Geology Reference
In-Depth Information
(a)
(b)
Soil dumped -
occasionally
disturbed samples
are taken
1
Hole advanced by clay cutter
(no sample)
- Heavy cylinder (clay cutter)
dropped to advance hole.
- Bailer used in sand/gravel.
- Chisel used to break boulders
'Undisturbed' sample depth
Hole advanced by clay cutter
(no sample)
2
Strength test and sample
- either SPT or vane
Steel casing installed if
necessary to support hole
casing
- Undisturbed samples or SPT or
vane tests every 1.5 metres
- May carry out strength test
directly below undisturbed
sample
3
'Undisturbed' sample taken
at predefined depth or
changed in stratum by
driving or pushing tube
Undisturbed samples are usually taken using a relatively thin walled
sampler of diameter 100mm (U100), and much of the published
empirical relationships that are relied upon by designers are based on
tests on samples achieved in this way. This sampling method does not,
however, meet the more stringent requirements of Eurocode 7 for class
1 sampling and testing, because of fears over disturbance. This is rather
naïve in that it implies that thinner sampling tubes can take an undis-
turbed sample, which is not the case. Any sample taken from depth,
squeezed into a tube and then extruded at the laboratory, will inevi-
tably be disturbed to some degree. Further disturbance occurs during
preparation of samples for laboratory testing and initial loading and
saturation, as expressed schematically in
Figure 4.24
and investigated
by Davis & Poulos (1967). The engineering geologist and geotechnical
engineer need to be aware of the likely disturbance to any tested
samples and take due care in interpretation. Furthermore, the scaling
up of results from laboratory to project scale requires careful consid-
eration because it must include the effect of mass fabric and structure,
including fractures and discontinuities. This is discussed further in
