Effects Of Overconsolidation Ratio Research Articles

The effect of overconsolidation on monotonic and cyclic behaviours of frozen subgrade soil

This paper presents an experimental investigation on the monotonic and cyclic behaviours of the overconsolidated subgrade soil used in the Lanzhou-Urumchi high-speed railway at the temperature of -3°C. The important mechanical parameters of the overconsolidated soil, i.e., failure strength, initial elastic modulus, accumulative permanent strain and resilientmodulus were experimentally studied by using triaxial tests. Testing results indicate that the failure strength, initial elastic modulus, dilatancy and stress–strain relationship of frozen sample are all dependent to the overconsolidation ratio (OCR). The failure strength is slightly more susceptible to the overconsolidation ratio than the initial elastic modulus. The anisotropically consolidated path (k0=0.5-1) has a negligibleimpact on the stress–strain-strength behaviors. The development characteristics of accumulative permanent strain and resilient modulus are collectively dominated by the overconsolidation ratio and shear stress amplitude. Axial permanent strain acceleratedly accumulates with the decrease of the overconsolidation ratio at the same shear stress amplitude. According to the shakedown theory, allowable cyclic stress ratio was introduced to analyze accumulative deformation characteristics of frozen sample. The overconsolidation ratio and loading cycle number have negligible influences on the allowable cyclic stress ratio of frozen subgrade soil. Four cyclic stress-paths with the same shear stress amplitude were adopted in triaxial cyclic tests with aim to investigate the path-dependent deformation behaviors of frozen subgrade soil. The relationships of axial permanent strain under different cyclic stress-paths are roughly proportional, regardless of the cyclic stress ratio (CRS) and loading cycle number. Those quantitativerelationships can be completely determined by the overconsolidation ratio and cyclic stress-path parameter. The effect of cyclic stress-path on resilient modulus is more than the effect of overconsolidation ratio and less than the effect of shear stress amplitude. Several empiricalmodelswere proposed for the overconsolidated subgrade soil at the negative temperature to characterize the evolution laws of the above-mentioned engineering properties. Those empiricalmodels are capable of giving well descriptions of the development of those engineering parameters for frozen subgrade soil over a wide range of the oveconsolidation ratio.

Undrained Spherical Cavity Expansion in Unsaturated Soils: Semianalytical Solution Coupling Hydraulic and Mechanical Behaviors

A coupled hydromechanical solution to undrained spherical cavity expansion problem in unsaturated soil is proposed with an elastoplastic model for unsaturated soil. The mechanical response and hydraulic behavior of soil during expansion are coupled together through a soil–water characteristic curve (SWCC) that evolves with the void ratio. A system of four first-order differential governing equations is derived based on the equilibrium equation, large stain theory and coupled hydromechanical constitutive relationship of soil. The present solution can reduce to unified hardening parameter model based solution under saturated condition, which validates the proposed solution. The effects of overconsolidation ratio and initial suction on expansion responses are explored based on extensive parametric studies. Special attention is paid to investigate the distributions of the principal stresses, suction, and saturated degree of soil around the expanded cavity; the stress trajectory of a representative element at the cavity surface; and cavity pressure–displacement curves. The proposed analytical solution can serve as a theoretical framework for modelling of some fundamental geotechnical problems in unsaturated soils using critical state soil models.

A cavity expansion–based solution for interpretation of CPTu data in soils under partially drained conditions

SummaryA cavity expansion–based solution is proposed in this paper for the interpretation of CPTu data under a partially drained condition. Variations of the normalized cone tip resistance, cone factor, and undrained‐drained resistance ratio are examined with different initial specific volume and overconsolidation ratio, based on the exact solutions of both undrained and drained cavity expansion in CASM, which is a unified state parameter model for clay and sand. A drainage index is proposed to represent the partially drained condition, and the critical state after expansion and stress paths of cavity expansion are therefore predicted by estimating a virtual plastic region and assuming a drainage‐index–based mapping technique. The stress paths and distributions of stresses and specific volume are investigated for different values of drainage index, which are also related to the penetration velocity with comparisons of experimental data and numerical results. The subsequent consolidation after penetration is thus predicted with the assumption of constant deviatoric stress during dissipation of the excess pore pressure. Both spherical and cylindrical consolidations are compared for dissipation around the cone tip and the probe shaft, respectively. The effects of overconsolidation ratio on the stress paths and the distributions of excess pore pressure and specific volume are then thoroughly investigated. The proposed solution and the findings would contribute to the interpretation of CPTu tests under a random drained condition, as well as the analysis of pile installation and the subsequent consolidation.

Effects of overconsolidation ratio on tunnel responses due to overlying basement excavation in clay

Due to shortage of usable spaces in congested urban cities, deep basements for buildings are frequently constructed above or at a side of existing tunnels. Although much attention was paid to the basement-tunnel interaction, previous studies mainly simplified it as a plane strain problem and focused on excavation-induced tunnel responses in sand. In this study, three-dimensional centrifuge tests were conducted to investigate long-term tunnel responses due to overlying basement excavation in lightly (overconsolidation ratio (OCR) = 1.7) and heavily overconsolidated (OCR = 6.0) kaolin clays. Immediately upon completion of basement excavation, the measured heave and tensile strain of tunnel in heavily overconsolidated clay are up to 25% smaller than those in lightly overconsolidated clay. This is because a smaller void ratio in a stiffer clay possesses large soil stiffness around tunnel lining. Due to dissipation of excess negative pore water pressure, the maximum heave and tensile strain of tunnel increase by up to 210% and 50%, respectively, in heavily and lightly overconsolidated clays. To ensure the safety and serviceability of existing tunnel, special attention should be paid to long-term rather than short-term tunnel responses.

Numerical Simulation of Soil Stress State Variations due to Mini-Pile Penetration in Clay

During installation of prefabricated piles in saturated clayey soils, excess pore water pressure (EPWP) is generated around the pile shaft and tip. The developed excess pore pressure is extremely important for evaluation of changes of stress state in clay and subsequent pile bearing capacity. Therefore, the main objective of this paper is to present a numerical finite-element model (FEM) to simulate a mini-pile installation and subsequent pore water pressure dissipation over time. The mini-pile is inserted from the ground surface into the soil to the desired depth. The numerical model is validated using the results of a physical model in which a piezocone is penetrated into a consolidation chamber containing saturated clay. The effects of Over-Consolidation Ratio (OCR), coefficient of lateral soil pressure (K 0), penetration rate (PR), and soil hydraulic conductivity (K i ) on soil stress state are investigated. The verification results have shown that the numerical model has successfully simulated the penetration of mini-pile, generation, and subsequently dissipation of the excess pore water pressure (EPWP). The parametric study revealed that increasing the OCR and K 0 has resulted in increase of the effective radial stresses on the pile skin compared to its initial value, after dissipation of EPWP. The results indicate that the variations in radial stresses at the end of dissipation are independent from the penetration rate. The dissipation of EPWP is more correlated with horizontal hydraulic conductivity of the soil, because the dissipation rate is accelerated in the radial direction.

Comparative Study of Linear and Non-Linear Theories of Radial Consolidation of Lightly OC Clays

Many clays in situ are lightly overconsolidated with overconsolidation ratio ranging between 1.0 and 4.0 due to various reasons. However, the conventional theories used for analysis of radial consolidation assume the soils to be normally consolidated. For lightly OC Clays, phase transition occurs from overconsolidated to normally consolidated state due to increase of effective stress beyond the preconsolidation stress during the consolidation process under external loading. This paper presents a comparison of the results of linear and non-linear theories of radial consolidation of lightly OC clays. The effects of overconsolidation ratio, stress increment ratio and recompression to virgin compression compressibility ratio are studied. While the pore pressure dissipation rate with time is relatively slower, the rate of change of degree of settlement is relatively faster in the case of non-linear theory compared to that in the linear theory of radial consolidation of lightly OC clays. The difference in the results of linear and non-linear theories is significant only in the intermediate stages of consolidation of lightly OC soils.

Effect of overconsolidation ratio on dynamic properties of binary mixtures of silica particles

Overconsolidated soils are ubiquitous in nature due to multiple mechanisms; however, the stress-history-based studies of small strain stiffness on binary mixtures, such as silty sand, are limited even though natural sand deposits are commonly mixtures of sand particles with varying amounts of fines. Consequently, this study quantified the stress-history-based dynamic properties of binary mixtures, such as sand-sand mixtures with different size small particles, and silty sand mixtures with small amounts of non-plastic fines, up to the critical fines content. By performing bender element tests on those mixtures according to fines content, size ratio, and overconsolidation ratio, the stress-history-based Gmax of binary mixtures was evaluated. For the relevant data analysis, the OCR (overconsolidation ratio) exponent in the Gmax formulation was expressed in terms of stress exponents during loading and unloading. It was found that the effect of OCR on the estimation of Gmax increased with a decrease in size ratio (or increase in size difference), since the stress exponents during loading increased more significantly with a decrease in size ratio due to the pronounced change in interparticle coordination between large grains. However, the variation of stress exponents during unloading of different mixtures was relatively small due to the prevalent elastic deformation. It was demonstrated that the maximum stress history effect of tested mixed soils was observed at a fines content of approximately 5%, which was smaller than critical fines content of silty sand. This behavior was attributable to the delay in critical fines content observed during unloading, when compared to that observed during loading.

Influential factors of loess liquefaction and pore pressure development

The liquefaction of loess under dynamic loading is studied experimentally with a dynamic triaxial test system. The effects of over-consolidation ratio (OCR), saturation degree and the frequency of dynamic loading upon loess liquefaction are investigated. The development of pore pressure within loess samples is also discussed. Based on the experimental results, the empirical relationship between pore pressure ratio and loading cycle number ratio is established for normal consolidated saturated loess.

AN ANISOTROPIC HARDENING MODEL FOR CYCLIC PLASTICITY OF SAND

A model introduced in the present paper is capable of describing cyclic behaviour of sand reasonably well. The salient feature of the proposed model is that a nonassociative relationship between stress and strain is used with the anisotropic hardening which undergoes a combination of isotropic and kinematic hardening. Generalized forms of Cambridge models are provided to represent yield function and plastic potential of sand. Translation rule of yield surface is specified by modifying Ziegler's kinematic hardening. The constitutive model is capable of accounting for expansion of yield surface with simultaneous translation. Several undrained cyclic tests for normally consolidated and over-consolidated sands are simulated by taking into account the membrane penetration. The effect of over-consolidation ratio on liquefaction is predicted and good comparison with experimental results is reported for the effective stress path and the stress-strain relationship.