prepared sand bed and drainage layer, and anchored at crest and toe.
Careful attention must be given to ensuring the integrity of the
seams and welds, and the completed membrane is normally provided
with a granular protective cover supporting conventional upstream
face protection.
Internal membranes, less commonly employed, are normally pro-
vided with protective transition zones on either side.- Filter and drainage layers (seepage control).Relatively thick geosyn-
thetics with high internal transmissivity are suitable for filters or
drainage layers. It is important to ensure that the anticipated stresses
and/or some degree of soil particle migration do not diminish the
transmissive capacity to an unacceptable degree. - Earth reinforcement (stability of slopes etc.).Geosynthetic reinforce-
ment materials, e.g. geogrids, can be used to permit construction of
steeper face slopes or to help to contain lateral deformation and
spread within the embankment or in a soft foundation. - Control of surface erosion (precipitation or limited overtopping
flows).The use of geosynthetics in conjunction with natural vegeta-
tion can considerably enhance erosion resistance and reduce ‘gully-
ing’ due to localized run-off on the downstream face. Geogrids and
mats have also been employed to provide a measure of security
against modest overtopping of limited duration (Section 4.4). - Separation interlayers.Geosynthetics can be employed to act as an
interlayer to ensure positive separation of fill materials at an inter-
face. Interface functions can require the geosynthetic to act as a sup-
porting or cushion layer, or as a low- or high-friction interlayer. An
example of use in this context would be separation of earthfill from
adjacent rockfill of different compressibility.
The general application of geosynthetics to dams is described by
Giroud (1990, 1992). A review of the use of geosynthetic membranes is
given in ICOLD Bulletin 78 (ICOLD, 1991) and of geotextiles for filter-
transition layers in Bulletin 55 (ICOLD, 1986).
2.13 Upgrading and rehabilitation of embankment dams
The upgrading and rehabilitation of older embankment dams is assuming
great importance within the older industrialized nations where, insofar as
dams are concerned, much of the national infrastructure was constructed
in the 19th and 20th centuries. The median age is therefore comparatively
high. A well-engineered embankment dam is a surprisingly tolerant and
durable structure. The majority of problems which arise with older dams
are associated with a contemporary lack of understanding with regard to: