Undrained Shear Strength Ratio Research Articles

Stress-Dependent Shear Strength of Resedimented Nile Silty Clay

The variation of clay undrained shear strength with consolidation stresses is of paramount importance to geotechnical engineers. For several decades, the normalized shear strength of clays has been the cornerstone of geotechnical design in many applications, such as staged construction of embankments. Recently, different trends of shear strength dependency on consolidation stresses have emerged indicating nonlinear variation. Resedimentation has been a useful and reliable method to investigate stress dependency by generating lab test specimens of fine-grained soils with a minimum sample to sample variation for benchmark studies. Previous work on resedimented Nile silty clay (RNSC) has shown unique features, such as high sand fraction and contradicting index properties used for soil classification. The shear strength of Nile silty clay (NSC) and the effects of the consolidation stresses are explored through a laboratory program on resedimented samples from material excavated within the Nile River flood plains. A portion of the material was processed using mechanical energy, which exhibited a significant increase in the liquid limit and a larger clay-sized fraction of particles from grain-size distribution tests. The measured engineering parameters are compared with comparable studied soils and established empirical correlations to provide insights on mechanical behaviors of the NSC. NSC is found to have a rapid decrease in the undrained shear strength ratio and a rapid increase in the lateral stress ratio with increasing consolidation stress.

Maximum Strain Effect and Secant Modulus Variation of Hemic Peat Soil at large Deformation due to Cyclic Loading

This study presents the findings obtained in post-cyclic behaviour and degradation of shear strength from the static triaxial test, cyclic triaxial test and post-cyclic monotonic triaxial test to study the dynamic loading relationships with the degradation of shear strength after cyclic loading to the maximum strain effect due for Hemic peat soil and aim of this research was to assess the post-cyclic loading condition that brought to the understanding of secant modulus by using dynamic triaxial apparatus. It begins with a visual inspection of fibre characteristics. This is followed by an analysis of static, cyclic, and post-cyclic loading with stress-strain behaviour. Shear strength decreased and notched lower strength than its initial strength. As a matter of fact, PNpt-25 kPa from 1, 2, and 3 Hz are accumulated in the adjacent maximum strain. With regards to this maximum strain, the undrained shear strength ratio shows sequent decreases from higher to lower frequency applied. For instance, PNpt-25 kPa-1Hz to PNpt-25 kPa-3Hz recorded 1.16 to 1.13 undrained shear strength ratios, respectively. The secant modulus (Esec) for all specimens reflects decrement. The secant modulus for BSpt at an effective stress of 100 kPa in static monotonic is about 18.74 MPa, while in post-cyclic, the secant modulus expanded to 19.630 MPa cyclically loaded with 1 Hz. Unfortunately, the secant modulus returned to decline position when higher frequency applied at 2 Hz, where the secant modulus is about 12.781 MPa and continues to decline with 3 Hz at 7.492 MPa. Doi: 10.28991/CEJ-2022-08-10-015 Full Text: PDF

A New Approach to Estimate Bearing Capacity of Strip Footings on Geogrid-Stabilised Granular Layer over Clay

In this study, the finite element method (FEM) is used to estimate the bearing capacity of strip footing on a non-stabilised and geogrid-stabilised granular layer over clay. The FEM model was developed using ABAQUS software that accommodates a range of angles of internal friction and thicknesses of overlying stronger granular layer and strength of underlying weaker clay layer. The results are used to develop design charts with dimensionless parameters in terms of linear-gradient (m) and the ratio of undrained shear strength of clay to effective vertical stress at the base of the granular layer ({{c}_{u}/p}_{0}). The proposed charts also contain trend lines with simple power-rule equations for different angles of internal friction of the granular layer. The charts are validated and compared with the published results in the literature and found to be closely similar. A design example is provided to illustrate the comparison of the design curves and to benchmark with other established design methods. The comparative results reveal that the proposed approach yields a comparable outcome and predicts reasonable bearing capacity. Moreover, the developed design charts and the associated equations are relatively simple and easy to use. A typical result of the analysis indicates that the use of a geogrid-stabilised granular layer provides a capacity improvement factor (CIF) of 1.1 to 1.6 depending on the soil parameters and the thickness of the granular layer.

Theoretical Equations for the Ratio of Undrained Shear Strength to Vertical Effective Stress for Normally Consolidated Saturated Cohesive Soils

Abstract Two theoretical equations are developed to calculate the ratio of undrained shear strength to the vertical effective stress (the ratio of (su/sv’)) for normally consolidated saturated cohesive soils. The effective stress approach is used as the basis in the development of the theoretical equations. The theoretical equations are developed by relating the total and the effective stress paths. The development of the excess pore-water pressure is quantified using Skempton A and B pore-water pressure parameters. The theoretical equations are developed for two initial stress conditions: (i) an initially hydrostatic condition and (ii) an initially Ko (non-hydrostatic) condition. The performance of the theoretical equations of this study is compared with field and laboratory measurement data obtained from the literature. The close results between the theoretical equations and the measurements show that the theoretical equations of this study can compute the ratio of (su/sv’) well. Using the theoretical equations, the values of the ratio of (su/sv’) commonly used in engineering practice can be explained from the soil mechanics framework. Keywords: Saturated cohesive soils, c/p ratio, normally consolidated soil, undrained shear strength, effective shear strength, theoretical equation. Abstrak Dua persamaan teoritis dikembangkan untuk menghitung rasio kuat geser tak teralirkan dengan tegangan efektif vertikal (rasio (su/sv’)) untuk tanah kohesif jenuh terkonsolidasi normal. Pendekatan tegangan efektif dijadikan dasar dalam pengembangan kedua persamaan teoretis ini. Persamaan teoretis tersebut dikembangkan menghubungkan lintasan tegangan total dan lintasan tegangan efektif. Kenaikan tekanan air pori ekses dikuantifikasi menggunakan parameter tekanan air pori A dan B dari Skempton. Persamaan teoretis dikembangkan untuk dua kondisi tegangan awal: (i) tegangan awal hidrostatik dan (ii) teganan awal Ko (non hidrostatik). Kinerja kedua persamaan teoretis tersebut dibandingkan terhadap data pengukuran lapangan dan pengujian laboratorium yang diperoleh dari literatur. Persamaan teoretis dari studi ini memiliki kinerja yang baik dalam memperhitungan rasio (su/sv’) yang ditunjukkan dengan dekatnya hasil perhitungan menggunakan persamaan teoretis dan hasil pengukuran lapangan maupun pengujan laboratorium. Dengan persamaan teoretis tersebut, nilai rasio (su/sv’) yang biasa digunakan dalam rekayasa praktis bisa dijelaskan secara mekanika tanah. Kata-kata Kunci: Tanah kohesif jenuh, rasio c/p, tanah terkonsolidasi normal, kuat geser tak teralirkan, kuat geser efektif, persamaan teoretis.

Bearing capacity of conical footings on clays considering combined effects of anisotropy and non-homogeneity

ABSTRACT A spudcan is one of the foundation systems widely used to support the load transferred from a jack-up platform for most offshore drilling operations. The spudcan is an axisymmetric footing with a conical shape. The study on the stability effects of anisotropic and non-homogeneous clays on the bearing capacity of this kind of footings is yet to be found in the current literature. This paper presents novel plastic solutions of the bearing capacity factor for conical footings on clays with linearly increasing anisotropic shear strength. The finite element limit analysis is employed to perform the bearing capacity solutions based on three dimensionless parameters including the cone apex angle, the strength gradient ratio, and the ratio between the undrained shear strength ratio obtained from the triaxial compression test and triaxial extension test. The proposed equations in this study provide a reliable solution to improve the current design standard for conical footings in anisotropic and non-homogeneous clays.

Relationships between strength properties and Atterberg limits of fine-grained soils

ABSTRACT Soil strength is responsible for all aspects of soil engineering regarding geotechnical underground constructions or slope stability. As Atterberg limits, are the easiest but at the same time very important properties to measure soil strength, this paper summarised and interpreted over 1600 data both from literature and novel experimental data. Correlations considering peak effective friction angle, undrained shear strength ratio and coefficient of earth pressure at rest with Atterberg limits are presented. Besides, critical state soil mechanics parameters were interpreted by correlating them with the plasticity index and the undrained strength ratio for normal consolidated soils. The goal of this research is to provide a general overview between Atterberg limits and strength parameters of fine-grained soils. Based on the overview, the existing predicted models in the literature need to be carefully considered if utilised for practical use due to some correlation discrepancies. One of the reasons for dissimilarities is due to the fact that such correlations are developed based on the data pertaining to particular sites and therefore a general validity is still missing.

A Random Forest Model for the Prediction of Spudcan Penetration Resistance in Stiff-Over-Soft Clays

Abstract Punch-through is a major threat to the jack-up unit, especially at well sites with layered stiff-over-soft clays. A model is proposed to predict the spudcan penetration resistance in stiff-over-soft clays, based on the random forest (RF) method. The RF model was trained and tested with numerical simulation results obtained through the Finite Element model, implemented with the Coupled Eulerian Lagrangian (CEL) approach. With the proposed CEL model, the effects of the stiff layer thickness, undrained shear strength ratio, and the undrained shear strength of the soft layer on the bearing characteristics, as well as the soil failure mechanism, were numerically studied. A simplified resistance profile model of penetration in stiff-over-soft clays is proposed, divided into three sections by the peak point and the transition point. The importance of soil parameters to the penetration resistance was analysed. Then, the trained RF model was tested against the test set, showing a good prediction of the numerical cases. Finally, the trained RF was validated against centrifuge tests. The RF model successfully captured the punch-through potential, and was verified using data recorded in the field, showing advantages over the SNAME guideline. It is supposed that the trained RF model should give a good prediction of the spudcan penetration resistance profile, especially if trained with more field data.

Stiffness and strength anisotropy of overconsolidated Bootlegger Cove clays

This paper presents the evaluation of the stiffness and strength anisotropy of overconsolidated (OC) Bootlegger Cove Formation (BCF) clays at the Port of Alaska, formerly known as the Port of Anchorage. The stiffness and strength anisotropic material response was evaluated based on triaxial samples equipped with internal instrumentation including a submersible load cell and three subminiature linear variable displacement transducers (LVDTs). Three sets of bender elements were used in this research to measure shear wave velocities for different propagation and polarization directions. The effects of reproducing the stress history of the soil deposit on the stiffness cross-anisotropic behavior of the material are discussed. The laboratory test results are compared with in situ measurements of shear wave velocities based on suspension logging and crosshole and downhole soundings. The results of the experimental program showed that BCF clay is a cross-anisotropic material. Mean stiffness anisotropy ratios ranged from 0.90 to 1.22 and 0.93 to 1.46 for lightly OC and OC conditions, respectively. Strength anisotropy ratios, defined as the ratio of undrained shear strength under triaxial extension to compression, varied between 0.8 and 0.5. It is found that reproducing the stress history of the OC soil deposit during the laboratory reconsolidation stage did not have a significant impact on the initial stiffness anisotropy ratios of the BCF clay.

Behaviors of drag embedment anchor in layered clay profiles

Layered soil profiles can complicate the installation of an offshore foundation and affect its performance. The goal of this study is to further understand the drag trajectory and capacity of a drag embedded plate anchor in layered clay profiles. Drag embedment tests using a model anchor were performed in various test beds made of marine clay with different profiles of undrained shear strength. The trajectory and orientation of the anchor during drag embedment were captured by a magnetometer. This innovative instrument is capable to measure the six-degree-of-freedom movements for buried objects in soil. A prediction model based on the simplified plasticity theory was calibrated against experimental results. The calibrated model can well predict anchor capacity, trajectory and pitch in single-layer soil profiles, including the constant and the linearly increasing shear strength profiles. For the layered clay profiles with a stiff layer interbedded in soft clay, one key factor determining whether or not the anchor can penetrate into the stiff layer is the pitch of the fluke when approaching the stiff layer, and a flattened pitch may be favorable for penetration. The stiff layer depth and the mooring line thickness significantly affect the pitch angle. Moreover, a lower undrained shear strength ratio of the stiff layer over surrounding soft soil facilitates the anchor to penetrate into the stiff layer.

Stress–strain behaviour of bio-desaturated sand under undrained monotonic and cyclic loading

Microbial (or bio-) desaturation is a new method for mitigation of the liquefaction hazard of sand. In this paper, a series of monotonic and cyclic triaxial tests was performed on bio-desaturated sand under undrained conditions to demonstrate that the bio-desaturation method is effective in enhancing the liquefaction resistance of sand under either monotonic or cyclic loading conditions. The test results showed that, under monotonic loading, the undrained shear strength increased by more than two times and the behaviour of soil changed from strain softening to strain hardening when the degree of saturation (Sr) reduced from 100% to 86·4%. The undrained shear strength ratio (su/p′0) has a unique relationship with the peak excess pore water pressure ratio (Δumax/σ′c) regardless of different effective confining stress conditions. Under the applied cyclic load, the liquefaction resistance showed a significant increase when the sample was slightly desaturated. For the same cyclic stress ratio and number of cycles, the accumulated excess pore water pressure (Δu) in the sample with a degree of saturation of 96% was only a quarter of that in the fully saturated sample. The instability behaviour of sand was also studied. The results showed that the slope of the instability line, which is a measurement of the resistance of sand to collapse, also increased with the reduction in degree of saturation under monotonic loading. The failure stresses obtained in cyclic triaxial tests conducted under different cyclic stress ratios coincide with the instability line obtained under monotonic conditions for sand with the same degree of saturation. This provides good support for the use of the instability line to predict the failure stress state of sand for both monotonic and cyclic loading conditions.

A constitutive model for clays and plastic silts in plane-strain earthquake engineering applications

A plasticity model for representing clays and plastic silts, as opposed to purely nonplastic silts or sand, in geotechnical earthquake engineering applications is presented. The PM4Silt model builds on the framework of the stress-ratio controlled, critical state based, bounding surface plasticity PM4Sand model, and is coded as a user defined material for use with the program FLAC. The model was developed to provide reasonable approximations of monotonic undrained shear strength, cyclic undrained shear strength, and shear modulus reduction and hysteretic damping responses. The model does not include a cap, and therefore is not suited for simulating consolidation or reconsolidation settlements (i.e., volumetric strains) or strength evolution with consolidation stress or seismic loading history. The primary input parameters are the undrained shear strength ratio (or undrained shear strength), the shear modulus coefficient, and the contraction rate parameter. All secondary input parameters are assigned default values based on a default calibration, but may be adjusted when calibrating against advanced laboratory test data or performing sensitivity studies. The calibration process is described and illustrated by calibrations for three different normally consolidated, fine-grained soils with plasticity indices ranging from 4 to 20. The model is shown to provide reasonable approximations of behaviors important to many earthquake-engineering applications and to be relatively easy to calibrate.

A predictive model based on an optimized ANN combined with ICA for predicting the stability of slopes

In this study, we optimized artificial neural network (ANN) with imperialist competition algorithm (ICA) for the problem of slope stability design charts. To prepare training and testing datasets for the ANN and ICA–ANN predictive models, an extensive number of limit equilibrium analysis modelings (e.g., for the lower bound, LB, limit analysis and upper bound, UB, limit analysis) was conducted. The analyses were conducted using OptumG2 computer software and implemented on two-layered cohesive soil layer sets. For each of the LB and UB limit analysis, the database consisted of 320 training datasets and 80 testing datasets. Variables of the ICA algorithm such as the number of countries, the number of initial imperialists and the number of decades were optimized using a series of trial-and-error process. The input parameters that used thorough the OptumG2 finite element modeling (FEM) analysis include depth factor (i.e., the ratio of first soil layer thickness to the slope height), slope angle, undrained shear strength ratio where the output was taken dimensionless stability number. The estimated results for both of datasets (e.g., training and testing) from ANN and ICA–ANN models were assessed based on three known statistical indices namely value account for (VAF), root means squared error (RMSE), and coefficient of determination (R2). To evaluate the performance of proposed models, color intensity rating (CER) and total ranking method (TRM), i.e., based on the result of statistical indices, was used. After 72 trial-and-error processes (e.g., sensitivity analysis on some neurons) the optimal architecture of 3 × 6 × 1 were found for both of the ANN–UB and ANN–LB models. As a result, both models presented excellent performance, however according to the introduced ranking system the ICA–ANN model could slightly perform a better performance compared to ANN. Based on R2, RMSE and VAF values of (0.9999, 0.0107 and 99.9924) and (0.9991, 0.0102 and 99.9913), respectively, were found for training and testing of the optimized ICA–ANN–LB predictive model. Similarly, for the ICA–ANN–UB predictive model, values of (0.9984, 0.0129 and 99.9659) and (0.9984, 0.01047 and 99.9915) were obtained for the R2, RMSE and VAF of training and testing datasets, respectively. However, in the ANN model, the R2 and RMSE for both of the training and testing datasets were (0.9982 and 0.01815) and (0.9972 and 0.01748), respectively. This proves a better performance of the ICA–ANN model in predicting the behaviors of slope stability of cohesive soils and consequently more reliable design solution charts provided herein.

Practical criteria for assessment of horizontal borehole instability in saturated clay

Horizontal borehole instability raises concern due to negative impacts on success of drilling, reaming and pipe installation during Horizontal Directional Drilling. Shear failure (blow-out) and tensile failure (hydraulic fracture) are the two main failure mechanisms controlling borehole instability in saturated clay. Criteria for categorizing shear failure versus tensile failure are given which are functions of the ratio of undrained shear strength to vertical effective stress Su/σv′ and the Lateral Earth Pressure Coefficient at Rest K0′. Given that Su/σv′ and K0′ can be expressed as a function of soil parameters like friction angle, Over Consolidation Ratio (OCR) and Liquidity Index (LI), new forms of these criteria are developed for situations where Su/σv′ and K0′ cannot be obtained directly. It is confirmed that tensile failure generally controls in brittle clays like heavily overconsolidated and/or low liquidity index materials. Cases from the laboratory and the field as reported in the literature are used to examine the effectiveness of the new criteria, with inconsistency between theory and the physical evidence explained.

Geotechnical characteristics and consolidation properties of Tianjin marine clay

Tianjin, which is located on the west shore of the Bohai Sea, is part of China\'s Circum-Bohai-Sea Region, where very weak clay is deposited. From the 1970s to the early 21st century, Tianjin marine clay deposits have been the subject of numerous geotechnical investigations. Because of these deposits\' geological complexity, great depositional thickness, high water content, large void ratio, excessive settlement, and low shear strength, the geotechnical properties of Tianjin marine clay need to be summarized and evaluated based on various in situ and laboratory tests so that Tianjin can safely and economically sustain more infrastructure in the coming decades. In this study, the properties of Tianjin marine clay, especially its consolidation properties, are summarized, evaluated and discussed. The focus is on establishing correlations between the geotechnical property indexes and mechanical parameters of Tianjin marine clay. These correlations include the correlations between the water content and the void ratio, the depth and the undrained shear strength, the liquid limit and the compression index, the tip resistance and the constrained modulus, the plasticity index and the ratio of undrained shear strength and the preconsolidation pressure. In addition, the primary consolidation properties of Tianjin marine clay, such as the intrinsic compression line (ICL), sedimentation compression line (SCL), compression index, Cc, coefficient of consolidation, Cv, and hydraulic conductivity change index, Ckv, are evaluated and discussed. A secondary consolidation property, i.e., the secondary compression index, Ca, is also investigated, and the results show that the ratio of Ca/Cc for Tianjin marine clay can be used to calculate Ca in secondary consolidation settlement predictions.

Characteristics of clay deposits in Saga Plain, Japan

The characteristics of clay soil in the Saga Plain in Japan, as well as the effect of the depositional environment on its engineering properties, have been investigated. While the clay was being deposited there were several large-scale volcanic eruptions in the region. A considerable amount of volcanic ash settled into the clay, which had a marked effect on its engineering properties. The soil is strongly structured and the sedimentation compression curves are well above the sedimentation compression line proposed by Burland, which indicates some degree of cementation. The compression index is high – even for soil samples with in situ vertical effective stresses of 200–300 kPa, the index can be as high as 2·3. The rate of permeability reduction with void ratio is higher than most reported data in the literature. The reason considered is that the compression process involves not only gradual compression of the pores but also breaks down relatively large pores into smaller pores, which can cause rapid reduction of permeability. The stiffness is lower, with results of unconfined compression tests showing that the ratio of undrained shear strength to secant modulus is about 150.

Multivariate correlation analysis of seismic piezocone penetration (SCPTU) parameters and design properties of Jiangsu quaternary cohesive soils

A recent trend of geotechnical correlation analysis is to construct a multivariate distribution model to describe the multiple dependency characteristics of different soil parameters. In this study, the feasibility of this multivariate method to model the multivariate dependency of seven soil parameters including the undrained shear strength ratio, overconsolidation ratio, sensitivity, normalized cone tip resistance, normalized sleeve frictional resistance, pore pressure parameter, and normalized shear wave velocity.is studied. A database containing 372 sets of observations of these seven parameters is collected from 25 sites in the Jiangsu province, China. Two data transformations including the Johnson system of distribution and Box-Cox method were used in the construction of the multivariate model for the Jiangsu cohesive soils. The correlations derived from these two local models were compared with the compiled Jiangsu database, data points at other testing sites in China, and database and correlations published in the literature for validation. It was shown that the local models developed in this study were capable of the multivariate dependency of the soil parameters for the Jiangsu cohesive soils, whereas the global model developed in the literature tends to overestimate the uncertainties associated with the design parameters for the Jiangsu soils. A potential explanation is that the soil parameters in a local region were less variable with stronger correlations than those over the global sites, due to the site-specific effect. However, caution shall be taken if the local models for the Jiangsu soils are used at other sites. Besides, the Johnson system of distribution is superior to the Box-Cox transformation in the construction of the multivariate distribution model in the sense that the former is more effective to achieve the standard normal distribution, especially for the soil parameters with a bounded distribution.

Determination of liquid limit of a low swelling clay using different cone angles

The fall cone test is one of the most popular methods used to determine the soil liquid limit. In this study, a series of laboratory tests were conducted on kaolin soil, which is considered representative of low-swelling clays, in order to determine the penetration depths for different cone parameters (angle and weight) under a liquid limit state. The required soil undrained shear strength of kaolin at liquid limit was determined using a hand-held vane shear device. Based on the analysis of the test results, the relationship between the angle of the fall cone and the ratio of undrained shear strength and fall cone penetration depth over cone weight was obtained and represented by a power function. The established relationship was then used to determine the penetration depth at liquid limit for different cone parameter values. A difference in liquid limit of less than 15% was recorded between tests using standard and alternative cone parameter values, respectively. Furthermore, a better and more reliable result was obtained when cone angles were below 90°, with a standard deviation of less than 2.5% from the standard method. Based on critical analysis of the test data, an equation with which to determine the penetration depth under a liquid limit state, as well as a one-point test method with which to determine the liquid limit for different cone parameter values are proposed for low-swelling clays.

Bearing capacity of strip footings on two-layered clay under combined loading

In this paper, a lower bound limit analysis in conjunction with finite elements and second-order cone programming (SOCP) is used to determine the bearing capacity of a rigid strip footing placed on two-layered clay subjected to inclined or eccentric loading. The footing is founded on the free surface of the soil mass with no surcharge applied. Two types of footing–soil interfaces are considered: (1) zero tensile capacity and (2) non-zero tensile capacity. The numerical results are presented in the form of failure envelopes in the loading plane, and the corresponding failure mechanisms are also presented. The size and shape of the failure envelopes are dependent on (1) the undrained shear strength ratio (cu1/cu2), where cu1 and cu2 are the undrained shear strength of the top and bottom clay, respectively, and (2) the normalized thickness D/B of the upper clay, where D is the thickness of the top clay layer and B is the width of the footing.

Discussion: Laboratory measurement of strength mobilisation in kaolin: link to stress history

CONTRIBUTION BY P. W. MAYNE While Vardanega et al. (2012) present an interesting empirical power law relationship for representing the stress– strain response of clays under constant volume, they must take care in the mixing and matching of different strength modes. In particular, the observed increase of normally consolidated undrained shear strength ratio (cu=s’v0)nc with plasticity index (Ip) given by equation (4) was specifically developed on the basis of raw (uncorrected) vane shear strengths measured in the field on natural clays. The general trend for the shear strength ratio from field vane tests (FVTs) varying with Ip is reported by Bjerrum (1972), Larsson (1980), Jamiolkowski et al. (1985), Chandler (1988) and Leroueil & Hight (2003), albeit using non-linear relationships such as power law rather than the original linear form. On the other hand, the general trend for the strength ratio (cu=s’v0)nc from triaxial compression (TC) tests on clays shows essentially no dependence on Ip (Larsson, 1980; Jamiolkowski et al., 1985; Ladd, 1991; Ladd & DeGroot, 2003). As a consequence, this fact led Chandler (1988) to recommend an Ip expression that inter-relates FVT strengths to equivalent TC values. Figure 9 shows the trends of (cu=s’v0)nc versus Ip using the original FVT dataset of Skempton (1957) and the laboratory dataset from TC, direct simple shear and triaxial extension modes presented by Ladd & DeGroot (2003). While the FVT data show a strong correlation with Ip (r 2 5 0?946), the TC data do not (r 5 0?0002). These trends have been supported by larger datasets including over 200 vane shear test (VST) data and over 200 laboratory strength data (Mayne, 2012). On the other hand, as per the theoretical link established by critical state soil mechanics, the ratio (cu=s’v0)nc for TC mode increases with the effective stress friction angle w9 of the clay, as verified elsewhere (Mayne, 1988, 2012).

Study on Influence of Unloading of Vacuum on Stability of Embankment Improved by Vacuum Combined with Preloading

In order to study influence of unloading of vacuum on stability of embankment whose soft ground is improved by vacuum combined with preloading, the change of effective stress of soft clay in the improved area is analyzed when vacuum is unloaded and the undrained shear strength of soft clay in the improved area after unloading of vacuum is deduced. Changes of undrained shear strength of soil and stability of embankment owing to unloading of surcharge are also analyzed for comparison. It is shown by theoretical analysis, laboratory tests and a project that the undrained shear strength of soft clay improved by vacuum combined preloading is decreased after unloading of vacuum and the stability of embankment after unloading of vacuum should be taken into account for design. Safety factor of stability of embankment increases after unloading of surcharge since decrease ratio of load of embankment is more than decrease ratio of undrained shear strength of clay.