8.4 Test procedure
8.4.1 Predrilling and pushing down the vane
(1)P Predrilling shall be made through eventually possibly dry crust or fill when a vane test is to be carried out in the soil below such layers.
(2)P When using an outer system with a casing protecting the vane, the water pressure in the casing system shall be the same as that in the soil at the test level.
(3)P The vane shall be pushed down, if possible without use of blows or vibration. Rotation is never permitted. The pushing rate shall be constant and not exceeding 20 mm/s.
(4) In stiff clays, silts and in clay till driving may be needed to get the vane to the desired depth.
(5)P The distance between investigation points shall be at least 2,0 m in plan in case of test depths greater than 5 m.
(6)P The first test shall be conducted at a depth of at least 0,5 m below the ground surface or at a depth of at least 5 times the diameter of a predrilled hole below its base.
(7)P The minimum vertical distance for two tests conducted in the same borehole shall be at least 0,5 m.
8.4.2 Vane shear test
(1)P The time from the moment when the desired test depth has been reached to the beginning of the vane test (waiting time) shall be at least 2 min and no more than 5 min.
(2)P The vane shall be loaded by application of torque in such a way that failure of the soil occurs in undrained conditions. The vane shall be rotated at a constant rate.
(3) A guiding value for the rotating rate of the vane to fulfil the criteria given above in cohesive soils is 0,1 °/s to 0,2°/s (6°/min to 12°/min). Rotation rates up to 0,5°/s may be used for soft cohesive soils with low sensitivity.
(4)P The test shall be run in such a way that the skin friction along the rods can be separated.
(5)P After failure has occurred and the maximum torque has been recorded, the vane shall be rotated rapidly at least ten turns in order to remould the soil at the failure surface, after which a new test is performed immediately according to the instructions above. The constant value of the torque at the remoulded state is recorded.
8.5 Interpretation of test results
(1)P For standard vanes with d/h = 1 : 2 and with the failure surface shown in figure 8.2, the undisturbed shear strength value of the soil shall be determined by using the formula:
Tmax;u is the maximum torque on the vane.
2)P The failure surface for a rectangular vane shall be assumed to be a cylindrical surface circumscribing the vane and two planes on the top and bottom of that cylinder respectively. The shear strength shall be assumed to be fully mobilized, constant and uniform at failure around the perimeter and across the ends of the cylinder, see figure 8.2.
(3)P The undrained shear strength (cfu) of the soil shall be obtained from the undisturbed shear strength value after correction with regard to the soil's liquid limit, plasticity index or the effective vertical stress.
(4)P Tmax;u shall be reduced by the friction along the rods.
(5)P The remoulded shear strength value (crv) shall be determined by the same formula replacing
Tmax;u with Tmax;r.
(6) By registering the torque as a function of torsion, information is obtained on the mode of the failure of the soil. By using different shapes of vanes, it is possible to evaluate the anisotropy of the soil.
8.6 Reporting of the results
(1)P In addition to the requirements given in 2.6 the test report shall include, for both manual and automatic recording, the following information:
- type of test equipment and of the torque measuring device;
- vane size;
- diameter of the extension rods;
- number of the torque measuring device (to check the calibration data);
- date of the last calibration and calibration factor for the instrument;
- test depth;
- rotation rate of the vane during the test;
- maximum torque on the vane in undisturbed and torque in remoulded state, a reading or a graph;
- time to failure (the time for the activated vane only);
- observations made in connection with the test, as well as events or details that may influence the test results.
8.7 Derived values of geotechnical parameters
(1)P If the bearing capacity of a spread foundation, the ultimate bearing resistance of piles or stability of slopes is evaluated based on vane test results, an analytical method shall be used.
(2)P In order to obtain derived values for the undrained shear strength cfu, the undisturbed shear strength value shall be corrected with consideration to empirical experience. The correction factor shall be determined based on local experience.
(3) Existing correction factors are usually related to the liquid limit or plasticity index and the effective vertical stress.
(4) In annex G examples are given of such correction factors.