3284
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
By using heavy compaction standard (modified Proctor
compaction), the compaction energy is increased from 604kJ/m
3
to 2704kJ/m
3
. Accordingly, the maximum dry density of the
material is increased from 2.23
∼
2.32g/cm
3
to 2.375g/cm
3
. The
permeability of the material is less than 1
×
10
-5
cm/s and the
deformation modulus is also remarkably increased.
For borrow soil mainly composed by fine grains, the material
usually cannot fit the stability and deformation requirement of
high dam. In this case, gravels or crushed rock should be added
to increase the content of coarse grains. The case for applying
this measure is Nuozhadu ECRD in China.
The borrow materials of Nuozhadu ECRD is mixture of
slope washed, residual soil and some strongly weathered rocks.
The average grain composition is: 24% gravels with the size
larger than 5mm, 44.3% fine grains with the size smaller than
0.074mm, 21.7% of the grains with the size smaller than
0.005mm. Most of the soils are classified as clay sand, low
liquid limit clay with sand. As most of the grains are weathered
sandstone and mudstone, grain particles are easily broken. After
compaction, the content of grins with the size larger than 5mm
could be reduced to 10%. The density, deformation parameters
and shear strength of the material are very low. Thus, it is
decided to add crushed hard rock to the nature borrow material.
The size range of crushed rock to be added in borrow
material is 5
∼
60mm. After optimization, proportion of the
adding material is 35%. From the research results, after adding
coarse particles, content of the grains with size larger than 5mm
is 50%, content of the grains with size smaller than 0.074mm is
23.6%, content of the grains with size smaller than 0.005mm is
10%. The classification of the mixed material is GC. It is an
idea impervious soil for high ECRD. Due to the breakage after
compaction, the content of grains larger than 5mm could be
36%. Compare with the unmixed material, the maximum dry
density could be increased from 1.7
∼
1.8g/m
3
to 1.9
∼
2.0g/m
3
.
The corresponding water content is about 10%
∼
15%. The
overall engineering properties of the material are greatly
improved.
(2) Gradation and permeability of gravelly soil
The permeability of gravelly soil has close relationship with its
gradation. For high ECRD, the general requirement is: the
content of grains with size larger than 5mm should not above
50% (or should below 60%), the content of grains with size
smaller than 0.075mm should not below 15%, and the content
of clay grains should not below 8%. But in practices, due to the
wide range gradation of natural gravelly soil, the above
principles could be adjusted according to the real situations. By
the analysis from the point of geotechnical engineering, the
requirement of content of grains with size larger than 5mm
should not above 50% is to guarantee the void of coarse grains
could be filled by fine grains. The requirement of content of
grains with size smaller than 0.075mm should not below 15% is
to guarantee the low permeability and to keep internal stability
of soil structure under seepage flow. As for the requirement of
the permeability of impervious soil, when it is in the quantity of
10
-5
cm/s, the actually leakage is quite small. It is unnecessary to
request the permeability to be 1
×
10
-5
cm/s. As for the seepage
stability, normally, the gravelly soil with wide range gradation
will have less clay grains. Thus, it has the same properties in
seepage deformation as non-cohesive soil. The hydraulic
gradient for seepage failure resistance of gravelly soil is mainly
depends on the filter at the exit. With the protection of filter, the
failure gradient can be improved significantly. Therefore, the
content of clay grains above 8% could not be an unchangeable
rule. For the case of Pubugou ECRD, the material has
17%
∼
48% grains with the size smaller than 1mm and 4%
∼
12%
clay grains. Under normal compaction, the tested maximum
hydraulic gradient could reach 90
∼
140.
(3) Compaction of gravelly soil
The two methods for soil compaction quality control are dry
density and compaction degree. In compaction, gravelly soil
presents the properties of both gravel and clay. For the
compaction of gravelly soil, it is required to get the maximum
dry density of the full material and also to check the dry density
of fine grains. By considering the variability of the soil,
compaction degree is more often to be used as the index for
quality control of gravelly soil compaction.
For the mixed soil with coarse and fine grains, besides the
compaction degree of full material, it is also has the compaction
degree of fine grins. As the compaction degree of fine grains
mainly controlled the permeability and mechanical property of
gravelly soil, it is unnecessary to conduct difficult large-scale
compaction test for full material. When content of coarse grains
is below 60%
∼
70%, the dry density of full material will be
increased with the increasing of coarse grains content. When
content of fine grains is below 20%
∼
30%, the coarse grains will
not take the function of soil skeleton. The fine grains are fully
compacted. Its dry density keeps unchanged. When content of
fine grains reaches 30%, the coarse grains start to take function
of skeleton. The more content of coarse grains, the stronger is
its skeleton function. Thus, the fine grains inside void cannot be
fully compacted. The dry density of soil will be reduced with
the increase of coarse grains. When content of coarse grains is
60%
∼
70%, the skeleton function of coarse grain is fully
realized. The dry density of full material and fine grains reduced
synchronously. All its mechanical properties are dropped in big
scope and the permeability of soil are increased rapidly.
Therefore, in the application of compaction degree control of
fine grains, when content of coarse grains is below 25%, the
compaction degree for fine grains could be controlled with
100%; when content of coarse grains is 25%
∼
50%, the
compaction degree control for fine grains could be reduced to
97%
∼
98%.
3.1.2 Filter material
The process of seepage failure in soil is always started from
exit, and gradually developed to the inside, then finally lead to
local failure or whole structure failure. Using filter to control
seepage exit is an effective seepage control measures in dam
engineering. It could be a drainage zone and also to prevent fine
grains flow out. For high ECRD with gravely soil as the core,
although gravelly soils have certain content of clay grains, it is
still belong to the soil without plasticity or with low plasticity.
The filter design should also follow the principle of no fine
grains been washed out. The paper submitted by S.
Messerklinger discussed the functions of filter in geotechnical
structures and summarized the main principles of filter design.
Figure 3 is the summary of the design criteria of filter design
(Messerklinger 2013). The paper submitted by R. Eerzariol
presented the test studies on filter protection of loess, a kind of
sandy silt that largely distributed in central Argentina. The
studies suggested that filter with fines content between 15% and
25% perform the best condition of seepage stability for
protecting silt core (Eerzariol 2013).
Figure 3 Design criteria of filter (Messerklinger 2013)
