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AMÉLIORATION DE LA STABILITÉ DE LA BARRAGE EDER PAR DES ANCRAGES PRÉCONTRAINTS PERMANENTS
Prof. Dr.-Ing. Dr.-Ing. E.h.
Prof. Dr.-Ing. W. Wittke Consulting Engineers for
TUNNELING and GEOTECHNICAL ENGINEERING (WBI)
Henricistrasse 50, D-52072 Aachen
Prof. Dipl.-Ing.
President of the district head quarter of the Federal administration for waterways and hydraulic structures
Am Waterlooplatz 5, D – 30169 Hannover Germany
The Eder dam was built between 1908 and 1914 as a curved gravity dam to
provide Water for the Mittelland canal (fig. 1). The dam is founded partly on
slate and partly on graywacke of the lower carboniferous. Graywacke and trass
cement were used for the masonry. The dam, which is 36 m wide at the base
and 6 m wide at the crest, is relatively thin. With its capacity of 202.4 Million
m³
the Eder reservoir is one of the biggest reservoirs in Germany.
Fig. 1: The Eder masonry dam
| Height: | 47 m |
| Base length: | 270 m |
| Base width: | 36 m |
| Crown length: | 400 m |
| Crown width: | 6 m |
As for many gravity dams built in Germany around the turn of the century, the
absence of pore water pressure in the dam foundation was assumed in the original
design. This assumption, however, cannot be upheld based on today’s knowledge
(fig. 2). Achieving stability was therefore only possible through either restricting
the storage level or using other measures to improve stability. Because the original
water level in the reservoir was to be maintained, extensive measures became necessary.
The works started in October 1991 and were finished on May 6, 1994.
Fig. 2: Reasons for remedial measures
After careful studies of several design alternatives, a solution involving
104
permanent rock anchors was given preference, whereby the dam would be
anchored into the bedrock (fig. 3). To distribute the anchor forces, a load
distribution beam was needed on the dam crest. The chosen type of anchor was
the Stump/VSL prestressed anchor which carries a design load of 4575 kN. The
design load on each anchor in the area between the superstructures was
calculated as 4500 kN, with an average spacing of 2.25 m. The length of the
load transfer section of the anchors in the rock was chosen as 10 m.
To construct the load distribution beam at the top of the dam, the existing crest
had to be dismantled. At the same time the spillways on the dam were brought
into line with the relevant standard (DIN 19702). This standard requires the dam
to be able to cope with the 1000 year flood event, calculated in this case to
be
1100 m³/sec. The existing spillway had been designed for the 100 year event of
590 m³/sec.
Fig. 3: Cross-section of the dam
Instrumentation installed in this dam during construction in 1908 included
12
water pressure gauges and 12 temperature gauges. The horizontal movements
of two points on the crown were monitored geodetically. Further instrumentation
was added in later years, especially in 1984, when the following devices were
installed (fig. 4):
| 3 | 7-fold and 5-fold, resp., inclined extensometers, located below the reservoir with their heads installed on the downstream side of the dam and in the cross adits, resp. |
| 2 | plumb lines, located between the crown and the inspection gallery |
| 2 | inverted pendulums, located between the inspection gallery and the bedrock |
| 6 | sliding micrometer tubes in 3 cross-sections. |
| 3 | reservoir water temperature gauges |
| 36 | dam temperature gauges |
| 1 | air temperature gauge |
| 40 | pore water pressure gauges located in the masonry dam and the bedrock. |
Fig. 4: Dam plan with measuring devices in cross-sections
To enlarge on the calculations done by the Federal Institute for Water
Engineering (Bundesanstalt für Wasserbau, BAW) using beam theory [2], Prof.
Dr.-Ing. W. Wittke Geotechnical Engineering Consultants (WBI) carried out finite
element (FE) analyses. These served as an aid to the interpretation of the
measured water pressures and displacements in the dam in order to thus help
determine the parameters for dam and bedrock. In addition to this, FE-analyses
were also carried out as a verification of stability analyses performed by the
client [5]. These analyses will be explained in the following sections.
The computation of the stresses and strains in dam and bedrock were carried
out with the program system FEST03 described in detail in [8]. For the dam and
the surrounding rock an elastic-viscoplastic stress-strain relationship was
assumed. Anisotropy in the elastic and viscoplastic range can be taken into
account. For the intact rock as well as for the discontinuities of the rock, the
Mohr-Coulomb failure criterion is applied, limited by a vertical line in the tension
region of the t-s- graph ("tension cut-off").
The latter assumption is also
relevant for the horizontal joints in the dam. The influence of the joints is
taken
into account by reducing the shear and tensile strengths parallel resp.
orthogonal to the discontinuity in the finite elements in the respective areas.
The most commonly used type of element is the three-dimensional
isoparametric element, the number of nodal points of which can be varied
between 8 and 21. Rock is usually simulated using an element with eight nodes,
while curved structures, such as arched dams, are modelled using more nodes.
For the modelling of arched dams the application of twenty-node elements has
proved necessary.
The piezometric levels resulting from seepage flow through dam and bedrock
were calculated with the program system HYD03. The basis of these
calculations is Darcy's Law. Anisotropic permeabilities resulting from the
orientation of the discontinuities can be taken into account. Variations in
permeability within the computation section as well as the topography and the
geometry of the construction project can be modelled as well.
The calculations result in the piezometric head distribution within dam and
bedrock. Furthermore uplift and seepage forces for use in stress-strainanalyses
(see above) are derived from these computations.
4.2.1 Area of Calculation, Element Mesh and Calculation Steps
As a verification of the existing stability analyses for the dam in the area
between the gate superstructures [3], two-dimensional FE-analyses were carried
out. Since no three-dimensional effects are present due to the considerable
length of the dam and the large radius of curvature, the calculations could be
carried out in one plane.
The 1m thick slab used for the calculations includes a 300 m long and 159 m
high section of the bedrock as well as the 47 m high dam (fig. 5). The FE-mesh
described here has 2757 isoparametric elements. As mentioned earlier it was
used for the hydraulic as well as the mechanical calculations.
The analysis for the load cases dead weight, anchoring and filling of the
reservoir was carried out in three steps (fig. 6). In the first step the stresses
and
strains resulting from the dead weight of the bedrock were calculated, taking
uplift into account. The uplift forces necessary to account for the ground water
conditions were determined in the hydraulic calculation. The second step
simulated the construction of the dam and the installation of the pre-stressed
anchors. The third step then considered the uplift and seepage forces resulting
from seepage through dam and bedrock for varying storage levels.
In the section of the element mesh shown in fig. 5 the fine meshed modelling of
the dam on the up and down stream sides can be seen. The purpose of this is
to calculate as accurately as possible the stresses in these areas with large
stress gradients. The inspection gallery and the elements used to simulate the
low permeability grout curtain are also shown. The effect of the prestressed
anchors was simulated by means of two equal and opposite forces with the
same line of action, one of which acting on the rock, the other in the upper
inspection gallery (fig. 5). This inspection gallery was not included in the FE-
mesh as the aim was to determine the overall stability of the dam and not the
local stress distribution in the vicinity of the dam crest.
Fig. 5: Detail of the 2D finite element mesh
Fig. 6: Computational steps
4.2.2 Parameters
In the area of the Eder masonry dam the rock in the valley and on the valley
slopes consists of carboniferous slate and quartzite. The rock strata on the righthand
side of the valley consist predominantly of slate, while quartzite prevails on
the left-hand side. The bedding dips at angles between 65 and 75 degrees and
strikes approximately from the upstream to the downstream side. Apart from the
beddig parallel discontinuities, two further joint sets exist in the rock.
The permeability of the ground can be considered approximately isotropic in
relation to seepage flow under and around the dam [2]. According to the results
from core drillings and Lugeon tests carried out during the rehabilitation the
bedrock can be divided into three zones (I-III) with respect to its permeability
(figs. 5 and 7).
Fig. 7: Parameters for seepage flow and stability analyses
The dam is also considered to be isotropic and homogenous with respect to its
permeability. The permeability assumed for the grouted area is within the range
of permeabilities usually achieved with standard cement grouting. The grout
curtain connects into zone III of the rock foundation. With respect to their elastic
behaviour, the quartzite layers can be considered isotropic, whereas for the
slate this is only an approximate assumption. The value of Young's modulus for
the rock in fig. 7 results from back analysis and an interpretation of measured
displacements for different water levels (fig. 8). Isotropic behaviour was also
assumed for the dam in the elastic range.
Fig. 8: Horizontal displacements due to raising the storage level from 217 to 241 m above MSL
For the rock discontinuities parallel to the dam, i.e. at right angles to the
plane of
fig. 5, and for the horizontal joints in the masonry dam it was assumed that the
tensile strength perpendicular to these planes is zero (fig. 7). Using these
assumptions, the weakening effects of the joints in the rock as well as the
horizontal planes of weakness in the dam were taken into account.
4.2.3 Results
In the first case examined the reservoir is empty and the only loading is
the
dead weight of dam and rock and the anchor forces. Fig. 9 shows the vertical
stresses in horizontal sections. Assuming that the tensile strength at right angles
to the sections is zero, cracks occur on the downstream side of the dam due to
the anchor forces. These cracks reach approximately 3,5 m into the masonry
dam and are unsignificant with respect to dam stability. In the same way, the
compression stresses of 1.6 MN/m2 on the upstream side of the dam are easily
sustained by the masonry.
For the filling of the reservoir to the maximum storage level, it has to be absolutely
ensured that the masonry dam is stable and the sealing remains functional. The
equi-potential lines resulting from the seepage flow calculation are shown in
fig. 10. The draining effect of an inspection gallery located at the foundation
level on the upstream side and the effectiveness of the grout curtain were taken
into account. The analysis lead to the result that the water pressure on the foundation
on the downstream side of the grout curtain amounts 40% of the hydrostatic pressure
(fig. 11). The seepage forces from this analysis together with the loading due
to dead weight and prestressed anchors produced the stress distribution shown
in fig. 12.
Fig. 11: Seepage analysis: Distribution of water pressure in the dam foundation
area
As mentioned earlier, the masonry cannot sustain vertical tension. Horizontal
cracks therefore develop at the upstream side of the masonry dam base.
However the area where these horizontal cracks occur is relatively small. Since
the cracks do not reach the grout curtain, its sealing function is not affected,
and
restrictions on the use of the dam do not have to be applied. The maximum
compressive stress of 1.6 MN/m² can easily be sustained by the masonry.
Fig. 12: Principal normal stresses: dead weight, anchor forces, storage level at el. 246,85 m
On the dam crest above the toes of the two slopes there are two 37 m long gate
superstructures. In the vicinity of these superstructures and the adjacent slopes
no prestressed anchors are located. It therefore had to be determined, wether
the anchors in the central part of the dam are sufficient to also reduce the
tensile stresses in the slope areas. To investigate this, three-dimensional FEanalyses
were carried out using the mesh shown in fig. 13. It consists of 3673
isoparametric elements. The length of the analysis region is 280 m, the width
232 m and the height 147 m. The masonry dam is modelled in the area of the
slopes, the superstructures and over a length of 50 m in the central part. This
section of 50 m length was chosen long enough to prevent effects from the
slope from having an influence on the central edge of this part. The small
curvature of the dam in plan was neglected in creating the FE-mesh.
The three-dimensional calculation was also carried out in three steps (fig. 6).
For
the calculation of the seepage flow under and around the dam, the maximum
storage level of 246,85 m (HQ1000) was assumed
because this loading case had
turned out to be the worst case in the two-dimensional analyses. The
assumptions about the effectiveness of the grout curtain correspond to those
from the two-dimensional analysis. The draining effect of the horizontal gallery
was neglected in the calculations.
Fig. 13: 3D finite element mesh
The three-dimensional load bearing behaviour is easiest to recognize in the
results of the second analysis including only the dead weight of the dam and the
anchor forces. Fig. 14 shows the size and orientation of the principal stresses
within slice 11 (fig. 13), located close to the upstream side of the dam.
Comparably large principal stresses occur in the lower slope area resulting from
the dissipation of the loading from the dead weight of the dam and the anchor
forces. Due to these increased compressive stresses, there are no tensile
stresses reaching the grout curtain even at maximum storage level.
Fig. 14: Principal normal stresses: dead weight and anchor forces,
longitudinal section on the upstream side of the wall
To construct the load bearing beam; a section of the original dam crest was
separated from the body of the dam by gap blasting. The parts that were to be
removed were prepared for demolition by loosening blasting. The downstream
facade remained in its original form. In order to retain the old sand stone arcs
above the spillways. In the next step, the load bearing beam and the upper
inspection gallery were constructed. After completion of the overflow crest the
piers for the road were concreted. The sand stone parapet was, as far as
possible, reconstructed with the original stones. Finally the front on the
upstream side of the crest was faced with quarry-stones.
From December 1991 to February 1992 extensive preliminary field tests were
carried out in order to determine the suitability of the anchors and to investigate
the transfer of prestressing forces into the bedrock.
In the stilling basin of the masonry dam three anchors were placed on the right
side of the valley in an alternating sequence of graywacke and claystone, while
three further anchors were placed into graywacke strata on the left side of the
valley.
The anchors were subjected to a suitability test, including pull-out-tests with
a
maximum loading of 8000 kN. The outcome confirmed the suitability of the
anchors for the envisaged construction measures.
The preparatory drillings for the anchors to an alternating depth of 68 and 73
m
were carried out from the newly constructed crest as wire line core drillings
(ø
146 mm) using two double tube core barrel drilling machines (fig. 15).
As the thickness of the masonry dam amounts to only about 2,5 m between the existing
face liner wall and the lower inspection gallery, the client demanded the borehole
deviations less than 1 % of the borehole length at the level of the lower inspection
gallery. This means that after 40 drilling meters the actual drilling center line
was to deviate no more than 40 cm from the envisaged center line. Typical for
core drilling boreholes deviations of 2 - 3 % are expected. The contractor achieved
an average borehole deviation of 0,36 % at the level of the lower inspection gallery
and of 0,45 % at the deepest point of the borehole. After drilling, the boreholes
were cement grouted. In total, 100 t of cement were injected into the masonry
dam and the rock during 2000 h of grouting works. In the next step, the injected
boreholes were re-drilled with a roller bit and subsequently expanded from 146
mm to 273 mm by means of a down-the-hole hammer. The hammer head was equipped
with a pilot spike in order to ensure the adherence to the original drilling center
line. After the expansion, the treatment of the boreholes was examined by means
of Lugeon tests. An absorption capacity of less than one liter per minute under
a pressure of 100 kPa for all of the bonding length of 10 m was used as treatment
quality. Only 8 out of 104 borings in which this was not achieved had to be injected
again.
Fig. 15: Borehole procedure as a preparation for the prestressed anchors
The most important elements of the reconditioning works are 104 permanent
rock anchors as described before. With this anchor type the anchor forces are
transmitted from the anchor head via a load bearing beam made of reinforced
concrete into the masonry. At the anchor foot, the forces are transmitted by the
grouted bonding section of the anchor into the bedrock. The load transfer from
the anchor head to the anchor foot is effected by 34 wire strands, ST (steel
quality) 1570/1770 with a 150 mm2 nominal cross-sectional area each
(fig. 16).
In the free moving anchor section each individual strand is enclosed in a mantle
consisting of anti-corrosive grease and a plastic coating (PE). The steel strands
are inserted in a 11.4 mm strong, smooth tube (ø 200 mm) in the free moving
anchor section and in a ribbed PE-tube in the bonding section. The transition
between the smooth PE-tube and the ribbed PE-tube is supported on the inside
by a steel socket and sealed by a shrunken-on plastic tube from the outside.
Fig. 16: Anchor cross-sections
As a length of 70 and 75 m and a weight of about 4 t prevented the anchors
from being transported by lorry or train, they were assembled on site (fig. 17).
Fig. 17: Anchor manufacturing scheme
From the production stage to the installation of the anchors the bonding section
was protected against mechanical damages by an additional foil coating. The
anchors were moved to the drilling site on the crest by means of rollers and
subsequently placed in the borehole by means of a mobile crane and an
installation frame that had been developed specifically for this purpose (fig.
18).
The tensioning of the anchors was carried out from the upper inspection gallery
by means of a hydraulic jack.
Fig. 18: Anchor transport and installation
The anchoring of the dam with prestressed anchors in the bedrock results in
the
masonry dam stability being in line with current requirements for a future design
life of 80-100 years. Two- and three-dimensional stress-strain and seepage flow
FE-calculations confirmed that applied anchor forces of 2000 kN/m reduce
tensile stresses in the dam to tolerable values. The reconstruction project
proved that high-tension anchors can be produced under site conditions. This
presupposes an expert and diligent execution of all steps as well as constant
supervision up to the very end of construction.
References:
[1] Aberle, B.: Sanierung der Ederstaumauer. in: Festschrift anläßl.
des 60. Geburtstages von Univ.-Prof. Dr.-Ing. W. Wittke. Veröffentlichungen
des Institutes für Grundbau, Bodenmechanik, Felsmechanik und
Verkehrswasserbau der RWTH Aachen, Heft 26, 1994.
[2] Bundesanstalt für Wasserbau: Eder Talsperre - Gutachten über den
Zustand und die Standsicherheit der Edertalsperre. BAW-Nr. 32.4490,
Februar 1998.
[3] Corinth, E.: Instandsetzung der Eder- und Diemelstaumauer.
Binnenschiffahrt, November 1993
[4] Edertalsperre 1994, Sonderdruck aus Anlaß der Wiederherstellung der
Edertalsperre, Wasser- und Schiffahrtsverwaltung des Bundes, 6. Mai
1994
[5] Edertalsperre, Prüfung der Berechnungen zur Ermittlung der
erforderlichen Ankerkräfte. Bericht Prof. Dr.-Ing. W. Wittke Beratende
Ingenieure für Grundbau und Felsbau GmbH (WBI), September 1991
[6] Feddersen, I., Mühring, W., Reimer, W.: Instandsetzung der Staumauer
der Edertalsperre. Wasserwirtschaft, Nr. 2, 1992.
[7] Grote, H.: Die Instandsetzung der Ederstaumauer. Die Weser, Nr. 1,
1993
[8] Wittke, W.: Rockmechanics, Theory and Applications with Case
Histories; Springer Verlag, 1990
[9] Wolff, R., Wesseler, H.: Überwachung von schweren Felsankern mit
Lichtwellensensoren. Felsbau, Nr. 5, 1993
[10] DIN 4125, Kurzzeitanker und Daueranker, November 1990
[11] ISO 9000, Qualitätsmanagements- und Qualitätssicherungsnormen,
Mai 1990
Summary
The 47 m high Eder masonry dam was built of graywacke quarry-stones and
lime trass mortar in 1908-1914. Some years ago the dam stability was
reinvestigated which the consequence that the storage level had to be lowered.
Prestressed permanent anchors, reaching from the top of the dam down to
approximately 30 metres into the bedrock, were installed in order to achieve
dam stability according to todays standards. The anchors, placed with 2,25 m
spacing, were prestressed with an anchorforce of 4500 kN each. The paper
describes the stability analyses and the installation of the prestressed anchors.
Resumé
Le barrage Eder (47 m de hauteur) est édifié comme mur en pierres
brutes de
carriére de grauwacke de 1908 á 1914. Une investigation du barrage
a montré
qu´on ne peut plus prouver la stabilité pour les charges supposées
actuelles et
au niveau actuel des consignes de sécurité. Afin d´élever
la stabilité, on a
installé des ancrages permanentes, qui descendent du couronnement jusqu´
à
30 m dans la fondation rocheuse. Les ancrages ont un éspacement de 2,25
m
et un effort de précontrainte de 4500 kN. Dans l´article, des calculs statiques
à
la méthode des éléments finis sont présentés
et le procédé de l´installation des
ancrages est décrit.
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