Key geological issues - Geotechnical Engineering of Dams - page 62

7. In the light of these assessments, what needs to be done, e.g. in the location and design
of structures, in treatment of the ground, or in operation of the storage?
The sections which follow discuss when and how Questions 1 to 5 should be answered,
and include suggested strategies for action, in respect to Questions 6 and 7.
2.11.1.1 Most vulnerable existing or proposed project features, and parts
of storage area? - Question 1
This question can usually be largely answered in Stage 1 (Pre-feasibility), by examining
contour plans showing existing infrastructure and buildings, the proposed storage in out-
line, and a preliminary layout of the proposed dam and associated works. From the lists
in 2.11 above it is clear that generally, the project features most vulnerable to the effects
of landsliding will be the dam, saddle dams, inlet and outlet works, spillways and roads.
Also, topographically, the most vulnerable parts of the storage area will be the highest,
steepest slopes within narrow sections ( Figure 2.42 ), and steep, narrow ridges forming its
rim ( Figure 2.43 ) . The exception may be existing landslides which are often not the steep-
est slopes.
2.11.1.2 Currently active or old dormant landslides? - Questions 2 and 4 to 7
Tentative answers to Question 2 should come also during Stage 1, from existing historical
records, examination of air photos, and from inspections from the air and on the ground.
Currently active landslides, and the scars or remnants of old, dormant landslides can usu-
ally be recognized and their plan boundaries delineated, during air-photo interpretation
(Section 5.2.2.2), and/or geomorphological mapping (Section 5.3) and geotechnical map-
ping (Section 5.4.2.2). Methods for their recognition and subsequent investigation are
available in Rib and Liang (1978), Sowers and Royster (1978), Hunt (1984), Stapledon
(1995), and Soeters and van Westen (1996).
In view of the ICOLD (2002) statistics quoted in Section 2.11, any pre-existing land-
slide feature found must carry a substantial risk of reactivation. The significance of a
major pre-existing slide of either type, in the rim of a storage area, will depend on its size
and location. Two possible situations are considered here.
Situation A - a slide which is judged to be sufficiently large and located so that its future
movements could endanger a vulnerable existing feature, or proposed project feature. For
this situation the following approach might be followed, in Stage 2, Feasibility and Site
Selection.
1. Do enough surface (and if necessary, subsurface) investigations to more accurately
assess the size of the mass and its likely movements during storage operation;
2. If this confirms the size of the slide and that its predicted future movements are unac-
ceptable, the following options should be considered:
- Relocation of the existing feature, or proposed project feature, to a proven stable
area, or
- Design of systems aimed at stabilizing the slide or limiting its displacements to
acceptable velocities/amounts. Such systems are listed and discussed Section 9.3 of
ICOLD (2002).
From experience, the authors believe that relocation is likely to be the better option, in
most cases.
Situation B - a slide which because of its apparent size and location (e.g. Figure 2.42)
is judged to present a serious risk to the feasibility of the whole storage project. For this
situation the following approach might be followed.