Pore Pressure Increment Research Articles

Study of the variation of B with Sr

Skempton coefficient B is commonly used to verify the saturation of a sample before triaxial testing. This coefficient is obtained during undrained isotropic consolidation and is defined as the ratio between the increment of pore pressure u measured and the imposed increment of isotropic stress. This coefficient varies between 0 for dry soils and 1 for saturated soils. Many studies on liquefaction of unsaturated soils were published using Skempton coefficient B to represent saturation degree Sr of soil. On the first hand, this variation of B coefficient with saturation degree is mostly due to the compressibility of air in the pores. On the second hand, we also know that the presence of air as a fluid phase gives birth to suction after equilibrium is reached inside the sample. The higher the suction, the stiffer the soil skeleton. These two phenomena are opposite. Their effects in laboratory testing depend on the experimental apparatus. For example, if we consider an unsaturated triaxial device, we will have to take suction into account. On the contrary if we plan to break the menisci just before measuring B, suction equilibration will not occur. Experimental tests were performed to show the difference between these two cases and to study the equilibrium phase. Based on these observations, this article presents new relationships that permit to calculate saturation degree with a given Skempton coefficient with different hypotheses and with different experimental devices. These results are confronted to the commonly used relation given by Lade and the difference between all these calculations is studied.

The Degradation of Saturated Clay’s Un-Drained Shearing Deformation Modulus under Cyclic Loading

The un-drained shearing deformation modulus of clay will degenerate under cyclic loading. This degeneration is attributed to two reasons, the first one is the reduction of effective pressure due to the increment of super pore pressure; another one is the change of soil’s structure due to the accumulation of plastic shearing strain. Firstly, based on the result of un-drained shearing tests, two formulations were obtained which include initial deformation modulus ratio versus over-consolidated ratio and initial deformation modulus ratio versus initial shearing strain. Then, the quasi-over-consolidated ratio was introduced to consider the effect of super pore pressure, and a fitting formulation was applied to consider the influence of shearing strain. At last, the degenerate formulation of saturated clay’s deformation modulus was deduced, which was proved reasonably by the test results.

A simplified numerical approach for lateral spreading evaluation

Los desarrollos ocurridos en las últimas décadas, tanto en los métodos numéricos de solución de problemas de ingeniería geo-sísmica, como licuación y sus efectos, así como en la tecnología computacional, ha permitido que el uso de técnicas rigurosas de modelado matemático sean más frecuentes en ingeniería aplicada. Sin embargo, para el caso particular de la estimación de desplazamientos laterales inducidos por sismos, (asociados a la licuación de áreas extensas de terreno), resulta complicado para llevar a cabo análisis detallados que involucren modelos bi y tridimensionales. Es común el recurrir a estimaciones estadísticas, o inclusive a predicciones obtenidas con sistemas neurodifusos. En este artículo, se introduce un enfoque simplificado para calcular los desplazamientos laterales inducidos por sismos cuando se presenta licuación. La respuesta dinámica de un perfil de suelo es simulada a través de elementos finitos unidimensionales acoplados a un esquema de generación de presiones de poro. La ecuación de movimiento se resuelve en el dominio del tiempo y con base en cada incremento de presión de poro. Los parámetros de resistencia y dinámicos del suelo se modifican, permitiendo la estimación de los desplazamientos del suelo que ocurren durante el evento dinámico. La formulación propuesta se aplica al análisis de varios casos de estudio en donde se realizaron mediciones de desplazamientos laterales.

How close to failure is a granite rock mass at a 5 km depth?

We discuss in this paper the mechanical significance of microseismicity induced by pore pressure variations. Particular attention is given to identifying whether microseismic events reflect only small effective stress perturbations, in a manner somewhat similar to the so-called Kaiser effect, or whether they outline the onset of large-scale failure. This issue is addressed in the context of the development of microseismic activity observed during large-scale water injections conducted in the 5000 m deep experimental geothermal reservoir at Soultz (France). Results from large hydraulic tests together with analysis of borehole images and induced seismicity are integrated to provide a well-constrained characterization of the complete stress field down to 5 km. It is shown that, for this site, pore pressure increments larger than 10% of the natural minimum principal stress magnitude are required for inducing large-scale shear failures that are characterized by a clear structuring of the microseismic events occurrence. But the onset of induced seismicity occurs for much smaller pore pressure variations that only reflect the “elastic” response to changes in local effective stresses. It is concluded that the linear variation with depth of principal stress magnitudes observed in this granite depends on the long-term rheology of this formation rather than on the frictional characteristics of the main faults that affect the massif.

Fluidization in insect constructions in soils

Fluidization, a special case of liquefaction, is a physical process occurring in unconsolidated, water‐saturated, sediments that can be verified in insect constructions in soils. Behavioral analysis of the bees Ptilothrix relata and Diadasina distincta shows that the fine‐grained material of the soil pellets, utilized to construct the chimneys and cells of the nest, have been subjected to fluidization. The increment of pore pressure transmited by the bee's appendages to the moulded soil pellets, produces the outward escape of water, which drags the fine‐grained material. Micro‐morphologically, the fluidization is reflected in the thin layers of clay and organic matter deposited on those surfaces of the constructions exposed to direct contact with the bee's appendages and in the higher content of fine‐grained material close to these layers. Along with the reorientation of the coarser grains of the soil, produced also by the moulding behavior of the bee, these micromorphological features, having high preservational potential, constitute important ichnotaxobases and valuable clues to aid in the recognition of insect trace fossils in paleosols.

A Reformation of the Equations of Anisotropic Poroelasticity

The constitutive equations of linear poroelasticity presented by Biot (1955) and Biot and Willis (1957) extended the description of rock behavior into the realm of saturated porous rocks. For isotropic material behavior, Rice and Cleary (1976) gave a formulation which involved material constants whose physical interpretation was particularly simple and direct; this is an aid both to their measurement and to the interpretation of predictions from the theory. This paper treats anisotropic poroelasticity in terms of material tensors with interpretations similar to those of the constants employed by Rice and Cleary. An effective stress principle is derived for such anisotropic material. The material tensors are defined, rigorously, from the stress field and pore fluid content changes produced by boundary displacements compatible with a uniform mean strain and uniform pore pressure increments. Such displacements and pore pressure increments lead to homogeneous deformation on all scales significantly larger than the length scale of microstructural inhomogeneities. This macroscopic behavior is related to the microscopic behavior of the solid skeleton. The tensors which describe the microscopic behavior of the solid skeleton would be difficult, even impossible, to measure, but their introduction allows relationships between measurable quantities to be identified. The end product of the analysis is a set of constitutive equations in which the parameters are all measurable directly from well-accepted testing procedures. Relationships exist between measurable quantities that can be used to verify that the constitutive equations described here are valid for the rock under consideration. The case of transverse isotropy is discussed explicitly for illustration.

Pore pressure predictions in finite element analysis

AbstractMany of the existing pore pressure models for FEM analysis are limited either to cases where the pore pressure component produced by the shearing stress is neglected (usually restricted to static analysis) or where the component produced by the mean confining stress is disregarded (usually restricted to cyclic loading, as in earthquake and offshore engineering). A pore pressure model including the simultaneous effects of shear and mean confining stresses is presented in this paper. The model is simple and applicable to static as well as cyclic loading. The conceptual relationship between strains and pore pressure is well recognized in soil mechanics; however, it is difficult to determine the strains by FEM with the necessary accuracy to ensure satisfactory pore pressure predictions, mainly in the range of strains where the concepts of stress path are best applicable. To overcome this limitation, the proposed technique introduces a pore pressure function, to be coupled to a potential and a loading function in an elastic—plastic constitutive model. For simplicity, it is assumed that the pore pressure response can be satisfactorily represented by a path dependent calibrating function determined to fit experiments over a given range of confining pressures. This procedure allows the independent determination of the pore pressure increment before the iterative elastic—plastic procedure is activated, thus supplying the constitutive model directly with the appropriate direction and magnitude of the effective stress increment. The advantages of this procedure during undrained loading are presented in the text.

The Influence of pore pressure and confining pressure on dynamic elastic properties of Berea sandstone

Compressional‐ and shear‐wave velocities of watersaturated Berea sandstone have been measured as functions of confining and pore pressures to 2 kbar. The velocities, measured by the pulse transmission technique, were obtained at selected pressures for the purpose of evaluating the relative importance of confining pressure and pore pressure on elastic wave velocities and derived dynamic elastic constants. Changes in Berea sandstone velocities resulting from changes in confining pressure are not exactly canceled by equivalent changes in pore pressure. For properties that involve significant bulk compression (compressional‐wave velocities and bulk modulus) an incremental change in pore pressure does not entirely cancel a similar change in confining pressure. On the other hand, it is shown that a pore pressure increment more than cancels an equivalent change in confining pressure for properties that depend significantly on rigidity (shear‐wave velocity and Poisson’s ratio). This behavior (as well as observed wave amplitudes) is related to the presence of high‐compressibility clay that lines grains and pores within the quartz framework of the Berea sandstone.