Effective Consolidation Stress Research Articles

Effect of vibration on flow properties of fine glass beads

AbstractAn estimation of cohesion C and angle of internal friction φ of fine glass beads, which have been previously subjected to small intensity vertical vibrations of controlled frequency and amplitude for a fixed period of time, is presented. In the range of vibration intensities tested previbration causes powder compaction. Experimental measurements to obtain C and φ have been performed by means of a centrifuge powder tester in which the previbrated powder bed is rotated around its vertical axis. At a critical rotation velocity the shear stress induced by rotation is large enough to drive material avalanches. From a theoretical analysis of these avalanches, based on the Coulomb's method of wedges, C and φ are derived. We have found a significant increase of cohesion, and a slight decrease of the angle of internal friction as the effective consolidation stress due to previbration is increased. The estimated interparticle cohesion force ft from the experimental measurements, and using a Rumpf's averaging equation, is in agreement with the calculated interparticle attractive force due to van der Waals and capillary interaction for the unconsolidated beds. Moreover ft increases as the effective consolidation stress is increased in agreement with estimations from theoretical models based on contact plasticity. This result suggests that, at the microscopic level of asperities, compaction due to previbration is able to induce plastic deformation of the contacts. © 2008 American Institute of Chemical Engineers AIChE J, 2008

Permeability and petrophysical properties of 30 natural mudstones

We present permeability and other petrophysical data (including pore size distribution, porosity, particle size distribution, grain density, specific surface area, total carbon content, organic carbon content, and sulphur content) for 30 deeply buried mudstones. Permeabilities were measured at different effective consolidation stresses ranging from 2.5 to 60 MPa with a 30,000 mg L−1 NaCl solution. Samples represent a wide spectrum of mudstone types with clay size particle contents ranging from 13 to 66%. Porosities range from 6 to 27%; pore size data show that porosity loss is driven primarily by collapse of the largest pores. Our results confirm and considerably extend previously reported results indicating the influence of clay content on pore size distributions and the way they evolve as a result of compaction. Vertical permeabilities, measured using the transient pulse decay technique, range from 2.4 × 10−22 m2 to 1.6 × 10−19 m2. Horizontal permeabilities range from 3.9 × 10−21 m2 to 9.5 × 10−19 m2, overlapping with but generally higher than vertical permeabilities. In general, permeability decreases logarithmically with porosity. The relationship between permeability and porosity is strongly influenced by clay content, especially at higher porosities. Ratios of horizontal to vertical permeability measured on four samples range from 1.7 to 11.8, implying the influence of both particle alignment and sedimentological heterogeneity. We have used the data to calibrate two permeability models. For the Kozeny‐Carman model, values of 200 and 1000 for the product of shape and tortuosity factors provide the best fit for the vertical and horizontal permeabilities, respectively. The calibrated Yang‐Aplin model predicts the permeability of almost all the samples to within a factor of ±3 over a 4 orders of magnitude range of permeability.

Dissipated strain energy method for determining preconsolidation pressure

The dissipated strain energy method (DSEM), a new method for determining the preconsolidation pressure, is presented in this paper. Compared with the energy method, the DSEM uses dissipated strain energy and the slope of the unloading–reloading cycle (in the strain energy – effective consolidation stress space) for the plot to minimize the sample disturbance effects and eliminate the effect of elastic deformation. Dissipated strain energy, in terms of micro mechanics, is directly related to the irreversible process of consolidation and can be supported by theories dealing with consolidation and compaction. The use of the unloading–reloading slope to simulate the elastic reloading for the recompression stage can minimize sample disturbance effects. Examples presented indicate that the proposed new method is less operator dependent than most of the existing methods.Key words: dissipated strain energy, preconsolidation pressure, consolidation, energy method, dissipated strain energy method.

Determining Hydraulic Conductivity of Soil-Bentonite Using the API Filter Press

AbstractThe American Petroleum Institute (API) filter press is commonly used to measure the hydraulic conductivity of soil-bentonite, both during mix design and as a part of construction quality assurance and quality control. However, interpretation of the test results is complicated by the fact that, during the test, the soil-bentonite specimen is consolidated by seepage forces, which produce a variation in effective consolidation stress from the top of the specimen to the bottom. This paper presents the results of a laboratory investigation undertaken to illustrate that seepage consolidation theory provides a logical way to interpret filter press test results. A description of seepage consolidation theory, which relates stress, compressibility, and hydraulic conductivity in soil consolidated by seepage forces, is presented. Following this is a description of the laboratory testing program that involved comparing the hydraulic conductivity of two soil-bentonite mixes, as measured in rigid wall consolidometer permeameter cells, a flexible wall permeameter, and an API filter press. Using seepage consolidation theory to interpret the filter press tests produces hydraulic conductivities consistent with those obtained from the other tests. The filter press test results are used to illustrate a method for estimating the hydraulic conductivity of soil-bentonite in a cutoff wall by taking backfill consolidation pressures into account.

Shear Strength and Stiffness of Silty Sand

The properties of clean sands pertaining to shear strength and stiffness have been studied extensively. However, natural sands generally contain significant amounts of silt and/or clay. The mechanical response of such soils is different from that of clean sands. This paper addresses the effects of nonplastic fines on the small-strain stiffness and shear strength of sands. A series of laboratory tests was performed on samples of Ottawa sand with fines content in the range of 5–20% by weight. The samples were prepared at different relative densities and were subjected to various levels of mean effective consolidation stress. Most of the triaxial tests were conducted to axial strains in excess of 30%. The stress-strain responses were recorded, and the shear strength and dilatancy parameters were obtained for each fines percentage. Bender element tests performed in triaxial test samples allowed assessment of the effect of fines content on small-strain mechanical stiffness.

Damping at Small Strains in Cyclic Simple Shear Test

Cyclic tests were conducted to study damping properties of two reconstituted sands and three laboratory-made clays at small cyclic shear strain amplitudes of γc≈ 0.001–0.04%, employing a recently developed constant-volume equivalent-undrained direct simple shear device for small-strain testing. The tests were strain-controlled with an approximately sinusoidal shape of cyclic straining. The effects of cyclic strain amplitude (γc), frequency of cyclic loading (f), plasticity index (PI), silt content, vertical effective consolidation stress (σvc′) and overconsolidation ratio (OCR) on the equivalent viscous damping ratio, λ, were investigated. The results show that, for a given γc, λ decreases with σvc′ and OCR, but both of these effects become smaller if PI increases. The effect of f on λ was not observed for f≈ 0.01–0.1 Hz. The results also show that below γc≈ 0.005%, λ for clays is larger than λ for sands, which is exactly the opposite of the trend above γc≈ 0.005% established previously. Such a reversal of the trend of λ with respect to the type of soil is explained by the relative contributions of soil nonlinearity and soil viscosity to the area of the hysteretic cyclic stress-strain loop at small versus large cyclic strains.

Reduction of Shear Modulus at Small Strains in Simple Shear

Cyclic tests were conducted to study the secant shear modulus, Gs, of two reconstituted sands and three laboratory-made clays at small cyclic shear-strain amplitudes γc≈ 0.0006%–0.02%. A recently developed direct simple shear device for small-strain testing was employed. The effects of cyclic strain amplitude (γc), type of soil and plasticity index (PI), vertical effective consolidation stress (σvc′), and overconsolidation ratio (OCR) were investigated. The results show that the ordinates of the normalized shear modulus reduction curve, Gs/Gmax−γc, generally increase as σvc′ and OCR increase. However, these effects of σvc′ and OCR become smaller and may eventually disappear as the PI of the soil increases.

Measurement of the Pore Pressure Parameter C Less Than Unity in Saturated Sands

Abstract Two saturated sands, Monterey No. 0/30 and Enewetak coral, tested undrained under one-dimensional strain loading, were found to have values of the pore pressure parameter C, less than unity. The C parameter for Monterey No. 0/30 sand, determined to be unity at an effective consolidation stress of 86 kPa, decreased with increasing effective consolidation stress and increasing relative density. A similar behavior was observed for the Enewetak coral sand. These trends are similar to those reported by other researchers for the pore pressure parameter B. The decrease in C parameter values for saturated specimens appears to be a direct result of increasing skeleton stiffness due to increases in effective stress and density. A theoretical analysis of skeleton stiffness based on porosity and pore pressure response predicts similar trends.

Normalized behavior of clay under irregular cyclic loading

A systematic analysis of the undrained stress–strain behavior of clay under irregular cyclic simple shear loading is presented. Seven specimens of an offshore clay consolidated to overconsolidation ratios of 1, 2, and 4 were subjected to different combinations of variable and nonsymmetric cyclic amplitudes using the Norwegian Geotechnical Institute (NGI) direct simple shear device. The test results show that (1) the behavior under such loads is influenced by several different factors, (2) among these factors the loading history and cyclic stiffness degradation are predominant, and (3) the irregular cyclic loading stress–strain curves can be described quite well by five rules that incorporate only these two factors. Four out of these five rules are the extensions of two original and two extended Masing rules to the behaviour of cyclically degrading clay. The fifth rule is new. The effects of the S-shaping of the stress–strain curves and the rate of loading on the applicability of the rules are also discussed. The stress–strain curves are presented in the normalized form with respect to the vertical effective consolidation stress. In this form they show quantitatively the same trends, indicating that such normalization is applicable to irregular cyclic loading. Key words: clay, earthquake loading, laboratory test, ocean soil, overconsolidation, simple shear test, soil dynamics, strain rate effect, repeated loading.

Effects of Anisotropic Versus Isotropic Consolidation in Consolidated-Undrained Triaxial Compression Tests of Cohesive Soils

Abstract This investigation examines effects of anisotropic versus isotropic consolidation in consolidated-undrained triaxial compression tests performed on specimens of two clays consolidated from a slurry in large-diameter consolidometers. Shear strength and corresponding deformation characteristics were developed in both normally consolidated and overconsolidated ranges. Test results indicate that volume changes during consolidation are not a unique function of the major principal consolidation stress but instead are related to the mean effective consolidation stress and the deviator stress during consolidation. Consequently, for a given major principal consolidation stress, the undrained strength of anisotropically consolidated specimens is lower than that of isotropically consolidated specimens.