8 FIELD VANE TEST (FVT)
8.1 General
(1)P The field vane test is an in situ test and it is carried out with a rectangular vane, consisting of four plates fixed at 90 ° angles to each other, pushed into the soil to the desired depth and rotated.
(2) This section covers the field vane test used in soft and very soft cohesive soils for the determination of the undrained shear strength and the sensitivity of the soil. Field vane test may also be used for the determination of the undrained shear strength in stiff clays, silts and clay till. The reliability of test results varies depending on the type of soil.
(3) After extensive rotation of the vane, whereby the soil along the failure surface becomes thoroughly remoulded, the remoulded shear strength value can be measured and the soil's sensitivity can be calculated.
(4)P The tests shall be carried out in accordance with requirements given in this section.
(5)P The test method used shall be reported in detail with the test results.
(6) The test method may be reported by reference to a standard.
(7)P Any deviation from the requirements given below shall be justified and in particular its influence on the results of the test shall be commented upon.
(8) Experience of deviations exists with respect to shape of vanes.
8.2 Definitions
(1)P maximum torque Tmax,u : torque required to obtain failure along the failure surface. Mmax is the torque for undisturbed strength value.
(2)P maximum torque for remoulded conditions Tmax : torque required tot obtain failure along the failure surface is remoulded conditions.
(3)P test depth: depth at mid height of the vane.
(4)P time to failure: time between the first application of torque to the vane until the moment when maximum torque is reached when measuring the undisturbed strength value.
(5)P undisturbed shear strength value (cfv): the shear strength value in undisturbed conditions.
(6)P remoulded shear strength value (crv): shear strength value after remoulding the soil.
(7)P sensitivity according to field vane test (Stv): relation between the undisturbed shear strength value and the remoulded shear strength value.
8.3 Equipment
8.3.1 Vane
(1)P The vane shall consist of four rectangular plates fixed at 90 ° angles to each other, see figure 8.1. The blades shall be parallel with the extension rods and no distortion is allowed.
(2)P The relation between the height (h) and the diameter (d) of the vane shall be 2,0 for standard vanes.
(3) A maximum vane size (d × h) of 100 mm × 200 mm for very soft soils and a minimum vane size of 40 mm × 80 mm for dense soils are commonly used for standard vanes.
(4)P The blade thickness (t) shall not exceed 3,0 mm but shall not be less than 0,8 mm. The diameter of the vane shaft, as well as possible welding seams in the centre of the vane shall be small enough to avoid disturbing effects to the measured shear strength value.
(5) In very sensitive clays the blade thickness should not exceed 2,0 mm to minimize the disturbance of the soil during pushing the vane into the soil.
(6)P If the vane is filled with a protective casing, the length of the protrusion at the test shall be at least 5 times the diameter of the vane.
(7)P The diameter of the vane shaft close to the vane should be less than 16 mm. However, vane shaft shall be of such rigidity that it does not twist significantly under full load conditions.
(8) It is recommended that the vane is filled with a device making it possible to separate the torque of the vane from that of the extension rods. A casing or slip coupling can be used.
8.3.2 Extension rods
(1)P Extension rods shall have a diameter and torsional stiffness large enough to transmit the torque generated during the test to the vane.
(2) The extension rods should have a diameter of at least 20 mm.
(3)P The rods shall be straight. The eccentricity of the threads at the rod joints shall be less than 0,1 mm. Then maximum permitted bending for rods or for two jointed rods is 2 mm over each 1 m of length, measured as height of arch.
(4)P If casing tubes are used in order to prevent buckling of the rods, the inner diameter of the tubes shall be large enough to minimize the friction along the rods. When using casing tubes the friction along the rods shall be measured.
8.3.3 Equipment for rotation and recording instrument
(1)P The equipment for rotation of the vane shall be designed to provide rotation at a given and constant rate.
(2)P The recording instrument shall be designed so that it is possible to read off accurately the maximum torque.
(3)P The recording instrument shall be calibrated at least once every six months or when it has been damaged, overloaded or repaired.
(4) Continuous automatic recording is recommended. For interpretation of the test results a graph of the measured torque versus the rotation angle should be made.
(5) The measuring range for the necessary measurements of the angle of rotation should be 360 °, with a reading of 1°.
