3283
Technical Committee 210 + 201 /
Comité technique 210 + 201
seepage pressure in advance for keeping the stability of
foundation and geotechnical structure. (3) Prevent seepage
failure of foundation and structure. Generally, the
corresponding engineering measures are also includes three
aspects:
·Seepage prevention: Put impermeable material in dike and
foundation to cut the seepage passage and to decrease the water
head in the dike and foundation.
·Drainage: Put permeable material as drainage at the certain
places in the dike and foundation where the hydraulic gradient
is relative large. To release the seepage pressure and let the
seepage water freely discharge to downstream by the drainage.
·Filter protection: Filter is an effective measure for preventing
seepage failure of soil. As it also has the drainage function, it is
often as one part of the drainage. The materials for filter are
natural sand and rock. But the material must satisfy following
principals:
�
filter material must be non-piping soil.
�
the
gradation relation between filter and protected soil must satisfy
filter principals.
�
the permeability coefficient of filter must
larger than the protected soil.
�
the coarse grains of the filter
must hard and weathering resistant.
Figure 2 gives the typical engineering measures of seepage
control applied in dike construction. The selection of theses
engineering measures depends on the different situations of the
dike.
Figure2 Engineering measures of seepage control
2.3 Bank collapse
In nature rivers, the interaction of water flow and riverbank
(upstream slope of dike) could cause bank erosion and bank
collapse, which are the common damage to the safety of dikes.
In USA, the total length of river channel is 5,600,000 km.
About 800,000km riverbank were suffered of flow erosion,
include bank collapse. In China, a severe bank collapse case
has caused the lost of 115,000m
3
land. About half of a town was
collapsed into river.
There are many factors that affect the occurrence of bank
collapse, which include hydraulic features, properties of the
materials of riverbed and river bank, features of river bank,
impact of wind and wave, impacts of climate, impacts of human
activities, etc. (Simons, 1982). For all these factors, water flow
and boundary are the basic conditions.
For the mechanism of bank collapse, there are different
viewpoints. Some scholars think water flow is the precondition
of bank collapse. When the main stream of river approach
riverbank, water flow will scour the bank and the bank slope
will be steep. Some scholars believe the liquefaction of soil the
cause of bank collapse. According to the force act on soil, the
liquefaction of soil could be classified to shear liquefaction,
seepage liquefaction and vibration liquefaction. If the dike is
composed by non-cohesive sand or soils with less cohesion, its
effective stress may drop to zero under the action of shear stress
or seepage force. Some scholars explain the mechanism of bank
collapse from the point of slope stability. Bank collapse
occurred when upstream slope of the dike lost its stability. In
flood season, soil of dike is submerged in water, which lead to
the reduction of its shear strength. With the drop of upstream
water level, the seepage force towards riverside and the reduced
c,
ϕ
value could cause riverbank lost the stability.
As bank collapse is the result of the interaction of river flow
and watercourse boundary condition, the engineering measures
for avoiding bank collapse will mainly focus on water flow and
watercourse boundary. For changing local water flow, groyne
works are commonly employed. For improving boundary
conditions, different bank protection method could be applied,
which include riprap, concrete protection, geosynthetics, etc.
3 GEOTECHNICAL PROBLEMS OF ROCKFILL DAM
Rockfill dam is a widely applied dam type of dam engineering.
The development of earth core rockfill dam in 1940s to 1960s is
mainly based on the progress of the theory of soil mechanics. In
recent years, more and more high rockfill dam will be
constructed. New challenges on geotechnical engineering
problems are encountered in the construction of those high
dams.
3.1 Construction material
The construction materials of rockfill dam include impervious
material, filter/transition material, and rockfill material. Proper
application of the construction material according to its
engineering properties is one of the key issues for rockfill dam
design.
3.1.1 Impervious material of earth core
For high ECRD, earth core will subject to high stresses.
Ordinary clay material could not meet the strength and
compressibility requirements of high dam. Therefore, for most
ECRD with the height above 200m, the core material uses
gravelly soil. As for the composition, gravelly soil is mixture of
clay and gravels with the grain size larger than 5mm (or 2mm).
Soil of weathered rock and glacier deposit are also a kind of
gravelly soil.
(1) Gradation adjustment for gravelly soil of core material
Generally, if the soil has more than 20% coarse grains content,
i.e. the grain size larger than 5mm, it could be classified as
gravelly soil. Those soils include various soils with gravels, clay
gravel and weathered rocks.
The composition of nature formed gravelly soil is very
inhomogeneous. When it is used as core material, its gradation
and water content are often need to be adjusted by the
requirement of design.
For nature gravelly soil with wide range of gradation, if the
material is basically applicable, the oversize particles could be
removed to increase the content of fine particles. The case for
applying this measure is Pubugu ECRD in China.
The impervious material for the central core of Pubugou
ECRD is the gravelly soil with wide range of gradation. The
coarse grain content is 50%
∼
65% and the content of particles
with the size less than 0.1mm is 8.8
∼
20%. In soil classification,
the material is GP. The permeability of the material after
compaction is 10
-4
∼
10
-5
cm/s, which is not fit the requirement of
impervious material of high dam. With series studies, two
measures were employed for improving the properties of the
material, which are: adjust gradation by removing the particles
with the size larger than 80mm (or 60mm) and use modified
Proctor compaction energy to increase its density.
After removing the particles of the size larger than 80mm
form the nature wide range gradation gravelly soil, the gradation
of the material was improved significantly. The content of
particles of the size less than 5mm was 50%, and the content of
particles of the size less than 0.1mm was 22%. Classification of
the material was change from GP to GC. Permeability of the
material reached to 10
-5
∼
10
-6
cm/s. With the protection of filter
material, the hydraulic gradient of seepage failure was 60
∼
100.
