Interpretation of Undrained Strength from FVT

The principal measurement obtained from the FVT is the maximum torque required to rotate the vane. In order to extract an estimate of the undrained shear strength, it is necessary to make a number of assumptions regarding the failure surface, the distribution of shear stresses on the ends and sides of the vane, etc. In addition to the factors listed in Table 5.2, a number of factors that may affect the interpretation of vane test results from an individual test are listed in Table 5.3.

Flaate (1966a) listed the following assumptions traditionally made for computing the undrained shear strength from the torque measured in the field vane:

• 1. The test is completely undrained, and no consolidation takes place during the installation and throughout completion of the test.
• 2. No disturbance is caused during advancement of the borehole or vane.
• 3. The remolded zone around the vane is very small.
• 4. The shear strength is fully mobilized and uniform over the entire cylindrical failure surface at peak torque (i.e., there is no progressive failure).
• 5. The maximum shear strength, s_u, is isotropic.

The undrained shear strength is calculated from the peak torque and based on the geometry of the assumed failure surface shown in Figure 5.2 and is obtained as follows:

Table 5.3 Factors that may influence interpretation of field vane results

Solving for s_u gives

where

T = torque

D = diameter of the vane H= height of the vane

a= a shape factor to account for assumed distribution of shear stress.

Assuming a = 3 for uniformly distributed end shear gives

For a standard vane with H/D = 2, Equation 5.7 reduces to

One of the assumptions used in the calculation of undrained shear strength is that the strength is isotropic; i.e., the same unit strength is developed along the sides as well as the top and bottom of the vane. Additionally, it is assumed that the shear stress distribution on the cylindrical (vertical) surface and on the top and bottom (horizontal) surfaces is uniform, giving a rectangular distribution. Results of a three-dimensional finite element analysis presented by Donald et al. (1977) showed that while there was generally uniform distribution (rectangular) along the sides or vertical surface of the vanes, the distribution along the vane top and bottom was not rectangular. Experimental results obtained by Menzies & Merrifield (1980) using an instrumented vane showed similar results.

Wroth (1984) considered the effect of this more realistic stress distribution on vane results assuming that the shear stress distribution on the top and bottom surfaces could be expressed by a polynomial. As a result of this analysis, Wroth (1984) showed that for H/D = 2,

Equation 5.9 suggests that most of the torque measured in the FVT is a result of the soil shearing resistance along the vertical surface, which means that the undrained strength obtained from the conventional interpretation (Equation 5.8) will be underestimated by about 9%.

Anisotropic Analysis

As described in Section 5.7, the normal assumption regarding the interpretation of FVT results is that the undrained strength is isotropic. A number of investigations have been performed using the FVT to evaluate the directional anisotropy of undrained shear strength (Table 5.4). In many cases, vanes of different geometries, as shown in Figure 5.15, have been used to determine the shear strength in different directions.

Table 5.4 Investigations of undrained shear strength anisotropy using field vane

The deficiency in the conventional approach to interpretation of FVT results suggests that vanes with different H/D give different s_u. This means that test data from early use of the field vane or with laboratory vane devices with H/D not equal to 2 (e.g., Skempton 1948; Bazett et al. 1961) may need to be reevaluated in relation to modern test results and interpretation. Analyses by the approaches suggested by both Aas (1967) and Wiesel (1973) using vanes of different H/D may be used to determine the undrained strength on the horizontal plane (s_uh) and vertical plane (s_uv). Figure 5.16 shows results obtained in marine clay using vanes of different geometries.

Several investigations (e.g., Bjerrum 1973; Richardson et al. 1975) have related the strength anisotropy ratio K_s = s_uh/s_uv to soil plasticity. However, other test results (e.g., Silvestri & Aubertin 1983) show that the anisotropy ratio does not always follow the trend suggested by Bjerrum (1973). The anisotropy ratio may be related to the P.I. in soft near normally consolidated nonstructured or brittle soils (e.g., Ladd et al. 1977) where K₍₎ is in the range of 0.5-0.7; however, there is also considerable evidence demonstrating that K_s is also related to K„ in overconsolidated soils (e.g., Garga & Khan 1992).