Friction pore pressure

While most SPT-V correlations were based solely on N, the CPT-V correlations sometimes included tip resistance, side friction, pore pressure, and effective stress. However, there are yet other factors which influence the relationship between CPT results and dynamic soil properties. Projects in which these factors were investigated are summarized below. 

Even very solid roeks contain pores, and many of these pores are interconnected. It is through such pores that water and oil move toward wells. Below the water table, all the pores are filled with water with no surface tension to eliminate. So it might seem that rock down there would not be affected by rainfall at the surface. As the rains come, however, the water table rises, and the additional water increases the pressure in the fluids in the pores below. This increase in pore pressure pushes adjacent rock surfaces apart, reducing the friction between them, which lowers the strength of the rock and makes it easier for fractures to develop. Elevated pore pressures are implicated in many dramatic mass wasting events. 

A variety of normalized indices are available to define the end-bearing, frictional, and pore pressure response recorded around the tip of an advancing piezocone. The dimensionless magnitudes of tip resistance, Q and sleeve friction, F may be defined as... 

Solutions are developed for the pore pressure fields that develop around penetrometers either advanced at constant or decelerating rate and under partially-drained flow conditions. The maximum pore pressure, recorded at the penetrometer is suggested as indicative of the permeability of the surrounding soil, since the peak-pressure is conditioned by the ability for fluids to escape. This full suite of solutions promotes a consistent framework to understand the processes operating during penetrometer penetration and arrest, and to enable the conventional sounding indices inclusive of tip resistance, Q friction factor, and pore pressure ratio, to be related to penetration-induced pore pressures, and therefore to permeability. 

According to Coulomb s law of friction, the relationship among shear stress, normal stress, coefficient of friction and pore-pressure at the moment of shear slip (critical condition) due to increasing of pore-pressure are described as follow. 

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. 

C is the cohesion (()a. is the internal angle of friction yt is the total unit weight Y is the buoyant unit weight Am is the pore pressure in excess of hydrostatic and % are the vertical and horizontal acceleration in fraction of respectively d is the depth of sliding surface a is the slope angle... 

Zuidberg, H.M., Schaap, L.H.J., and Beringen, F.L. 1982. A penetrometer for simultaneously measuring of cone resistance, sleeve friction and d5mamic pore pressure. In Penetration Testing, Verruijt, A., Beringen, F.L., and de Leeuw, E.H., eds., A.A. Balkema, Rotterdam, the Netherlands, pp. 963-970. 

Where, A = normal stress on the sliding surface of the slice, kN/m T = shear stress on the sliding surface of the slice, kN/m T = pore pressure on the slice, kN/m L = sliding surface length of the slice, m = effective internal friction angle, ° c = effective internal cohesion, kN/m IF = slice weight, kN/m Yw = unit weight of water, kN/m ... 

Determination of pore pressures in sandcastles is a test of fundamental soil mechanics. The analyses require construction of Mohr circles of total and effective stress to show how negative pore pressure, coupled with friction, develops unconfined compressive strength. The tests and their analyses also illustrate relationships between water content, suction and grading. 

This situation is physically impossible and it transpired that it was caused by a bug in CRISP. It had the signifieant effect of reducing pore pressures in the layer adjacent to the wall and so increasing the available wall friction. The importance of inspeeting eomputer output and questioning whether it is physi-eally reasonable, is again emphasised, partieularly in relation to water pressures. 

Because CPT soundings provide continuous records of tip and sleeve resistances (and frequently pore pressure) versus depth (Figure 7.4), they provide a continuous indicator of soil and subsurface conditions, which is useful in defining soil stratification. Numerous correlations between the CPT measurements have been developed to define soil type and soil classification. In addition, empirical correlations have been published to relate the cone tip and sleeve friction resistances to engineering behavior, including undrained shear strength of clay soils and relative density and friction of granular soils. 

When CPT measurements are available, there is a wide variety of classification systems based on the measured cone resistance, friction ratio and pore pressure. Reference is made to specialized literature. 

Boreholes can be drilled to sample the fill material throughout the entire layer thickness of the fill. Alternatively CPT testing can be used after calibration with the borehole results. The advantage of CPT testing is the speed of execution and the continuous recording of the Cone resistanee, the friction ratio and the pore pressure. 

The friction ratio (i.e., sleeve friction/cone resistance x 100%) can give an indication of sandy versus clayey nature of the penetrated soil. Starting from these two parameters it is possible to determine the soil type. Measurement of the pore pressure provides additional info about the encountered soil type. 

The relatiOTiships between V Go, Eq, and Mq were presented in the SPT section of this chapter. The research summarized here compares CFT results to various soil moduli, but other properties of interest can be determined from the relationships above. In the section describing the CPT equipment, procedures, and results, /, and u were introduced. Some relationships described in this section use dimensionless, normalized cone parameters. The normalized cone tip resistance, Q,-, normalized friction ratio, Fn, and normalized pore pressure, Bg, were proposed by Robertson (1990) and are given by... 

The effect of decreased normal stress on a shallow imbedded crack makes it more likely to mp-ture to the surface above a critical depth as shown by Rudnicki and Wu (1995). The Coulomb friction condition is introduced such that slip occurs when Icr l = kapparent coefficient of friction, including effects of pore-pressure differences in the fault zone. The right-hand side of integral equation above is modified such that... 

The limit equilibrium method determines the forces required for equilibrium, taking into account the geometry of the slope, the pore pressure u, and the loads. When layers of reinforcement are introduced, it is possible to define the envelope of the maximum strength available in each layer, which depends on the friction between the reinforcement and the soil, the characteristics of the reinforcement, and the compatibihty of deformations with the functionality of the project. The diagram of the available forces must overlap that of the required forces, as shown in Fig. 15.4. 

Basically all formations penetrated during drilling are porous and permeable to some degree. Fluids contained in pore spaces are under pressure that is overbalanced by the drilling fluid pressure in the well bore. The bore-hole pressure is equal to the hydrostatic pressure plus the friction pressure loss in the annulus. If for some reason the borehole pressure falls below the formation fluid pressure, the formation fluids can enter the well. Such an event is known as a kick. This name is associated with a rather sudden flowrate increase observed at the surface. 

II is via penetration of hot combustion products into the existing pores of the expl. Propagation in Region III is via convective flow between the charge surface and its confinement. This regime is claimed to be affected by confinement expansion (due to pressure), and by fragmentation of the peripheral portions of the expl column. The phenomena in Regions IV and V have already been described in Section VIII under Initiation by Impact Friction... 

A pressure dependence of the internal angle of friction is known to represent the quantification of the pore space dependence of internal friction [3], Such a dependence has also been observed in wall friction experiments for the bed friction angle [5, 13] of soil. So, we must assume 6 = 6(p) and pressure dependence of friction angle with pressure [3,5,13], the simplest parameterisation is linear and we choose... 

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