Initial Pore Water Pressure Research Articles

Comparative Study of the Hydromechanical Behavior of Intact, Horizontally Jointed, and Vertically Jointed Rocks under Undrained Conditions

AbstractThis paper investigates the influence of the presence of a joint (vertically or horizontally oriented) on the hydromechanical behavior of sandstone under undrained triaxial conditions. A series of undrained triaxial experiments was performed on intact and horizontally and vertically jointed sandstone samples under different confining pressures and initial pore-water pressures. Joints in jointed samples were artificially created with rough and irregular surface profiles. An increase in pore-water pressure with increasing deviatoric stress was observed for all tests. Maximum induced pore-water pressure values were higher and occurred later in the jointed rock samples when compared to the intact samples at all confining pressures. This may be related to the efficiency of the response of the joint pore volume to loading when compared to that of the matrix pore volume. Moreover, the significance of new pore volume creation (by microcracking during failure) on sample pore-water pressure is dependent on ...

Do joint geometrical properties influence the fracturing behaviour of jointed rock? An investigation through joint orientation

Failure of brittle rock is often progressive and understanding the fracture development process within rock masses is important to identify the precursors of failures. Joints in rock masses influence the fracture progression behavior and we studied the influence of joint orientation on fracture progression in jointed rock. Undrained triaxial tests were conducted on 54 mm diameter and 108 mm high intact and singly-jointed sandstone specimens. The joints in jointed specimens were made rough (joint roughness coefficient = 10–12) and embedded in six different orientations—0°, 30°, 45°, 60°, 75° and 90°—and three different combinations of confining and initial pore-water pressures were considered (confining pressure/pore-water pressure = 4/1, 10/4 and 25/10 MPa). An acoustic emission (AE) monitoring system was employed during all tests to monitor the fracture development within the test specimens. The post-failure patterns of specimens displayed three different failure mechanisms—shearing through intact material, sliding along the joint and a mixture of both shearing and sliding—depending on the joint orientation and confining pressure. The fracture progression behavior of the specimens showed a strong correlation with these failure mechanisms. Intact specimens and specimens with joints oriented at 0°, 30°, 90° failed by shearing. Nevertheless, the AE patterns of intact specimens and specimens with 90°-oriented joints contrasted with those of specimens with joints oriented at 0° and 30°. Specimens with joints oriented at 60° failed by sliding with a constant rate of acoustic event generation starting from the beginning of deviatoric loading. A mixed failure mechanism was observed for specimens with joints oriented at 45° and 75°. While the characteristics of both sliding and shearing failure were observed from the AE patterns of both 45° and 75° orientations, the shearing failure component of the 75° case was found to be a result of experimental limitations rather than real behavior. Finally, we propose a family of typical AE curves to assist with the characterization of the fracture progression behavior of jointed rock by taking confining pressures and failure mechanisms into consideration.

Water-weakening behavior of Hawkesbury sandstone in brittle regime

This paper investigates the effect of water on the mechanical behavior of intact Hawkesbury sandstone, a massive rock formation in Sydney, Australia. Two sets of cylindrical samples — air-dried and water-saturated — were tested under the confining pressures of 4, 10, 18 and 25MPa. Water-saturated samples were tested under undrained conditions with initial pore-water pressures of 1, 4, 7.2 and 10MPa, respectively.Initial pore-water pressure was observed to increase with increasing deviatoric stress during undrained tests and the maximum induced pore-water pressure was observed to increase with increasing effective confining pressure. Peak effective strength showed an increasing trend with increasing confining pressure for both sample sets and the strengths of dry samples were always greater. Peak strength drops of 13.36%, 25.27%, 34.70% and 38.12% were observed due to water at the confining pressures of 4, 10, 18 and 25MPa, respectively. Results for the residual strength of tested samples displayed that it increased with increasing confining pressure for both sample sets. The volumetric strain response revealed that the volume reduction due to compaction increases with increasing confining pressure, and dilatancy-related volume increase close to sample failure was not significant at higher confining pressures for both sample sets. The samples tested under dry conditions showed a considerably higher volume reduction by compaction than that for the samples tested under undrained conditions. Analysis of failure mechanisms indicated that all samples failed mainly by shear localization, where the angle of the failure plane, measured from the minor principal stress direction, was varied from c.55° to c.45° for dry samples and from c.50° to c.40° for the samples tested under undrained conditions, at 4 and 25MPa confining pressures, respectively. Fracture propagation behavior was studied using an acoustic emission detection system and the results demonstrated that the micro-crack initiation occurred very close to the failure stress under low confining pressures, leading to a more brittle sudden failure, w\\hereas under higher confining pressures it occurred relatively earlier, compared to the failure stress, showing more quasi-brittle characteristics.

Consolidation settlement due to ramp loading

In one-dimensional consolidation analysis, it is generally assumed that the load is applied instantaneously. In reality, the load is applied over certain duration which has an effect on computations. This paper presents some ramp loading tests using a standard oedometer to support some simple theoretical developments. It is shown that during ramp loading, the plot of settlement v. time, normalized with respect to the end of ramp loading values, is relatively independent of the clay type and loading duration. This normalized plot can be used to predict the settlement pattern during the construction period and the coefficient of consolidation, using a single measurement at any time during construction. Post-construction settlements are predicted separately by assuming a sinusoidal initial pore water pressure distribution at the end of the construction period that is applied instantaneously. Laboratory oedometer test data show very good agreement with the theory.

Consolidation Behavior of a Cylindrical Soil Layer Subjected to Nonuniform Pore-Water Pressure Distribution

Nonuniform, initial, excess pore-water pressure distributions in the horizontal direction will have a different impact on the consolidation behavior of the soil layer, when compared with the uniform pore-water pressure distributions discussed in the literature. In the current study, various practical instances in which the nonuniform, axisymmetric, horizontal distributions of pore-water pressure occur are identified. The influence of these distribution patterns on the consolidation response of a cylindrical soil layer draining only at its peripheral face is analyzed analytically using a simple series solution method and by finite-element analysis using PLAXIS. From the Uavg-T curves and pore-water pressure isochrones developed, it is observed that the uniform pore-water pressure assumption overestimates the degree of consolidation achieved for various pore-water pressure distribution conditions and important information about pore-water pressure redistribution is overlooked. Incorporating appropriate initial pore-water pressure distribution will enable realistic estimates of the degree of consolidation to be made.

Incorporating hysteresis in one-dimensional seepage modeling in unsaturated soils

To understand better the influence of soil-water characteristic hysteresis on rainfall infiltration and pore-water pressure distributions in unsaturated soils, an analytical solution to the one-dimensional governing partial differential equation considering hysteresis is derived using a Fourier integral transformation. The analytical solution considers time-dependent and arbitrary initial pore-water pressure distributions, as well as a time-varying rainfall flux process at the ground surface. An exponential function is used to represent the hysteretic Soil-Water Characteristic Curve (SWCC) and the relationship between hydraulic conductivity and pore-water pressure. The calculated results demonstrate that the critical point, the intersection between wetting and drying domains in one-dimensional unsaturated seepage, is influenced by many factors. The hysteresis in soil-water characteristics is an important factor in infiltration process influencing the pore-water pressure profiles in unsaturated soils. The effects of hysteretic parameters on pore-water pressure profiles are also analyzed. The value of α in the hysteresis model is found to be the most significant factor influencing the pore pressure distributions.

Method Study on Generation-Dissipation of Seismic Pore Water Pressure in Saturated Sands

With considering the effect of the initial pore water pressure and the variety of seismic pore water pressure generation, a two-dimensional inhomogeneous compound diffusion equation for describing generation and dissipation of seismic pore water pressure are given by coupling the Terzaghi’s consolidation theory and the increasing mode of seismic pore water pressure in saturated sands. The analytical expression of the diffusion equation is obtained by using the method of the generalized Green function and the finite Fourier transformation.

Finite Element Analysis on Instantaneous Deformation and Consolidation Deformation of Saturated Soft Clay Foundation

Deformation analysis of foundation is an important research field in geotechnical engineering, especially analysis on instantaneous deformation and consolidation deformation of saturated soft clay foundations. In this paper, a computation model of instantaneous deformation and consolidation deformation was built on the basis of Biot Consolidation Theory and consideration of mechanism of instantaneous deformation and consolidation deformation and consideration of interrelation of both deformations. A method was proposed to determine initial pore water pressure in soil. Nonlinearity of soil deformation under drainage conditions was considered by combining Duncan-Zhang model with compaction curve of soil, and failure soil elements were processed by Mohr-Coulomb Failure Criteria. By numerical simulation, a nonlinear axi-symmetric FEM program was complied to solve foundation deformation and applied in calculation of soft ground tank settlement. Results show that the method presented in this paper is efficient and suitable to analyze instantaneous deformation and consolidation deformation.

METHOD OF DETERMINATION OF INITIAL PORE WATER PRESSURE OF SATURATED CLAYED GROUND UNDER EMBANKMENT

Base on proposed formulation and testing results, method of determination of initial excess pore water pressure, which is used in filtration consolidation and other geotechnical problems in case of plane and spatial problems. Unlike supposition in initial time t = 0, σ = uwo, i.e. accepting a value of coefficient of initial pore water pressure β=1, by the laboratory testing results in triaxial cell, value of coefficient of initial pore water pressure of weak saturated clayey soils in HoChi Minh City and Mekong delta is lower 1. Output computation and research results show, that initial excess pore water pressure depends on water saturation, value of compression stress and groundwater level. Using the suggested formula allows determining coefficient of initial excess pore water pressure and applies to estimate initial settlement, distribution initial pore water pressure in consolidation and long term settlement.

Spatial Distribution of Excess Pore-Water Pressure due to Piezocone Penetration in Overconsolidated Clay

This paper presents the results of an analysis of the spatial distribution of the excess pore-water pressure induced by piezocone penetration into overconsolidated clays. From the experimental results obtained for moderately and heavily overconsolidated clays, it was observed that the excess pore-water pressure increases monotonically from the piezocone surface to the outer boundary of the shear zone and then decreases logarithmically, approaching zero at the outer boundary of the plastic zone. It was also found that the size of the shear zone decreases from approximately 2.2 to 1.5 times the cone radius with increasing overconsolidation ratio (OCR), whereas the plastic radius is about 11 times the piezocone radius, regardless of the OCR. The expressions developed in this study based on the modified Cam clay model and the cylindrical cavity expansion theory, which take into consideration the effects of the strain rate and stress anisotropy, provide a good prediction of the initial pore-water pressure at t...

Mechanical behavior of a volcanic ash soil (Typic Hapludand) under static and dynamic loading

Andisols are often subject to landsliding and coinciding liquifaction of the existing soil structure because they dominate at steeper slopes and show often an insufficient hydraulic conductivity. Thus, soil erosion at those sites is considered as one of the major risk. In order to quantify the proportion of the mechanically and of the pore water dependent soil strength, we evaluated their dependency during compression (divergent process) and shear tests as a function of the pore water pressure in a volcanic ash soil (Typic Hapludand). Undisturbed samples collected at two depths (0–15, 40–55 cm) were equilibrated at two pore water pressure values (−60 and −300 hPa). Thereafter they were compressed for either 10 or 30 min, respectively at each total stress applied. Shear tests with a shear speed of 0.2 mm min −1 were performed at the same two initial pore water pressure values, after the samples had been statically pre-stressed with identical loads all smaller than 400 kPa. The pre-compression stress was similar between time intervals when the samples were dried at −60 hPa, but on average for the two depths the pre-compression stress was 32% lower when 30 min steps were performed because of a more complete settlement due to the time dependency of water flow during soil deformation. There was an increase in cohesion of 30% in topsoil and 900% in subsoil when the pore water pressure changed from pF 1.8 to pF 2.5. Irrespective of the loading time interval, pore water pressure increased to even positive pore water pressure values when mechanical stresses of >300 kPa were applied, being more important in samples after pre-drying at −60 hPa than at −300 hPa. In the shear test the behavior was similar, but positive values of pore water pressure already occurred when the vertical stress exceeded 200 kPa. Thus, we can conclude, that hydraulic properties are specially important in soils showing thixotropic behavior and/or when shear stresses are applied resulting in a more pronounced weakening by increasing the destabilizing pore water pressure as a reason for an enhanced particle rearrangement and soil homogenization compared with the original soil structure.

Infiltration characteristics of two instrumented residual soil slopes

This paper presents the analysis of a 12 month long field study of the infiltration characteristics of two residual soil slopes in Singapore. The field measurements consist of rainfall data, runoff data of natural and simulated rainfall events, and pore-water pressure changes during infiltration at several depths and at several locations on the two slopes. The analysis of the field measurements identifies the total rainfall and the initial pore-water pressures within the two slopes as the controlling parameters for the changes in the pore-water pressures within the slopes during infiltration.Key words: infiltration, rainfall, runoff, pore-water pressure, field measurements.

Analytical Cavity Expansion-Critical State Model for Piezocone Dissipation in Fine-Grained Soils

After the arrest of cone penetration in clays and silts, excess porewater pressures decay with time until Δu = 0 and hydrostatic conditions prevail. A dissipation model is developed and initial porewater pressures are formulated in terms of cavity expansion theory and critical-state components, indicating the derived coefficient of consolidation (ch) is a function of stress history (OCR), effective friction (M), and rigidity index (Ir), as well as the probe diameter. Both OCR and Ir are assessed theoretically from the CPTu results. The governing rate of dissipation can be expressed by a second order differential equation and solved explicitly in closed-form. The framework is unique in that both monotonic decay and dilatory response (initial increase and then decrease of Δu with time) are handled by the approach. The model results show good comparison with laboratory data, as well other currently accepted methods of ch determination.

The undrained shear strength of soils containing large gas bubbles

Measurements of undrained shear strength are reported from a series of triaxial tests on reconstituted soil samples containing large bubbles of methane gas. The experimental results indicate that the undrained strength can be either increased or reduced by the presence of large gas bubbles, depending upon the values of the consolidation pressure and the initial pore water pressure. A conceptual model for soils containing large gas bubbles, developed in the companion Paper, is analysed to provide theoretical predictions for the undrained shear strength. The analysis leads to theoretical upper and lower bounds for the undrained strength, which show good agreement with the experimental data. The experimental and theoretical developments have led to a better understanding of the strength of soils containing large gas bubbles. This could be of considerable benefit to the offshore construction industry, if platforms, pipelines and other structures are to be built in areas of gas-bearing sediments. L'article présente des mesures de résistance au cisaillement non-drainé effectuées au cours d'une série d'essias triaxiaux sur des échantillons de sol reconstitués qui contenaient de grandes bulles de méthane. Les résultats de ces expériences indiquent que la résistance dans l'état non-drainé peut être augmentée ou diminuée par la présence de grandes bulles de gaz selon les valeurs de la pression de consolidation et de la pression initiate de l'eau interstitielle. On analyse un modèle conceptuel qui est présenté sur un autre article de ce numéro et qui concerne les sols qui contiennent de grandes bulles de gaz. Cette analyse, dont le but est de fournir des prédictions théoriques pour la résistance dans l'état non-drainé, conduit à des limites supérieure et inférieure théoriques pour la résistance dans l'état non-drainé qui s'accordent bien avec les données expérimentales. Les développements expérimentaux et théoriques ont conduit à une meilleure compréhension de la résistance des sols qui contiennent de grandes bulles de gaz dont pourrait profiter beaucoup la construction en mer là où des plateformes, des pipelines ou d'autres structures doivent se construire dans des régions de couches gazifères.

Modelling non-darcian flow in one-dimensional consolidation

A numerical investigation has been worked in order to better understand the mechanism of consolidation of fine-grained solis with very low permeability. By using a simple one-dimensional finite difference scheme it has been possible to assess the effects of laws different from Darcy's law upon the consolidation phenomenon of such materials, all other hypotheses worked out by Terzaghi being unchanged. The deviation from the classical theory of the hydraulic behaviour of soils increases the more the gradient dependent flow rate deviates from Darcy's law and the more the initial pore water pressure dissipates. First comparisons with experimental data have also been made, in order to verify the reliability of the numerical results.