Environmental Engineering Reference
In-Depth Information
It is important during the planning stages (Stages 1 to 3) that the site investigation stud-
ies should proceed in phase with other engineering work being carried out at those times,
e.g. hydrological and topographic surveys, design and preparation of specifications. This
is because most decisions affecting the project design or feasibility from the geotechnical
viewpoint will affect the work requirements in the other fields. Conversely, decisions
made from the results of these other studies will affect the scope of the geotechni-
cal work.
It is desirable that the dam designer takes a keen interest in the geotechnical work and that
field and office consultations with the geotechnical team are frequent. Design drawings
(e.g. cross-sections through structures) should include the main features of the foundation
(i.e. the portion which Nature has made) as well as details of what Man proposes to build.
It is also vital that the geotechnical investigator be given adequate time to do his or her
work. The whole process of geotechnical investigation, and indeed of design, requires
'thinking', not just the mechanical production of drill hole logs, pit logs and the like. Too
often, time is reduced by the project planners to a minimum, without realising the risk
that action could put the whole project in.
Timing and co-operation are just as critical during the construction stage. During this
stage the foundation rock or soil is exposed better than ever before or (hopefully) after,
and if the site investigator arrives too late, the exposed area showing a critical clue can be
covered by the first layer of fill or concrete. It goes without saying that unless the site
investigator has the confidence and co-operation of the construction team, this phase of
the work will not be as fruitful as it otherwise could be. In extreme cases, lack of a com-
petent geotechnical observer and advisor during the construction stage can prove disas-
trous - resulting in either expensive contractual disputes or later failure of the structure.
Similarly, during the operation stage inspections must be made at regular intervals,
to ensure that any malfunctions are discovered while there is still time to remedy them.
Such inspections are carried out as part of a surveillance programme as discussed in
Chapter 20.
4.3
AN ITERATIVE APPROACH TO THE INVESTIGATIONS
During each of the project planning stages (
Table 4.3
, Stages 1 to 3) the geotechnical studies
should follow the iterative approach shown on
Figure 4.1
, which is modified from ISRM
(1975) and Stapledon (1983). The following notes relate to the 4 phases on Figure 4.1.
Phase 1
. First, the objectives of the work, or questions to be answered, are defined. As
discussed in Section 4.1, these will include both geotechnical engineering and geological
questions. The number of geological questions which can be defined clearly at this time
might be quite small, if little is known of the geology of the region or site.
Phase 2
. Existing geological, geotechnical and other data relevant to the site are col-
lected and compiled to give a tentative geotechnical model (or models). Tentative answers
to the questions asked in Phase 1 are obtained where possible, from local knowledge or
from rapid analyses, or both. New questions are usually added during this phase, arising
from the understanding acquired while assessing the existing data.
Phase 3.
The investigations are planned to confirm the tentative answers and tentative
geotechnical model and to answer the outstanding questions. The plan is based on the
'broad to particular' approach and types of activities set out in Section 4.4.
It is usual (and desirable) for the work to proceed in stages. For each stage the proposed
activities are related to time and money respectively, in an activity chart and cost estimate.
The proposals are set out in a report to the client, seeking approval to proceed.
Phase 4
. The investigations proceed in stages as planned. A cooperating team of engi-
neering geologist(s) and geotechnical engineer(s) is able to complete most of the studies in
