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Introduction
The inspection vane is used to measure in-situ undrained shear strength in clays.
It is primarily intended for use in trenches and excavations at depths up to 3m.
This can be extended with additional optional extensions.
The range of the instrument is from 0 to 260kPa when three different size vanes are
used. Accuracy should be within 10% of the reading.
Description
The measuring part of the instrument is a spiral-spring, (max. torque transmitted 30
kgcm). When the handle is turned, the spring deforms and the upper part and the
lower part of the instrument get a mutual angular displacement. The size of this
displacement depends on the torque required to tum the vane. By means of a
graduated scale shear strength of the clay is obtained.
The lower and upper halves of the instrument are connected by means of threads.
The scale is also supplied with threads, and follows the upper part of the instrument
by means of two lugs. The O-point is indicated by a line on the upper part. When
torque is applied, the scale-ring follows the upper part of the instrument, and when
failure in the clay is obtained, the scale-ring will remain in its position due to the
friction in the threads.
Four sizes of four-bladed vanes are supplied:
20 x 40mm (standard): Reading is direct-Range is 0 to 130kPa (1.3 TSF)
16 x 32mm (extra vane): Multiply reading by 2-Range is 0 to 260kPa (2.6 TSF)
25.4 x 50.8 (extra vane): Multiply reading by 0.5-Range is 0 to 65kPa (0.65 TSF)
50.8 x 101.6mm (extra vane):, Multiply reading by 0.0625-Range is 0 to 8.125kPa
(0.08TSF).
• Note that each mark on graduated scale equals 1kPa. Therefore, the mark 1
actually represents 10kPa when using the standard (20 x 40mm) vane. The
maximum possible reading is 130kPa using the standard vane and 260kPa when
using the smallest (16 x 32mm) vane.
The “area ratio” of the vanes are 14, 16.5 and 24% (ratio of cross sectional area of
vane to the area to be sheared).
The vane blades are soldered to a vane-shaft, which can be extended by one or
more 0.5m long rods. The connection between the shaft-rods and the instrument is
made by threads. To make the connection as straight as possible, the rods have to
be screwed tight together and with threads free from dirt.
The maximum shear strength that can be measured with the inspection vane is
260kPa. In clays with this shear strength, a force of about 40 to 50 kilos is required to
press the vane down into the clay. The vane-shaft is designed to take this -force, but
if extension rods are used, precautions against buckling are suggested.
