The dam authority of Saxony (LTV) is responsible

Transcription

1 Three innovative dam rehabilitation schemes in Saxony U. Müller, Landestalsperrenverwaltung des Freistaates Sachsen, Germany Three recent rehabilitation schemes in Saxony involved a number of innovative strategies. The almost 100 year-old Neunzehnhain II dam was refurbished by having an impermeable diaphragm wall attached, including an inspection gallery. At the Carlsfeld masonry dam, the upper part of the structure was sealed with bituminous concrete and precast elements in only nine weeks. The inspection gallery was concreted with an internal formwork of Styrodur. At Bautzen earthfill dam, m 2 of asphaltic concrete sealing was renewed in only one construction season, and a new system was used to coat the intake tower. Fig. 1. View and basic layout of the Neunzehnhain dam (after rehabilitation), where: 1 = entrance; 2 = seepage control measures; 3 = spillway;4 = foundation; 5 = bottom outlet gallery; 6 = diaphragm wall; 7 = joint; 8 = inspection gallery; 9 = valve chamber; 10 = intake shaft; 11 = intake works; 12 = installation shaft; 13 = crest building; and, 14 = stilling basin. The dam authority of Saxony (LTV) is responsible for planning, constructing, operating, monitoring and maintaining rivers, local dams, reservoirs and retention basins, including the associated works. The LTV facilities are mainly to provide flood protection, drinking and industrial water supply and also flow regulation. The LTV is thus responsible for 69 main dams and reservoirs with altogether 52 secondary dams, approximately 50 km of artificial canals, approximately 60 km of tunnels, 2973 km rivers of first order, 118 km of border waters, 652 km of flood protection dykes and 195 weirs. Three recent rehabilitation projects, which will be described here, involved designs based on some innovative ideas. 1. Neunzehnhain II dam The Neunzehnhain II dam was constructed near Lengefeld (Erzgebirge) between 1911 and It is mainly used for drinking water supply in conjunction with the downstream Neunzehnhain I dam (built between 1905 and 1908). The dam is now classified as a historical monument. The dam is a masonry structure with a curved axis. It has a maximum height of 38 m, and a crest length of 235 m. Flood flows are discharged by a free overflow crest spillway in the middle of the masonry dam. The spillway consists of nine bays and has a total width of 45 m. Its discharge capacity is 65 m 3 /s. Energy is dissipated in a stilling basin. The dam also has two bottom outlets with a nominal diameter of 600 mm and a total discharge capacity of 7.2 m 3 /s. The storage capacity of the reservoir is approximately m 3. The reservoir has two small secondary dams. The rehabilitation of this dam was necessary for the following reasons: Based on present regulations, the stability of the dam was not theoretically verifiable. The structure, especially the upstream sealing element, had deteriorated considerably after 90 years of operation. The dam equipment no longer conformed with generally recognized codes of practice. The Neunzehnhain II dam was therefore virtually reconstructed between 1996 and The main work, which was designed by Salveter GmbH and carried out by the contractor Hochtief AG, comprised: renewal of the dam crest; renewal of the upstream sealing element as a prefixed reinforced concrete facing; construction of an upstream inspection gallery; renewal of hydraulic conduits and equipment; renewal and expansion of monitoring devices; and, installation of a modern system for monitoring and control. General view of the Neunzehnhain II dam. 88 Hydropower & Dams Issue Four, 2001

2 Fig. 2. Cross-section of the Neunzehnhain masonry dam and foundation (original condition before rehabilitation), where: 1 = maximum water level; 2 = spillway; 3 = concrete apron; 4 = loam; 5 = concrete bed; 6 = masonry; and, 7 = stilling basin. The rehabilitation process, involving the provision of an upstream diaphragm wall of watertight reinforced concrete attached, as shown in Fig. 3, was chosen to increase the dam s stability and to ensure upstream sealing of the dam body. The new diaphragm wall extends 1 m deeper into the rock than the existing masonry. To protect the old dam body, the necessary excavations had to be carried out very carefully without excessive vibrations. An inspection gallery made of watertight reinforced concrete was built at the upstream toe of the dam. The diaphragm wall extends above the inspection gallery, up to the dam crest. It is 0.4 m thick, and is divided into blocks 8.5 m wide, like the inspection gallery. The block joints are sealed with expansion water bars of PVC-P-NBR (nitrile butadiene rubber). To connect the polygonal diaphragm wall to the curved dam body, a compensation and reinforcement layer was built (see Fig. 3). This layer increases from a thickness of 0.2 m at the dam crest to 1.85 m at the upstream toe. The compensation and reinforcement layer consists of plain concrete (with an elasticity modulus of masonry) and is connected to the dam body by anchors. Throughout the entire height of the dam, a separation layer has been attached to the reinforcement layer to enable vertical movement between the diaphragm wall and the dam body. This separation layer is made of elastomer bituminous welding strips, which are stuck together by hot bitumen, without any joints. Every 14 m 2, an anchor (40 mm in diameter) is installed 1.4 m deep into the masonry of the existing dam to prevent buckling of the diaphragm wall. So as to be able to check the effectiveness of the diaphragm wall, an 11.5 cm-thick drainage layer made of frost-resistant permeable bricks was provided between the diaphragm wall and the separation layer. This drainage layer drains into the new inspection gallery so that seepage can be monitored. Uplift pressure in the subsoil is released by 2 to 3 m- deep drainage holes. Because of the good foundation conditions, a grout curtain is not considered necessary so far. If required, a grout curtain could be installed at any time, working from the inspection gallery. The test impoundment has shown that the rehabilitation project is successful. 2. Carlsfeld dam Carlsfeld dam was built near Eibenstock (Erzgebirge) between 1926 and Its main purposes are drinking water supply and flood control. Carlsfeld is classified as a historical monument, and its location is at the highest elevation for a drinking water supply dam in Germany. Fig. 3. Cross-section of the Neunzehnhain dam after rehabilitation, where: 1 = diaphragm wall; 2 = surface drainage; 3 = bituminous separation layer; 4 = compensation and reinforcement layer; 5 = masonry; 6 = anchor; 7 = raw water intake; 8 = gate shaft; 9 = spillway; 10 = crest building; 11 = valve chamber; 12 = stilling basin; 13 = rock; 14 = subsoil drainage; and, 15 = inspection gallery. Fig. 4. Basic layout of the Carlsfeld dam. View of the Carlsfeld dam. Hydropower & Dams Issue Four,

3 The Carlsfeld dam with an upstream wall of precast elements. Fig. 5. Cross-section of the Carlsfeld dam with its foundation horizon (condition before rehabilitation), where: 1 = normal water level; 2 = masonry apron; 3 = cement plaster with a coat of black sealant; 4 = granite masonry; 5 = gutter, shingle concrete; 6 = clay Intze wedge; 7 = manifold, 300 mm dia.; 8 = ground level; and, 9 = elevation dates. Fig. 6. Cross-section of the Carlsfeld dam after rehabilitation, where: 1 = crest slab; 2 = apron anchor; 3 = bituminous concrete; 4 = gutter, shingle concrete; 5 = apron, precast concrete; 6 = prefixed beam; 7 = base beam; 8 = swivel arm intake; 9 = gate chamber; 10 = reinforced concrete diaphragm wall; 11 = adjustment layer; 12 = inspection gallery; 13 = base slab; 14 = compensation concrete; 15 = double grout curtain; 16 = dam axis; 17 = connection beam; 18 = manifold 300 mm dia.; and, 19 = granite with many joints. The Carlsfeld dam is a gravity masonry structure with a curved axis, like the Neunzehnhain II dam. It has a maximum height of 32 m and a crest length of 206 m. There is a free overflow spillway, with a discharge capacity of 12 m 3 /s, in the centre of the dam crest. The total width of the spillway, which consists of eight bays, is 33.2 m. Flood waters are discharged via a cascade into a stilling basin. The dam has two bottom outlets with a nominal diameter of 600 mm and a total capacity of approximately 4 m 3 /s. The gross capacity of the reservoir is approximately m 3. Rehabilitation of the dam was necessary for the following reasons: On the basis of present-day regulations, the stability of the structure was not theoretically verifiable. The construction, especially the upstream sealing element, had deteriorated considerably after more than 70 years of operation. The dam s equipment did not conform with generally recognized codes of practice. The dam was reconstructed between 1996 and The following main work was designed by the Engineering company Wasser- und Tiefbau mbh and carried out by Züblin GmbH: renewal of the dam crest; renewal of the upstream impermeable element (partly with bituminous concrete) including the provision of a reinforced concrete facing; installation of a grout curtain in the foundation of the dam; construction of a new upstream inspection gallery; construction of a new upstream valve chamber; renewal of pipework including construction of an extraction device with infinitely variable height adjustment; renewal and extension of the monitoring equipment for the structure; and, installation of modern control and monitoring equipment. To increase the stability and to seal the upstream facing, the rehabilitation arrangement as shown in Fig. 6 was chosen. Similar to the reconstruction project at Neunzehnhain, a diaphragm wall (of waterproof reinforced concrete) was erected at the lower part of the dam. A double row grout curtain was installed beneath the base slab. For the concreting of the inspection gallery, internal formwork made of polystyrene blocks was used. It was thus possible to shorten the construction process as stripping of the whole inspection gallery could take place immediately after concreting. The upper part of the new sealing wall consists of asphaltic concrete behind pre-fixed precast reinforcing concrete elements. Thus a considerable reduction in the construction time for this process was also possible. It was not necessary to strengthen the exposed masonry at the upstream face with additional concrete. 90 Hydropower & Dams Issue Four, 2001

4 The Bautzen dam during rehabilitation. The 6 m-long, 1 m-high and 0.25 m-thick precast elements were joined to the old dam. Two stainless steel anchors were used for each element. The precast elements remained at the dam as lost formwork. Subsequently the space between the old dam and the precast elements (0.4 to 0.8 m) was filled in with watertight asphaltic concrete. It was necessary for the asphaltic concrete to meet the current typical quality standards for asphaltic hydraulic engineering. The asphaltic concrete was manually compacted using electric tampers. Attention focused especially on adequate and even compaction, the maintenance of appropriate temperatures and the coating of the anchors with a binder, as specified. To design the correct dimensions for the precast elements, which were to be used first as formwork and then as a mechanical protection wall, a full-scale insitu test was carried out in advance. During this process the complete procedure for placing the asphaltic concrete was tested and optimized. The completed upstream face of the dam is shown in the photograph on p90. The 496 stainless steel anchors, which had previously been installed in the masonry and tested individually, were connected to the precast elements by coupling and bolting to external anchor plats. The whole construction process involved 617 m 3 of asphaltic concrete and 245 pre-fixed precast elements. With this method it was possible to seal the upstream face (1200 m 2 ) of the Carlsfeld dam within nine weeks. Seepage measurements at the refurbished drainage system confirm the success of the sealing system. During the renewal of the existing equipment, a raw water intake with a completely variable height adjustment system was installed. Now it is always possible to extract raw water from the best elevation for drinking water processing. At present, the dam is at the test impoundment stage. The results indicate the success of the rehabilitation project. 3. Bautzen dam Bautzen dam was built near the town of the same name between 1968 and It is mainly used for flood control, to increase low flows in the river Spree, recreation and releases to fill exhausted brown coal open cast minings. The Bautzen development consists of two earthfill dams (known as line I and line III). Both dams have asphaltic concrete facings. Line I has a maximum height of 18 m and a crest length of 1652 m. Line III has a maximum height of 19 m and a crest length of 426 m. Flood flows are discharged over a duckbill spillway. This consists of a collecting channel (180 m long), a chute and a stilling basin. The spillway capacity is 225 m 3 /s. The Bautzen dam has two bottom outlets with a nominal diameter of 1400 mm and a total capacity of 26.5 m 3 /s and one by-pass pipe (with a nominal diameter of 600 mm). This equipment is installed in an intake tower on the right-hand shore of the reservoir near dam line III. The storage capacity of the reservoir is approximately m 3. Two secondary dams have been constructed upstream of the Bautzen dam. Rehabilitated shaft of the intake tower at Bautzen. Fig. 7. Layout of Bautzen dam, where: 1 = dam line I; 2 = dam line III; 3 = plant building; 4 = intake tower; 5 = bottom outlet tunnel; and, 6 = saddle spillway. Hydropower & Dams Issue Four,

5 Fig. 8. Cross-section of the Bautzen dam (line III) before rehabilitation, where: 1 = cofferdam; 2 = loess clay (transition element); 3 = cutoff wall; 4 = weathered rock; 5 = loess clay foundation; 6 = sand and gravel only for a width of 3 m; 7 = gravely sand and sandy gravel; 8 = foundation of dam; 9 = the same as (7); 10 = gravel; 11 = rubble; 12 = top soil; 13 = wave wall (concrete); 14 = max. water level; 15 = max. operating level (retention water level);16 = normal water level; 17 = asphalt concrete; and, 18 = abutment block. Rehabilitation of the dam was necessary for the following reasons: The asphaltic concrete face had become seriously damaged during the years of operation. The failures originated from technological problems during original placement of the sealing. These included: inadequate gripping between the two sealing layers leading to the formation of bubbles; and, reopening of vertical joints because of inadequate gripping between two adjacent sealing strips. Parts of the concrete structures (for example, the spillway) were considerably damaged as a result of alkali aggregate reaction (AAR). The development of cracks several centimetres wide, and corrosion. Depth of carbonation of up to 1.5 cm caused corrosion of reinforcement because the concrete surface was too thin. The Bautzen dam was refurbished between 2000 and The project was designed by Salveter GmbH and carried out by the contractor Strabag, together with some other companies. The main works comprised: renewal of m 2 of the asphaltic concrete facing; renewal of the dam crest including the wave wall (at both dams); modification to the downstream slopes of the dams; coating of the intake tower using a new system; removal and reconstruction of the stilling basin; rehabilitation work on the forebay dam and other concrete components; and, removal of m 3 of sediment deposits from the reservoir of the Oehna secondary dam. With respect to the dam rehabilitation, it was only possible to lower the reservoir water level for a maximum period of one year. It was thus necessary to divide the work into that which was dependent on the water level and that which was not. All the work which depended on the low water level was carried out in This included: re-commissioning of the diversion tunnel during the construction period; dewatering around the intake tower; site preparation; rehabilitation of the asphaltic concrete sealing; coating of the intake tower shaft; rehabilitation of the concrete for the stilling chamber and the bottom outlet tunnel; partial renewal and replacement of equipment components; rehabilitation work at the pre-dam; removal of sediment deposits from the secondary dam reservoir; and, closure of the diversion tunnel with concrete. All the work independent of the new water level must be finished by the end of November this year. This involves: installation of additional surveying and monitoring devices; removal and reconstruction of the stilling basin; renewal of the dam crest including the wave walls; modification of the downstream slopes of the dams; rehabilitation of the intake tower head; and, any remaining work. Adaptation to current technical standards should be achieved with the renewal of the asphaltic concrete sealing element. The original sealing element of the Bautzen dam had no drainage layer to check the sealing efficiency. After 25 years of operation, the dam bodies had become consolidated. Therefore it was intended to find a rehabilitation solution without touching the dam body and the loam connecting element, which joins the asphaltic concrete facing to the cutoff wall. The following procedure was chosen: removal of the two 4 cm-thick sealing layers up to the loam connecting element; milling of the 14 cm-thick structural layer up to the loam connecting element; placing of an 8 cm-thick asphalt binder up to the loam connecting element; placing of optical fibres for seepage monitoring; application of a coating of mastic; and, extending the loam connecting element to cover the overlapping joint between the new and the old sealing elements. The asphaltic concrete sealing was placed horizontally by a bridge paver to minimize the number of joints. The sealing on dam line I was placed as one strip with a length of 1652 m and a width of 18.5 m, and with only a few vertically staged joints. On dam line III, the sealing was placed as two horizontal strips with widths of 5 and 18 m. To ensure that the work was completed in time, the bridge paver was moved between the dam lines without being dismantled. Optical fibres were placed for seepage monitoring. This solution was much more economical than the provision of a drainage layer. The optical fibres were placed in selected areas, especially near to joints. Altogether more than 4000 m of optical fibres were placed. Leaks can be located with a precision of 0.5 m. During the erection of the intake tower shaft, the external surface was laminated (with glass fibre reinforced polyester-resin). The upper part of the coating (zones exposed to direct atmospheric influences and zones where there is an alternating water level) had been damaged during operation of the dam. It is necessary to protect the concrete of the intake tower against permanent water contact because the concrete is threatened by alkali aggregate reaction (AAR). The following main works were carried out for the rehabilitation of the tower shaft: stripping of the old coating; preliminary work on the substrate (concrete); provision of a new seal (the first such application in Germany); protection of the edges of the coat with stainless steel clamps; closure of the construction joints in the plinth section; reconstruction of local surface flaws of the concrete; corrosion protection of the steel components; and, 92 Hydropower & Dams Issue Four, 2001

6 installation of a new device to keep the outside of the tower ice-free. Fig. 9 shows the new coating on the intake tower shaft. It consists of a multi-layer lining system on a base of polyurethane. It is known as the PP-Dam system and is made by ISO Permaproof AG of Switzerland. The liquid material is applied in several layers to the pretreated concrete. When the coating has solidified, it will be a smooth coat with the following properties: extensive and space-free compound to the substrate and between the several layers; no underseepage; ability to bridge structural and joint movements and high permanent elasticity; waterproof, even at high pressures; vapour-permeable; and, good resistance against mechanical stress. At present, the dam is at the test impoundment stage. Results so far indicate the success of the rehabilitation project. 5. Conclusion The examples which have been described show how solid engineering and construction work can help to solve complex tasks in hydraulic engineering. Thanks to new ideas, a considerable shortening of the time for completion was possible in all of these cases. Good cooperation between the engineer, contractor and client is important to obtain satisfactory results. Without the teamwork of these three groups, operative modifications or special proposals are unthinkable. Bibliography Landestalsperrenverwaltung des Freistaates Sachsen, Talsperren in Sachsen ; Fig. 9. Cross-section of the Bautzen dam (line I) after rehabilitation, where: 1 = wave wall; 2 = new asphalt concrete sealing; 3 = gravel layer; 4 = old sealing; 5 = apron; 6 = normal water level; 7 = loam connecting element; 8 = cutoff wall; 9 = electric water pressure dispenser; 10 = dam levelling surface; 11 = gravel sand filling; 12 = underground gauge;13 = toe drain; and, 14 = dam gauge. Gläser, E., and Jüngel, E., Innovative Methods for the Renovation of Masonry Dams classified as Historic Monuments illustrated by the case of the Carlsfeld Barrage, Paper for ICOLD Annual Meeting, Dresden, Germany; Lengfeld, M., and Zschammer, Ch., Rehabilitation of gravity dams; the example of Neunzehnhain II Dam, Paper for ICOLD Annual Meeting, Dresden, Germany; Müller, U., and Salveter, G., Innovative techniques for the rehabilitation of Bautzen dam, Paper for ICOLD Annual Meeting, Dresden, Germany; Dr. Ing. Uwe Müller is Head of the Department of Civil Engineering/Stability at Landestalsperrenverwaltung des Freistaates Sachsen (the Dam authority of Saxony). Landestalsperrenverwaltung des Freistaates Sachsen, Bahnhofstraβe 14, Pirna, Germany. I. U. Müller Hydropower & Dams Issue Four,