STRENGHTENING THE SPULLERSEE DAMS Lombardi

8/10/2019 STRENGHTENING THE SPULLERSEE DAMS Lombardi

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Our Ref.: 102.2-R-155 Minusio, March 2004

Article for The International Journal on

Hydropower and Dams

STRENGHTENING THE SPULLERSEE DAMS

R. Bremen and F. AmbergLombardi Engineering Ltd, Switzerland

G. Lehmannsterreichische Bundesbahnen (BB), Austria


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STRENGHTENING THE SPULLERSEE DAMS - Article for Hydropower and Dams

Lombardi Ltd. - I - March 2004102.2-R-155 Bre/AF

TABLE OF CONTENTS

page

1. SUMMARY 1

2. INTRODUCTION 1

3. THE EXISTING DAMS 3

4. MAIN PROJECT FEATURES 4

5. STABILITY ANALYSES 7

5.1 Generalities 7

5.2 Loading conditions and evaluation of earth pressure 7

5.3 Dynamic analysis of gravity dams 9

6. CONSTRUCTION WORKS 11


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Lombardi Ltd. - 1 - March 2004102.2-R-155 Bre/AF

1. SUMMARY

The strengthening works of the Spullersee dams in Austria started in 2002 after a

relatively extensive design phase. The finally adopted solution involving the

placement of a rockfill shoulder on the downstream faces of the gravity dams is

presently under construction. The rather uncommon project features and some

particular analyses are presented.

2. INTRODUCTION

After World War I, and in order to meet the growing electricity demand, the Aus-

trian Railways (sterreichische Bundesbahnen, BB) decided the construction of

the Spullersee powerplant located in the Vorarlberg region. Two gravity structures

built between 1922 and 1926 formed the original reservoir with a N. W. L. at el.

1825.00 m a.s.l.

Figure 1: General layout of the Spullersee scheme.

In 1965 both dams have been heightened to increase the available storage capacity

to 15.7 mio. m3. The heightening project included the pouring of 3.3 m high con-

crete blocks on the crests of the dams and the installation of vertical pre-stressedanchors associated with a 4.6 m raise of the N. W. L. The BBRV-type anchors had


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been grouted on the entire length, a common practice in this period, in order that

no reliable control of their efficiency and of the corrosion protection are presently

possible.

The lack of reliable control methods combined with some signs of corrosion does

no longer allow considering the contribution of the pre-stressed anchors to the

dams stability. Remedial measures had thus to be developed to provide proper

stability conditions of the dams under static and dynamic loads.

Various alternatives and proposals have been developed in the last years for the

rehabilitation of both gravity dams. However, for various reasons none of these

projects has come to the construction phase.

The finally proposed solution, presently under construction, consists to replace

the stability contribution of the 40 years old anchors with a rockfill shoulder

placed on the downstream faces of the dams. This relatively uncommon solution

was preferred to other alternatives mainly for the positive effect on the environ-

ment and the rather low costs.

Figure 2: View of the Spullersee reservoir with the north dam in front.


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3. THE EXISTING DAMS

Both gravity dams have similar cross sections as shown in Figure 3 with a maximum

height of 38.4m for the south dam and 27.6m for the north one. With nearly verti-

cal faces for the upper 8 m dam section, the ratio between the dam width and the

height of the water column is below 0.70 immediately above the downstream berm

in order that the dams stability can only be achieved with the contribution of the

prestressed anchors.

Figure 3: Typical cross section of the south Spullersee dam after the heightening in1965.

The concrete volume is 68600 m3 for the south dam and 27400 m 3 for the north

one. The overall properties of the prestressed anchors are summarized in Table 1.

After the anchor tensioning, the entire free length was grouted with cement mor-

tar in order that no load tests can be carried out. Some corrosion damages on the

anchors could be identified within an extensive investigation program carried out

in 1986 although a quantitative prediction of the corrosion evolution could not be

achieved.


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Properties of Spullersee anchors Unit South dam North dam

Anchor-type [-] BBRV BBRV

Anchor force per dam meter [t/m] 40 15

Working load [t] 132 63

Average anchor distance [m] 3.30 4.40

Total anchor length (max.) [m] 47.4 32.2

Anchored length [m] 2.80 2.80

Number of anchors [-] 75 43

Number of strands (7.0 mm diam.) [-] 35 17

Total strand section [mm2] 1'348 655

Section of borehole [mm] 101 76Ultimate tensile stress (U.T.S.) [MPa] 1740 1740

Working tensile stress [MPa] 980 980

Table 1: Main features of the Spullersee anchors installed in 1965

The lack of reliable control methods of the anchors in terms of working load and

corrosion resulted finally to consider remedial measures in order to provide satis-

factory stability conditions of the dams without taking into consideration the con-

tribution of the existing anchors.

Various strengthening alternatives have been investigated including the construc-

tion of new higher dams, the replacement of the existing anchors with new post-

tensioned anchors or the pouring of a 3-4 m thick concrete slab on the downstream

faces to increase the weight of the dams.

However due to cost, durability, seismic behaviour or environmental reasons, the

previous alternatives have been successively abandoned and a new strengthening

concept had to be developed. The finally adopted concept consists basically in theconstruction of a downstream rockfill shoulder using the locally available materi-

als as described hereafter.

4. MAIN PROJECT FEATURES

The main project requirements to be taken into consideration for the strengthen-ing project are:


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long term and maintenance free strenghtening of the dams,

no interference of the works with the normal reservoir operation, and

limitation as much as possible of material transports and construction nuisances

since the dam is located in a touristic relevant and environmentally sensitive

area.

Following the analysis of various alternatives, the finally selected concept is

shown in Figure 4. The stabilizing forces of the prestressed anchors are replaced

by the weight of the rockfill shoulder. The purpose of the rockfill is thus only to

increase the weight of the gravity dams without any other stability contribution or

seepage control. Since only the weight contribution is relevant, relatively poor

quality fill materials as locally available can be used for the embankment. Only a

sufficienly high permeability of the embankment material is required in order that

rainfall or melting snow will not built-up any uplift at the contact surface between

the gravity dam and the embankment.

Figure 4: Typical cross section of the south Spullersee dam after strengtheningworks.

The embankment will result in a significant decrease of the temperature variations

in the dam concrete with favourable effects on ageing. Furthermore the crest of

the embankment will be enlarged and equipped with a new crest road to be used

by local traffic.

In addition to laboratory tests carried out on material samples, a large scale in-

situ test was performed to determine the compaction procedure and the finally re-

sulting material properties. Some of the obtained results are listed in Table 2. The


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obtained material parameters were significantly better than the adopted design

parameters. To mention that a cohesion of 9 kN/m2was obtained with a Standard

Proctor ranging between 75 and 81%, whereas for a 95% Standard Proctor value a

cohesion of 87 kN/m2was measured.

Main project features Unit South dam North dam

Embankment volume [m3] 55000 30000

Crest el. of embankment [m a.s.l.] 1820.0 1827.0

Embankment slope (H:V) [-] 1.5:1

Specific weight s [kN/m3] 28.5

Gross weight (dry) d [kN/m3] 21.7

Friction angle of rockfill [] 34.1

Cohesion [kN/m2] 9-87

Permeability [cm/s] 7.8.10-24.9.10-4

Number of reinforcement bars [-] 96 32

Diameter of reinforcement bars [mm] 50 40

Max. length of reinforcement bars [m] 21 12

Minimum distance between bars [m] 1.65 4.4

Table 2: Main characteristics of the embankment material and reinforcement barsfor the strengthening of the Spullersee dams.

To provide a sufficient resistance of the dam crests under dynamic loads, passive

anchors are installed in the upper dam portion. These anchors do not provide any

contribution to the dam stability under static loads, but improve significantly the

dynamic behaviour of the dam crests.

Finally since the south dam was equipped with an auxiliary spillway located on the

right dam abutment at the centre of the dam crest, the capacity of the main

spillway had to be increased and overflow sill on the dam crest abandoned.


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5. STABILITY ANALYSES

5.1 Generalities

The proposed strengthening concept has to offer adequate stability conditions of

the dams without considering the contribution of the pre-stressed anchors. The

stability analyses were performed for the concrete dams and for the rockfill em-

bankments. Different geometrical and computational models were considered tak-

ing into account various dam sections and embankments profiles. Only the general

aspects of the computational models used and some typical results will be dis-

cussed hereafter.

For the gravity dams a general safety analysis was carried out. In addition a dy-

namic seismic analysis was performed in order to evaluate the structural safety

under extreme seismic loads. The stability of the embankments was finally ana-

lysed with the standard Bishop method.

5.2 Loading conditions and evaluation of earth pressure

In the stability analyses normal and exceptional loading cases have been consid-

ered. The relevant loading conditions and parameters taken into consideration are

the following:

unit weight of concrete : 24 kN/m3,

uplift : 85% and triangular distribution

normal water level : 1829.60 m a.s.l.,

maximum water level : 1830.40 m a.s.l.,

ground peak acceleration for OBE : 0.6 m/s2

ground peak acceleration for MCE : 1.2 m/s2

ice load : 60 kN/m

earth pressure of by the downstream rockfill embankment.

The ground peak accelerations for the OBE (Operating Basis Earthquake) and the

MCE (Maximum Credible Eearthquake) were defined according to the Austrian

earthquake guidelines. The vertical acceleration was assumed equal to 2/3 of the

horizontal one.


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An essential feature of the strengthening project is undoubtedly the earth pres-

sure induced by the rockfill on the downstream faces of the gravity dams. Since

the gravity dams are significantly more rigid than the embankment, the fill pres-

sure is mainly due to gravity and can be considered independent from the geo-

technical properties of the rockfill.

The earth pressure on the gravity dam was determined using the Culmann [1]

method. This method is based on the assumption that the earth pressure results

from a stability analysis, similarly to the standard analytical Rankine method used

for the evaluation of the active and passive earth pressures. The graphical method

of Culmann can be applied to very complex geometries.

For different rigid bodies a stability analysis is performed assuming a limit equilib-

rium on the slip surface, i.e. the reaction force R acts under the full friction angle

as shown in Figure 5. The active earth pressure corresponds to the maximum pres-

sure by assuming limit equilibrium with positive shear forces along the sliding sur-

face (the rigid body tends to move towards the concrete dam). Similarly the pas-

sive pressure corresponds to a minimum by assuming negative shear forces (the

rigid body is pushed downstream). The critical sliding surface, thus the value of

the earth pressure, is identified by an iterative process.

Figure 5: Earth pressures resulting from the equilibrium of a rigid body delimitedby an iteratively defined slip surface.

For the evaluation of the earth pressure a unit fill weight of 20 kN/m 3was consid-

ered.


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Within the dynamic analysis of the dams the earth pressure was also determined

with a finite elements analysis. This calculation showed earth pressure values 5 to

15% higher then the simplified Culmann method.

The FEM analysis was also used to assess the effect of different friction angles be-

tween the dam face and the rockfill. The effect on the dam stability was however

very limited in order that this effect can be neglected.

5.3 Dynamic analysis of gravity dams

In addition to the conventional stability analyses of the dams under static loads,

the dynamic behaviour was investigated. The stability of the dams under seismic

loads was initially analysed, using a simplified pseudo-static method. However due

to the type and dimensions of the dams a response spectra dynamic analysis was

required. The purpose of the dynamic analysis was the integrity assessment of the

concrete dams, while the evaluation of the seismic stability of the rockfill was

carried out using pseudo static approaches completed with a displacements analy-

sis by Newmark [1].

For the dynamic analysis, the fill pressure was considered as added masses. This

computational artifice was necessary for a correct determination of the higher

modal frequencies of the concrete dam.

The selected approach is rather conservative since both the high damping effect

and the plastic behaviour of the rockfill, absorbing part of the dynamic energy are

neglected.

The reservoir was modelised as added mass distributed according to the Wester-

gaard law.

The modal frequencies have been determined until 30 Hz and at least 95% of the

total mass. The modes were superposed according to the SRSS method (square root

of sum of squares).

The response spectra of the rock foundation and a 5% damping factor was defined

according to the Austrian earthquake guidelines. For shorter period than 0.03 s the

response acceleration corresponds to the ground peak acceleration of 1.72 m/s

2

,


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while between 0.1 and 0.45 s the maximum response acceleration of 4.7 m/s 2 are

reached.

Some results are summarised in Table 3and illustrated in Figure 6 for the highersouth dam.

Participating mass

Mode F [Hz] T [s] Horizontal Vertical

1 6.1 0.165 57.3 % 0.4 %

2 12.5 0.080 30.4 % 0.1 %

3 17.5 0.057 1.2 % 95.2 %

4 22.4 0.045 9.4 % 1.4 %Table 2: South dam Modal analysis results with frequency F, period T and per-

centage of participating mass.

Figure 6: South dam with rockfill at elevation 1820 m a.s.l. Modal analysis, 1stand 2nd mode shapes.

The first two modes produce mainly horizontal displacements, in particular at the

dam crest, while the third mode is clearly a vertical excitation. Moreover the first

mode is in the frequency domain of the maximum response acceleration. This re-

sults in a significant increase of the acceleration at the dam crest with a value of

about 14 m/s2, i.e. 10 times the ground peak acceleration.


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The combination of the MCE with the static loads shows maximal tensile stresses

up to 2 MPa at the south dam. This maximum value is reached at the upstream

dam face were reinforcement bars will be installed.

6. CONSTRUCTION WORKS

The works started in Spring 2002 following the project approval by the Regional

and National Authorities and the award of the construction contract to a JV of

Austrian companies (Teerag-Asdag, Porr, Prantl).

The works carried out in 2002 included mainly the modification of the spillway

structures and the preparation of the foundation for the rockfill at the south dam.

Following the interruption of the works during winter, the construction program

continued with the new access and drainage adits as well as with the placement of

the fill material at the south dam. Some difficulties with the construction of the

new access and drainage galleries resulted in a delay of the construction works com-

pared to the program schedules. Figure 7 shows the south dam during Summer 2003

with the completed supports for the drainage gallery at the downstream dam toe.

Figure 7: Construction works on the south Spullersee dam in Summer 2003.

The construction of the embankment will resume in spring 2004 with a completionof the works planned in 2005.


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Lombardi Ltd. - 12 - March 2004102 2 R 155 Bre/AF

Roger Bremen graduated in civil engineering from the SwissFederal Institute of Technology of Lausanne (Switzerland) in 1987.After receiving a PhD in hydraulics from the same institute he

has been involved in the design, construction and rehabilita-tion of numerous dams and hydro power projects in Switzer-land and abroad. He is currently head of the Hydraulic Struc-ture Department at Lombardi Engineering Ltd.

Francesco Amberggraduated in 1995 in civil engineering fromthe Swiss Federal Institute of Technology of Zurich (Switzerland).He has been working since 1995 at Lombardi Engineering Ltd.,mainly in numerical analyses of concrete dams in Switzerlandand abroad as well as for problems related to rock mechanics.

He was involved in the rehabilitation works from the prelimi-nary design to the stability and dynamic analyses of the Spull-ersee Dams in Austria.

Georg Lehmann graduated in civil engineering from the Tech-nical University of Vienna (Austria) in 1981. He has been work-ing at the design, construction and supervision of tunnels andhydro power plants in Austria. He is also involved in rehabilita-tion project and surveillance of large dams of the AustrianFederal Railways.

Lombardi Engineering Ltd., via R. Simen 19, 6648 Minusio-Locarno, Switzerland

sterreichische Bundesbahnen, Geschftsbereich Kraftwerke, Claudiastrasse 2A-

6020 Innsbruck, Austria

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STRENGHTENING THE SPULLERSEE DAMS Lombardi