Environmental Engineering Reference
In-Depth Information
Areas of investigation where the
correct use
of geophysical methods have provided
valuable information include:
-
Delineation of boundaries between the underlying
in situ
rock and transported materi-
als such as alluvium, colluvium, glacial debris and landslide debris;
-
Delineation of boundaries between residual soil, weathered rock and fresh rock;
-
Delineation of boundaries between sandy and clayey soils;
-
Location of anomalous foundation features e.g. igneous dykes, cavities, deeply weathered
zones, fault zones, buried river channels;
-
Assessment of rippability, depth of foundation excavation, depth of cutoff excavation,
liquefaction potential;
-
Location in existing structures of seepage paths, low density zones, cavities.
It is the authors' experience that no surface geophysical method has been able to con-
tribute to the location of thin, weak seams in the rock mass. This information is often an
issue of great importance especially for concrete dams.
The different geophysical methods and their application to dam engineering are briefly
discussed below with reference to papers containing detailed method descriptions.
Further discussion is included in Whiteley (1983, 1988), Stapledon (1988b), Fell (1988a),
Fell et al. (2000) and Joyce et al. (1997).
5.5.1
Surface geophysical methods
The application and the limitations of the different geophysical methods which can be
carried out from the surface without the aid of boreholes (other than to assist interpreta-
tion) are summarised in
Table 5.1
.
5.5.1.1
Seismic refraction
This method utilises the fact that seismic waves travel at different velocities in different
materials; in rock and soil masses the velocity increases with increase in substance
strength and compactness. Whiteley (1988) provides details of the method and interpre-
tation of the results.
Profiles of apparent seismic “P” wave velocity are produced as shown on
Figure 5.7
, a
section along the line of Sugarloaf Reservoir Inlet Tunnel. Air photo interpretation had
shown two lineaments crossing the line of the tunnel. Seismic refraction traverses in com-
bination with trenching and core drilling indicated that the lineaments are the surface
expressions of a deeply weathered rock unit and a minor fault. This interpretation was
confirmed during the driving of the tunnel.
Seismic refraction using the “P” waves is the method most commonly used for delin-
eation of boundaries between soil and weathered rock, and within weathered rock pro-
files. It is the authors' experience that, in a residual weathered profile, the base of the
lowest velocity layer is usually a reasonable approximation of the probable general foun-
dation stripping level for an embankment dam, and the base of the second layer is a rea-
sonable approximation of the cutoff excavation level.
When combined with geological information about the rock mass, “P” wave velocities
can be used to estimate rock rippability (MacGregor et al., 1994). Seismic velocities alone
should not be used for predicting rippability.
Some limitations and requirements of the method are:
-
The method cannot distinguish between sandy and clayey soils, and between soil and
weathered rock where the boundary is gradational;
-
The accuracy of the method is affected by poor velocity contrast between “layers” and
where there are “cliffs” in the rock profile;
