Undrained Shear Strength

The vane test has proved to be a very useful method of determining the undrained shear strength of soft clays and silts. It has been shown to give strength values similar to the undrained unconsolidated triaxial test (Richards, 1988). Generally, a four-bladed vane is 

Very simple approximates certain field conditions 

Most common test for studing stress-strain and strength properties 

The laboratory miniature vane shear test consists of inserting a four-bladed vane into a sediment section and rotating the vane at a constant rate of rotahon until a peak torque is reached. Details on computing vane shear strength has been previously presented in Chapter 5 for a field test. The same equations are used for the laboratory test. 

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Shear Wave Velocity, Vs feet/second (m/s) Standard Penetration Test, Af [or NchIot cohesionless soil layers] (blows/foot) Undrained Shear Strength, s psf (kPa)... 

Province Grain Size (%) Water Content (%) Wet Bulk Density (g/cm3) Undrained Shear Strength (kPa) Accumulation Rate (B)... 

For clayey sod, the undrained shear strength (SJ, preconsolidation pressure (pf), and the overconsolidation ratio (OCR) can be correlated with CPT residts using empirical correlations (Mayne and Kemper, 1988) ... 

The vane shear test is a wireline tool used during the drilling operation to determine the in situ (field) undrained shear strength (S ) of soft to medium stiff clay soils. The vane shear apparatus consists of four blades placed on the end of a rod (Figure 4.20). The height (H) of the vane is standardized as twice the diameter (D). The vanes can be either rectangular or tampered. To use in the field, the vanes of the apparatus are pushed into the sediment at the bottom of the borehole. This operation produces very little soil disturbance. 

Su is undrained shear strength o is effective vertical stress... 

Chandler (1988) recommended that the field vane be allowed to rest for 5 min after insertion to a depth of 4D below the end of borehole. In addition, he also recommended that the rotation rate be 6° per minute, H/D = 2. He also indicated that the undrained shear strength can then be determined using the following equation ... 

Su is the undrained shear strength of sediment Nc is the bearing capacity factor (usually 9)... 

A formulation that varies the strain rate factor (S e) as a function of penetrometer velocity, diameter, and undrained shear strength of the soil was developed by True (1975). 

Hand penetrometer Torvane Fall cone Miniature vane Undrained shear strength (C ) ... 

Unconfined compression Triaxial compression Stress-strain curve Undrained shear strength (CJ Young s modulus (E)... 

The undrained shear strength as measured by unconsolidated (UU) and consolidated undrained (CU) tests behaves in a similar pattern to the compressive strength on disturbance. The maximum undrained strength, still lower than the field strength, is usually obtained in the second quarter from the bottom of a tube sample (De Lory and Salvas, 1969). 

Undrained shear strength of remolded material may be estimated on the basis of the index test results. Using the theories of critical state soil mechanics (Schofield and Wroth, 1968), and assuming the undrained shear strength at the plastic limit is 70 times that at the liquid limit, it may be shown (Weaver and Schultheiss, 1990) that... 

Meimon and Flicher (1980) correlated the post-cyclic undrained shear strength of two laboratory-prepared clays (with OCR = 1 and OCR = 4), with maximum permanent axial strain. They found that if the maximum permanent strain did not exceed 5%, the reduction in undrained shear strength was lower than 8%. With increasing strain level, the reduction could reach up to 40%. Experimental results also showed that the reduction did not necessarily depend upon OCR. The effect of loading rate has also been shown to influence... 

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. 

Thiers and Seed have shown that the deterioration of undrained shear strength would be minimized when the cyclic strain is kept below one-half the pre-cyclic undrained shear strain at failure. The strength after cyclic loading is shown in Figure 9.45 (Lee and Focht, 1976). 

Clay Available pile load test data in clay show that the average frictional resistance expressed as a percentage of the average undrained shear strength or average effective overburden pressure decreases with increasing pile penetration. Three different procedures have been developed based on this database. The first two, alpha and lambda methods, are based on total stress concepts. The third, beta method, is based on effective stress concepts. 

Alpha method The alpha procedure was derived by Drewry, Weidler, and Hwong (1977) from the same data set used for the original lambda correlation. For pile-capacity calculations the procedure utilizes a multiplier a on the undrained shear strength (Su) of the clay. The API code has traditionally recommended a value of 1.0 for Gulf of Mexico-type clays or a value that varies with strength for other types of clays. The API (2000) code varies the a value with the ratio of undrained soil strength to effective soil overburden ( / = S /a ) is as given in Equation 10.21 ... 

Undrained shear strength = Effective unit weight of soil = Strain at 5 - level = Pile diameter = Penetration = Correction coefficients = Effective angle of friction 9 ... 

P is the an empirical factor that is a function of soil type, stress level, and OCR Cu is the undrained shear strength found from friaxial compression tests... 

SuB is the undrained shear strength of the lower soil stratum H is the thickness of upper layer in two-layer system D is the depth of widest cross-sectional area... 

R is defined as the undrained shear strength ratio Then rearranging Equations 11.8 and 11.9 will give the following ... 

Since for normally consolidated (NC) marine sediments the undrained shear strength ratio R = (S /cj vo) 0.2 therefore ttui, 12°. Higher slope angles can be achieved for overconsolidated sediments and those with cementation. 

Larsson, R. 1980. Undrained shear strength in stability calculation of embankments and foundations on clay. Canadian Geotechnical Journal, 17 591-602. 

Koutsoftas, D.C., and Fischer, J.A. 1976. In-situ undrained shear strength of two marine clays. Journal of the Geotechnical Engineering Division, 102(9) 989-1005. 

De Lory, F.A., and Salvas, R.J. 1969. Some observations on the undrained shear strength to analyze failure. Canadian Geotechnical Journal, 6(2) 97-110. 

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