c. Other Types. Other piezometers used for special investigations
include electrical, air pneumatic, oil pneumatic and water pressure types.
4. MULTIPLE INSTALLATIONS. Several piezometers may
be installed in a
single boring with an impervious seal separating the
measuring zones.
However, if measurements are needed in zones with 10
feet or less of
vertical separation, it is generally best to install
piezometers in
separate borings.
5. MEASUREMENTS. water levels can be measured to within 0.5 inch, using
several devices, including the plumb bob, cloth or metal surveyors' tapes
coated with chalk, or commercially available electrical indicators for use
in small tubes.
6. SOURCES OF ERROR. Major sources of error are due to gas bubbles and
tube blockage. Some are shown in Figure 9. The magnitude of errors can be
controlled by proper piezometer selection, installation, and de-airing
techniques.
Section 9.
MEASUREMENT OF SOIL AND ROCK PROPERTIES IN SITU
1. SCOPE. A great number of tools and methods have been devised for
measuring in situ engineering properties of soil and rock. The most common
tools, the split spoon sampler and the cone penetrometer, have been
previously discussed. This section describes other methods commonly used in
exploration programs or during construction control.
2. SHEAR STRENGTH BY DIRECT METHODS. Several devices are available to
obtain shear strength data in the field as a supplement to laboratory tests
or where it is not possible to obtain representative samples for testing.
a. Pocket Penetrometer.
Used for obtaining the shear strength of
cohesive, non-gravelly soils
on field exploration or construction sites.
Commercial penetrometers are
available which read unconfined compressive
strength directly. The tool
is used as an aid to obtaining uniform
classification of soils. It
does not replace other field tests or
laboratory tests.
b. Torvane Shear Device. Used for obtaining rapid approximations of
shear strength of cohesive, non-gravelly soils on field exploration. Can be
used on ends of Shelby tubes, penetration samples, block samples from test
pits or sides of test pits. The device is used in uniform soils and does
not replace laboratory tests.
c. Vane Shear Apparatus. Equipment setup for the vane shear test is
illustrated in Figure 10 (Reference 15, Acker Soil Sampling Catalog, by
Acker Drill Company, Inc.). In situ vane shear measurements are especially
useful in very soft soil deposits where much of the strength may be lost by
disturbance during sampling. It should not be used in stiff clays or in
soft soils containing gravel, shells, wood, etc. The main equipment
components are the torque assembly, which includes a gear reduction device
capable of producing constant angular rotation of 1 degree to 6 degrees per
minute, a calibrated proving ring with a dial gage for torque measurement
within 5%, a means of
7.1-97
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