Environmental Engineering Reference
In-Depth Information
Table 5.8.
Deficiencies in drilling and sampling.
Problem
Consequences
Remedy
Only drilling vertical holes where jointing is near Joint spacing and rock mass permeability incorrectly
Angle holes to intersect joints, bedding and other
vertical
assessed
features
Poor identification of orientation of joints in
Lack of knowledge of actual joint orientation affecting
Careful orientation in boreholes and mapping in
boreholes
slope stability
trenches
Water table not measured, not measured often
Dewatering problems with resultant contractual claims;
Routine measurement and or recording. Install
enough or data not recorded
lack of information for design, e.g. pore pressures for
casing and or piezometers
slope stability
Drill water inflows or outflows not recorded
Poorer understanding of reasons for high/low water tables
Employ good drillers and supervise full time
Poor identification of layering in soil deposits
Soil strata are mixed together, hole may collapse causing
Use rotary drilling with mud and/or casing,
particularly when using augers below water table, greater mixing. Leads to under/over-estimation
sample systematically with thin wall tubes
or wash boring without sampling, e.g. see
of horizontal/vertical permeability, potential
and SPT. Use static cone penetrometers
Figure 5.12 which compares log from augered
overestimation of strength by mixing clay layers
borehole and static cone penetration test
“Blowing” in boreholes in silty sand,
Low SPT values in silt and sand and disturbed samples
Use drilling mud and excess head of mud in the
sand and soft clay i.e. flow of material
(in clay) leads to over-estimation of settlement,
borehole to prevent blowing
towards borehole
underestimation of strength
Only drilling holes, not test pits, in
Failure to recognise the structure usually leads to
Dig backhole and excavator pits and have
“structured” clays, e.g. fissured or lateritised
overestimation of strength and underestimation of
experienced personnel log them
soils, soils with root holes
permeability (often by orders of magnitude)
Drilling in gravel and gravelly sands with
Gravel is broken up to finer particles, mixed with sand
Recognise the problem. No real drilling solution.
percussion drill
to give the impression of uniform sandy gravel. Fines
Test permeability with pumpout tests
may be lost in the drilling process. Horizontal
permeability is under-estimated, possibly by orders of
magnitude (see Figure 6.55)
Ground surface level and location of
Errors in plotting sections, plans misinterpretation
Survey all investigation location
boreholes not surveyed
of conditions
Copyright © 2005 Taylor & Francis Group plc, London, UK
