Frictional shear strength, defined

The ratio F/W defines the coefficient of friction behavioristically the ratio a/p defines it in terms of the shear strength of the metal in the asperity junctions (a) and the yield pressure of the bulk metal (Pij ), and each asperity contact is treated as though it were being deformed independently by a rigid flat plane. In the aggregate the true area of contact is the statistical mean of the individual asperity areas over the time of sliding and remains constant because ideally is a... 

The coefficient of friction n is defined as the force fF required to initiate and maintain sliding divided by the load "W applied perpendicular to the surface l = fF/W. The coefficient of friction is found to be independent of the area of contact over a wide range of loads that cause plastic deformation. The reason for this is that both force and load are proportional to the same area of interface contact. For plastic deformation, we have already shown that W = Ap, where p is the average pressure over the contacts and A is the area of contact. At the contacts, chemical bonds are made that must be sheared or broken as one solid surface slides over the other. If the shear strength 5 is constant, the force S required to shear these contacts equals the area of contact times the shear strength (S = /4s). If materials of comparable hardness are brought into contact, the sliding force is equal to the shear force and proportional to the area of contact. Therefore we can write /A = (F/ W = s/p this observation that the frictional force is independent of contact area is known as Amontons law. 

N. Barton (1973) considered that the shear strength of the joint surface is subject to the residual friction angle (p and the incremental friction angle K

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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. 

Cohesion of a powder is defined as the shear strength at zero normal load. The tester is thus designed to assess the cohesive strength of powder samples at varied states of compaction, from lightly settled conditions to firm uniaxial compressed compacts. The bench-moimted equipment consists of a radially finned vane mounted on a vertical spindle, which in turn is carried on low friction bearings (Figure 1.21). 

Having defined the correlation between relative density, stress level and shear strength for a certain sand material, the specification for the hydraulic fill comes back to a relative density requirement. Alternative, a direct correlation between the shear strength (or angle of internal friction) and the adopted monitoring test can be used, (reference is made to Appendix C, Correlations and correction methods). 

Robertson (2010) added three flow failure case histories to the Olson and Stark (2002) database and proposed a correlation that utilizes cone penetration test measurements (normalized net tip resistance, Qt , and normalized friction ratio, F ) to define a contractive-dilative boundary and a correlation to estimate liquefied strength ratio using equivalent clean sand normalized net tip resistance, Qm.cs- These correlations are shown in Fig. 20. Robertson (2010) also recommended that the liquefied shear strength be limited to the drained shear strength of the soil. 

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