Environmental Engineering Reference
In-Depth Information
5.13
IN SITU PERMEABILITY TESTS ON SOIL
The permeability of soil in a dam foundation may be of importance if the cutoff is
founded in the soil, as in dams on alluvial, colluvial and glacial deposits and on some
deeply weathered residual or lateritised soils.
In general the structure of the soil controls water flow, for example:
- sandy layers in alluvial soils;
- root holes and fissures in residual and alluvial soils;
- worm burrows in alluvial and residual soils;
- eached zones, infilled root holes and relict joints in lateritised soil and extremely weath-
ered rocks.
Hence it is seldom possible to obtain a realistic estimate of soil mass permeability from
laboratory testing. Some of the difficulties associated with laboratory testing are discussed
in Chapter 6.
Because the soil structure tends to be blocked or smeared by the drilling action, pump-
in type permeability tests in boreholes can also give quite misleading results, with meas-
ured permeabilities one or two orders of magnitude lower than the actual soil mass
permeability.
Where possible pump out tests should be conducted with a pump well and observation
wells. Figure 5.29 shows a typical arrangement.
In a pump out test the inflow of water removes the soil blocking or smearing the hole
side and realistic estimates of permeability can be obtained.
The design of the pump well, observation wells and the test program analysis need to
recognise the effect of different vertical and horizontal permeabilities in the soil. The
results are often difficult to assess and it is recommended that a groundwater hydrologist
be employed in the planning, execution and analysis of such tests.
In the event that the groundwater level is below the depth of interest, it is necessary to
carry out pump-in tests. Even when considerable care is taken to clean the hole, it should
be understood that the results of such tests give lower bound estimates and that the actual
permeability may be up to one or two orders of magnitude greater than indicated by the
test result.
Figure 5.29.
Typical arrangement for a well pumping test with observation wells.
 
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