Geotechnical News • June 2016
31
GEOTECHNICAL INSTRUMENTATION NEWS
Table 7. Some instruments that can be considered for monitoring landslides in rock
Possible geotechnical questions
Measurement
Some instruments that can be
considered
What are the post-landslide conditions? As in Table 6 for “What are the initial
site conditions?”
As in Table 6 for “What are the initial
site conditions?”
Is the slope stable in the long term?
As in Table 6 for “Is the slope stable in
the long term?”
Rainfall, for possible correlation with
any displacement
Load in tiebacks
As in Table 6 for “Is the slope stable in
the long term?”
Rain gauges
Load cell
Case History V
extract from Suit is a Four-letter Word
(Hugh Nasmith, 1986)
This case history illustrates the hazard
of filling a report with an excess of
detail and comments.
A major high-rise office building
with several levels of underground
parking was planned for an urban
development. A geotechnical firm was
employed to carry out and report on
subsurface conditions. Test drilling
established bedrock (a horizontally
bedded sedimentary rock) at a shallow
depth and the borings were extended
to the full depth of the proposed exca-
vation. A professor of geology was
retained by the geotechnical consultant
and asked to examine and describe the
core. His report was very thorough
and comprehensive. The age, lithol-
ogy, structural discontinuities, miner-
alogy, jointing, bedding, and fossils
were described and discussed in detail,
even though the report was based on
an examination of discontinuous small
diameter core. The entire geological
description was incorporated in the
geotechnical report which became part
of the contract documents.
In the course of drilling and blast-
ing the bedrock to excavate for the
basement and footings, considerable
overbreak occurred which the contrac-
tor was obliged to backfill with lean
concrete. Blasting was carefully con-
trolled by an explosives expert hence
the overbreak could not be attributed
to poor procedures.
The contractor claimed for an extra
as a result of the overbreak and his
“expert” claimed that the contractor
relied on the geological description
of the core as thinly “bedded” and
accordingly made little allowance for
overbreak. Photographs taken during
construction showed horizontal beds
1.0 to 1.5 meters thick which in terms
of mass rock would not be regarded as
thinly bedded.
It was concluded that the contrac-
tor had a valid claim and was paid
an extra. This claim might not have
been allowed if the report had merely
reported the rock type, elevation and
percentage of core recovery (RQD)
and included representative photo-
graphs of the core.
From a practical point of view the
small diameter of the core made it
unsuitable for determining the spacing
of the bedding planes. It is unlikely
that the geologist or the geotechni-
cal engineer anticipated that the rock
description would be used to predict
the behaviour of the rock when exca-
vated. If the professor had realized
the importance that would be attached
to the term “thinly bedded” he would
no doubt have considered the use of
the term more carefully. An examina-
tion of a nearby rock outcrop would
probably have been more informative
than the core fragments. A cynic might
suspect that the contractor only stud-
ied the description of the rock in detail
when he realized the extent and cost of
the overbreak.
When you pad out a report with a
mass of extraneous detail and com-
ments, you are providing answers to
questions which have not been asked
and the answers you have given may
well be wrong or misleading.
