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Prof. Dr.-Ing. W. Wittke, Geotechnical Consultants
Henricistrasse 50, D-52072 Aachen,
Phone: +49 241 88987-0 Fax: +49 241 88987-33
Wasserverband Eifel-Rur
Eisenbahnstraße 5, D-52321 Düren,
Phone: +49 2421 494-13 50
The Urft dam, with a height of 58 m and a width at the foundation level of approximately
65 m, was built from 1900 until 1904 according to Prof. Intze's plans. It is curved
towards the upstream side, and the 226-metre-longdam crest has a curvative with
a radius of 200 m ( Fig. 1).
Since the heightening of the Rur dam Schwammenauel in 1959, the Urft dam hasa pondage of 12 m on it's downstream side.
Fig. 1: Urft Reservoir
Lime efflorescences on the downstream face of the dam in 1980 gave rise to carry
out an extensive drilling and investigation programme in order to determine the
substance of the dam and the foundation rock. After this programme had provided
satisfying results, the dam's stability could preliminarily be proven by three-dimensional
finite element analyses. For a conclusive assessment of the stability, however,
as well as for an adjustment of the dam to the generally acknowledged technical
standards, an appropriate remediation programme had to be developed and submitted
to the supervising authority.
Most important elements of the rehabilitation measures were the excavation of
two inspection galleries by blasting and the installation of a monitoring programme
( Fig. 2). Moreover, Lugeon, dilatometer and flat jack tests have been carried
out in order to determine the permeability and deformability of the dam and the
foundation rock. The reservoir level could not be lowered duringexecution of these
works.
Figure 3 shows the geological conditions in the area of the masonry dam. The rockmasses
belong to the upper Rurberger layers of the Lower Devonian. There are massivesandstone
layers as well as alternating sequences of silt, sand- and clay-stonelayers at
the left slope, whereas silt and clay-stones are prevailing inthe middle of the
valley. The right hillside mostly consists of alternating sequences of silt and
sandstone layers.
Fig. 2: Measuring Devices and Locations (Longitudinal Section)
Fig. 3: Geological section
The orientation of the families of discontinuities, which are important for
thestability of the dam and the permeability of the underlying rock are shown
in figure 4.
A more precise knowledge of the composition of the foundation rock and the concrete
dam was to be gained with the aid of core drillings. In the following the essential
results gained from all core drillings are illustrated on the basis of drilling
KB 7.
Drill cores, obtained from this core drilling, are shown in Fig. 5 for the masonry.
It consists mainly of greywacke rubble stones, which are embedded in a mortar
of lime-trass-concrete. The mortar is in good condition and shows no leaching
damage, which is probably due to it's high lime content. This high lime contentled
to efflorescences on the wall's downstream face.
Fig. 4: Discontinuity fabric
Lugeon tests have shown low coefficients of permeability for the masonry. In drilling
KB 7 a kf-parameter ranging from 10-6 up to 10-7 m/s was
determined. Higher coefficients of permeabilities, however, accur adjacent to
the foundation. Dilatometer tests, which have been carried out in the masonry,
have led in case of KB 7 to Young's moduli reaching from 3000 MPa to 6000 MPa.
These values are within the expected range.
The cores further reveal sandstones, prevailing down to a depth to 40 m. Theseare
followed by alternating sequences of sand- and clay-stones and layers, mainly
consisting of clay-stones, which commence in a depth of approx. 50 m.
Lugeon tests have shown that the permeability is becoming smaller with greater
depth. Only small permeabilities, in the order of kf = 10-7 m/s, occur
in depths³ 20 m below the foundation level.
With the exception of the area situated directly under the foundation level, the
dilatometer tests have provided Young's moduli ranging from 4.000 to 8.000 MPa.
They did not show any dependence of depth.
In the masonry, two flat jack tests were carried out ( Fig. 7). The Young's modulus,
deduced from test results for the rubble stone masonry, ranges from 8.000 to12.000
MPa and is therefore higher than expected. The verifying analyses carried out
later on do, however, confirm these values.
Fig. 5: Drill core KB 7 (masonry)
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Fig. 6: Drill core KB 7 (rock) |
Fig. 7: Flat Jack Test 1 (horizontal) carried out in masonryYoung'smoduli for crack widths of t=0 and t=0,2 m
The data gained from the extensive monitoring programme ( Fig. 2) were interpretedby
means of three-dimensional finite element analyses using the three-dimensional
FE-mesh shown in figure 8. Seepage flow analyses as well asstability analyses
were carried out for the load cases dead weight of the dam impounding and temperaturechanges.
Fig. 8: FE-mesh, view from downstream
By means of a comparison of monitoring and analyses results and a variation ofcharacteristic
parameters for the masonry and the foundation rock, the finite element model was
calibrated. Then, the calibrated model and realistic characteristic parameters
were used to carry out the final stability analysis.
As an example for the pore water pressure measurements, figure 9 shows the monitoring
results from measuring cross section VI for two different storagelevels. These
results illustrate the fast potential reduction from the water-side tothe axisof
the inspection galleries, which coincides with the axis of the fan of drainageholes,
which was carried out in the dam. Moreover, it becomes clear that the foundationwater
pressure on the downstream side of the lower inspection gallery is determined
by the storage level downstream of the dam.
Carrying out the seepage flow analysis for the calibration of the FE-model, the
permeabilitiesof dam and rock have been varied until a good correspondence between
measuring and analysis results was obtained. The result of this parameter variation
is shown in figure 10 in form of potential lines and assumed permeabilities. A
permeability coefficient of kf = 1 × 10-6
m/s was determined for the dam. For the underlying rock a graduation of permeability
was defined.Up toa depth of 10 m below foundation level, a kf-value of 8 ×10-6
m/s was determined, for the rock in greater depth the kf-value wasset to 3 ×
10-7 m/s.
Fig. 9: Pore Water Pressure Measurements MQ VI
Fig. 10: Seepage Flow Analysis Equipotential Lines
After calibration of the model for seepage flow analysis and calculation of the
resultants of the water pressure, the finite element model for the stability analyses
was calibrated. For this, the displacement measurements (measurements of the displacements
along the crest, pendulum and invetted pendulum as well as extensometers and inclinometers)
were interpreted and, thereby, the dependance of the displacements on storage
leveland temperature changes was determined. Figure 11 shows the displacements
derived frommeasuring results for a rise of the storage level from 295 m above
sea levelto315 m above sea level and for a temperature increase of 10 °C (mean
value of air and masonry temperatures). The rise of the storage level leads to
a displacement of the dam's crest of 7 to 8 mm towards the downstream side. The
rise in the dam's temperature leads to a displacement of 5 to 7 mm towards the
upstream side.
The stability analyses were based on the characteristic values shown in figure12.
The Young's moduli of masonry and foundation rock were varied assuming, analogously
to the seepage flow analysis, two rock zones with different compressibilities.The
best coincidence with monitoring results was obtained assuming a Young's modulus
of 10.000 MPa for the masonry, of 4.000 MPa for the rock down to a depth of 10m
below foundation level and of 10.000 MPa for the rock in greater depth. Figures13
and 14 show the displacements of the damin the valley section and on the crest
computed for this case. The computed displacements of the dam's crest of 6 mm
approximately correspond with the measured values of 7 to 8 mm ( Fig. 11).
Fig. 11: Displacements of the Dam in Valley Midths caused
by a Raise of Storage Level from 295 mNN to 315 mNN and by a temperature increase
of 10° C
Fig. 12: List of characteristic parameters for the stabilityanalyses
Figure 15 shows the principal normal stresses in the dam for the design storagelevel,
which is treated at 322,5 m above sea level.
Fig. 13: Stability Analysis: Rise of Storage Level from
295 mNN to 315 mNN,
Displacements caused by Rise of Storage Level
Fig. 14: Stability Analysis: Rise of Storage Level from295 mNN to 315 mNN,
Displacements caused by Rise of Storage Level, Top View on theDam's Crest
Fig. 15: Stability Analysis: Rise of Storage Level up to322,5 mNN,
Principal Normal Stresses in Valley Midth
In order to enable the numerical determination of the deformations of the dam
causedby temperature changes, which amount to more than 20 °C throughout the year
( Fig.16), the distribution of temperature in the dam is to be determined. For
this, a total of 29 temperature gages were installed, equally distributed over
the dam's section, in a measuring location in the middle of the valley ( Fig.
17). Then, the temperature distribution in the dam was deduced from the monitoring
results. Figures 17 and 18 show the distribution of temperature in the dam during
winter (lowest temperatures) and summer (highest temperatures). These temperature
distributions were inserted into the finite element model and, thus, the deformations
and stresses for the load case temperature were determined. Figures 19 and 20
illustrate the displacements resulting from the rise intemperature from winter
to summer. The largest calculated displacement at the dam's crest of 6,5 mm coincides
very well with the measured displacements of 5to 7 mm ( Fig. 11).
Fig. 16: Storage Level and Masonry's Temperature in the
Area of theDam's crest
Fig. 17: Temperature Distribution
Fig. 18: Temperature Distributionin the Dam during Winterin the Dam during summer
The extensive monitoring equipment installed in the Urft dam enables a reliable
supervisionof the structure as well as the calibration of the three-dimensiona
lnumerical model used to proof the stability of the dam. The stability analysis
was carriedout taking under consideration the loads resulting from dead weight,
impounding and temperature changes. Moreover, analyses for the load case earthquake
were carried out. Considering the threedimensional load-carrying action of the
dam and using realistic characteristic values which had been confirmed by measuring
results, it could be shown with each analysis that there quired stability of the
dam is given. The realistic verification of the dam's loadcarrying action and
the good condition of the masonry enabled an economy priced rehabilitation of
the dam.
Fig. 19: Stability Analysis: Displacements in Valley Midth
caused by Temperature Rise from Winter to Summer
Fig. 20: Stability Analysis: Displacements at the Dam's
crest caused by Temperature Rise from Winter to Summer
[1] DNK/DVWK: Talsperren in der Bundesrepublik Deutschland. Systemdruck GmbH,
Berlin 1987.
[2] Wittke, W.: Rock Mechanics, Theory and Applications with Case Histories,
Springer-Verlag, Berlin 1990.
[3] -WBI: Urfttalsperre, Zwischenbericht zu den Ergebnissen der Versuche und
Messungen im Zeitraum September 1992 bis Oktober 1997, unveröffentliches
Gutachten, Aachen, April 1998.
[4] WBI: Urfttalsperre, Standsicherheitsnachweis auf der Grundlage des Zwischenberichts
vom April 1998, unveröffentlichtes Gutachten, Aachen, Mai 1998.
The 58 m high masonry dam of the Urft reservoir, which has a storage volume
of approximately 45 Mio. m³, was built around the turn of the century. Examining
the dam's stability under consideration of today's standards, the stability
was preliminary proven by means of a three-dimensional finite element analysis.
In order to enable a concluding and reliable judgement of the dam's stability,
inspection galleries were driven in the dam. Further monitoring devices were
installed in various cross sections of the dam, and tests for determining mechanical
and hydraulical parameters of the dam and the rock were carried out. Moreover,
the existing drainage system was renewed and connected to the above mentioned
galleries.
Based upon the test results and the comparison of monitoring and analysis results,
a finite element model was elaborated, which enabled a realistic simulation
of the load-carrying action of the dam and the underlying rock. Using this finite
element model, the stability of the dam could be proven.
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