Environmental Engineering Reference
In-Depth Information
Cone resistance, q c (ksf)
300
0
100
200
400
500
600
700
800
900
100
100
120
130
140
150
160
Legend
Clay
Sand
170
FIGURE 2.45
Plot of cone resistance data. (Courtesy of
Fugro.)
180
either naturally or with additives used without casing, or air pressure in highly
fractured or cavernous rock where circulating water is lost and does not return
to the surface.
Hole Stabilization
Some form of stabilization is often needed to prevent hole collapse. None is required in
strong cohesive soils above GWL.
Casing , used in sands and gravels above the water table, in most soils below GWL, and
normally in very soft soils, is usually installed by driving with a 300 lb hammer, although
140 lb hammers are often used with lighter tripod rigs. The hollow-stem auger serves as cas-
ing and wireline and Odex methods install casing by drilling.
Driven casing has a number of disadvantages:
Installation is slow in strong soils and casing recovery is often difficult.
Sampling at stratum changes is prevented unless they occur at the end of a
driven section, and in situ testing is limited.
Obstacles such as boulders cannot be penetrated and require removal by blast-
ing or drilling. The latter results in a reduced hole diameter which restricts sam-
pling methods unless larger-diameter casing is installed before drilling.
Loose granular soils below GWL tend to rise in casing during soil removal,
resulting in plugged casing and loosened soils below.
Removal of gravel particles is difficult and requires chopping to reduce particle
sizes.
Casing plugged with sand or gravel prevents sampler penetration adequate for
recovery of undisturbed samples and representative SPT values. (The drill rod
length and sampling tool must be measured carefully to ensure that the sampler
rests on the bottom slightly below the casing.)
Mud slurry , formed naturally by the mixing of clayey soils during drilling, or by the
addition of bentonite, is a fast and efficient method suitable for most forms of sampling
and in situ testing.
 
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