TM 5-818-1 /AFM 88-3, Chap. 7
where
hole is pre-drilled and then the device is inserted. The
d
=
diameter of vanes
method is subject to wide interpretation and should not
h
=
height of vane
normally be employed in conventional investigations.
ϕ
=
2/3 for uniform end-shear (usual
4-4.
Boring logs. The results of the boring program
assumption) distribution
shall be shown in terms of graphic logs of boring. The
=
3/5 for parabolic end-shear
logs of borings shall be prepared in accordance with
distribution
governmental standards. A typical log of boring is shown
=
1/2 for triangular end-shear
in figure 4-4.
distribution
The vane shear is best adapted to normally consolidated,
4-5.
Groundwater observations. In many types of
sensitive clays having an undrained shear strength of
construction it is necessary to know the position of the
less than 500 pounds per square foot. The device is not
groundwater level, its seasonal variations, how it is
suitable for use in soils containing sand layers, many
affected by tides, adjacent rivers or canals, or the water
pebbles, or fibrous organic material. Vane tests should
pressures in pervious strata at various depths. Possible
be correlated with unconfined compression tests before
future changes in groundwater conditions, such as those
they are used extensively in any area. Strength values
resulting from irrigation or reservoir construction, should
measured using field vane shear tests should be
be anticipated.
corrected for the effects of anisotropy and strain rate
using Bjerrum's correction factor, λ, shown in figure 4-3.
a. Boreholes. With many fine-grained soils it
This value represents an average and should be
may be necessary to wait for long time periods before
multiplied by 0.8 to obtain a lower limit. The correction is
water table equilibrium is reached in boreholes.
based upon field failures.
Observations made in a borehole during or shortly after
drilling may be misleading. Even with pervious soil, a
d. Borehole
pressuremeter
test.
A
water level
pressuremeter can be used to obtain the in situ shear
modulus and/or K0. Several versions of the device exist
including self-boring equipment, which tends to avoid the
loss of Ko conditions caused by soil relaxation when a
(Courtesy of J. H. Schmertmann, "The Measurement of In Situ
Shear Strength, " Proceedings, Conference on In Situ
Measurement of Soil Properties. Raleigh. N. C., Vol 2, 1975,
pp 57-180. Reprinted by permission of the American Society
of Civil Engineers. New York.)
Figure 4-2. Rough correlation between effective friction angle, standard blow count, and effective overburden pressure.
4-6
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