Site investigation techniques - Geotechnical Engineering of Dams - page 221

water will result in clogging of fractures and lower Lugeon values than are correct for
the rock.
A centrifugal pump or reciprocating pump with surge chamber is necessary to ensure
constant pressure. Pressure and water flow is measured by meters which should be
recently calibrated. Before testing the water supply rods and packer should be calibrated
for friction losses at different flow rates.

5.14.4.3 Selection of test pressures
The object of the test is to:

- measure the natural permeability;

- indicate the probable water flows under the expected hydraulic loading by the proposed

structure.

The pressures applied during the test should not be sufficient to produce hydraulic frac-
turing (jacking) of the rock around the test section. In weak rocks near the ground surface
this fracturing does take place at relatively low pressures and is usually indicated by a
unexpected increase in water loss following a raising of test pressures. It is recommended
that, to avoid potential “jacking”, maximum effective test pressures be limited below
overburden pressure (approximately 22 kPa/m). In low strength weathered rocks, lower
values will probably be necessary.

5.14.5 Test procedure

The test involves:

- measurement of ground water level;

- washing out of drill hole. Circulation of drilling water should be continued for at least 15

minutes after the water appears clear;

- installation of packer at the selected level;

- connection of the water supply system;

- application of the test pressures and measurement of water loss;

- removal of equipment.

The testing is carried out in several stages with different pressures. Commonly at least
three test pressures are used (five are desirable). Pressures are applied in an increasing
and then decreasing sequence. For example with three pressures a, b, c, the water loss
is measured at stages with pressure successively at a, b c, b, a. Each stage should be
continued until a constant rate of water loss (within 10%) for a 5 minute period is
recorded.

5.14.6 Presentation and interpretation of results

The results are best plotted with the effective test pressure at the centre of the test section
against the flow rate. The effective test pressure is the gauge pressure corrected for the ele-
vation difference between gauge and water table and for friction losses in the system. (i.e.
H T in Figure 5.30 ). Where the water level is not known, or suspected to be locally elevated
due to the effects of the testing, the results are plotted using gauge pressure only. Typical
test results ( Table 5.6 ) plotted in this way are shown in Figure 5.35 . Note that the data
from the increasing pressures do not plot identically to those on the decreasing pressures.