Environmental Engineering Reference
In-Depth Information
5
Site investigation techniques
Several techniques or “tools” may be used in a dam site investigation designed to follow
the broad framework explained in Chapter 4. This chapter discusses some of the most
common techniques, their applicability and limitations. It is emphasised that use of several
techniques is always required, as restriction to a single investigation method would be
unlikely to yield correct answers to the site questions in an economical manner.
5.1
TOPOGRAPHIC MAPPING AND SURVEY
A fundamental requirement for the investigation and design of any project is accurate
location and level of all relevant data. Topographic maps at suitable scales are essential with
establishment on site of clearly identified bench marks. All features recorded during the
investigation should be located and levelled, preferably in relation to a regional coordi-
nate system and datum. A local system may be established provided that at some stage dur-
ing the investigation the relation between the regional and local survey systems is determined.
Survey control for regional studies can utilise Global Positioning System (GPS) tech-
niques but detailed project studies should be based on standard ground survey methods.
These statements may appear obvious but, in many instances, interpretation, construc-
tion and related contractual problems have been shown to have originated from poor sur-
vey control.
Topographic maps at several scales are required. The actual scale will depend on the
size and complexity of the area but the following common scales are given as a guide:
- Regional maps, 1:250 000 with 20-50 m contours to 1:25 000 with 10 m contours;
- Catchment area, 1:25 000 with 10 m contours to 1:2000 with 2 m contours;
-Project area, 1:1000 with 2 m contours to 1:200 with 1 m contours;
- Individual engineering structures, 1:500 with 1 m contours to 1:200 with 0.5 m contours.
Regional topographic maps issued by government agencies in most countries are pub-
lished at several standard scales which range from 1:250 000 with contours at 20-50 m to
1:25 000 with contours at 10 m interval. The user of these maps should consider their origi-
nal purpose and the accuracy of the information plotted on them in relation to the project
requirements. The notes on the map which indicate the method of compilation - whether
aerial photogrammetry or ground survey - and the date of preparation of the map may be
relevant.
Photogrammetry can be used in the preparation of project specific plans provided that
the aerial photographs have been flown at low level and that accurate ground control
points can be identified on the photographs and that vegetation is not dense.
Photogrammetry is often not adequate in steep, tree-covered areas. The authors are
aware of two projects where errors of up to 20 m in elevation have occurred due to inad-
equate allowance for tree cover. This resulted in the requirement for substantially larger
saddle dams than estimated in the feasibility studies and significantly affected the viability
of the projects.
