Part 1: Special cones: geotechnical cones
Push-in shear vane
PUSH-IN SHEAR VANE 4.6
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The shear vane was developed to measure, in-situ, the undrained
shear strength, Su, of clays generally with a strength consistency
between very soft and firm.
Vane apparatus used by Lankelma is the Geonor penetration
vane manufactured by the Norwegian Geotechnical Institute
(Figure 24). This apparatus incorporates a protection shoe at its
leading edge which protects the vane and cleans it automatically
before each measurement. Therefore there is no need to withdraw
the vane between measurements to check its condition or to clean it.
In its simplest form, four blades are arranged in a cruciform and
attached to the end of a rod. The rods are pushed into the soil and the assembly is rotated. For each test the relationship between the turning moment, or torque, and the angular rotation is determined.
Undrained shear strength
When the vane is rotated, the soil fails along a cylindrical surface
passing through the edges of the vane as well as along the horizontal
surfaces at the top and bottom of the blades. From the dimensions of
the vane and from the measured torque the undrained shear strength of
the soil can be determined.
Experience has shown that the shear strength will vary according to the
time elapsed between installing the vane and the start of shearing. In
general the strength will increase with time and it is therefore important
that the test procedure is standardised. It is recommended that
shearing should commence between three and five minutes after the
vane has been installed. The rate of shearing should be between 0.1°
and 0.2°/sec which usually corresponds to a test duration of between
two and fifteen minutes. Increasing the rate of strain will result in an
overestimate of strength.
After quickly rotating the vane twelve times, the remoulded strength
should be measured again between three minutes and a maximum of
five minutes after the peak strength has been reached.
Two vane sizes are available for testing: 110 x 55 mm which applies to
soils of firm consistency; and 130 x 65 mm for soils whose consistency
is between soft and very soft. The ratio of the initial peak strength and
the remoulded strength is a measure of the sensitivity of the soil.