Section 8.
GROUNDWATER MEASUREMENTS
1. UTILIZATION. The groundwater level should be measured at the depth at
which water is first encountered as well as at the level at which it
stabilizes after drilling. If necessary, the boring should be kept open
with perforated casing until stabilization occurs. On many projects,
seasonal groundwater fluctuation is of importance and long-term
measurements can be made by converting the borings to standpipe
piezometers. For certain construction projects, more sophisticated
pneumatic or electrical types of piezometers may be used.
2. TYPICAL INSTALLATION.
The three basic components of a piezometer
installation are:
a. Tip. A piezometer tip consisting of a perforated section, well
screen, porous tube, or other similar feature and, in fine-grained or
unstable materials, a surrounding zone of filter sand;
b. Standpipe. Watertight standpipe or measurement conduit, of the
smallest practical diameter, attached to the tip and extending to the
surface of the ground;
c. Seals. A seal or seals consisting of cement grout, bentonite
slurry, or other similarly impermeable material placed between the
standpipe and the boring walls to isolate the zone to be monitored.
The vertical location, i.e., depth and elevation of each item must
be accurately measured and recorded.
3. PIEZOMETER TYPES. All systems, except the open well, have a
porous filter element which is placed in the ground. The most common types
used for groundwater measurements are described below (see Table 14).
a. Open Well. The most common groundwater recording technique is to
measure water level in an open boring as shown in Figure 7(a). A
disadvantage is that different layers of soil may be under different
hydrostatic pressures and therefore the groundwater level recorded may be
inaccurate and misleading. Thus, this system is useful only for relatively
homogeneous deposits.
(1) Open Standpipe Piezometer. Most of the disadvantages of the
open borehole can be overcome by installing an open standpipe piezometer in
the borehole as shown in Figure 7(b). This system is effective in isolating
substrata of interest.
b. Porous Element Piezometer. As shown in Figure 8, a porous element
is connected to the riser pipe which is of small diameter to reduce the
equalization time. The most common tip is the nonmetallic ceramic stone
(Casagrande Type). The ceramic tip is subject to damage and for that reason
porous metal tips or other tips of the same dimension are now available.
Pores are about 50 microns size, so that the tip can be used in direct
contact with fine-grained soils.
7.1-93
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