and the time required for the initial (filtration) consolidation. Professor of the Scientific-Research Institute of Engineering Surveys and Design in the City of Shenzhen Liu xiao min discussed projects on which consolidation of soft saturated clayey soils in the beds of buildings and structures was carried out under a vacuum. As a result, consolidation of soils by an accelerated method during which vigorous evacuation of water takes place, and effective stresses in

Performance of Sand-Treated Clay Subgrade Supporting a Low-Volume Flexible Pavement

Low-volume roads constitute a major portion of the world's transportation infrastructure and hence require an economical and effective pavement design. When constructed on unstable and soft subgrades, these roads are susceptible to premature pavement failures and require subgrade stabilization. The presented research focuses on mechanical stabilization of soft clay with graded sand that has been adopted as a base course for pavements. For this study, a local soft clayey soil from Dallas was mixed with various fractions of a graded sandy soil, and the stiffness behavior of the compounded material was studied. The soil mixtures were then studied for their basic soil characterization, including standard Proctor compaction studies. Statically compressed soil specimens were prepared and then evaluated for their unconfined compressive strength and resilient moduli. Resilient moduli results were used in the design of low-volume roads, both with aggregate base and with no aggregate base. Results show that the variation of sand content in soft soil considerably affects the strength and stiffness properties of the subgrade. The effects of improved subgrade performance on the aggregate base and surface layers of the low-volume pavement are highlighted.

Stability analysis of homogenized stone column reinforced foundations using a numerical yield design approach

This paper deals with the ultimate bearing capacity of soft clayey soils, reinforced by stone columns, analyzed in the framework of the yield design theory. Since such geotechnical structures are almost impossible to analyze directly due to the strong heterogeneity of the reinforced soil, an alternative homogenization approach is advocated here. First, numerical lower and upper bound estimates for the macroscopic strength criterion of the stone column reinforced soil are approximated in a rigorous way with convex ellipsoidal sets, which makes the approximated criteria much easier to handle than the initial ones. Then, both static and kinematic approaches are carried out numerically on the homogenized problem using the above approximated macroscopic strength domains in an adapted finite element method. The whole numerical procedure is applied on one classical geotechnical problem: the ultimate bearing capacity of stone column reinforced foundations. The strength capacity of the structure is rigorously framed and the efficiency of the proposed numerical method is highlighted in terms of accuracy and calculation time.

Settlement prediction and behaviour of pile foundations in deep clayey soil deposits

A new measurement technique is used to determine the settlement of bridge pile foundation and the thickness of deep compressed soft layer. The finite element Plaxis 3D foundation program is used in the analysis with a proposed empirical equation to modify the input parameters represented by the soil compression modulus. The results of the numerical analysis using the proposed empirical equation provide insight to the settlement analysis of pile groups in soft clayey soils; consequently, the finite element Plaxis 3D program can be a useful tool for numerical analysis. The numerical analysis is modified by adjusting the calculation of compression modulus from those obtained under pressure between 100–200 kPa by which the results of the settlement are modified and thus matching the realistic measurements. The absolute error is 3 mm which is accepted compared with the last researches. This scenario can be applied for the similar problems in the theoretical applications of deep foundations.

A Change of Undrain Shear Strength of Soft Ground during Consolidation Process

The purpose of this study is to investigate the undrain shear strength increment during consolidation process of soft clayey soils. Thirty kinds of laboratory triaxial tests have been performed using undisturbed and remolded Ariake clay samples with different degree of consolidation and 5 kinds of confining pressure. Test results show that well known linear equation proposed by Yamanouchi et al.(1982) is overestimated the strength of undisturbed soft clay ground in the process of consolidation. A new simple and reasonable exponential equation proposed in this paper.

Несущая способность забивных свай в слабых водонасыщенных грунтах с учетом фактора времени

Несущая способность забивных свай в слабых водонасыщенных грунтах с учетом фактора времени

Experimental Study on the Behavior of Skirted Foundation Rested on Soft Clayey Soils

Experimental Study on the Behavior of Skirted Foundation Rested on Soft Clayey Soils

Building damage during nearby construction: Forensic analysis

The paper presents a case history of a 5-storey building, seriously damaged during construction of a nearby structure. Founded on spread footings on soft clayey soil, the building was functional for nearly 30years. When construction of an adjacent 4-storey RC building begun, its infill walls sustained shear failure. The shear cracks were inclined at 45°, being characteristic of differential settlements, and the neighboring construction was deemed to be the cause of failure. During the present forensic investigation, additional cracks were detected, also inclined at 45° but towards the opposite direction. Internal floor measurements were conducted, revealing that the maximum differential settlement of 5cm is accompanied by 2.4cm of differential settlement to the opposite direction. A number of construction defects were detected, with the most important being the superficial foundation of the building and the absence of tie beams. To quantify the contribution of each factor to the observed failure, the neighboring structures are simulated through finite elements, modeling nonlinear soil response with a Cam-clay model, validated against the conducted measurements. It is shown, that the shear failure of the infill walls is primarily due to construction defects, with the neighboring construction playing a secondary role.

Ground improvement efficiency and back-analysis of settlements

The ground improvement works carried out for the construction of five oil tanks at the Chiriqui Grande Phase II Oil Terminal, Chiriqui Grande, Panama, and their results are presented herein. The site is underlain by some 28 m of loose/soft, poorly graded, silty sands to clayey sands and silts. Ground improvement was used to provide the required bearing capacity and reduce the expected total and differential settlements. To fulfil the technical specifications, two ground improvement techniques were used in association with preloading: wick drains to full depth of the soft layers and stone columns in the upper 18 m. Stone columns are commonly installed as partial replacement to improve a soft ground, increasing its bearing capacity and accelerating its primary consolidation. Wick drains are used to accelerate the consolidation of soft clayey soils. Particular emphasis was given to the behaviour of the treated ground under the preloading fill: a back-analysis was carried out on measured settlements and pore pressures; the improvement of the ground and the degrees of consolidation achieved at the end of the preloading are highlighted; some correlations were derived and they provide an interpretation of the overall behaviour of the improved ground; estimates of the settlements for the short and long term are presented, the short term being that of the hydrotest and the long term being the design life of the oil terminal. They are based on results of preloading for their primary consolidation part, and on reasonable assumptions for their long-term creep (secondary consolidation) part. The estimates are compared to the actual hydrotest survey results.

Cyclic Elastoviscoplastic Constitutive Model for Clay Considering Nonlinear Kinematic Hardening Rules and Structural Degradation

AbstractA cyclic elastoviscoplastic constitutive model for clayey soils is proposed based on the nonlinear kinematic hardening rules and considering the structural degradation. The performance of the model is verified through the undrained triaxial test simulation of soft clay samples under cyclic and monotonic loading conditions and the cyclic compression test. The simulated results are compared with the experimental data through stress-strain relations and stress paths. The simulated results have shown a good agreement with the experimental data, which indicates the capability of the proposed model to reproduce the cyclic behavior of soft clayey soils.

Consolidation settlement of floating-column-improved soft clayey deposit

The interaction behaviour between a floating column and the surrounding soil, with or without a surface cement stabilised slab, has been investigated by laboratory model tests as well as finite-element analysis (FEA) using an axisymmetric unit cell model. Based on the test and the FEA results, a method for calculating the consolidation settlement–time curve of floating-column-improved soft clayey subsoil has been developed. This method has been modified from the earlier method of Chai and Pongsivasathit by treating a part of the column improved layer with a thickness of Hc as an unimproved layer in settlement calculation to consider the effect of possible penetration of the column into the soft soil layer below the column. An explicit equation for calculating the value of Hc has been proposed as a function of area improvement ratio (α), depth improvement ratio (β), load intensity (p) and the undrained shear strength (su) of the soft clayey soil. The validity of the method has been checked using the laboratory model test results as well as four field case histories in Japan.

Deep Foundation Numerical Analysis Using Modified Compression Modulus

This paper is a study on the behavior and prediction of settlement values of bridge pile foundations due to con-struction loads. Field test results show new field technique using the single point of account settlement meter to estimate the thickness of compressed layer in the deep soft soils which is considered as a difficult task in the field. The settlement is predicted using hyperbolic model and statistical regression. The statistical models indicate that the structure would re-main safe for a long period of time, and the field measurements are compared with the hyperbolic model results and those predicted by the statistical regression. The finite element Plaxis 3D Foundation program is used in the analysis with a new empirical equation to modify the input parameters represented by the soil compression modulus. The foundation soils are modeled with the Mohr-Coulomb plasticity material model. The piles are represented with pile elements, between the pile and the surrounding soil, interface elements are automatically generated by the program. In the analysis, the effects due to soil stiffness, pile length and pile spacing are considered. The calculated results for the simulation of the pile installation sequence are compared with the measured results obtained from the field monitoring. The results of the numerical analy-sis using the proposed empirical equation provide insight to the settlement analysis of pile groups in soft clayey soils and the finite element Plaxis 3D program can be a useful tool for numerical analysis. In this paper, the numerical analysis cal-culations are modified using a new empirical equation to calculate the compression modulus from those obtained from the test which modify the results of the settlement and thus become close to the reality. Finally, the numerical finite element analysis produced logical and conservative results as compared to the statistically derived equations and those calculated by hyperbolic model analysis. This scenario can be applied to the similar problems in the theoretical applications of bridge foundations.