Environmental Engineering Reference
In-Depth Information
the possible locations, type, and size of ground subsidence and collapse. From this data
base explorations are programmed.
Explorations
General
The location of all important cavities and the determination of their size and extent is a
very difficult and usually impossible task. Explorations should never proceed without
completion of data collection and landform analysis.
Geophysics (see
Section 2.3.2)
Explorations with geophysical methods can provide useful information, but the degrees of
reliability vary. Seismic refraction surveys may result in little more than “averaging” the
depth to the limestone surface if it is highly irregular. Seismic-direct cross-hole surveys
may indicate the presence of cavities if they are large. Electrical resistivity, at times, has
indicated shallow cavity development.
Gravimeter surveys were found to be more useful than other geophysical methods (seis-
mic cross-hole surveys or electrical resistivity) in a study for a nuclear power plant in
northwestern Ohio (Millet and Moorhouse, 1973).
Ground-probing radar (GPR) may disclose cavities, but interpretation of the images is
difficult.
Test Borings and Pits
Test and core borings are programmed to explore anomalies detected by geophysical
explorations and terrain analysis as well as to accomplish their normal purposes. Voids are
disclosed by the sudden drop of the drilling tools and loss of drilling fluid. The material
in a sinkhole is usually very loose compared with the surrounding materials, and may
overlie highly fractured rock where a roof has collapsed. One should be suspicious when
low SPT
N
values, such as 2 or 3, are encountered immediately above the rock surface, fol-
lowing significantly higher
N
values recorded in the overlying strata.
Test pits
are useful to allow examination of the bedrock surface. Although the normal
backhoe reach is limited to 10 to 15 ft, on important projects, such as for dams or con-
struction with heavy foundation loads, deeper excavations, perhaps requiring dewatering,
may be warranted.
Rotary Probes and Percussion Drilling
If cavity presence has been confirmed, it is usually prudent to make either core borings,
rotary probes, or pneumatic percussion drill holes at the location of each footing before
final design, or before construction. The objective is to confirm that an adequate thickness
of competent rock is present beneath each foundation.
Percussion drilling with air track rigs
(
Figure 2.55)
is a low-cost and efficient method of
exploring for cavities and the rock surface, especially in dipping limestone formations.
Typical air track logs are shown in
Figure 2.56;
a void is noted on “Log A9.” Core borings
usually drill 20 to 40 ft/day, whereas air track probes readily drill 300 to 500 ft/day.
During investigation several core borings should be drilled immediately adjacent to air
track holes for rock-quality correlations.
Proof testing
with air drills is an alternate to rotary probes at each footing location, particu-
larly for drilled piers where installations are relatively deep. Proof testing the bottom of each
pier founded on rock with air drills is much less costly than rotary probes or core borings.
