Pore Pressure Build-Up

Generation of excess pore water pressures under cyclic loading has been shown to cause marked reduction in undrained strength and stiffness of clay soils. Theoretical and empirical expressions have been developed that relate excess or residual pore pressures with factors such as cyclic stress and strain level, number of cycles of loading, and OCR. An empirical expression has been developed by Van Eekelen and Potts (1978) for the rate of generation of excess pore pressures. Another has been developed by Matsui et al. (1980) for the residual pore pressures  

max is the single amplitude maximum cyclic shear strain OCR is the over consolidation ratio P = 0.45 (found experimentally) 

Togrol and Guler (1984) suggested an empirical relation for normally consolidated clay that related the deviatoric stress at failure to excess pore pressure developed during cyclic loading  

Using experimental values of maximum excess pore pressure developed and Equation 9.7, they found that the maximum reduction in undrained shear strength of the soil would be on the order of 35% under repeated load application. 

K is the rebound or recompression index on the natural logarithmic scale (= Q/2.3) 

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Hydraulic breaching occurs when pore fluid pressure exceeds the total minimum confining stress and the tensile strength of the rock (Eq. (3)). In order to understand how this process works, the difference between pore-pressure build-up within the cap-rocks and underlying reservoir needs to be clarified. 

Abstract The Canadian Nuclear Safety Commission (CNSC) used the finite element code FRACON to perform blind predictions of the FEBEX heater experiment. The FRACON code numerically solves the extended equations of Biot s poro-elasticity. The rock was assumed to be linearly elastic, however, the poro-elastic coefficients of variably saturated bentonite were expressed as functions of net stress and void ratio using the state surface equation obtained from suction-controlled oedometer tests. In this paper, we will summarize our approach and predictive results for the Thermo-Hydro-Mechanical response of the bentonite. It is shown that the model correctly predicts drying of the bentonite near the heaters and re-saturation near the rock interface. The evolution of temperature and the heater thermal output were reasonably well predicted by the model. The trends in the total stresses developed in the bentonite were also correctly predicted, however the absolute values were underestimated probably due to the neglect of pore pressure build-up in the rock mass. 

Sections 9.4.1 through 9.4.3 discuss about (1) strength determination, (2) pore pressure build-up, and (3) reduction and degradation of stiffness. 

The relationships reported by Ansal and Erken were established through a series of cyclic simple shear testing of normally consolidated kaolinite clay. In this model the pore pressure build-up is expressed as... 

As the boundary conditions are extremely important in the results observed, the container where the models were prepared need to be carefully chosen. Thus, the models were prepared and tested within an Equivalent Shear Beam (ESB) container, described by Schofield and Zeng (1992), having flexible walls intended to replicate the soil dynamic behavior and minimize boundary effects. However, due to the large degradation of soil properties during liquefaction caused by pore pressure build-up and subsequent effective stress reduction, the container cannot exactly match the soil behavior at all times during the test. 

The pore-pressure profiles for the wells belonging to hydraulic compartments II and III show a rapid increase versus depth for the lowermost Cretaceous-Upper Jurassic interval (Figs. 4 and 14 red trend line). This pressure increase versus depth is higher than lithostatic, and is taken to suggest that the primary cause of overpressure for this interval is related to the pulse of increased maturation and pressure build-up within the Spekk Formation source rock caused by increased maturation and kerogen transformation to liquid hydrocarbons, again primarily in response to the Pliocene to Recent subsidence. 

Extremely low permeability of gels hinders expulsion of expanded liquid from the pores as a result, pressure builds up inside the solid. 

In summary, liquid displacement is another method to determine the pore size distribution in microporous and mesoporous materials. The advantage of this method is that only active pores are characterised. A drawback may the occurrence of swelling due to the stagnant liquid that changes the pore sizes. Moreover, the set up is rather comple.x and a pressure build-up may occur which interferes with the measurements. 

Using also a metal alkyl in the catalyst system, initiation is so rapid that the polymer in the pore has no time to crystallize while filling the pore. Since there is no locking effect, further pressure build-up in pores and fragmentation is instantaneous. 

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