Environmental Engineering Reference
In-Depth Information
4
Planning, conducting and reporting of
geotechnical investigations
4.1
THE NEED TO ASK THE RIGHT QUESTIONS
The French detective, Bertillon, considered by many to the father of modern crime detec-
tion, is reputed to have made the following statement.
“We only see what we observe, but we can only observe that which is already in the
mind”
. Experience from analysis of many case histories (Stapledon, 1976, 1979 and 1983)
shows that this principle (that we will only find that which we recognise) is equally true
in engineering site investigations. In almost every foundation failure and contractual dis-
pute over “changed geological conditions”, it is found that a major contributing factor has
been the failure of project planners and site investigators to fully understand and define
all of the geotechnical questions which needed to be answered by the site investigations.
There are two types of questions, namely:
-
engineering questions
, which relate essentially to the design, construction and opera-
tion of any structure of the type proposed, and
-
geological questions
, which arise from understanding of the site geological environ-
ment and its likely influence on the design, construction and operation of the project
(see Chapters 2 and 3).
4.1.1
Geotechnical engineering questions
For dams which are intended to store water, or water plus solids, it is obvious that important
questions must relate to the permeability of the foundations. However, there are many
other equally important questions, because construction and operation of a dam causes
much greater changes to a site environment than any other type of engineering activity.
Four main processes involved in dam engineering, their principal effects on the site envi-
ronment, and some resulting questions for the designer and site investigator, are as follows;
1. Excavation: To reach suitable levels for founding the dam, and also for the spillway, out-
let works and for construction materials. Excavation causes removal of support from and
increases in shear stress in the surrounding material and hence raises questions of stabil-
ity of the excavations themselves, during construction and/or operation, and of the sta-
bility of the adjacent valley slopes.
2. Foundation loading: Imposed by the dam structure, raises questions of compressibility
of the foundation and its shear strength against sliding upstream or downstream,
before and after filling of the storage and under flood and earthquake loading.
3. Inundation - Filling the storage: Causes changes to the groundwater regime, lowering
of strengths of cohesive soils, weak rocks and joint cements, and decreases in effective
stress. These effects all add to the questions of stability of the dam and its foundation and
also they raise the question of stability of the reservoir sides. These stability questions
are more serious when water storage levels are required to fluctuate widely and rapidly.
