Geology Reference
In-Depth Information
Figure 4.24
Potential sources of
sampling
disturbance leading
to much lower
strengths being
measured in the
laboratory
compared to those
in situ.
Actual strength
of soil or rock
in situ
Least sensitivse
(rock)
Strength in undistrubed ground (may vary
naturally with moisture content)
Reduction in strength due
to disturbance during
sampling (whatever method
is used)
Range of
measured
strengths
Loss of strength
due to disturbance
in preparing
sample for testing
Most sensitive
(weakly structured
soil)
in-situ
strength
Measured strength
Lower bound,
remoulded strength
τ
remoulded
strength
envelope
true
cohesion
φr
σ
Rotary drilling is used in all rocks but can also be used to obtain good
samples in weaker materials, including colluvium (mixed rock and
soil), weathered rock and soil. In weaker ground, a similar investiga-
tion strategy is often adopted as for soils, whereby sections are cored
followed by SPT tests, although as for soils there is the risk that
important geological features may be missed.
A drilling rig rotates a string of drilling rods whilst hydraulic cylin-
ders apply a downward force. At the lower end of the drilling string
there is a hollow annulus bit, usually coated with diamonds or tung-
sten carbide. As the bit is rotated, a stick of core enters into a core
barrel at the bottom of the drilling string. The retained core is pre-
vented from falling out as the barrel is brought back to the surface, by
some form of core-catching device. Air, water, mud or foam is used to
cool the bit and carry rock cuttings back to the surface (
Figure 4.25).
Where cored samples are not required over a particular length of hole,
it can be advanced more quickly using rock roller bits, down-the-hole
hammers and water jets, as used in much oil and gas drilling.
At the most basic level, a single-barrel well-boring rig can be used to
take core samples but these are often highly disturbed (
Figure 4.26).
Most drilling is carried out using double-barrel systems in which the
outer barrel rotates around an inner barrel that takes in the core. A
