2.5.2 Construction
The construction operations which follow initial site development fall into
four principal groups of activities, relating to (1) material source develop-
ment, (2) foundation preparation and construction, (3) fill construction
and control and (4) ancillary works construction.
Material source developmentactivities involve the opening out of
borrow areas or quarries, including the installation of fixed plant, e.g.
crushers, conveyors, etc. Access and haulage roads are also constructed
between the various borrow areas and the embankment site, and excava-
tion and haulage plant is mobilized.
Foundation preparation activities, including river diversion, can
proceed concurrently with the development of the fill sources. Temporary
river diversion is commonly effected by driving a flanking tunnel, which in
most cases subsequently houses the outlet works. (Where an outlet culvert
through or under the embankment is planned rather than a flanking tunnel
the culvert may be used temporarily for river diversion purposes.)
Topsoil and weathered surface drift deposits etc. are removed. In the
case of a soft, compressible foundation, strength can be enhanced and con-
struction accelerated by preconsolidation and/or the installation of sand
drains, as for the Derwent dam, constructed on a difficult site in the UK
(Ruffle, 1970). Foundation instrumentation is also installed at this stage to
monitor pore pressures and cut-off performance (Chapter 7). Foundation
construction is completed with the laying of the drainage blankets which
will underlie the downstream shoulder.
Fill constructionis an exercise in efficient plant utilization within the
terms of the specification requirements as to materials compliance and
compaction technique. Placing operations may be subject to the influence
of weather conditions and to subtle changes in material characteristics.
Control of placing is centred upon supervision of water content, layer
thickness and compaction procedure. The quality and uniformity of the
compacted core fill are critical. Recent UK practice is to employ a statisti-
cal approach to test-ing and quality control and to require that the
undrained shear strength, cu, should lie within specific limits. A specifica-
tion requirement of cu55–100 kN m^2 is typical for UK clays (Kennard et
al., 1979). Control of construction porewater pressures and acceleration of
consolidation in cohesive fill materials of low permeability can require the
installation of horizontal blanket drains in both shoulders, at vertical inter-
vals of 3–6 m (Fig. 2.10(a) and (b); Gibson and Shefford (1968)). Knight
(1989) reviews aspects of fill construction and control in terms of recent
practice on a number of projects.
The installation of instrumentation in the core and shoulders pro-
ceeds in parallel with the placing of fill. Fill construction is concluded with
the completion of upstream rock armouring or other face protection works.
76 EMBANKMENT DAM ENGINEERING
