(2) Fine-Grained Strata. Extend borings in potentially
compressible fine-grained strata of great thickness to a depth where stress
from superposed load is 50 small that corresponding consolidation will not
significantly influence surface settlement.
(3) Compact Soils. Where stiff or compact soils are encountered at
shallow depths, extend boring(s) through this material to a depth where the
presence of an underlying weaker stratum cannot affect stability or
settlement.
(4) Bedrock Surface. If bedrock surface is encountered and general
character and location of rock are known, extend one or two borings 5 feet
into sound, unweathered rock. Where location and character of rock are
unknown, or where boulders or irregularly weathered material are likely
geologically, increase the number of borings penetrating into rock to
bracket the area. In cavernous limestone areas, extend borings through
strata suspected of containing solution channels.
(5) Check Borings. In unfamiliar areas, at least one boring should
extend well below the zone necessary for apparent stability, to make sure no
unusual conditions exist at greater depth.
d. Sealing Boreholes. Borings made in foundation areas that eventually
will be excavated below groundwater, or where artesian pressures are
encountered, must be plugged or grouted unless they are used for continuing
water-level observations. In boreholes for groundwater observations, place
casing in tight contact with walls of holes, or fill annular space with
sand/gravel.
e. Cavernous Limestone. In limestone areas suspected of containing
solution channels or cavities, each column location should be investigated.
For smaller structures, locate boring or probe at each planned column
location. For large structures and area investigation use indirect methods
noted below, followed by borings or probes in final column locations, and on
close centers (25 ft. under walls or heavily loaded areas). Aerial
photographs have been used effectively by experienced geologists for
detecting sinkholes and the progress of cavity development by comparing old
to new photographs. Geophysical methods are used to detect anomalies in
subsurface resistivity, gravity, magnetic field or seismic velocities and to
correlate such anomalies with cavity presence (see Reference 5, The Use of
Geophysical Methods in Engineering Geology, Part II, Electrical Resistivity,
Magnetic and Gravity Methods, by Higginbottom, and Reference 6, Bedrock
Verification Program for Davis Besse Nuclear Power Station, by Millet and
Morehouse).
2. TEST PITS. Test pits are used to examine and sample soils in situ, to
determine the depth to groundwater, and to determine the thickness of
topsoil. They range from shallow manual or machine excavations to deep,
sheeted, and braced pits. See Table 8 for types, uses, and limitations of
test pits and trenches.
Hand-cut samples are frequently necessary for
highly sensitive, cohesive soils, brittle and weathered rock, and soil
formation with honeycomb structure.
7.1-71
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