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evaluated through back calculations on strains measured during operation to lie
between 30 and 45 MPa, correspond fairly well with their location in the diagram.
Figure 3.19. Scale effects in rockfill Rigidity Modulus at end of construction -
correlation with grain size (from Hunter &Fell, 2003)
As a counter-example, we can cite the Karahnjukar Dam in Iceland at a height of
198 m which did not suffer any excessive deformations during the commissioning
phase. This can be explained by its characteristics, corresponding to a well
compacted granular material with D 80 < 80 mm, which induced a higher rigidity
modulus value measured between 80 and 120 MPa, in fairly good correspondence
with the trend displayed in Figure 3.19.
Figure 3.20, published by Johannesson in 2007, gives a practical correspondence
between the apparent rigidity modulus measured in various rockfill dams and layer
thickness during construction, [JOH 07]. The layer thickness usually being chosen to
equal the size of the biggest blocks, the scale effect due to grain breakage may also
play a role in this diagram. If we now consider the thickness of the layer rather than
the value of D Max , the corresponding trend lines display a similar pattern to those in
Figure 3.19, leading to the same observations:
− most of the negative correlation pointed out by Johannesson between the
apparent rigidity modulus and the layer thickness can be explained by the scale
effect;
− for materials set in place in very thick layers (right-hand side of the diagram),
the decay of the rigidity with the increase of layer thickness appears to be more
pronounced than predicted by the scale effect rule. This difference can be attributed
to the loss of efficiency of the usual compacting techniques when the layer thickness
becomes too big.
 
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