scholarly journals Observations of heterogeneous pore pressure distributions in clay-rich materials

2012 ◽  
Vol 76 (8) ◽  
pp. 3115-3129 ◽  
Author(s):  
R. J. Cuss ◽  
J. F. Harrington ◽  
C. C. Graham ◽  
S. Sathar ◽  
A. E. Milodowski
Keyword(s):  
High Pressure ◽  
Pore Pressure ◽  
Effective Stress ◽  
Gas Flow ◽  
Pore Water Pressure ◽  
Water Pressure ◽  
Opalinus Clay ◽  
Boom Clay ◽  
Pore Pressures ◽  
Stress States

AbstractThe concept of effective stress is one of the basic tenets of rock mechanics where the stress acting on a rock can be viewed as the total stress minus the pore water pressure. In many materials, including clay-rich rocks, this relationship has been seen to be imperfect and a coefficient (χ) is added to account for the mechanical properties of the clay matrix. Recent experimental results during the flow testing (both gas and water) of several rocks (Callovo-Oxfordian claystone, Opalinus Clay, Boom Clay) and geomaterials (bentonite, kaolinite) has given evidence for stable high pressure differentials. The design of the experiments allows multiple measurements of pore pressure, which commonly shows a complex distribution for several different experimental geometries. The observed stable high pressure differentials and heterogeneous pore pressure distribution makes the describing of stress states in terms of effective stress complex. Highly localized pore pressures can be sustained by argillaceous materials and concepts of evenly distributed pore pressures throughout the sample (i.e. conventional effective stress) do not fit many clay-rich rocks if the complexities observed on the micro-scale are not incorporated, especially when considering the case of gas flow.

An effective stress model for creep of clay

1995 ◽  
Vol 32 (5) ◽  
pp. 819-834 ◽  
Author(s):  
Mohammed M. Morsy ◽  
D.H. Chan ◽  
N.R. Morgenstern
Keyword(s):  
Finite Element ◽  
Pore Pressure ◽  
Effective Stress ◽  
Pore Water Pressure ◽  
Water Pressure ◽  
Surface Model ◽  
Interpolation Technique ◽  
Stress Integration ◽  
Integration Algorithms ◽  
Double Yield

An effective stress constitutive model to study the problem numerically of creep in the field is presented. A double-yield surface model for the stress–strain–time behaviour of wet clay is described. The model adopts the concept of separating the total deformation into immediate and delayed components. The yield surfaces employed are the modified Cam-clay ellipsoid and the Von Mises cylinder inscribed in the ellipsoid. The proposed numerical scheme incorporates the pore pressure based on field observations into a finite element analysis. An interpolation technique is used to determine the pore pressure at every element. A field example is presented to illustrate the interpolation technique procedure. The scheme not only avoids the complexity of making predictions of pore-water pressure, but also allows the analysis to be carried out in terms of effective stresses based on the actual observed pore pressure. Two stress integration algorithms based on the implicit calculation of plastic strain are implemented and tested for the double-yield surface model. A numerical simulation of stress-controlled drained creep tests confirms the numerical procedure. Key words : constitutive equations, creep, finite element, stress integration algorithms, effective stress approach, pore-water pressure.

Excess Pore Pressures During Undrained Clay Creep

1973 ◽  
Vol 10 (1) ◽  
pp. 12-24 ◽  
Author(s):  
Thomas L. Holzer ◽  
Kaare Höeg ◽  
Kandiah Arulanandan
Keyword(s):  
Pore Pressure ◽  
San Francisco ◽  
Pore Water ◽  
Pore Water Pressure ◽  
Water Pressure ◽  
Excess Pore Pressure ◽  
Time Behavior ◽  
Pore Pressures ◽  
Undrained Creep ◽  
Excess Pore

The objective of this presentation is to examine experimentally how the excess pore-water pressure is related to the mechanism for undrained creep of San Francisco Bay mud. The results are discussed in the context of creep mechanisms previously suggested in the literature and based on laboratory testing.It is found that shear strains occurring during undrained creep are directly related to a gradual but significant increase in excess pore pressure and, hence, reduction in effective stresses. The increase in magnitude of the pore pressure is, except immediately after the creep shear stress is applied, solely a function of the initial consolidation stress and consolidation period. The magnitude of the long-term build-up may be related to the amount of secondary compression which would occur during drained conditions. It increases with the organic content of the soil and decreases with the degree of remolding. The mechanism for the increase in pore-water pressure may be explained by drainage of water from micropores in the microstructure into the macrostructure.Unless one accounts for the increase in pore pressures during undrained creep, it is unlikely that one will be successful in formulating a generally valid mathematical model for stress–strain–strength–time behavior based on laboratory testing.

The influence of high pore-water pressure on the strength of cohesionless soils

1977 ◽  
Vol 284 (1318) ◽  
pp. 91-130 ◽  
Keyword(s):  
Pore Pressure ◽  
Effective Stress ◽  
Quartz Sand ◽  
Pore Water Pressure ◽  
Pore Space ◽  
Water Pressure ◽  
Experimental Studies ◽  
Interparticle Contact ◽  
Particulate Materials ◽  
Shearing Resistance

The influence on the mechanical properties of saturated particulate materials of the component of stress carried by the water filling the pore space is fundamental to both theoretical and experimental studies in soil mechanics. The rôle of pore pressure in controlling compressibility and shear strength is expressed in Terzaghi’s principle of effective stress to a degree of accuracy which is sufficient for most engineering purposes. However, the precise significance of the small but finite area of interparticle contact has remained uncertain in the application of this equation to shearing resistance. In the present paper the possible errors associated with the use of current expressions for intergranular stress and effective stress are examined. These errors are of significant magnitude at high values of pore pressure and low values of the yield stress of the solid forming the particles. A very accurate experimental investigation has been carried out into the sensitivity of shearing resistance to large changes in pore pressure (up to 41.4 MN/m 2 ), using particulate materials ranging in strength from Quartz sand to lead shot. The results indicate that the simple Terzaghi effective stress equation a' - o - u is consistent with all the observations, though for Quartz sand a range of pore pressure changes an order of magnitude higher is desirable for additional confirmatory evidence.

Simulation of creep deformation in the foundation of Tar Island Dyke

1995 ◽  
Vol 32 (6) ◽  
pp. 1002-1023 ◽  
Author(s):  
Mohammed M. Morsy ◽  
N.R. Morgenstern ◽  
D.H. Chan
Keyword(s):  
Pore Pressure ◽  
Effective Stress ◽  
Creep Deformation ◽  
Pore Water Pressure ◽  
Soil Mechanics ◽  
Water Pressure ◽  
Pressure Change ◽  
Sensitivity Study ◽  
Stress Model ◽  
Oil Sand

Tar Island Dyke is a 92 m tailing dyke for retaining oil sand tailings and has been operated by Suncor in Fort McMurray, Alberta. Construction of the dyke began in the mid-1960's adjacent to the Athabasca river. The foundation of the dyke consists of a layer of interbedded silts and clay overlying a basal sand stratum. Stresses imposed by the dyke on the foundation clay have been causing continuing movement of the structure over 30 years. Movements of the dyke have been monitored for over 25 years and show significant creep deformation of over 1 m in the foundation clay. Pore pressure in the clay was monitored, with little pore pressure change during this period. Therefore the movement was mostly due to creep rather than consolidation. The unique feature of this case is that the loading due to the dyke has been essentially constant for over 15 years but movement has continued. An effective stress model for creep is adopted to simulate the construction of the Tar Island Dyke. The model is based on critical state soil mechanics and uses secondary consolidation and the Taylor Singh-Mitchell creep relationships. The model is able to capture the movement of the dyke and its foundation, and good agreement is obtained between the calculated and measured deformations. A sensitivity study has been carried out to study the effect of varying the creep parameters on the results of the analysis. Key words : Tar Island Dyke, creep mechanism, finite element, clay foundation, effective stress model, pore-water pressure.

Type of Slope Failure Identified from Pore Water Pressure and Moisture Content Measurements

2015 ◽  
Vol 744-746 ◽  
pp. 690-694
Author(s):  
Muhammad Rehan Hakro ◽  
Indra Sati Hamonangan Harahap
Keyword(s):  
Moisture Content ◽  
Pore Pressure ◽  
Slope Failure ◽  
Pore Water Pressure ◽  
Water Pressure ◽  
Failure Process ◽  
Small Scale ◽  
Content Increase ◽  
Sudden Increase ◽  
Model Slope

Rainfall-induced landslides occur in many parts of the world and causing a lot of the damages. For effective prediction of rainfall-induced landslides the comprehensive understanding of the failure process is necessary. Under different soil and hydrological conditions experiments were conducted to investigate and clarify the mechanism of slope failure. The failure in model slope was induced by sprinkling the rainfall on slope composed of sandy soil in small flume. Series of tests were conducted in small scale flume to better understand the failure process in sandy slopes. The moisture content was measured with advanced Imko TDR (Time Domain Reflectrometry) moisture sensors in addition to measurements of pore pressure with piezometers. The moisture content increase rapidly to reach the maximum possible water content in case of higher intensity of rainfall, and higher intensity of the rainfall causes higher erosion as compared to smaller intensity of the rainfall. The controlling factor for rainfall-induced flowslides was density of the slope, rather than intensity of the rainfall and during the flowslide the sudden increase in pore pressure was observed. Higher pore pressure was observed at the toe of the slope as compared to upper part of the slope.

scholarly journals Mechanical response and pore pressure generation in granular filters subjected to uniaxial cyclic loading

2018 ◽  
Vol 55 (12) ◽  
pp. 1756-1768
Author(s):  
Jahanzaib Israr ◽  
Buddhima Indraratna
Keyword(s):  
Cyclic Loading ◽  
Pore Pressure ◽  
Pore Water ◽  
Mechanical Response ◽  
Pore Water Pressure ◽  
Axial Strain ◽  
Water Pressure ◽  
Internal Erosion ◽  
Excess Pore Water Pressure ◽  
Excess Pore

This paper presents results from a series of piping tests carried out on a selected range of granular filters under static and cyclic loading conditions. The mechanical response of filters subjected to cyclic loading could be characterized in three distinct phases; namely, (I) pre-shakedown, (II) post-shakedown, and (III) post-critical (i.e., the occurrence of internal erosion). All the permanent geomechanical changes such, as erosion, permeability variations, and axial strain developments, took place during phases I and III, while the specimen response remained purely elastic during phase II. The post-critical occurrence of erosion incurred significant settlement that may not be tolerable for high-speed railway substructures. The analysis revealed that a cyclic load would induce excess pore-water pressure, which, in corroboration with steady seepage forces and agitation due to dynamic loading, could then cause internal erosion of fines from the specimens. The resulting excess pore pressure is a direct function of the axial strain due to cyclic densification, as well as the loading frequency and reduction in permeability. A model based on strain energy is proposed to quantify the excess pore-water pressure, and subsequently validated using current and existing test results from published studies.

scholarly journals Implications of the principle of effective stress

2020 ◽  
Author(s):  
Gerd Gudehus
Keyword(s):  
Critical Phenomena ◽  
Pore Water ◽  
Effective Stress ◽  
Critical Points ◽  
Pore Water Pressure ◽  
Numerical Models ◽  
Water Pressure ◽  
Shear Bands ◽  
Triaxial Tests ◽  
Water Saturated

AbstractWhile Terzaghi justified his principle of effective stress for water-saturated soil empirically, it can be derived by means of the neutrality of the mineral with respect to changes of the pore water pressure $$p_w$$ p w . This principle works also with dilating shear bands arising beyond critical points of saturated grain fabrics, and with patterns of shear bands as relics of critical phenomena. The shear strength of over-consolidated clay is explained without effective cohesion, which results also from swelling up to decay, while rapid shearing of water-saturated clay can lead to a cavitation of pore water. The $$p_w$$ p w -neutrality is also confirmed by triaxial tests with sandstone samples, while Biot’s relation with a reduction factor for $$p_w$$ p w is contestable. An effective stress tensor is heuristically legitimate also for soil and rock with relics of critical phenomena, particularly for critical points with a Mohr–Coulomb condition. Therein, the $$p_w$$ p w -neutrality of the solid mineral determines the interaction of solid fabric and pore water, but numerical models are questionable due to fractal features.

scholarly journals Mechanism of Fracturing in Shaft Lining Caused by High-Pressure Pore Water in Stable Rock Strata

2019 ◽  
Vol 2019 ◽  
pp. 1-9 ◽  
Author(s):  
Jihuan Han ◽  
Jiuqun Zou ◽  
Weihao Yang ◽  
Chenchen Hu
Keyword(s):  
High Pressure ◽  
Rock Mass ◽  
Pore Water ◽  
Expansion Coefficient ◽  
Pore Water Pressure ◽  
Water Pressure ◽  
Surrounding Rock ◽  
Shaft Lining ◽  
Rock Strata ◽  
Expansion Effect

With the increase in shaft depth, the problem of cracks and leakage in single-layer concrete lining in porous water-rich stable rock strata has become increasingly clear, in which case the mechanism of fracturing in shaft lining remains unclear. Considering that the increase in pore water pressure can cause rock mass expansion, this paper presents the concept of hydraulic expansion coefficient. First, a cubic model containing spherical pores is established for studying hydraulic expansion, and the ANSYS numerical simulation, a finite element numerical method, was used for calculating the volume change of the model under the pore water pressure. By means of the multivariate nonlinear regression method, the regression equation of the hydraulic expansion coefficient is obtained. Second, based on the hydraulic expansion effect on the rock mass, an interaction model of pore water pressure–porous rock–shaft lining is established and further solved. Consequently, the mechanism of fracturing in shaft lining caused by high-pressure pore water is revealed. The results show that the hydraulic expansion effect on the surrounding rock increases with its porosity and decreases with its elastic modulus and Poisson’s ratio; the surrounding rock expansion caused by the change in pore water pressure can result in the outer edge of the lining peeling off from the surrounding rock and tensile fracturing at the inner edge. Therefore, the results have a considerable guiding significance for designing shaft lining through porous water-rich rock strata.

Experimental verification of the theory of consolidation for unsaturated soils

1995 ◽  
Vol 32 (5) ◽  
pp. 749-766 ◽  
Author(s):  
Harianto Rahardjo ◽  
Delwyn G. Fredlund
Keyword(s):  
Water Content ◽  
Pore Pressure ◽  
Pore Water ◽  
Unsaturated Soils ◽  
Pore Water Pressure ◽  
Water Pressure ◽  
Matric Suction ◽  
Water Volume ◽  
Loading Tests ◽  
Undrained Loading

An experimental program was designed to study the behavior of unsaturated soils during undrained loading and consolidation. A Ko cylinder was designed and built for the testing program. Simultaneous measurements of pore-air and pore-water pressures could be made throughout a soil specimen using this Ko cylinder. Four types of tests were performed on a silty sand. These are (1) undrained loading tests where both the air and water are not allowed to drain, (2) constant water content tests where only the water phase is not allowed to drain, (3) consolidation tests where both the air and water phases are allowed to drain, and (4) increasing matric suction tests. Undrained loading tests or constant water content loading tests were conducted for measuring the pore pressure parameters for the unsaturated soil. Drained tests consisting of either consolidation tests or increasing matric suction tests were conducted to study the pore pressure distribution and volume change behavior throughout an unsaturated soil during a transient process. The experimental pore pressure parameters obtained from the undrained loadings and constant water content leadings agreed reasonably well with theory. The pore-air pressure was found to dissipate instantaneously when the air phase is continuous. The pore-water pressure dissipation during the consolidation test was found to be faster than the pore-water pressure decrease during the increasing matric suction test. The differing rates of dissipation were attributed to the different coefficients of water volume change for each of the tests. The water volume changes during the consolidation test were considerably smaller than the water volume changes during the increasing matric suction tests for the same increment of pressure change. Key words : consolidation, Ko loading, matric suction, pore-air pressures, pore-water pressures, unsaturated soils

Horizontal pressure on barricades for backfilled stopes. Part II: Submerged conditions

2009 ◽  
Vol 46 (1) ◽  
pp. 47-56 ◽  
Author(s):  
Li Li ◽  
Michel Aubertin
Keyword(s):  
Pore Pressure ◽  
Pore Water Pressure ◽  
Experimental Testing ◽  
Water Pressure ◽  
Companion Paper ◽  
Simple Method ◽  
Horizontal Pressure ◽  
Undrained Conditions ◽  
Backfilled Stopes ◽  
Submerged Conditions

This paper presents a method to calculate the pressure generated by submerged backfill on barricades (or bulkheads) located in drifts at the base of mine stopes. The paper complements Part I (see companion paper, this isue), which presents an analytical solution for the pressure on barricades when the backfill is in drained conditions (after the pore-water pressure has dissipated). The solution presented here applies shortly after backfill deposition, for undrained conditions. In this case, the effect of pore pressure cannot be neglected as it may be critical for the response of barricades. The solution is developed for totally or partly submerged backfill (with the water table at various elevations). Experimental testing and numerical modelling results are used to validate the proposed equations. Both numerical and analytical results show that the total pressure on barricades can be significantly increased by pore pressure, while the effective stress is decreased in the access drifts (compared to dry or drained conditions). The proposed solution provides a simple method to obtain a realistic estimate of the total and effective stresses, and can thus be used as a basis for barricade design.

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