Environmental Engineering Reference
In-Depth Information
equipment on site may not be justified by the benefit of the additional information
obtained. However in some situations some techniques are valuable e.g. borehole imaging
can provide invaluable data on joint and bedding parting openings greatly complement-
ing the logging of core, and water pressure testing.
5.5.3
Crosshole and Uphole seismic
The use of seismic waves in combination with boreholes has provided a range of methods
to identify subsurface conditions both in the dam foundation and within the dam struc-
tures. In crosshole seismic a hydrophone array is lowered into one borehole and an energy
source - a specially developed drop hammer - into a second borehole. By changing the
location of the source and also the position of the hydrophone array the arrival times of
the direct and refracted seismic waves can be computed into a tomographic image of the
section between the two boreholes.
An alternative method using a single borehole involves the placement of the hydrophone
array in the borehole, with the energy source at different points on the surface. The results
can be computed to provide information on a cone of material up to 100 m deep with a
radius up to twice the depth of the hydrophones.
The methods are described by Joyce et al. (1997), Whiteley (1998) and Van Aller and
Rodriguez (1999).
5.6
TEST PITS AND TRENCHES
5.6.1
Test pits
Test pits excavated by a rubber-tyred back-hoe or tracked excavator are effective in pro-
viding information on subsurface conditions in dam foundations or existing dams, for the
following reasons:
-
They are relatively cheap and quick;
-
The subsurface profile is clearly visible and can be logged and photographed;
-
Material types, the nature and shape of their boundaries and structure can be observed
and recorded in three dimensions;
-
The absence or presence of groundwater is indicated and the sources of inflows can
usually be observed and their flow rates recorded;
-
Undisturbed samples can be collected;
-
In situ tests can be carried out;
-
The resistance to excavation provides some indication of excavation conditions likely
to be met during construction.
It may be possible to leave pits open for inspection by design engineers and representa-
tives of contractors. In this case fencing is usually required. The presence of fissures in clays
can be assessed by leaving the pit walls to dry for some time e.g. overnight. Fissures can be
classified using the systems shown in Figure 6.28.
Figure 5.8 shows a log of a test pit which includes a sketch of one wall with descriptions
of each material type, traces of their boundaries, and traces of defects. Some limitations
of test pits are:
-
Pit depth is limited by the reach of the machine and its ability to dig the material. The
maximum depth of a pit dug by backhoe is 4 m but the machine commonly refuses on
very weak rock. Larger excavators can reach to depths of more than 6 m and, depend-
ing on the nature of the rock defects, can excavate into weak rock;
 
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