Normal Consolidated Clay Research Articles

Strength and Dilatancy of Overconsolidated Clays in Drained True Triaxial Tests

AbstractThe long-term mechanical behavior of overconsolidated (OC) clays in three-dimensional (3D) stress states plays an important role in many geotechnical problems, such as slope failures. However, previous experimental studies of OC clay were carried out under undrained conditions. This paper presents the results of a series of drained true triaxial tests on normally consolidated (NC) and OC clays. The stress paths were maintained in the same π-plane with Lode angles of θ=0–60°. The test results show that OC clays exhibit positive dilatancy tendencies and greater shear strengths and smaller strains to failure than NC clays. The shear strength and dilatancy are largely influenced by the Lode angle. As the Lode angle increases, the shear strength and dilatancy decrease. The decreasing gradients of shear strength and dilatancy versus Lode angle are almost the same, indicating that the constitutive models for OC and NC clays can share the same rules for strength and dilatancy. The formation of shear bands...

Attenuation-type and failure-type curve models on accumulated pore water pressure in saturated normal consolidated clay

Based on dynamic triaxial test results of saturated soft clay, similarities of variations between accumulated pore water pressure and accumulated deformation were analyzed. The Parr’s equation on accumulated deformation was modified to create an attenuation-type curve model on accumulated pore water pressure in saturated normal consolidation clay. In this model, dynamic strength was introduced and a new parameter called equivalent dynamic stress level was added. Besides, based on comparative analysis on variations between failure-type and attenuation-type curves, a failure-type curve model was created on accumulated pore water pressure in saturated normal consolidation clay. Two models can take cycle number, coupling of static and dynamic deviator stress, and consolidation way into consideration. The models are verified by test results. The correlation coefficients are more than 0.98 for optimization of test results based on the two models, and there is good agreement between the optimized and test curves, which shows that the two models are suitable to predict variations of accumulated pore water pressure under different loading cases and consolidation ways. In order to improve prediction accuracy, it is suggested that loading cases and consolidation ways should be consistent with in-situ conditions when dynamic triaxial tests are used to determine the constants in the models.

Automated True Triaxial Apparatus and its Application to Over-consolidated Clay

AbstractA better understanding of long-term mechanical behavior of over-consolidated clay (OC) is important to numerous geotechnical problems. Most previous experimental studies, however, have used sand or normally consolidated clay (NC); only limited reports of the drained true triaxial test of OC are available. In this paper, an electro-pneumatic (E/P) regulator based automatic control true triaxial apparatus is developed. A mixed boundary loading device designed by Nakai Research Group is used in the apparatus. Unlike other loading devices, the membrane covers not only the vertical but also the horizontal rigid platens, which can apply arbitrary three different principal stresses to the block specimen. The loading pattern that can reduce the corner effect and the interference problem is also analyzed. The method to increase the pressure control precision of the E/P regulator as well as the automatic control system is presented. An extra volume change due to water-absorption of membrane during long time drained test is discussed and a simple solution is proposed. The apparatus is used to study the mechanical properties of NC and OC clays under drained condition. The test results show that the initial stiffness, shear strength and dilatancy are largely influenced by the Lode angle. Furthermore, both the NC and OC clays obey the Matsuoka-Nakai criteria well.

Attenuation-Type and Failure-Type Curves of Cyclic Accumulated Deformation Models in Saturated Normal Consolidated Clay

Based on dynamic triaxial test results of saturated soft clay, influence factors and variations on accumulated deformation were analyzed. The Parr’s equation on accumulated deformation was modified to create an attenuation-type curve model on accumulated deformation of saturated normal consolidation clay. In this model, dynamic strength was introduced and a new parameter called equivalent dynamic stress level was added. Besides, based on comparative analysis on variations between failure-type and attenuation-type curves, a failure-type curve model was created on accumulated deformation of saturated normal consolidation clay. The two models can take cycle number, coupling of static and dynamic deviator stress, consolidation way into consideration. The models were verified by test results. The correlation coefficients are more than 0.96 for optimization of test results based on the two models, and there is good agreement between optimized and test curves, which show that the two models are suitable to predict variations of accumulated deformation under different loading cases and consolidation ways. In order to improve prediction accuracy, it is suggested that loading cases and consolidation ways should be consistent with in-situ conditions when dynamic triaxial tests being used to determine the constants in the models.

The Transport of Escherichia coli Through Freeze-Fractured Clay Soil

Little is known about the transport of microorganisms through freeze-fractured clay soils. Normally consolidated clay (NCC) and compacted clay (CC) columns (representing a natural clay barrier and a compacted barrier, respectively) were exposed to six consecutive freeze–thaw cycles and permeated for 21 days with an Escherichia coli cell suspension (approximately 1 × 107 colony forming units (CFU)/mL) containing a 2.1-mM bromide tracer. An unfractured sand column was also examined for comparison with the clay columns. While no E. coli was detected in the effluent of both untreated NCC and CC control clay columns, a relatively low density of E. coli (between 228 and 270 CFU/mL compared to 1 × 107 CFU/mL in the influent) was first detected in the effluent of the freeze-fractured NCC and CC columns at 0.29 and 0.31 pore volumes (or at 5.4 and 4.1 h), respectively. It took 11 min for a full breakthrough of E. coli through the sand column, but only about 0.1% of the influent E. coli density was detected in the effluents of the freeze-fractured NCC and CC columns at day 21. These observations show that despite the high bacterial retention capacity of the freeze-fractured clay columns, the fractures were large enough for the E. coli to flow through. Based on batch sorption tests and the permeation data, it is estimated that 18%, 7%, and 84% of the freeze-fractured NCC, CC, and sand columns would be exposed to the influent, respectively, under a full E. coli breakthrough condition. Our data show that the high bacterial retention capacity of clay barriers can be compromised by freeze–thaw conditions.

Constitutive Model for 1-D Cyclic Soil Behaviour Applied to Seismic Analysis of Layered Deposits

ABSTRACT A phenomenological constitutive model, 'BWGG', is developed for the non-linear one-dimensional ground response analysis of layered sites. The model reproduces the nonlinear hysteretic behaviour of a variety of soils, and possesses considerable flexibility to represent complex patterns of cyclic behaviour such as stiffness decay and loss of strength due to build-up of pore-water pressure, cyclic mobility, and load induced anisotropy. It also has the ability of simultaneously generating realistic modulus and damping versus strain curves, by a simple calibration of only three of its parameters. The model is implemented through an explicit finite-difference algorithm into a computer code which perform integration of the wave equations to obtain the nonlinear response of the soil. The code, 'NL-DYAS', is then applied to study the seismic response of a soft marine normally-consolidated clay. The results are compared with those of widely used codes. Finally, the records of the Port Island array during the Kobe 1995 earthquake, are utilized, and the model is shown to “predict” the observed response with sufficient accuracy.

UNDRAINED SHEAR STRENGTH OF CLAY OBTAINED FROM MINIATURE PRESSUREMETER TEST

A series of miniature pressuremeter tests were carried out in a triaxial cell to evaluate the undrained shear strength obtained from the pressuremeter tests on normally or over-consolidated clay. To compare with these results, triaxial compression and extension tests were conducted simultaneously. Four kinds of the evaluating methods which have been already proposed were used to estimate the undrained shear strength related with the pressuremeter. As a result, the undrained strength obtained from the Stress-Strain Method, in which no functional form of stress-strain relation is assumed, was found to be in a best accordance with those obtained from the triaxial extension tests and estimated from the elasto-plastic constitutive model. Semi-Log Method which is simpler if compared with the Stress-Strain Method was found to be effective for normally-consolidated clay.

Cyclic Behavior of Clays in Undrained Simple Shear

Undrained cyclic shearing of normally consolidated (NC) clays develops positive excess pore pressures and causes a reduction in effective stresses. As a result, initially cycled NC clay subjected to subsequent undrained monotonic shearing behaves as an overconsolidated clay. This observation is extended herein to form the basis of the apparent overconsolidation (AOCR) hypothesis and to develop a framework for predicting the undrained cyclic behavior of slightly overconsolidated clays from the NC behavior. In addition, the AOCR framework allows predictions of clay response due to cyclic shear with variable intensities. Comparisons of predictions with results of cyclic direct simple shear tests on Boston blue clay show generally favorable agreement.

Finite-strain axial analysis of piles in clay : Nystrom, G A Proc of the Conference on Analysis and Design of Pile Foundations, San Francisco, California, 1–5 Oct 1984P1–20. Publ New York: ASCE, 1984

In the late 1970's, a series of research projects developed simple effective-stress methods to predict the axial capacity of piles in normally-consolidated clay. The method known as ESM2 was built around a simplified, one-dimensional, cavity expansion solution. The ESM2 boundary-value approach has been extended analyse pile installation, consolidation, and axial loading. This model was used to evaluate the effects of both ends of the pile, pile loading direction, and pile loading rate. Other improvements include use of a full finite-strain theory and extension of the modified Cam-clay constitutive relations which accounts for the effect of the third stress invariant. The paper compares the results from one-dimensional and axisymmetric models.

UNDRAINED SHEAR OF SOFT CLAY AFTER A CYCLIC STRESS HISTORY

Effects of undrained cyclic stress-strain history on clay behavior were studied by cyclic triaxial compression tests. An apparently-overconsolidated clay produced by undrained cyclic loading decreases its undrained strength because of the decrease in effective stress in clay. while a clay subjected to drained cyclic loading gains the strength similarly to the long-termed consolidated clay. Undrained strengths of a reconstituted marine soft clay were predicted by a method which was formerly extended from the empirical relation existing between overconsolidated and normally-consolidated clays. In terms of the effective stress analysis, the critical state parameter in clay has nothing to do with undrained cyclic loading histories.

Increase in Undrained Shear Strength Due to Secondary Compression

Undrained shear strength of clay is generally increased due to secondary compression or delayed compression. Though this increased strength is regarded to be derived from densification of clay particles, metamorphosis of clay minerals and dissolution of cemented matters into free water, mechanism of increase in strength during long term consolidation has not completely been clarified. In the present paper, described is a method for estimating the amount of increase in undrained shear strength of clay due to secondary compression. The most important postulate in derivation of the basic equation is that the quasi-overconsolidation of clay induced by secondary compression should be equivalent to overconsolidation due to release of over-burden pressure. In this way, the simplified relation for estimating the secondary compression-induced increase in strength is expressed by means of comparison with the strength of normally-consolidated clay. The relation suggests the increase in strength to be a power function of the elapsed time during secondary compression, including three parameters which can be obtained from oedometer tests and triaxial compression tests. The proposed method is proved to be valid for estimating the increase in shear strength of a reconsolidated clay and an undisturbed clay in direct shear tests. Additionally, the strength increment ratio defined by means of the equivalent consolidation pressure of Hvorslev (1960) is shown to be independent of the elapsed time of secondary compression.

The engineering properties of mudrocks

Summary The ‘rock’ and ‘soil-like’ properties of British mudrocks are shown to be influenced by: (a) their lithology; (b) their geological history of loading (especially during exhumation); (c) the type and method of testing; and (d) the degree of weathering. In particular, unloading and weathering leads ultimately to a normally-consolidated clay of much the same undrained shear strength, irrespective of age and origin of the parent material. For this reason, the engineering properties of the unweathered mudrocks are illustrated separately in terms of classification indices, undrained and effective shear strengths and deformability in relation to geological age.

Discussion: Stress-Probe Experiments on Saturated Normally-Consolidated Clay

Discussion: Stress-Probe Experiments on Saturated Normally-Consolidated Clay

The consolidation of clays by gravitational compaction

Sedimentation compression curves, relating void ratio to effective overburden pressure, are presented for a wide lithological range of argillaceous deposits. These curves show the progressive changes from recently deposited muds on the sea floor, to Quaternary clays at depths of several tens of metres, and finally to hard clays and mudstones of Pliocene age at depths extending to about 3 000 m. Twelve localities are examined in some detail and information is also given from another eight previously published sites. In all cases the data are derived from ‘normally-consolidated’ deposits, strata which have never been under greater pressures than those existing at the present time. This procedure eliminates the difficulties of estimating the effect of pressure reduction by erosion. Clays containing high proportions of carbonates and organic matter are not included in this study. The water content (or void ratio or porosity) of any particular clay in the normally-consolidated condition is controlled by the effective overburden pressure p 0 , given by Terzaghi's law p 0 = σ − u, where σ is the total vertical pressure exerted by all the material (particles and water) above the point considered, and u is the pore water pressure at that point. This law is shown to hold good even at porosities as low as 15 per cent. At any particular effective overburden pressure the water content of a normally-consolidated clay is directly related to the amount of clay minerals present and to their colloidal activity. The combined influence of these two factors can be indicated quantitatively by the Atterberg limits; and at a given value of p the water content is found to be a function of the Atterberg (liquid and plastic) limits for all inorganic non-calcareous clays except those with an extremely unstable microstructure, such as the so-called ‘quick clays’ of Scandinavia. Moreover, the water contents of muds on the sea bed or in tidal flats can also be expressed approximately by single-value parameters in terms of these limits. Thus if the water content, effective overburden pressure and Atterberg limits are known for an individual layer of normally-consolidated clay, it is possible to reconstruct the entire sedimentation compression curve for that clay with a reasonable degree of certainty; and hence an estimate can be made of the compaction which has occurred in the clay under its own weight and under the load of any overlying strata. At some of the sites, in addition to data relating to compaction, information is given on the increase in strength with depth and the rate of deposition as deduced from radiometric dating.

THE PORE PRESSURE – STRAIN RELATIONSHIP OF NORMALLY CONSOLIDATED UNDISTURBED CLAYS: PART I. THEORETICAL CONSIDERATIONS

This paper presents a critical review of the current theories of predicting pore pressures in saturated clays. These theories are generalized and examined and their limitations discussed. It is shown mathematically that the pore pressure induced by shear may be expressed as a sole function of major principal strain, and this approach eliminates the anomalies inherent in the "stress theories" for pore pressure. A hypothesis is also developed to provide a possible mechanism of pore pressure set up in undisturbed normally-consolidated clays.

On the Volume Changes of Normally-Consolidated Clays

Changes in volume of soils are a function of changes both in normal stresses and shearing stresses. Development of mathematical expressions for the change in volume would be of aid for the understanding of the failure criteria for clays and would permit more reliable estimates for the settlement of structures founded on massive clay deposits.This paper describes an investigation of the stress-volume change characteristics of a normally-consolidated clay. The test procedure was similar to that in the case of drained compression test, except that the loading was arranged so that the mean principal effective stresses might be maintained constant throughout the test.The nomenclature used in this paper is as follows: C: coefficient of compressibility. D: coefficient of dilatancy. V0: initial volume corresponding to σ0'. ΔV: total volume change. ΔVc: change in volume due to the change in mean principal stress. ΔVd: change in volume due to the change in shearing stress. σ1' σ3': principal effective stress. σm': mean principal effective stress. σ0': pre-consolidation pressure. σc: critical stress, below which dilatancy is zero. σm'-const.-test: drained compression test in which the mean principal effective stress is maintained constant.The analysis of the test data shows that the volume change behaviour can be adequately described by the following conclusions:1) The volume of normally-consolidated clays decreases when the mean principal effective stress is increased, and vice versa. The virgin branch of a semi-logarithmic plot of the consolidation diagram is usually straight (Fig. 1) and can be expressed by the equationΔVc/V0=C·logσm'/σ0'2) The volume decreases when normally-consolidated clays are subjected to an increase in shearing stress. Fig. 6 shows that the volume change during a σm'-const.-test ΔVd/(V0-ΔVc) is a unique function of (σ1-σ3)/σm' and correlations between them can be established.ΔVd/(V0-ΔVc)=D{(σ1-σ3)-σc/σm'}3) The total volume change ΔV of normally-consolidated clays is ΔV=ΔVc+ΔVd, then ΔV is given by the expressionΔV/V0=C·logσm'/σ0'+D(1-C·logσm'/σ0'){(σ1-σ3)-σc/σm'}

Shearing Strength of a Soft Clay Deposit Near Rio De Janeiro

Synopsis Tests are reported on a deposit of normallyconsolidated clay near Rio, Brazil. The clay is about 35 feet thick, and until recently its surface was a tidal marsh. The average liquid and plastic limits of the clay are 154 and 38 per cent respectively. The strength, as determined by undrained tests on undisturbed samples, increased uniformly with depth (except near the surface where some drying out had occurred subsequent to the land being reclaimed). Below this zone of partial drying, the ratio of shear strength c to effective overburden pressure p was found to be 0·59. This resultis compared with those for other normally-consolidated clays, and additional tests on the clay are summarized. L'article donne un compte-rendu des essais effectés sur un gisement d'argile normalement consolidée près de Rio en Brazil. Ce gisement a une épaisseur d'environ 11 mètres, et sa surface était jusqu'au temps récent un marais soumis à la marée. L'argile a des limites de liquidité et de plasticité moyennes de 154 et 38 pour cent, respectivement, et sa résistance, déterminée par des essais sans drainage sur des échantillons intacts, augmentait en proportion directe avec la profondeur (sauf à proximité de la surface, où le séchage partiel s'était produit à la suite de la réclamation du sol). L'on a trouvé qu'au-dessous de cette zone de séchage partiel, le rapport c/p était 0,59, dans lequel c représente la résistance au cisaillement et p la pression réelle des terrains de recouvrement. Ce résultat est comparé avec ceux établis sur d'autres argiles normalement consolidées, et l'article comprend un sommaire des essais supplémentaires sur l'argile.