General features to be identified and defined in the course of the
site investigation include the interface between soil and rock, groundwater
conditions, unstable and caving ground, e.g. karstic formations etc., and
all significant discontinuities, i.e. rock faults, shatter zones, fissured
or heavily fractured rock and the spacing and other characteristics of joint-
ing and bedding surfaces etc. within the rock mass. Reference should
be made to Attewell and Farmer (1976) and/or to Bell (1993) for a com-
prehensive perspective on engineering geology in relation to dam and
reservoir sites.
Key features of this phase of the investigation include
(a) meticulous logging of all natural and excavated exposures and bore-
hole records, etc.,
(b) careful correlation between all exposures, boreholes and other data,
and
(c) excavation of additional trial pits, boreholes, shafts and exploratory
adits as considered necessary.
It is at this stage that more extensive geophysical and in situtesting
programmes may also be conducted, with the primary intention of extend-
ing and validating borehole and laboratory data. A further purpose of field
testing at this time is confirmation of the natural groundwater regime, e.g.
through installation of piezometers, pumping tests, etc.
Extensive use is made of rotary drilling and coring techniques to estab-
lish the rock structure at depth and to confirm its competence. Core recovery
is a crude but useful index of rock quality, e.g. in terms of rock quality desig-
nation (RQD) (i.e. total recovered core in lengths of over 10 cm as a percent-
age of total borehole depth; RQD70 is generally indicative of sound rock).
In situtests, e.g. for permeability, strength and deformability, are used to
estimate rock mass characteristics in preference to small-scale laboratory
sample testing wherever possible. All cores are systematically logged and
should ideally be retained indefinitely. Drilling, sampling and testing tech-
niques are essentially those employed in conventional site investigation prac-
tice. A comprehensive review of the latter is presented in Clayton, Simons
and Matthews (1995) and in the CIRIA site investigation manual (Weltman
and Head, 1983). More specialist techniques, e.g. for large-scale in situtests,
are illustrated in Thomas (1976) and in Fell, MacGregor and Stapledon
(1992). The applicability of different equipment and exploratory methods in
the context of site investigation for dams are reviewed concisely in Wakeling
and Manby (1989).
Evaluation of seismic risk for an important dam requires identification
of the regional geological structure, with particular attention being paid to
fault complexes. Activity or inactivity within recent geological history will
require to be established from study of historical records and field reconnais-
sance. If historical records of apparent epicentres can be matched to key