Environmental Engineering Reference
In-Depth Information
17.1.3
Definition of foundation requirements in geotechnical terms
In all cases it is important to define the requirements in geotechnical terms which can be
identified in the field during construction. This is discussed in Sections 17.5 and 17.8.
It is likely that in most cases excavation will proceed in stages, with sufficient clean up
to identify whether the geotechnical requirements have been achieved.
17.2
GENERAL FOUNDATION PREPARATION FOR EMBANKMENT DAMS
The requirements for preparation of general foundations for embankment dams are
described below.
17.2.1
General foundation under earthfill
17.2.1.1
Rock foundation
- Remove topsoil and weak compressible soil. In most cases this will involve removal of
colluvial soil and rock, including boulders, to expose an
in situ
rock foundation which
may be extremely weathered, variably weathered or in some cases fresh (unweathered).
An adequate foundation can usually be identified by near blade refusal of a small bull-
dozer or excavator;
- Where there are unfavourably oriented weak seams in the rock, e.g. bedding surface
shears or landslide rupture (slide) surfaces, these will influence stability and may need
to be removed, or the design modified to accommodate them;
- Slope modification may be necessary as described in Section 17.6.
The surface should be cleaned of loose soil and rock before placing earthfill, e.g. with a
grader, backhoe or excavator. Intensive cleanup is not generally required. It may be desir-
able to moisten the surface prior to placing earthfill to maintain adequate moisture in the
earthfill for compaction, and treat open joints to prevent erosion of the soil into the joint.
17.2.1.2
Soil foundation
- Remove topsoil and weak compressible soil consistent with the assumptions made for
stability and settlement analysis and design;
- Where soils are fissured, or have landslide rupture (slide) surfaces present, these will influ-
ence stability and may need to be removed, or the design modified to accommodate them;
- Slope modification may be necessary as described in Section 17.6;
- The surface may be proof rolled with a steel drum or tamping foot roller to assist in locat-
ing weak and compressible soil. Rolling to a specified compaction requirement should
not be necessary.
The surface should be cleaned of loose soil and rock prior to placing earthfill, e.g. with
a grader, backhoe or excavator. Intensive cleanup is not generally required. It may be
desirable to scarify and moisten the surface prior to placing the first layer of soil to assist
in “bonding” the embankment to the foundation.
17.2.2
General foundation under rockfill
Rockfill will generally be underlain by a rock foundation (or by a horizontal filter drain,
in which case refer to
Section 17.2.3
):
- Remove topsoil, soil and weathered rock which has a strength lower than the rockfill.
In most cases this will involve exposing distinctly weathered rock which is very weak or
