Granular Materials in Civil Engineering: Recent Advances in the Physics of Their Mechanical Behavior and Applications to Engineering Works - Multiscale Geomechanics: From Soil to Engineering Projects - page 72

These results can explain the accident that occurred in the Campos Novos Dam
in Brazil when it entered into operation in 2006. The main characteristics of this
dam correspond to the ones used in the above example for the extrapolated project:
H = 202 m, slope 1.3 /1, D Max = 1 m. The problem involved a set of fractures by
compression that appeared in the watertight upstream concrete slab during
impounding, and led to a considerable increase in water flow through the dam body
of up to 1,450 l/s. These fractures were discovered after an accidental emptying of
the reservoir (see Figure 3.18). They were due to excessive deformations of the
material constituting the dam body under service loads incompatible with the
rigidity of the concrete slab. Other accidents of this type have occured recently in
Brazil, but also in China, Lesotho, etc. (see [PIN 07]).
3.3.3.2. Scale effects in rockfill apparent rigidity modulus
The deformability of embankment works made of granular materials, such as
rockfill dams, is a key parameter deserving careful monitoring during construction.
As they explain most of the compressibility of rockfill under increasing loads, the
laws governing grain breakage are applicable here. The stiffness - or rigidity
modulus − of granular materials coming from the same mineral stock, with similar
grain size distributions and the same density, should be inversely proportional to the
square root of D Max or of any other characteristic size, D X% .
Figure 3.19 illustrates the statistical correlation proposed by Hunter and Fell
[HUN 03, FEL 05] between the apparent rigidity modulus during construction and
the coarse fraction characteristics D 80 . It results from a detailed analysis of data
coming from a set of 35 well-documented rockfill dams. The dotted lines added in
the diagram correspond to the trend in behavior obtained by the grain breakage laws.
They demonstrate that
− the scale effect due to grain breakage appears to explain most of the negative
correlation obtained by Hunter and Fell between the apparent rigidity modulus and
the coarse grading fraction;
− for rockfills with large blocks (right-hand side of the diagram), the decay of the
modulus with block size appears to be more pronounced than predicted by the scale
effect. This may be attributed to a loss of efficiency in the compaction techniques
when dealing with large blocks of rock.
If in this figure we plot the points corresponding to four high dams having
suffered from excessive deformations leading to significant problems during the
impounding phase (Barra-Grande, 185 m and Campos-Novos, 202 m in Brazil,
Mohale, 145m in Lesotho and Tian Sheng Qiao I, 179 m in China), we can observe
that they are located to the far right of the diagram. They were all built with very
coarse rockfills ( D 80 > 800 mm) and their particularly small rigidity modulus,