ase study of a complete 3D stress field diagnosis in
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a large gravity dam
NERIS performed successfully 3D stress measurements in a large gravity dam using the overcoring
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method. Representative test zones were investigated to compare measured stresses to those computed
with an advanced numerical model including swelling rheology of the concrete. The applied methodology
proved successful to yield a complete reliable picture of the 3D stress field in the geostructure, reinforcing
the quality of the prediction of dam movements in the future and improving decision making.
ontext. To evaluate the impact of the ageing of dams, operators monitor closely the long-term
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evolution of physical key parameters. In particular, significant strains and apparent disorders as
disseminated cracks are forensics of unexpected stresses developing in the geostructure. In this case, the dam,
built in the Alpine area during the early 30’s and now operated by Electricité de France (EdF), suffers from Alkali
Aggregate Reaction (AAR) of its constitutive cement, leading to a swelling pressure and then to pervasive cracking.
Therefore reliable characterization of the stresses exerting in the geostructure was needed to ensure the diagnosis
of the dam integrity and to assess its overall safety. However, field stress measurements remain a challenging
issue, especially in this context where the most interesting zones to investigate revealed to be located at the heart
of the structure close to the rock-concrete interface. Heterogeneity of the host material at the scale of a stress cell
and the low temperature of the dam were challenging conditions to succeed such measurements.
olution. The methodology chosen was to assess the stress field of the dam following two
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independent approaches: Finite Element
(FE) analysis of the geostructure and in situ stress
measurements. These approaches were performed independently to avoid any bias in the field measurement
interpretation and ensure reliable comparison.
irst, concerning the FE approach, effects of AAR were considered by EdF staff taking into account the
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interactions between AAR pressure and creep, the swelling anisotropy induced by oriented cracking and the
moisture, all three parameters governing the AAR swelling effect on the stress field. Once this AAR rheological
model fitted on laboratory tests on core samples, a FE inverse analysis was undertaken to calibrate the best
numerical model of the dam constrained on its well-known vertical displacement monitored over the past decades.
From there, numerical analysis were computed to assess the 3D stress field and then to select the four best zones
of interest to undertake in situ measurements. The model is developed by EdF in collaboration with LMDC
Toulouse and is implemented in EdF Code_Aster.
econd, INERIS performed overcoring stress tests based on the CSIRO
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HI Cell at these four locations with a quality control criterion of at least
two successful tests on each of them. All tests were run in the concrete
near the rock foundation, that is 20 meters far from the dam surface, with
successive tests along the same borehole distant of one meter from each
other.
Overcored sample
Recorded strains during an overcoring test (red
curve shows the temperature).
ince the concrete temperature was naturally too
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low for a correct hardening of the epoxy glue,
or all four locations, excellent data quality was
heated water was forced to circulate to warm the pilot
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obtained between successive overcoring tests,
hole hosting the cell. Moreover, heating resistors
yielding very coherent stress estimates. In a general
were adapted to the rod system to reduce the cooling
manner the major principal stress σ1 was found to be
locally following the set up of the cell. This procedure
parallel to the dam axis and perpendicular to the rock
limited the epoxy curing time to
20 hours while
foundation. With principal stresses ranging from 1 to 6
yielding excellent bonding of the gauges. The
MPa, with a global average near
2 MPa, the
automatic monitoring of all strains, cell temperature
comparison of the results with numerical simulations
and drilling parameters as well as the performance of
showed an excellent consistency.
the biaxial test on each core offered a thorough
checking of the data quality.
essons learned. The objective of
he concrete being melted with gyps could not be
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the study was to quantitatively explain the
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considered as homogeneous. Indeed at the
geotechnical disorders observed in the dam and to
scale of the rosettes of strain gauges, one rosette
obtain a comprehensive diagnosis of the integrity and
could be in contact with gyps, another with concrete.
safety of the swelling geostructure for the next future.
This heterogeneity was revealed through the different
The parallel approaches used advanced numerical
responses of the three orthoradial gauges during the
modeling and in situ measurements, which was
biaxial test. A correcting factor was determined for
proved necessary to yield a reliable assessment of
each rosette and introduced in the SYTGEOstress
the integrity and safety of the dam.
software to balance the measured strains according
to the local Young modulus compared to the average
he methodology and experience developed
one.
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during the project provide dam operators with a
trusty procedure to diagnose accurately large dams
suffering AAR, offering a great help in the decision
making for an ascertained maintenance strategy.
www.ineris.fr
Left : principal stresses computed and projected on a lower hemisphere. Uncer-
tainty is graphically represented as centered colored stains. Gray-dotted line
shows the estimated orientation of the rock foundation below zone 1. Right : 3D
elevation view of the dam with the major principal stress σ1 contours mapped
on its downstream face. Modelling by EdF-CIH.