Initial Consolidation Stress Research Articles

Research on the rebound rule of a pit bottom caused by excavation of typical strata

In practical engineering, the magnitude of soil unloading rebound is closely related to the physical and mechanical properties of the soil. Therefore, there are significant differences in geological conditions among the different regions. As such, targeted research on the rebound law and calculation methods of foundation pits is needed. This article reports indoor experiments and numerical simulation methods which are used to study the trends and calculation methods of foundation pit rebound based on typical geological conditions in South China. Our findings are as follows. 1) At maximum consolidation stress ranging from 100 kPa to 400kPa, the maximum rebound rate of plain fill soil in typical soil layers is 0.0539–0.0704, the rebound rate of silty clay is 0.0373–0.0528, the rebound rate of coarse sand is 0.0296–0.0343, the rebound rate of gravelly cohesive soil is 0.0159–0.0305, the rebound rate of fully weathered granite is 0.0175–0.0344, and the rebound rate of strongly weathered granite is 0.0170–0.0379. 2) The rebound indices do not change with changes in the unloading ratio or initial consolidation stress. The rebound indices of the soil layer from top to bottom are 0.0143, 0.0119, 0.0077, 0.0096, 0.0083, and 0.0076, respectively, and a formula for calculating the rebound modulus of typical soil layers in South China was proposed. 3) The pore ratio of the soil after the end of the recompression process is lower than that which occurs after the first compression. The difference between the compression porosity ratio of the soil layer from top to bottom and the compression porosity ratio is 0.1, 0.08, 0.02, 0.06, 0.02, and 0.03, respectively. 4) The calculation of the depth of influence by the self-weight stress offset method is based on the theory of eliminating self-weight stress and unloading stress. The calculation depth is not affected by geological conditions, the formula for calculating the rebound modulus is consistent with the formula obtained from experimental research, and the calculation results are in good agreement with the numerical values.

Investigation on Calculation Method of Permeability Coefficient in the Consolidation Process of Cohesive Soil

In order to study the permeability characteristics of cohesive soil in the consolidation process more accurately, in this work, the pores occupied by the combined water were defined as invalid pores, and the effective pores and ineffective pores were decoupled to study the relationship between effective pores and permeability coefficient. The widely used empirical formulas of the permeability coefficient were modified, and the empirical formulas of the permeability coefficient suitable for the consolidation process of cohesive soil were obtained. Combined with the e − lg σ relationships of normally consolidated soil, underconsolidated soil, and overconsolidated soil, the formulas of permeability coefficient, which considering the influence of initial consolidation state and consolidation stress, in the consolidation process of cohesive soil, were proposed. Finally, the rationality of the new formulas proposed in this paper was analyzed by indoor consolidation-permeability test and existing research. The results showed that the permeability coefficient of cohesive soil in the consolidation process predicted by the new formulas were more consistent with the measured average value. This study possesses practical engineering significance to solve the problem of foundation consolidation settlement.

Liquefaction of sands subjected to principal stress rotation caused by generalized seismic loading

A comprehensive experimental study that quantifies the influence of coupled compression and shear wave loading on liquefaction susceptibility of sands is presented. Such loading is typical in situ, and leads to complex principal stress rotation, which in turn impacts the potential for liquefaction in soils even if the cyclic loading intensity remains constant. The nature and degree of principal stress rotation caused by this coupled loading are significantly influenced by the initial consolidation stress state, and the cyclic shear (ΔS), cyclic normal (ΔN) stress increments, the ratio ΔS/ΔN, and the phase shift (δ) between the waves. Cyclic hollow cylinder torsional shear tests were carried out on Fraser River sand specimens isotropically consolidated to different effective mean normal stress [Formula: see text] and subjected to coupled cyclic loading with representative ΔS/ΔN. For a given cyclic stress ratio (CSR) and initial [Formula: see text], the liquefaction resistance decreases with increasing s-wave intensity relative to p-wave intensity, which are proxies to stress increments ΔS and ΔN, respectively. Liquefaction resistance decreases with an increase in ΔS/ΔN up to a limiting value of about 2 beyond which increasing ΔS/ΔN does not significantly influence the cyclic resistance. The finding that cyclic resistance ratio [Formula: see text] decreases with increasing ΔS/ΔN is consistent with the understanding that the cyclic resistance is lower under simple shear loading mode compared to triaxial shear. Tests results also demonstrate that the liquefaction resistance of sand decreases with increasing initial effective confining stress regardless of the nature of the cyclic shear. This indicates that the correction Kσ factor (ratio of cyclic resistance at [Formula: see text] to resistance at [Formula: see text] = 100 kPa) can be considered even under generalized coupled loading conditions.

A Hollow Cylinder Torsional Shear Device to Explore Behavior of Soils Subjected to Complex Rotation of Principal Stresses

Abstract A hollow cylinder torsional shear device that is modified to simulate more generalized earthquake loading, involving simultaneous compression and shear wave loading in situ, is presented. The nature and degree of stress rotation due to coupled action of compression waves and shear waves are discussed, and illustrative experiments conducted to simulate different loading scenarios in the laboratory are presented. The stress rotation due to this coupled loading is significantly influenced by the initial consolidation stress state and shearing parameters such as the ratio between shear stress and normal stress increments (ΔS/ΔN) and the phase shift (ϕ) between the waves. The representative (ΔS/ΔN) ratios were obtained from the numerical simulations and used in hollow cylinder torsional shear apparatus to investigate the cyclic response of Fraser River sand under the simultaneous action of normal and shear stresses. Typical cyclic test results demonstrating the capability of a hollow cylinder torsional shear apparatus commissioned at Carleton University in following complicated cyclic loading paths are presented in this paper. It is noted that loading under such nonconventional stress paths, such as elliptical and circular paths, could be initiated along different pathways. Cyclic test results along these pathways demonstrate that soil response is dependent on the overall path including the initial stress state. Test results also highlighted the significance of stress rotation and cyclic stress paths in affecting the liquefaction susceptibility of sands.

Effect of Initial Consolidation Stress Path on Shear Behavior of Calcareous Sand

Calcareous sand is widely used as backfill material for land reclamation, but due to its special mechanical characteristics such as easily broken particles, calcareous sand foundation is facing more complex engineering problems. In this study, drained and undrained shear tests were carried out on calcareous sand samples from the South China Sea using K0 consolidation and isotropic consolidation conditions. It was found that the particle breakage of calcareous sand has obvious dependence on the initial consolidation stress path and drainage condition, thus showing different shear strength behavior. The particle breakage under drained shear is greater than that under undrained shear, and the particle breakage under isotropic consolidated shear is greater than that under K0 consolidated shear. The larger the particle breakage, the smaller the dilatancy and internal friction angle of calcareous sand. The dilatancy and internal friction angle of K0 consolidated specimen are larger than those of isotropic consolidated specimen. It is suggested that the actual stress path of calcareous sand should be considered in the engineering.

A quasi-equal strain solution for the consolidation of a rigid pile composite foundation under embankment loading condition

Based on the deformation characteristics of rigid pile, a quasi-equal strain assumption was proposed to analyze the consolidation properties of a rigid pile composite foundation. By considering the parabolic distribution of the horizontal permeability of the surrounding soil, the initial consolidation stress difference in the underlying stratum and the time and depth dependent additional stress, analytic solutions for the consolidation degree were derived. The analytical solution was compared to the numerical simulation obtained from the standard finite element method which validates the accuracy of the proposed analytical method. Furthermore, the factors that affect consolidation properties were systematically analyzed. The results show that the consolidation rate for a rigid pile composite foundation was slower than that for a composite foundation with a granular column but was more rapid than that for a natural double-layered ground. The consolidation rate increases with the relative reinforced depth, the permeability coefficient, the ratio of the constrained modulus of the pile to that of the surrounding soil, the ratio of the area replacement and the ratio of the constrained modulus of the cushion layer to that of the surrounding soil. The consolidation rate increases with the decrease in the loading period and the pile-end resistance coefficient.

Evaluation of Liquefaction Potential in Sand–Tire Crumb Mixtures Using the Energy Approach

This research explores performance of the energy approach for evaluating the liquefaction potential of sand–tire crumb mixtures. Thirty-six stress-controlled undrained cyclic torsional tests were performed under three different confining pressures on samples with different rubber contents. The effect of such parameters as the rubber content and confining pressure on liquefaction behavior of sand–rubber mixtures was studied using the energy approach. Test results indicated that, unlike the observations made on the pure sand sample, in the case of sand–rubber mixture with 25% rubber content, when the initial liquefaction is triggered, the dissipated shear energy increases with continued cyclic loading. The increasing trend is stopped when the pore water pressure reached the initial consolidation stress (Ru = 1). This observation indicates that for the case of sand–tire crumb mixture, the dissipated energy per volume is associated only with the progression of pore water pressure. Moreover, the results show that the required energy for liquefaction occurrence decreases with the increase in the rubber content. The minimum amount of required energy is determined for mixtures with 10% rubber content. As the result, the inclusion of crumb rubber decreases the liquefaction resistance of sand. However, when the rubber content increases from 10 to 25%, the resistance to liquefaction improves. The generation rate for mixture with 25% rubber content is somewhat faster than that of the clean sand and the mixture with 10% tire crumbs, as it is expected.

A method to analyze shear cell data of powders measured under different initial consolidation stresses

PurposeThe shear cell test has been widely used to characterize flow properties of powders and granular materials. The purpose of this study is to address the gap between the extensive usage of the test and the limited methods available to analyze the data, and to introduce methodologies for comparing results for different initial consolidation stresses, materials, and testing devices. MethodA library of shear cell data was established. Forty-one powders were included, and each material was tested under four different initial consolidation stresses. For each initial consolidation stress, 3 sampling replicates were performed. ResultsA dimensionless cohesion, C*, was defined as the cohesion divided by the initial consolidation stress. By identifying a correlation between the flow function coefficient (ffc) and C*, the effects of the initial consolidation stress and the testing device were separated. In addition, by identifying a mathematical correlation between the unconfined yield strength and the cohesion, the yield loci from different initial consolidation stresses could be collapsed into a single material characteristic line, enabling one to characterize each material by a single number (the characteristic slope). This approach can be used to economically compare different materials, or different testing devices. ConclusionThe proposed method augments shear cell data analysis and significantly reduces the complexity of the shear cell data.

Humidification vibration deformation behavior of intact loess

A parameter, known as the parameter of humidification vibration deformation, was proposed, describing quantitatively the impact of water content on vibration settlement deformation, and its relationship with humidification water content, dynamic shear stress peak value, initial consolidation stress and vibration frequency was built. The result shows that 1) the parameter of humidification vibration deformation increases with the vibration shear stress peak value increasing. 2) The humidification water content has significant influence on the curve of the parameter of humidification vibration deformation and the peak vibration shear stress. When the humidification water content is low, the curve increases slowly. However, when the humidification water content is high, the curve increases rapidly. 3) Initial consolidation stress has significant influence on the humidification vibration deformation coefficient. When initial consolidation stress is not large enough to destroy the loess structure, with initial consolidation stress increasing, the humidification vibration deformation coefficient decreases. On the contrary, the humidification vibration deformation coefficient increases with initial consolidation stress increasing. 4) With the increase of vibration time, the parameter of humidification vibration settlement shows an increasing trend overall. The initial dynamic shear stress peak value and humidification water content all have significant effects on the curve of the parameter of humidification vibration settlement and vibration time. However, the humidification water content is even more significant.

Effect of Complex Initial Stress Conditions on Dynamic Deformation Behaviors of Compacted Loess

In practical projects, foundations or roadbeds soil is usually in complex initial stress states, and deformation and settlement may happen caused by earthquakes or other dynamic loadings. Since conventional laboratory soil test cannot authentically reproduce dynamic behaviors of soils under complex initial consolidation stress conditions, dynamic deformation behaviors of compacted loess is conducted under complex initial stress conditions through remodeled DTC-199 torsional cyclic load triaxial apparatus. The paper systematically investigated the effects of different initial stress state, such as average principal stress p0, the angle of initial principal stress α0and the initial intermediate principal stress coefficient b0, etc. on dynamic shear modulus Gdand damping ratio λ. Results show that: (1) To the same γd, Gdincreases with the increase of p0but the α0and b0have no influence on Gd. (2) The α0has a little influence on λ and the p0and b0 have no influence on λ.

On the undrained shear strength of gassy clays

The presence of methane gas will affect a soil’s strength, which will alter its resistance to submarine sliding and in turn impact offshore developments. This paper describes a constitutive model for determining the undrained shear strength of fine-grained gassy soils. The model results show that although the presence of gas has a beneficial effect on the soil strength in compressive loading, the peak strength is achieved at larger deformations. In addition, the model results showed that the undrained shear strength of gassy soils increases with increasing initial consolidation stress, increasing volumetric coefficient of solubility, and increasing initial void ratio.

Effect of Fines and Confining Stress on Undrained Shear Strength of Silty Sands

A silty sand is considered as a delicate composite matrix containing a sand-grain-matrix and a silt-matrix. Large strain undrained shear strength (Sus) data in triaxial compression for a particular host sand mixed with different amounts of nonplastic fines are presented. Results indicate that the intergranular void ratio, es, which is the void ratio of the sand-grain-matrix [given by (e+fc)/(1 −fc), where fc is the silt content fraction by weight], plays an important role on Sus of silty sands. At the same void ratio, e, a silty sand shows low Sus compared to that of the host sand. However, when compared at the same es, provided that it is less than the maximum void ratio of the host sand, emax,HS, both the silty sand and the host sand show similar Sus that is fairly independent of the initial confining stress. When es of the silty sand is in the vicinity of or exceeds emax,HS, the Sus depends on the initial effective confining stress. At such “loose” states, Sus normalized with respect to the initial consolidation stress, (Sus/σvo′), is very low and it decreases with a further increase in es. At a fines content greater than about 30%, a silty sand or sandy silt is expected to behave as a silt at an interfine void ratio, ef, defined as the void ratio of the silt-matrix (given by e/fc), unless the silty sand or sandy silt is very dense. A criterion is presented for the undrained shear strength behavior of silty sands in terms of relative percent compaction for field applications. Preliminary recommendations are presented for the choice of undrained strength for seismic stability evaluation against large deformations.

Static liquefaction of sands under multiaxial loading

A fundamental study of the undrained behaviour of sands under multiaxial loading is presented. The investigation was carried out using Fraser River and Syncrude sands in a hollow cylinder torsional shear device. Shear loading was carried out under strain control to capture the postpeak strain-softening characteristics of loose sands. It is shown that the undrained response of loose sands is highly dependent on the loading direction. The friction angle mobilized at phase transformation and steady state is a unique material property, independent of the mode of loading, direction of principal stress and initial consolidation stress, and void ratio state. There is no unique relationship between steady state strength and void ratio which is independent of stress path and the level of initial confining stress.Key words: anisotropy, hollow cylinder torsional shear, liquefaction, sand, simple shear, triaxial.

Anisotropic Plasticity Model for Undrained Cyclic Behavior of Clays. II: Verification

A constitutive model for the stress‐strain‐pore pressure behavior of saturated cohesive soils under both monotonic and cyclic loadings has been presented in the companion paper. The model has been implemented into a numerical program to facilitate evaluation of the model's predictive capability. Procedures for model parameter determination are given. Numerical simulations are compared with a set of data encompassing a wide range of stress history, consolidation‐stress ratio, and stress path. In addition, both strain‐controlled and stress‐controlled cyclic tests are considered. The comparisons show that the model is capable of capturing the essential features of undrained cyclic behavior of clays, including: (1) Degradation of undrained strength and stiffness; (2) development of transient and residual pore pressure and deformation; (3) influence of initial consolidation stress condition and stress history; and (4) effect of loading path involving rotation of principal stress direction.

Factors Affecting Liquefaction and Cyclic Mobility

Liquefaction is a phenomenon wherein a saturated mass of sand loses a large percentage of its shear resistance and flows in a manner resembling a liquid until the shear stresses acting on the mass are as low as its reduced shear resistance. Cyclic mobility is the progressive softening of a saturated sand specimen when subjected to cyclic loading at constant water content. Cyclic mobility has been observed in the laboratory. The writers believe that most observed cyclic mobility deformations in dilative clean sands are due to a test error, redistribution of void ratio, which is not representative of in-situ behavior. The manner in which soil type, confining stress, and initial consolidation stress ratio affect both liquefaction and cyclic mobility are shown by means of laboratory test results, for the purpose of permitting the reader to develop rational procedures for designing against the effects of earthquake loadings on soils.

Excess Pore Pressures During Undrained Clay Creep

The objective of this presentation is to examine experimentally how the excess pore-water pressure is related to the mechanism for undrained creep of San Francisco Bay mud. The results are discussed in the context of creep mechanisms previously suggested in the literature and based on laboratory testing.It is found that shear strains occurring during undrained creep are directly related to a gradual but significant increase in excess pore pressure and, hence, reduction in effective stresses. The increase in magnitude of the pore pressure is, except immediately after the creep shear stress is applied, solely a function of the initial consolidation stress and consolidation period. The magnitude of the long-term build-up may be related to the amount of secondary compression which would occur during drained conditions. It increases with the organic content of the soil and decreases with the degree of remolding. The mechanism for the increase in pore-water pressure may be explained by drainage of water from micropores in the microstructure into the macrostructure.Unless one accounts for the increase in pore pressures during undrained creep, it is unlikely that one will be successful in formulating a generally valid mathematical model for stress–strain–strength–time behavior based on laboratory testing.