Excess pore pressure

Several investigators [159, 115, 108] have examined other aspects of the lacunar-canalicular porosity using simple circular pore models and have attempted to analyze its possible physiological importance. These studies have primarily emphasized the importance of the convective flow in the canaliculi between the lacunae as a way of enhancing the supply of nutrients between neighboring osteocytes. Previous studies on the relaxation of the excess pore pressure have been closely tied to the strain generated potentials (SGPs) associated with bone fluid motion. The SGP studies are briefly reviewed below. 

It is unlikely, however, that the lithification of chalk will go on without consolidation, in which the volume of chalk material is reduced in response to a load on the chalk. Consolidation can lead to a reduction in porosity up to about 40%, and an increase in the effective stress (Jones et al., 1984). The increased effective stress is required to instigate the process of pressure solution. Pressure solution provides Ca2+ and HCO3 for early precipitation of calcite cement in the chalk. However, the inherently low permeability of chalk would inhibit the processes of consolidation and pressure solution/cementation unless some permeable pathways are opened up to permit the dissipation of excess pore pressure created by the filling of pore space by calcite cement. Pressure solution will cease if the permeable pathways are blocked by cement. Thus, it appears that the development of fractures, open stylolites and microstylolitic seams (Ekdale et al., 1988) is necessary to permit pressure solution to continue and lead to large rates of Ca2+ and HC03 mobilization. 

Most of the excess pore pressure dissipates during primary consolidation. Secondary consolidation involves the movement of particles as they adjust to the increase in effective pressure and the dissipation of excess pore pressure from very small pores. The pore water extracted during squeezing is mainly because of primary consolidation. 

When a load is applied to a saturated fine-grained soil, it is instantaneously carried by the water as excess pore pressure. The water immediately begins to flow away from the point of maximum stress, and as it does so the load is transferred to the soil matrix, which deforms under the load. The finer the pores in the soil, the longer it takes for the pore water pressure to approach zero, and for settlement under the load to become negligible. In the field, the process could take months or years. 

The consolidation state of the sediment below this cap of apparent of overconsolidation material appears to be dependent on the sedimentation rate if data from the East Bermuda Rise are deleted. In areas of low sedimentation such as the North Pacific, the sediments below the cap are NC. By contrast, for areas of high sedimentation such as the Eastern SOHM Abyssal Plain and the North Bermuda Rise Plateau, the underlying sediments are UC due to the presence of the cap restricting drainage (Silva et al., 1976). The decrease in the thickness of the apparent OC zone in the areas of high sedimentation is possibly due to the development of excess pore pressures. 

Creep is the long-term continuing deformation due to sustained deviatoric stress (od =Oi - Os) conditions that occurs as a function of time after dissipation of consolidation excess pore pressures. Thus, creep behavior of sediments is a function of the type of sediment, its physical properties, stress-strain history, and time. Mitchell (1976) distinguished between creep and secondary compression by noting that the former is referred... 

When T becomes equal to fN, gross sliding of the particle contacts occurs. This gross sliding is required for permanent particles reorientation as shown in Figure 9.5. This reorientation of particles results in either volume changes (drained conditions) or excess pore pressures (undrained conditions). If particle reorientation does not occur then neither volume change nor excess pore pressure will occur. 

Pore pressure as a function of cyclic shear strain illustrating a threshold strain of about 0.01%, below which no excess pore pressure are developed. (After Dobry, R. et al.. Prediction ofPorewater Pressure Buildup and Liquefaction of Sands during Earthquakes by the Cyclic Strain Method, NBS Building Science Series 138, U.S. Department of Commerce, National Bureau of Standards, 1982.)... 

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 ... 

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. 

Seed and Rahman (1978) developed a procedure for evaluating the magnitude and distribution of wave-induced pore pressures in ocean floor deposits. The generation of excess pore pressure in terms of the number of cycles Ml required to cause initial... 

Settlements from less than 1 ft to several feet occurred in areas subjected to slumping and sliding as excess pore pressures dissipated. Nearly all major valleys in south central Alaska showed evidence of fissmes and flows. Numerous sand boils occinred depositing material in cones, and (2-3 ft high and 100 ft long) ridges up to 6 ft wide and in one case a 1-acre area 3 ft deep. The sand in one cone was reportedly pumped from a depth of 12 ft... 

After pile installation an excess pore pressure exists around the pile. In soft clay the excess pore pressure adjacent to the pile surface is positive. By contrast, in stiff clay the excess pore pressure is negative. At a distance from the pile surface the pressure is positive and equals the increase in total stress. 

The excess pore pressure dissipation results in changes in the effective normal stress acting on the pile surface. This in turn results in changes in shaft resistance. This is known as pile setup. How fast the pore water pressure dissipation occurs is dependent on the soils permeability. In a clay the dissipation of excess pore water pressure may take months to occur. In silty clays, the dissipation may take only minutes. 

The other shear strain component is caused by the cyclic shear stresses from the wave forces. These repetitive shear stresses cycle around the permanent shear stresses due to the initial in-situ stresses and stresses due to platform weight. The soil elements are thus subjected to unsymmetrical cyclic shear stresses and this will cause a permanent shear strain increase in the direction of permanent shear stress even if there is no excess pore pressure. 

Slides that occur at almost the same time as the deposition that were caused by rapid sedimentation results in the generation of excess pore pressures and the resulting instability of the slope. This phenomenon typically occurs in deltas of large rivers or canyons. 

Fully drained condition In this case excess pore pressure does not exist. 

Partially drained condition In this case some pore water pressure dissipation has occurred but some excess pore pressures still exist. This type of case may occur due to gas generation from eifher mefhane gas coming out of solution or decomposition of biological maferial. 

Using the above exact upper and lower bound solutions can be determined for drained and undrained loading. An intermediate solution for the case where excess pore pressures exist can be determined explicitly. In Sections 11.4.2.1 through 11.4.2.3 each of these three cases will be considered. 

Therefore the limiting angle of a submerged slope with no excess pore pressure is Uui, = ( )tr. For Tuit = c + a vo tan< )cr... 

The normal effective stress on a failure plane at a depth z beneath the seabed is reduced by an excess pore pressure (mJ as shown in Figure 11.11c, represented by the following equation ... 

The above equation is plotted as maximum slope angle (a) as a function of the excess pore pressure ratio (ue/y z) and effective angle of friction (( )) (Figure 11.13). For a cohesionless material the maximum slope angle (a) increases with increasing effective angle of friction ((()) but decreases with increasing excess pore water pressure ratio (rj. 

Firm and Lee (1979) presented results from a more general effective stress method based on Sarma s (1973) model, which in addition to wave loading also included earthquake loading and excess pore pressures. An example of such an analysis is shown in Figure 11.16. 

Robertson, P.K., Campanella, R.G., and Gillespie, D. 1988. Excess pore pressure and the flat dila-tometer test. Proceedings of the 1st International Symposium on Penetration Testing, March 20-24, Orlando, FL. 

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