Degree Of Overconsolidation Research Articles

A simple 3D elastoplastic constitutive model for overconsolidated soils based on the improved Hvorslev envelope

Soils encountered in geotechnical engineering exhibit a certain degree of overconsolidation and are in a three-dimensional (3D) stress state. The overconsolidated ratio (OCR) and the intermediate principal stress play a significant role in the strength and deformation of overconsolidated soils. By considering the variation of the peak stress ratio with OCR, and taking the zero-tension line and the critical state line as asymptotes, a continuous and smooth Hvorslev envelope is deduced to describe the peak strength of overconsolidated soils. Based on the proposed Hvorslev envelope, a simple 3D elastoplastic constitutive model for overconsolidated soils is directly established through the characteristic stress method. The constitutive relation of the proposed model is presented by the calculation of the total strain increment. All material parameters have clear physical meanings and can be determined by conventional laboratory tests. The isotropic compression path, the triaxial drained shearing path with constant p, the triaxial undrained shearing path, and the true triaxial drained shearing path with constant p and θ are adopted to demonstrate the essential features of the proposed model. The effectiveness of the proposed model is confirmed by comparing the experimental data from Lower Cromer till, Kaolin clay, and Fujinomori clay with corresponding model predictions.

Constitutive Modelling of Temperature-Dependent Behaviour of Soft Rocks with Fractional-Order Flow Rule

This paper regards soft rock as a heavily overconsolidated clay and proposes a new fractional elastoplastic model to describe its temperature-dependent mechanical behaviour. Together with the critical state mechanics, the subloading surface concept is adopted to capture the irreversible plastic deformation developed inside the normal yield surface and provides a smooth transition between the elastic and plastic zones. In addition, the proposed model uses a fractional-order flow rule to account for the nonorthogonality between the plastic flow direction and the yield surface without introducing an extra plastic potential. The evolution law of the fractional-order is affected by the degree of overconsolidation and temperature. The proposed model is verified by the drained triaxial test data of Ohya rock under various confining pressures and temperatures with satisfactory performance. It can be found that an increase in the temperature will reduce the peak strength of soft rock and lead to a ductile failure pattern with a smaller tangent modulus.

Effect of loading conditions and stress history on cyclic behavior of Kutch soil

ABSTRACT Cyclic behavior of cohesionless soils at cyclic strains and frequencies resembling strong ground motion is highly non-linear. Recent developments in numerical analysis tools for the non-linear dynamic response of soils necessitate the determination of soil properties at different cyclic deformations. In this study, liquefaction characteristics and dynamic properties of medium dense silty-sand at different cyclic loading conditions (frequency and amplitude) and various overconsolidated states are investigated. The soil was collected from Fatehgarh dam, located in high seismicity Kutch region of Gujarat (India). At given strain amplitude, increased cyclic shear strength response was observed at higher loading frequencies during the 1st cycle; after that, the strength was essentially similar at all the frequencies. However, at a given frequency, liquefaction resistance decreased with increasing amplitude. The stress history had a strong influence on the cyclic response of soil the soil. The findings revealed a significant increase in cyclic shear strength and a decrease in pore water pressure leading to increased liquefaction resistance at higher overconsolidation ratios. The gain in cyclic strength was observed to follow a power law relationship with the degree of overconsolidation. Heavily overconsolidated states displayed improved soil behavior exhibiting higher shear modulus, higher degradation index, and lower damping ratio.

Dilatancy Relation for Overconsolidated Clay

Abstract A distinct feature of overconsolidated (OC) clays is that their dilatancy behavior is dependent on the degree of overconsolidation. Typically, a heavily OC clay shows volume expansion, whereas a lightly OC clay exhibits volume contraction when subjected to shear. Proper characterization of the stress-dilatancy behavior proves to be important for constitutive modeling of OC clays. This paper presents a dilatancy relation in conjunction with a bounding surface or subloading surface model to simulate the behavior of OC clays. At the same stress ratio, the proposed relation can reasonably capture the relatively more dilative response for clay with a higher overconsolidation ratio (OCR). It may recover to the dilatancy relation of a modified Cam-clay (MCC) model when the soil becomes normally consolidated (NC). A demonstrative example is shown by integrating the dilatancy relation into a bounding surface model. With only three extra parameters in addition to those in the MCC model, the new model and t...

Glacial geological implications of overconsolidated sediments on the Lomonosov Ridge and Yermak Plateau

With the coupled use of multibeam swath bathymetry, high-resolution subbottom profiling and sediment coring from icebreakers in the Arctic Ocean, there is a growing awareness of the prevalence of Quaternary ice-grounding events on many of the topographic highs found in present water depths of <1000 m. In some regions, such as the Lomonosov Ridge and Yermak Plateau, overconsolidated sediments sampled through either drilling or coring are found beneath seismically imaged unconformities of glacigenic origin. However, there exists no comprehensive analysis of the geotechnical properties of these sediments, or how their inferred stress state may be related to different glacigenic processes or types of ice-loading. Here we combine geophysical, stratigraphic and geotechnical measurements from the Lomonosov Ridge and Yermak Plateau and discuss the glacial geological implications of overconsolidated sediments. The degree of overconsolidation, determined from measurements of porosity and shear strength, is shown to result from consolidation and/or deformation below grounded ice and, with the exception of a single region on the Lomonosov Ridge, cannot be explained by erosion of overlying sediments. We demonstrate that the amount and depth of porosity loss associated with a middle Quaternary (∼790–950 thousand years ago – ka) grounding on the Yermak Plateau is compatible with sediment consolidation under an ice sheet or ice rise. Conversely, geotechnical properties of sediments from beneath late Quaternary ice-groundings in both regions, independently dated to Marine Isotope Stage (MIS) 6, indicate a more transient event commensurate with a passing tabular iceberg calved from an ice shelf.

Comparison Between Shear Behaviors of Overconsolidated Nakdong River Sandy Silt and Silty Sand

From this research, overconsolidated undrained and drained behaviors of specimens with high sand content were highly dilatant. According to the comparison results of laboratory tests, the deviator stresses of silty sand were greater than sandy silt due to high sand content under increasing OCRs, and both silty sand and sandy silt were presented strain softening tendency after failure under undrained loading. The pore water pressure increased with increasing fines content under increasing OCRs. Silty sand exhibited more dilatancy and increasing shear strength than sandy silt because pore water pressures of silty sand were lower than sandy silt under higher OCRs. In overconsolidated drained tests, silty sand is higher strength than sandy silt because silty sand has a lower volumetric strain and higher deviator stress than sandy silt under increasing OCRs. As the degree of overconsolidation increased, similar behaviors of silty sand and sandy silt observed that volumetric strain decreased to negative values due to dilatancy effect and low-cohesion under current effective confining pressures.

A Fully Analytical Solution of the Wellbore Stability Problem under Undrained Conditions Using a Linearised Cam-Clay Model

This paper presents a linearised version of the Cam-Clay model fully integrated in the scope of the general theory of poroplasticity. The constitutive law which is developed in the scope of the effective plastic stress concept, only contains two plastic parameters (hardening modulus and slope of the critical state line). To be validated, the model is integrated over homogeneous stress paths (hydrostatic, drained triaxial and undrained triaxal) then compared with experimental data issued from conventional laboratory triaxial tests. In the second part, a simplified version of the model is applied to the wellbore boundary problem (vertical well) in an axisymmetric horizontal stress field and under undrained conditions. Given the linearity of the constitutive law and the a priori knowledge of the shape of the plastic region, the solution (stress, strain and pore pressure) is fully analytical. The solution shows that for an overconsolidated material (overconsolidation degree less than 2) the hoop stress is strongly relaxed in the plastic zone. The higher the compressibility of the saturating fluid, the larger the relaxation of the hoop stress. In terms of stability, the more compressible the fluid saturating the porous medium is, the more stable the well will be. Finally the larger the overconsolidation ratio is, the less stable the well will be.

Change in Pore Size Distribution of Peat in Shear Processes

ABSTRACT Saturated remolded specimens of peat sample were used in this investigation. After the specimens were normally consolidated and overconsolidated under an isotropic stress condition, drained triaxial compression shear tests were carried out. After the ends of shear tests, all the specimens were instantly dried on vacuum freezing and then the measurements of pore size distribution were performed by using a mercury intrusion porosimetry apparatus. Based on the measured results of pore size distribution, the relationships between change in pore structure and drained shear behavior were discussed. Furthermore, undrained triaxial compression shear tests were carried out on normally consolidated specimens and the effect of strain rate on pore structure was also investigated. It was found that the changes in pore structure of peat corresponded to the volume changes during shear and were especially influenced considerably by the degree of overconsolidation.

Analysis of geosynthetic reinforced soil using a simple transform function

The use of geosynthetic to reinforce soft-soils or peat in order to improve the load-settlement response is quite common. In such cases a layer of engineered fill, reinforced with a geosynthetic, is placed on the soft ground and the load is supported by the granular fill. In the present paper a procedure is outlined for the analysis of such a geothynthetic reinforced soil system. The granular fill is assumed to be rigid-strain hardening plastic and the soft soil is modeled by a Winkler type foundation. The grid is considered to be “rough” on its surfaces and be linearly elastic when subjected to tensile stresses. A simple transform function is used in the analysis. Through the analysis performed the influence of various factors such as the degree of overconsolidation (through compaction) of the fill, its dilatational properties, and the tangent modulus of the geogrid are investigated and discussed.

Permafrost Beneath the Beaufort Sea: Near Prudhoe Bay, Alaska

The occurrence and properties of subsea permafrost near Prudhoe Bay, Alaska, were investigated by drilling and probing. Nine holes were drilled and 27 sites were probed with a cone penetrometer. The deepest drill hole was 65.1 m below the seabed, while a depth of 14.1 m was reached with the cone penetrometer. Engineering and chemical properties were determined from core samples and point penetration resistance data were obtained with the penetrometer. Thermal profiles were acquired at both the drill and probe sites. Temperatures below 0° C were observed in all the drill and penetrometer holes logged, although frozen sediments were encountered only occasionally. Seasonally frozen sediments were observed near the seabed at each site. The degree of ice bonding, or strength, could be related to seabed temperature and was greatest in shallow water (&lt;2 m). The penetrometer resistance and thermal data indicated that deeper ice-bonded sediments occur, for example approximately 12.7 m below the seabed in 2 m of water off the Sagavanirktok delta. Of eight holes drilled offshore, it appeared that four encountered bonded permafrost. In general, the position of the ice-bonded permafrost interface was extremely irregular. The depth below the seabed to this interface at various distances from shore along the line studied was 28.8 m at 1 km, 65.1 m at 3.5 km, 44.1 m at 6.8 km, and 29.5 m at 17.2 km. Shallow, over-consolidated marine sediments were found in the upper fine-grained section at all of the drill sites investigated; the degree of over-consolidation varied considerably among the sites. This fine-grained section was up to 10 m thick and covered sands and coarse gravels.

Nonlinear Settlement Analysis by Finite Element

A nonlinear constitutive equation for soil is used in conjunction with the finite element method to compute the settlement of oil storage tanks supported on clay. The nonlinearity is achieved by assigning bulk and shear moduli to each element, consistent with the soil type and the prevailing state of stress. Solutions for the bulk modulus were obtained by isotropic compression tests, as a function of void ratio and mean normal stress. Solutions for the shear modulus were obtained by constant mean normal triaxial compression tests as a function of void ratio, imposed state of stress relative to failure stress, and degree of overconsolidation. A comparison of the long-term settlement of oil storage tanks with that obtained by the finite element method showed good agreement.

Field and Laboratory Behaviour of a Lightly Overconsolidated Clay

Synopsis An extensive series of flood levées up to 10 ft high is being constructed in Launceston, Australia. The levées rest on a soft clay and a major failure occurred on raising the levées to 7 ft height. An investigation has been made to analyse the failure and recommend on future construction procedure. Laboratory testing showed the soil to be lightly overconsolidated and observations on a full size instrumented test levée indicate that the degree of overconsolidation, although small, influences field behaviour to a marked extent. Total stress analyses gave a factor of safety against general shear failure of 1·60, and an effective stress analysis gave slightly below unity assuming c′ = 0, indicating that critical state conditions may apply. A four stage construction procedure has been recommended on the basis of results from the investigation. Une série étendue de digues de hauteurs allant Jusqu'à 3,04 m (10 pieds) est en train d'être construite à Launceston en Australie. Les digues reposent sur un argile mou et une rupture de première importance s'est produite lorsque l'on éleva les digues à une hauteur de 2,13 m (7 pieds). Une enquête a été menée pour faire une analyse de la rupture et pour faire des recommendations sur les procédés futurs de construction, Des essais en laboratoire ont fait apparaitre que le sol était légèrement trop consolidé et des observations portant sur une digue d'essai de grandeur nature pourvue d'instruments indiquent que le degré de surconsolidation, quoique réduit, a une influence sur le comportement sur place à un degré notable. Des analyses de la contrainte totale donnèrent un facteur de sécurité par rapport à un défaut de cisaillement général s'élevant à 1,60 et une analyse de la contrainte effective a produit un chiffre légèrement inférieur à l'unité en supposant que c′ = 0, indiquant que des conditions d'état critique e peuvent s'appliquer. Un procédé de construction en quatre stages a été recommandé à partir des résultats de l'enquête.

The Empirical Evaluation of the Coefficient K0 and K0R

The coefficient of lateral earth pressure at rest is obviously important in numerous engineering problems. Its value appears to be governed by the soil strength and the degree of overconsolidation. It is possible to determine the coefficient in the laboratory but, in order to establish its dependence on stress history, the procedure would be quite time-consuming—particularly in view of the elaborate equipment required. The problem is analyzed in this paper and empirical correlations, using index properties, are applied to suggest a reasonably reliable procedure to estimate the value of K0 for primary compression and unloading conditions.

Stress-Strain Modulus of Clay in Undrained Shear

Stress-strain data from laboratory shear tests on clays are used in stress distribution and soil deformation problems. The soil is frequently assumed to be an isotropic linear-elastic material having a unique Young's modulus, E. The many factors that influence the value of E, determined from undrained triaxial tests on undisturbed and remolded clays, are examined. The level of applied shear stress, the type of shear test (UU versus CU), the quality of sampling, the degree of overconsolidation, the effects of time (thixotropy, aging, and rate of strain), and the type of stress system applied prior to and during shear (direction of major principal stress, values of intermediate and minor principal stresses) are all shown to have important effects on modulus. CU tests, rather than UU tests, are generally recommended for correlating laboratory and field values of modulus, because they are usually less influenced by sample disturbance and can be better suited for evaluating the effects of different stress systems.