42 Vane test

COMMENTARY ON CLAUSE 42

Vane testing in the field usually takes the form of either the "field vane" or "hand vane" for use in fine grained soils. The vane test comprises a cruciform vane on the end of a solid rod which is forced into the soil and then rotated at a constant rate of between 6°/min and 12°/min. In the field vane test the torque required to rotate the vane is recorded as the vane rotates; in the case of the hand vane a maximum reading is recorded. The field vane test is continued for a given rotation and can be related to the shear strength of the soil. The test can be extended to measure the remoulded strength of the soil. This is done by turning the vane through 10 complete rotations. A period of 5 min is permitted to elapse after which the vane test is repeated in the normal way.

The field vane test can be performed either in the base of a borehole or by penetration from the ground surface and should be carried out in accordance with BS EN ISO 22476-9, BS EN 1997-2 and NA to BS EN 1997-2; the friction on the rods above the vane should be kept to a minimum.

NOTE 1 The degree of disturbance caused by rotating the vane differs from that obtained by remoulding a sample of clay in the laboratory and the numerical value of the sensitivity of the clay determined by these procedures is not strictly comparable with the results obtained from laboratory triaxial tests.

The penetration technique may be used ahead of the borehole where the vane and a protective casing are forced into the ground by jacking. At the required depth, the vane should be advanced a short distance ahead of the protective casing, the test conducted and the casing and vane subsequently advanced to the next required depth. With this type, it is not always possible to penetrate to the desired stratum without the assistance of pre-boring.

The test should normally only be carried out in homogeneous uniform, fine, fully saturated soils, having an undrained shear strength up to about 100 kPa. The results are questionable in stronger clays or if the soil tends to dilate on shearing or is fissured. Results are unreliable in materials with significant coarse silt or sand content. When operating in a borehole then the borehole should be kept topped up with water in soft, layered or sensitive soils.

NOTE 2 The undrained shear strength determined by an in-situ vane test is normally not equal to the average value measured at failure in the field, e.g. in the failure of an embankment on soft clay. The discrepancy between field and vane shear strengths is found to vary with the plasticity of the clay and other factors (see Bjerrum, 1973 [81] and Aas, Lacasse, Lunne and Hoeg, 1986 [82]). Various corrections are considered in BS EN 1997-2:2007, Annex I.

NOTE 3 The main advantage of the vane test is that the test itself causes little disturbance of the ground and is carried out below the bottom of the borehole or vane housing in virtually undisturbed ground. This is particularly apparent in sensitive clays, because higher shear strengths tend to result from the in-situ vane test than from laboratory tests on samples obtained with the general purpose sampler described in 25.4. If the test is carried out in soil that is not uniform and contains only thin layers of laminations of sand or stiff to very stiff silt, the torque might be misleadingly high. The presence of rootlets in organic soils, and also of coarse particles, can lead to erroneous results.

Small hand-operated vane test instruments are available for use in the sides or bottom of an excavation. The hand vane tests should be regarded as index tests.

NOTE 4 Hand vane instruments are produced by several manufacturers. Some do not have the vane height to width ratio of 2 'standardized' for field vanes and the means of calibrating the torque measuring device and/or the basis for correlating with shear strength varies between the instruments is not always apparent.

NOTE 5 Neither the hand vane equipment nor the test procedure is explicitly covered by BS EN ISO 22476-9 or BS 1377-9:1990. For a useful review of the test, see the New Zealand Geotechnical Society's guidelines [83], which includes a suggested specification item and test method.