Environmental Engineering Reference
In-Depth Information
Mines, it was designed for near-surface or in-mine testing of coal and other fractured rocks
that are difficult to core. It has been used in all soft to medium-hard rocks.
Procedure
The apparatus is shown in
Figure 3.46.
A shear head, consisting of opposing plates with
two carbide teeth, is lowered into a 3 in. (75 mm) borehole. Normal stress is applied
by pushing the shear plates into the sides of the hole using a hand-operated pump.
The pressure is then valved off so it remains constant, while the same pump is used to
pull the expanded shear head a short distance upward along the hole by means of a hol-
low-ram jack. Both the expansion pressure and the pulling resistance are recorded, and
the test is repeated with different preselected normal stresses. Up to four tests can be con-
ducted at the same depth by rotating the shear head 45° between tests. A plot of each test
is made to obtain a Mohr's envelope of shear stress vs. normal stress providing meas-
urements of the angle of internal friction (
) and cohesion (
c
) (Handy et al., 1976).
Comparison with data from
in situ
direct shear tests indicate that cohesion values from
the BST are lower although the friction angles are close (R. L. Handy, personal commu-
nication, 2004).
φ
Applications
The apparatus is used in both rock (RBST) and soil (BST) in vertical, inclined, or horizon-
tal boreholes. The entire apparatus, including the shear head and pulling device, is easily
portable. A very significant advantage over other methods to measure shear strength is
that many tests can be run in a short interval of time, as many as ten per day, and yield
results on-site that can indicate if more tests are needed.
Threaded rod
Hollow jack
Dial gage
Half nut clamp
Shear hose
Retract hose
Tripod
Jack base
plate
Normal hose
Rod coupling
Hose connector
RW drill rod
NX hole
RW adapter
Shear plates
FIGURE 3.46
Schematic of borehole shear strength tester (BST) in
the borehole. (From USBM, Bureau of Mines, U.S.
Department of Interior, New Technology No. 122,
1981. With permission.)
BST body
