Unique Challenges Influencing the Design and Construction Of Three Recent Australian RCC Dams

Transcription

1 Unique Challenges Influencing the Design and Construction Of Three Recent Australian RCC Dams R. Herweynen Principal Consultant Civil, Entura, Australia T. Griggs Senior Engineer, Entura, Australia ABSTRACT: All dam sites are unique, they have different access conditions, different topography, different foundation geology, different available materials, different flow characteristics, different climatic conditions and different aquatic environments. In addition to this, construction contractors have preferred methodologies which will influence the final design of the dam. The end result is that we have specific challenges at each dam site, resulting in a different optimum design at each site and thus often leading to a unique design for that specific dam site. This paper will look at three recently completed RCC dams in Australia Paradise, Meander and Wyaralong dams, all of similar height, but each one unique in its design due to the specific characteristics of each site. This paper will demonstrate how the specific characteristics and challenges at these three dam sites, led to three unique RCC dam designs. Although it is important that we take our experience and learning from other projects and apply them to any new dam site, it is equally important that we do not force a past solution to a new dam site, without considering the uniqueness of that particular site. Keywords: RCC, Aggregate, Precast, Membrane, Spillway 1. INTRODUCTION Over the past decade the authors of this paper have been involved in three RCC dams within Australia, namely Paradise, Meander and Wyaralong dams. Although each of these dams were similar height, and the preferred dam type for each of these dam sites was an RCC dam, the adopted design for each site was significantly different. In each case, the experience and learning from the previous project was taken and applied to the subsequent project. However, all three dam sites were unique, they had different access conditions, topography, foundation geology, available materials, river flow characteristics, climatic conditions and aquatic environments. In addition to this, the different clients had their own business drivers, and the construction contractors had their own preferred methodologies, both of which influenced the decisions leading to the final designs. A key influence to the Paradise Dam design was the innovative approach to the diversion strategy providing both a solution to a foundation issue and an efficient source of RCC aggregate material. As a lean RCC mix, utilizing the natural pozzolan of the crushed basalt, it required no forced cooling, and ensured that permeability was not an issue by adopting an upstream PVC membrane sandwiched behind a precast concrete panel. With 400,000 m 3 of RCC, the most cost effective method of RCC delivery was a full conveyor system. In contrast, Meander Dam was located in a narrow valley, with low RCC volume (85,000 m 3 ). As a result a simple, yet effective RCC delivery method was developed, as high production rates were not critical. For this project, overhead costs were significantly reduced by keeping site plant and labour to a minimum by utilizing precast concrete solutions for all of the external faces, including the spillway crest. Finally, the Wyaralong Dam project utilized an inferior, onsite sandstone aggregate in the RCC, with a basalt conventional concrete skin to provide the necessary durability. The natural topography of the left abutment provided a natural ramp for an articulated truck delivery system. This paper will discuss in more detail the specific designs developed as a result of the unique characteristics of each of these sites. 2. PARADISE DAM Paradise Dam, which was previously called Burnett Dam, is an irrigation dam located on the Burnett River in

2 Queensland, Australia. It is a 50m high RCC dam, with an RCC volume of 400,000m 3. The project was delivered as an Alliance, with the designer, contractor and client all sharing the risks. Construction commenced in November 2003, and practical completion was achieved in December Due to drought conditions, the dam did not reach full supply level until February At Paradise Dam there were a number of specific site conditions, which had a significant impact on the final dam design arrangement (Herweynen et al, 2004 & 2006). These site conditions and their impact on the design and construction are discussed in detail below Diversion Arrangement The basement rock at the Paradise Dam site is a mudstone. The right abutment featured a basalt flow underlain by up to 2m of alluvial above the mudstone. This alluvial material posed both seepage and settlement issues and the final design solution was to locate the diversion channel through this problem area, thus: Addressing this foundation issue (for further details on this refer to Herweynen et al, 2004). Providing an efficient source material for RCC aggregate, utilising the basalt material. Enabling the diversion conduits to be utilised for the permanent outlet works. less critical, as the upstream PVC membrane provided this. Another benefit of the upstream membrane, along with the drainage system behind it, was that it was highly efficient at controlling the uplift pressures in the body of the dam. This has been confirmed by the piezometers within the body of the dam (Herweynen & Griggs, 2007). As a result of the very close proximity of the aggregate source to the dam, a very efficient RCC aggregate production process was developed, consisting of bulk excavation of the diversion channel, a crushing plant to produce an all-in-one aggregate stockpile, feeding a continuous mixer which supplied a high capacity conveyor system for RCC to the dam. Due to the clients concern that an exposed upstream membrane may be damaged by debris or vandalism, a decision was made to place the membrane behind an upstream precast concrete panel, which also provided the upstream formwork for the RCC. With the membrane not exposed, this solution also provided the necessary design life required by the client. Figure 2. Paradise Dam completed 2.3. Secondary Spillway and Downstream Facing Figure 1. Diversion channel excavation at Paradise 2.2. RCC Mix and Membrane For Paradise Dam adopting a lean RCC mix with an upstream PVC membrane was a very cost effective decision for this particular site (Herweynen et al, 2004). This lean RCC mix utilised the natural pozzolan generated from the crushed basalt, with the required RCC strengths being achieved with only 65 kg/m 3 of cement and no flyash. As a result of this, the transportation and storage of cementitious material was kept to a minimum, and no forced cooling was required to control thermal stresses. Required strengths were easily obtained with the adopted RCC mix (Lopez et al, 2005), while the permeability was The catchment area of Paradise Dam is 33,000 km 2, giving a peak estimated outflow for the PMP Design Flood of approximately 94,000 m 3 /s. There is very little flood routing effect provided by the storage, thus the inflow hydrograph is only slightly greater than the outflow hydrograph. For these extreme flood events the tailwater levels downstream of the dam are very high. As a result of these flood flow characteristics a key design decision was made to pass the flood rather than hold it back. This was achieved by allowing the right abutment to act as a secondary spillway, which was demonstrated to be possible through physical hydraulic modelling, predominantly due to the high tailwater levels (Herweynen & Griggs, 2006). During the maximum design flood, the surcharge over the primary spillway was over 18 metres, with high unit discharges and energy. To ensure the long term durability of the primary spillway it was decided to provide a

3 reinforced concrete skin on the downstream face, which was anchored into the RCC. These anchors were placed between RCC lifts, and the reinforced concrete was constructed as a second stage operation. In contrast to this, the secondary spillway was designed to operate relatively infrequently, with discharge commencing at approximately the 1:1,000 AEP flood. As a result an RCC surface for the downstream face was considered to be acceptable. Where the section was relatively high the RCC downstream face was formed with bedding mix squeezed up against the form. While for the lower sections of the right abutment the RCC downstream face was over-placed and trimmed back with an excavator (refer to Fig. 3). As vertical faces in the foundation could cause the dam to crack, a number of options for the treatment were considered including: trimming to flatten the slope, providing dental concrete infill to flatten the slope, relocating the adjacent dam block monolith joint to the top of the face and reinforcing the RCC. The option most favoured was reinforcing the roller-compacted concrete. Reinforcing was placed on the RCC layers above vertical faces to prevent cracking between the top of the vertical face and the adjacent monolith joint (Griggs & Gibson, 2007). Similar to Paradise Dam, in order to pass the design flood a secondary spillway was utilized on the right abutment. However, unlike the foundations at Paradise Dam, the dolerite rock had a very high strength and was considered to be resistant to erosion. Therefore it was decided not to concrete line the spillway aprons. Dental concrete and rock anchors were provided in areas of concern but generally no additional treatment was required. Figure 4 is a view of the completed dam from downstream showing the lowered section of the central primary spillway and the unlined spillway apron of the secondary spillway on the right abutment. A feature of the arrangement is that a spillway training wall was only provided on the left abutment to protect the outlet works and mini-hydro station and not on the right abutment. Figure 3. RCC excavator trimmed face at Paradise 3. MEANDER DAM Meander Dam is an irrigation dam located on the Meander River in Tasmania, Australia. It is a 50m high, 85,000 m 3 RCC dam and used a low cementitious mix of 70kg cement per m 3 of RCC with no flyash. It was delivered through a single design and construction contract. Construction occurred over a two year period, with river diversion and foundation preparation occurring in 2006, and RCC placement in The dam reached practical completion in November The key issues that influenced the design at Meander was the foundation rock structure, the narrow topography and the need to keep project overheads low. The influence of these elements will be discussed in detail below Foundation Treatment and Spillway Arrangement The dam foundation was dolerite rock that was characterized by vertical columnar jointing as well as sheet jointing parallel to the abutments. Due to the nature of the jointing, the resultant excavation surface tended to be blocky, with some relatively high vertical faces on the abutments. Figure 4. Meander Dam spillway arrangement 3.2. RCC Delivery System Due to the narrow valley, the RCC volume was relatively small at 85,000 m 3. As a result it was not economical to install a high capacity delivery system for the RCC. The delivery system adopted was simple and consisted of a conveyor from the pugmill to a bin on the dam surface; with a loader from the bin to the required position. Figure 5 shows the conveyor system on the left abutment and bin on the dam surface. The conveyor was supported by feet on the dam surface. The lowest section was lifted up each layer until it was horizontal and could be

4 removed. This sequence was repeated until the top of the abutment was reached. the same time as the downstream face units and thus eliminate the second stage process. The spillway crest was originally design as a mass concrete structure with surface reinforcement for crack control. The design was modified to a precast structure with mass concrete infill to provide a simpler construction process. The spillway crest precast units consist of 1 m wide sections that form the top of the spillway crest. The units have reinforcement protruding on the underside to tie into the mass concrete below. The mass concrete is, in turn, anchored into the RCC. Fig. 7 shows a number of precast crest units in position. Figure 5. Simple RCC delivery system at Meander Dam 3.3. Use of Precast Concrete The contractor had a strong preference for precast concrete to minimize the cost and time involved in construction (Griggs & Gibson, 2007). Details of the key precast items are discussed below. The upstream facing system used precast concrete panels similar to Paradise Dam. However, in this case a membrane was placed on the upstream side after the dam was complete. This eliminated the interface between RCC placement and membrane welding that had caused some issues on Paradise Dam (Griggs & Herweynen, 2007). The downstream facing system consisted of L-Shaped precast concrete units. A second staged grouting operation was performed to fill the void between the horizontal portion and the RCC. Details of the precast downstream blocks are shown in Fig. 6. Figure 7. Precast spillway crest units at Meander Dam The abutment crest is shown in Figure 7 and consisted of a cast in situ slab anchored into the RCC, with 1.5 m high upstream and 1.24 m high downstream parapets. Using precast parapets reduced safety issues with pouring them in situ, in addition to saving time and cost. 4. WYARALONG DAM Wyaralong Dam, a water supply dam located on a tributary of the Logan River in Queensland, Australia, is a 48m high RCC dam, with an RCC volume of 190,000 m 3. It was delivered as an Alliance, with design and construction occurring over a 2 year period. Early closure of river diversion was achieved in December 2010, with the dam filling within one month. Practical completion was achieved in June Figure 6. Precast downstream facing at Meander Dam It was originally planned to construct the left-hand spillway training wall as a second stage process by leaving a recess in the RCC and then forming and pouring an in situ reinforced concrete wall. This design was revised to allow the use of precast units that could be installed at The key site specific conditions that influenced the design were the foundation bedding defects, the hydrology, the onsite aggregate and the left abutment topography. These, along with some innovations, are discussed in detail below River Diversion Flows in the Teviot Brook are generally very low, although there are years when there are sharp high flow events. However, given the generally low flows and the

5 ability to sequence the construction activities to limit the exposure period, a cost effective river diversion was designed, which had a high likelihood of overtopping. The final diversion arrangement consisted of a 6m high cofferdam which was designed, with reno mattresses, to be overtopped and had a sheetpile cutoff down to the foundation rock through the more permeable alluvial materials in the main river channel. The river was diverted into a 2.4m diameter steel pipe which was concrete encased in the dam footprint. The maximum capacity of the diversion arrangement was 25 m 3 /s (Herweynen et al, 2010). The diversion closure procedure consisted of cutting the steel pipe, attaching a blank flange to it, and pumping in the plug concrete from the upstream face. It was particularly exposed to river inflows, requiring it to be planned well and completed in a single day, which occurred in December 2010 (Herweynen et al, 2011) Dam Cross Section and Spillway The bedrock at the Wyaralong Dam site is Gatton Sandstone overlain by alluvium. The Gatton Sandstone predominantly consists of feldspathic to lithic-feldspathic sandstone with a clay matrix. The average dip of the bedding at the site is approximately 14 degrees, generally dipping downstream and into the right abutment. Several sub-vertical joint sets are also present at the site. Weak layers identified at the site generally followed bedding. The rock at the damsite was classified based on the degree of weathering. Weathering classifications used for classifying the rock were: distinctly weathered with seams, distinctly weathered without seams, and slightly weathered to fresh. The foundation excavation profile was defined to remove all of the distinctly weathered with seams, as the cost to treat this foundation would have been excessive. Similar to both Paradise Dam and Meander Dam, there was significant savings with a spillway arrangement that adopted a secondary spillway. As a result the general arrangement adopted for Wyaralong Dam consisted of a 135m long primary spillway, with a smooth downstream face and stilling basin at the base; and a 150m long secondary spillway located on the left abutment, with a stepped downstream face and an apron channel directing the flow back to the river (refer to Fig. 9). The secondary spillway was designed to operate for floods greater than the 1:100 AEP flood, with the secondary spillway apron channel able to contain the 1:2,000 AEP flood. For more extreme floods, the capacity of the secondary spillway apron channel would be exceeded and the secondary spillway discharge would flow over the left abutment. A detailed erodibility assessment study, which took a holistic approach to dam safety, indicated that this solution was acceptable for the Wyaralong Dam site (Herweynen & Stratford, 2010). A key design feature of the secondary spillway was the anchoring of the apron slab to the foundation rock. These anchors were important to the structural integrity of the dam should the downstream rock erode. As a result double corrosion protection was specified. Figure 9. Wyaralong Dam secondary spillway 4.3. RCC Aggregate Figure 8. Wyaralong Dam spillway cross-section It was the weaker bedding planes that dictated the stability of the dam and the corresponding dam cross-section that was adopted (refer to Fig. 8). There were significant advantages in using the on-site sandstone for RCC aggregate due to greater control of aggregate supply, reduced cost and reduced haulage; resulting in fewer impacts to the existing road infrastructure and on the local community. However, at the feasibility stage of the project the on-site aggregate was reported as being unsuitable for RCC due to: High water absorption ( %) A 68% wet/dry strength variation Clay index values of %, with some swelling Not meeting Australian Standards for concrete aggregate Potential durability issues However, the initial trial mix program showed very positive results, leading the design team to go through an extensive evaluation process (Herweynen et al, 2010a). The conclusions of this extensive testing and evaluation

6 process was that the on-site sandstone could be used for RCC aggregate and that any long term durability concerns were eliminated. The only outstanding concern was the abrasion resistance of the RCC, and as a result a decision was made to use conventional facing concrete using a basalt aggregate. Further structural analysis indicated that the difference in modulus between the sandstone RCC and the basalt conventional concrete would not cause any bond issues between the two. A photo of the trial embankment is given in Fig CONCLUSION Using the case study of three similar height RCC dams, this paper has demonstrated that all dam sites are different due to their unique combination of site specific characteristics. As a result there is no one standard RCC dam solution, as these specific site characteristics will lead to a unique design. Therefore, although it is important that we take our experience and learning from other projects and apply them to any new dam site, it is equally important that we do not force a past solution to a new dam site, without considering the uniqueness of that particular site. REFERENCES Figure 10. Trial RCC embankment at Wyaralong 4.4. Truck Delivery down Left Abutment The natural slope of the left abutment is approximately 1V:6H, providing an ideal ramp for an articulated truck delivery system. This proved to be a very cost effective solution for this particular site. As a result, the quarry was located on the left side of the river, with a suitable quarry located downstream of the dam. The crushing plant and all-in-one stockpile was located on the left abutment, just above the final dam crest level. The all-in-one stockpile was considered to be critical for the sandstone aggregate as it ensured that point-to-point contact of the larger aggregate did not occur. The ramp for the trucks was positioned along the secondary spillway apron, thus protecting the sandstone dam foundation from deteriorating due to the high traffic associated with the articulated trucks delivering the RCC from the continuous mixer to the dam lift surface. Initially it was proposed to form the drainage gallery in the dam, however, this created significant restriction with the truck delivery system, resulting in very low RCC production rates. An innovative solution developed at Wyaralong was to excavate the gallery with a rock-trenching machine once the RCC reached the roof of the gallery level. The solution involved carving the 120m long horizontal gallery through the previously placed RCC using a rock trenching machine. With a 450mm cutting edge capable of cutting 3.6m, the trencher was required to form the 2m wide gallery in three longitudinal passes. With the high tolerance that could be achieved the gallery floor and drain could be directly cut into the RCC (Herweynen et al, 2011). Deible, J., Herweynen, R. and Dow, G. (2010): Challenges associated with identifying and analysing potential failure mechanisms in dam foundations Taum Sauk upper reservoir dam & Wyaralong dam case studies, Proceedings of ANCOLD Conference 2010, Hobart, Nov Griggs, T. and Herweynen, R. (2007): Hydro Tasmania Consulting s Recent RCC Dam Experience, Proceedings of the 5 th International Conference on Dam Engineering, Lisbon, Feb Griggs, T. and Gibson, G. (2007): Design and construction of Meander Dam, Tasmania, Proceedings of NZSOLD ANCOLD Conference 2007, Queenstown, Nov Herweynen, R., Griggs, T., Schrader, E. and Starr, D. (2004): Burnett RCC dam design an innovative approach to site specific conditions, Proceedings of ANCOLD Conference 2004, Melbourne, Nov Herweynen, R. and Griggs, T. (2006): Burnett RCC dam an innovative approach to floods, ICOLD 22 nd Congress on Large Dams, Q. 84 R. 18, pp Herweynen, R. and Griggs, T. (2007): Effectiveness of an upstream membrane on an RCC dam Paradise dam case study, Symposium ICOLD 75 th Annual Meeting, St. Petersburg. Herweynen, R., Stratford, C. and Deible, J. (2010): Construction of the Wyaralong RCC dam in Australia, The International Journal in Hydropower & Dams, Journal of Dam Engineering, Issue 3, pp Herweynen, R. and Stratford, C. (2010): A unique and holistic approach to the erodibility of dam foundations, Proceedings of ANCOLD Conference 2010, Hobart, Nov Herweynen, R., Montalvo, R. and Ager, J. (2010a): Using a clay cemented sandstone as RCC aggregate a major breakthrough at Wyaralong dam, Proceedings of ANCOLD Conference 2010, Hobart, Nov Herweynen, R., Stratford, C. and Watson, A. (2011): An innovative gallery solution for an RCC dam, The International Journal in Hydropower & Dams, Issue 4, pp Lopez, J., Griggs, T., Montalvo, R.J., Herweynen, R. and Schrader, E. (2005): RCC construction & quality control for Burnett dam, Proceedings of ANCOLD Conference 2005, Fremantle, Nov