Effect Of Overconsolidation Research Articles

Effects of Overconsolidation and K0, Conditions on the Liquefaction Characteristics of Sands

When overconsolidation is induced in a horizontal deposit of sand by the removal of surcharge pressure or by other events, the lateral deformation is prevented and the coefficient of earth pressure at rest, K 0 , increases. To evaluate effects of the overconsolidation ratio as well as increased K 0 -value on the liquefaction resistance of sand deposits, several series of cyclic torsion shear tests were carried out on samples of clean sand overconsolidated to different degrees under various K 0 -conditions. The results of these tests were summarized in an empirical formula. This formula is used to estimate the cyclic strength of sand at any given OCR and K 0 -values, based on the cyclic strength for isotropically and normally consolidated samples.

Creep behaviour of an undisturbed lightly overconsolidated clay

To fill an important gap in the knowledge of creep phenomena, the creep behaviour of the intact, overconsolidated Saint-Alban clay has been investigated by means of drained and undrained triaxial tests as well as of odometer tests.Creep deformations can be broken into volumetric and shear components. The development with time of both volumetric and shear strains can be represented by means of the phenomenological equation proposed by Singh and Mitchell in 1968. However, the stress function in that equation must be defined separately for each strain component and by reference to the limit state of the clay.A general model of the time dependent behaviour of clays might be obtained by combining the concepts of limit state and isotaches, as implied in the YLIGHT model proposed by Tavenas and Leroueil in 1977 but the effect of overconsolidation on the shape of the isotaches requires further investigation.

Influence of Sample Disturbance on Sand Response to Cyclic Loading

The factors influencing the cyclic liquefaction characteristics of saturated sands are reviewed and shown to be profoundly influenced by the relative density, the sand structure, a prolonged period of pressure application, the past seismic history, and the effects of overconsolidation. A study is described in which the cyclic liquefaction characteristics of samples extracted carefully from a bed of sand having a prior seismic history are determined and compared with the characteristics from an otherwise identical bed having no seismic history. Although the sand beds had characteristics differing by 50%, the characteristics of samples taken from both beds were essentially the same. It is suggested that the effects of many factors affecting the stability of natural deposits of dense clean sand lost during the sampling process and that special care is required to arrive at a reasonable evaluation of the cyclic liquefaction characteristics of such soils.

Undrained Behaviour of Overconsolidated Saturated Clays during Shear

ABSTRACT A laboratory investigation is made on the undrained behaviour of two heavily overconsolidated saturated clays during shear. The effect of heavy overconsolidation on undrained behaviour of saturated clays is markedly different as compared to that for low degree of overconsolidation and it is function of clay type. The strength envelope is highly nonlinear for heavily overconsolidated clays at low stress levels. Increase of qf/σc' is much pronounced at high overconsolidation ratios. The pore water pressure parameter A at failure is very much dependent on the type of clay and the stress system used. For the first time development of shear stress, pore water pressure with strain and OCR is discussed. The pore water pressure parameter A increasing with strain at low OCR values converges to a low strain-independent value at a critical OCR value for each clay and then decreases with strain for high OCR values leading to a steady, strain-independent, A value. Tangent modulus values are given as a function of OCR and confining pressure for use in analysis of foundation engineering problems in the clay deposits under undrained condition. The investigation provides valuable information in the much needed high OCR range of low stress levels where very little data is available.

Stress-History Effects on Shear Modulus of Soils

ABSTRACT The effects of time duration of the confining pressure and stress-history patterns were studied through resonant column tests of soil samples. For each soil the dynamic shear modulus obtained at small shearing strain amplitudes was the quantity evaluated as the testing conditions were changed. Under uniform confining pressures an increase in shear modulus occurred which was approximately linear with log time after about 1000 minutes of pressure application. This rate of shear modulus increase per log cycle was expressed as a per cent of the numerical value of the shear modulus at 1000 minutes, to minimize the influence of the pressure magnitude for each test. For soils with median grain size larger than about 0.04 mm the per cent increase in shear modulus per log cycle was about 3% or less. For dry and saturated samples of kaolinite, this increase averaged about 6% to 13%. The effect of overconsolidation was to reduce slightly this rate of shear modulus increase per log cycle. Soils with median grain size of 0.04 mm or greater showed little change in the small amplitude shear modulus as the time of applied confining pressure increased, or little influence of previous preconsolidation. For finer soils (D50 ≪ 0.04 mm) the effects of time of loading and of preconsolidation were significant. Consequently, extrapolation of laboratory shear moduli of fine-grained soils to field applications requires careful study.

Stability of Cuts in Soft Soils

For the case of excavations in normally consolidated sediments, slope-stability analyses were made for undrained and long-term conditions. Submerged slopes and dry slopes having different seepage patterns were investigated, and for long-term conditions the overconsolidation effect resulting from excavation was studied and found to be generally of negligible importance. The results of these analyses are presented in the form of design curves showing the required slope inclination for different values of s\Du\N/p’ for undrained conditions and different values of \a/\Dd\N for long-term conditions. Available field evidence is presented and none of this evidence conflicts with the results of the stability analyses. It is concluded that both the undrained case and the long-term case can provide the most critical conditions for slope stability of open excavations.

The Shear Strength Properties of Calcium Illite

Synopsis The shearing properties of a chemically purified calcium illite were studied using consolidated-undrained triaxial tests with pore-water pressures measured. Tests were performed on artificially sedimented samples and on samples remoulded at a liquidity index of 0·56. The effective stress failure envelope was found to be nearly independent of the voids ratio. The remoulded samples had much lower void ratios than the sedimented samples but the failure envelopes were similar. The remoulded samples were slightly stronger when the peak (σ1 – σ3) was used to define failure but the sedimented samples were stronger when failure was defined at peak (σ1,/σ3). The effect of overconsolidation varied with the failure criterion and whether the samples were remoulded or sedimented. When peak (σ1/σ3) was used to define failure the effective stress failure envelope was found to be unique, being independent of the degree of overconsolidation and of the initial water content of the sample and therefore independent of voids ratio. The pore-water pressures were markedly higher for sedimented samples at all degrees of over-consolidation. The stress-strain properties of the remoulded and sedimented samples also differed. The shearing characteristics are explained in terms of the soil structure. The soil structure analysis was based on observations of drying cracks, and on shrinkage limit and X-ray diffraction studies. Les caractéristiques de cisaillement d'illite de calcium purifié chimiquement ont été étudiées grâce á des essais triaxiaux consolidés a teneur en eau constante en mesurant les pressions d'eau interstitielles. Les essais ont été faits sur des échantillons á sédimentation artificielle et des échantillons remoulés á un index de liquidité de 0·56. L'enveloppe de rupture de contrainte effective s'est révélée presque indépendante de l'indice de vide. Les échantillons remoulés, avaient des indices de vide bien inferieur que les échantillons sédimentés, mais les enveloppes de rupture étaient pareilles. Les échantillons étaient légérement plus solides quand on a employé la valeur de pointe (σ1 – σ3) pour déterminer la rupture mais les échantillons sédimentés étaient plus solides lorsque la rupture étaient determinée á valeur de pointe ((σ1/σ3) L'effet de surconsolidation variait avec le critere de rupture et selon que les échantillons étaient remoulés ou sédimentés. Lorsque la valeur de pointe (σ1/σ3) était employée pour déterminer la rupture, l'enveloppe de rupture de contrainte effective s'est révélée unique, étant independante du degré de surconsolidation et de la teneur en eau initiale de l'échantillon et par conséquent indépendant de l'indice de vide. Les pressions de l'eau interstitielle étaient nettement plus élevées pour les échantillons sédimentes á tous les degres de surconsolidation. Les propriétés déformation-contrainte des échantillons remoulés et sédimentés variaient aussi. Les caractéristiques de cisaillement sont expliquées en termes de structure du sol. L'analyse de structure du sol était basée sur les observations de fissure en train de sécher, et sur les études de limite de retrait et de diffraction des rayons X.

The Effect of Overconsolidation on the Behaviour of Clays During Shear

Synopsis A series of drained and undrained triaxial compression tests have been made on samples of two clays in order to study the effects of consolidation history. The manner in which the strength and pore pressure or volume changes during shear vary with the degree of overconsolidation are recorded, and the differences in strength in the drained and undrained tests are compared. Afin d'étudier les effets du développement dû à la consolidation, on a fait sur des échantillons de deux espèces d'argile une série d'essais de la compression triaxiale drainée et non drainée. On a noté la façon dont la résistance et la pression interstitielle ou les changements de volume pendant le cisaillement varient en proportion avec le degré de surconsolidation; et on a comparé les variations de la résistance dans les essais drainés et non drainés.