9. RCC ARCH DAMS
9.3. DESIGN OF RCC ARCH DAMS
The design of an RCC arch dam can follow essentially the same approach as that for a CVC arch dam, with a cantilever section that varies from the crown to the flanks and fully accommodating mass gradient thermal effects through post-cooling and joint grouting, or it can follow a substantially different approach, particularly when the design seeks to take advantage of the primary benefits associated with RCC construction.
While the RCC arch dams constructed to date in China generally indicate a comparatively simple geometry, many of the structures have low base thickness/height (B/H) ratios and more recent dams have been constructed with double-curvature (Wang, 2007). By contrast, all RCC arch dams constructed to date elsewhere in the world have been structures with a significant level of cantilever function and would accordingly be classified as arch-gravity structures, or thick arch dams. In all cases, the design of an RCC arch dam needs to recognise the fact that RCC construction is most efficient when simplicity of construction is maintained, which generally requires wider sections with large placement areas and easy access.
In all cases of RCC arch dams, a number of fundamental differences compared to CVC arches must be recognised, related to the following:
• The application of horizontal, rather than vertical construction;
• The inclusion of induced, rather than formed joints, considering the related function of shear keys, grouting effectiveness, re-grouting, etc;
• The increased inconvenience/complication and partly reduced effectiveness of post- cooling;
• The observance of simplicity in design and construction.
Furthermore, the following factors are relevant to RCC arch dams:
• The cementitious materials content of RCC for arch dams is typically higher than for gravity dams and often similar to that for CVC arch dams;
• Placement rates of RCC for arch dams are generally lower than for gravity dams as a consequence of various of the above issues; and
• The unit cost of RCC for an arch dam will generally be higher than for a gravity dam.
9.3.2. RCC arch dams in China
With the first concepts developed in 1987, construction trials for RCC arch dams in China were initiated with curved gravity cofferdam structures at the Yantan and Geheyan dams in 1988. Thereafter, three RCC arch cofferdams were constructed at the Shuidong, Jiangya and Dachaoshan dams, each resulting in reduced construction time and cost. With a concrete volume of 75 000 m³, the Dachaoshan RCC arch cofferdam was completed in 88 days and was overtopped after only three months.
In the early 1990s, the National Program for Science and Technology Development in China addressed the "Development of Structural Design Methods and Novel Materials for RCC Arch Dams of around 100-m Height", in association with the design and construction of the 132 m high Shapai RCC arch dam. Involving researchers, designers and construction engineers across numerous disciplines and the entire nation, this research programme gave rise to a major breakthrough in the design and construction of RCC arch dams, with the development of various innovative related technologies and construction methods.
After the completion of Puding thick RCC arch dam in 1993 all types of arch dam have been constructed in China, from arch-gravity to double-curvature thin arch dams. Examples of some of the significant RCC arch dams completed in China by 2017 are listed in Table 9.1.
Table 9.1
Selected RCC arch dams completed in China to 2017 Name of Dam Type Height
(m)
Length
(m)
Concrete Volume
(m³x10³)
Shape of
valley
Ratio of
L/H
Ratio of
B/H
Year of
Completion
Puding D.C 75 196 137 V 2.61 0.38 1993
Wenquanpu S.C 49 188 63 U 3.84 0.28 1994
Xibing S.C 63 93 33 V 1.47 0.19 1996
Hongpo A.G. 55 244 78 U 4.44 0.47 1999
Shapai S.C. 132 238 200 V 1.80 0.24 2002
Shimenzi D.C 109 187 373 U 1.72 0.27 2001
Longshou D.C 82 196 275 U 2.39 0.34 2001
Linhekou D.C. 96.5 311 295 V 3.22 0.28 2003
Xuanmiaoguan D.C 65.5 191 95 U 2.92 0.22 2004
Zhaolaihe D.C. 107 198 204 V 1.82 0.17 2005
Dahuashui D.C. 134.5 198 290 V 1.47 0.17 2006
Maobaguan S.C. 66 120 106 V 1.82 0.42 2007
Yunlonghe III D.C. 135 143 175 V 1.06 0.13 2008
Huanghuazhai D.C. 108 244 300 V 2.62 0.24 2009
Tianhuaban D.C 113 159 305.9 V 1.41 0.22 2010
Qinglong D.C. 137.7 116 212 V 0.84 0.17 2011
Sanliping D.C. 133 284.6 448 U 2.14 0.17 2011
Shankouyan D.C. 99 268 260 V 2.71 0.30 2012
Lijiahe D.C. 98.5 352 370 V 3.57 0.32 2013
Lizhou D.C. 132 201.8 380 V 1.53 0.20 2015
Wanjiakouzi D.C. 168 413.2 1140 U 2.47 0.21 2017
Legend: S.C. = single curvature; D.C = double curvature; A.G. = arch-gravity;
RCC arch dams have been successfully constructed in China on sites where the geological conditions are not ideal. For example, the rock mass at Shimenzi Dam indicated a deformation modulus of 4 GPa (Wang, 2007).
9.3.3. Layout
The primary design, structural and geotechnical requirements and considerations for an RCC arch do not differ from those applicable for a CVC arch dam. Nevertheless, a number of fundamental differences that affect the most appropriate layout must be recognised. In general, the following issues must be given careful consideration in establishing the optimal layout and configuration of an RCC arch dam:
• Materials conveyance and construction management require detailed planning;
• The restricted working space associated with an arch dam will impact construction efficiency more in the case of RCC than CVC;
• Steep abutments, which can complicate vehicle and equipment access necessary for construction;
• Necessary work, such as consolidation grouting on steep slopes, may impact construction efficiency;
• The dam is constructed, in principle, horizontally;
• Arch action is developed as the dam increases in height;
• The specific stress relaxation creep characteristics of a particular RCC mix will influence early thermal and consequential stress behaviour;
• Specific construction techniques are used to develop transverse contraction joints;
• The contraction joint grouting system is installed together with/as part of the transverse joint inducing system; and
• Post-cooling systems and related special construction means and methods are often required.
To take advantage of the rapid construction benefits of RCC technology, the overall layout, geometry and design of RCC arch dams must be kept as simple as possible and appurtenant works must be separated from the dam where feasible or be designed for minimum interference with the RCC placement process. Recognising the importance of maintaining constructional simplicity through appropriate design is a key factor in the success of RCC arch dams.
RCC arch and arch-gravity dams commonly use GEVR for the upstream facing. In Chinese practice, a smaller maximum size aggregate RCC is often applied in a zone at the upstream face in combination with GEVR, to improve seepage resistance and surface finish quality.
9.3.4. Arch geometry and cross section
The optimal horizontal geometry and cross section of an RCC arch dam is dependent upon site conditions, similar to a CVC arch dam. However, as a result of the fact that the stress state and structural behaviour are particularly affected by the construction approach and methodology in the case of an RCC arch dam, these aspects must be given particular consideration in design and the arch geometry and vertical cross-section should be optimised for economy within the following constraints:
• Consider the adverse impact of thermal stresses;
• Facilitate rapid construction, in particular, adapting the geometry of an RCC arch dam to the construction procedures and equipment operation;
• Limit undercutting of the upstream heel on the cantilevers to 1V:0.2H, maintaining a smooth upstream face to avoid stress concentrations.
Respecting these constraints, various different arch configurations have been successfully applied in China, with single and multi-centre circular and parabolic horizontal curvatures, with and without undercut upstream faces and in many cases with double curvature. While most of the early arches in China adopted simple, thick arch configurations, almost all of the more recent examples are double curvature arch dams, with no distinct difference in arch thickness between RCC and CVC arches.
While the same requirements for simplicity of design and construction that apply to RCC gravity dams are generally even more relevant in the case of RCC arch dams, due to a typically reduced placement surface area, specific requirements of RCC arches are:
• To limit the degree of both horizontal and vertical curvature;
• To arrange induced joints and joint grouting systems for least interference with RCC placement;
• To rationalise and minimise the inclusion of post-cooling systems; and
• To design post-cooling systems for suitability with RCC construction and for least interference with RCC placement.
Within the above constraints, an RCC arch dam can range from a relatively thin structure with some double curvature, to a curved gravity structure, which relies only on arching for an additional factor of safety under extreme loading. While the former will most probably require pre-cooling, post-cooling and comprehensive transverse joint grouting, the latter may not. At one extreme, the thinnest possible and most complex geometry might give rise to the least concrete volume, but will incur impacts of lost RCC construction efficiency and increased time and cost for post-cooling and subsequent joint grouting.
At the other extreme, the elimination of post-cooling and joint grouting will require increased concrete volume, which would in turn allow increased RCC efficiency and imply impoundment is independent of any constraints associated with joint grouting. In between, a range of possible solutions exist, which can make use of pre-cooling, limited (or partial) post-cooling, re-injectable grouting systems, grouting systems containing multiple injection loops and/or grouting over limited sections of the arch, etc.
Located in temperate climates, the thick arch Portugues Dam (67 m) (Nisar, 2008) was constructed with induced joints, but without a grouting system, while the arch-gravity Changuinola 1 Dam (105 m) (Shaw, 2013) was constructed with a grouting system installed only in areas where arching occurs, although no grouting finally proved necessary, due to the low level of stress-relaxation creep apparent in the RCC. The optimal RCC arch configuration is consequently dependent on the applicable site-specific conditions and should be identified through an optimisation exercise considering topography, geology, temperature (climate), available cementitious materials, contraction joint spacing, placing temperature and construction.
9.3.5. Short structural joint
Structural analysis demonstrates that high tensile stresses often develop in RCC arch dams at the upstream face against the abutments and in the lower sections of the crown cantilever on the downstream face, as the arches deflect under load. An innovative solution was found in providing short structural joints at these locations to release the tensile stresses (Liu, Li & Xie, 2002), as first applied at the Xibing thin RCC arch dam. Short structural joints are only included at locations of high tensile stress and extend into the dam structure typically only 1 to 4 m from the upstream and downstream faces, on a radial alignment. The short structural joints on the upstream abutments more or less follow the alignment of the foundation, with some smoothing over irregularities. On the central downstream face, short structural joints are usually vertical. In principle, short joints allow an improved stress distribution on a single-curvature arch dam.
To avoid these joints extending deeper into the concrete, steel channels are installed on a perpendicular orientation at the inner end and a sealing element (waterstop) is installed at the outer end (on the upstream face) to prevent the ingress of water pressure.
9.3.6. Hinge joint
The hinge joint structure was developed during the design of the Shimenzi RCC arch dam (Liu, Li & Xie, 2002). Shimenzi Dam was constructed in two halves, with a conventional formed, transverse
downstream irrigation, the slot was filled with a shrinkage-compensating concrete (with MgO) prior to partial reservoir impoundment, to form a plug, or the so-called “hinge joint”. Expansion of the concrete plug creates pre-compression arch stresses, compensating for stress loss as the concrete cools with hydration heat dissipation. The inclusion of the hinge joint allows the grouting of the central contraction joint, downstream of the plug, to be delayed until full cooling to the joint closure temperature has been achieved, while also eliminating the requirement for post-cooling of the RCC prior to impoundment.
Post-cooling was, however, applied to reduce the peak hydration temperatures during the hottest period of the year.
Fig 9.2
Shimenzi Dam during construction (China) (Photo: Conrad, 2001)
9.3.7. Arch dams
While the majority of arch dams constructed in China would fall into the category of thick arch, with a base length/height ratio (B/H) exceeding 0.20, the most slender to date indicates a B/H ratio of 0.13 and more than half, and the significant majority of recent arch dams, indicate a B/H ratio of less than 0.25. Outside the application of Chinese technology, the only thick arch to have been constructed to date is Portugues Dam in Puerto Rico (67 m), which indicates a B/H ratio of 0.5. A thick arch, however, probably represents the solution with the greatest development potential for RCC. For a site where the topography and/or geological conditions will not allow the realisation of the full efficiencies of a thin CVC arch, greatest opportunity will often exist to achieve the maximum efficiencies associated with an RCC arch dam. Optimising the arch geometry and materials for applicable thermal loads, for example, can eliminate the need for post-cooling and joint grouting, while retaining a simple configuration suitable for RCC construction, as constructed at Portugues Dam (LIU, LI & XIE 2002 p. 68-77). Similarly, the configuration can be optimised to allow natural cooling and subsequent grouting prior to impoundment of the thinner sections only. An approach of localised and strategic post-cooling and grouting can also be developed in some cases, gaining the benefit of significantly reduced concrete volumes without any (or minimal) additional time requirements associated with post-cooling, or joint grouting.
9.3.8. Arch-gravity dams and curved gravity dams
1. General
As a result of their simple configuration and modest stress levels, arch-gravity and curved- gravity dam structures are generally well suited to construction in RCC. While an arch-gravity dam will offer advantages over a gravity dam in reduced concrete volume, consideration must be given to some additional complexities, with potentially increased time and cost, as a consequence of a possible requirement for groutable induced transverse joints and increased pre-cooling and/or post-cooling.
An arch-gravity structure will typically function as a series of rigid cantilevers, with deflections under load causing the cantilevers to make contact in the upper elevations and subsequently transfer load laterally. The structural behaviour is part gravity and part arch whereby certain zones of the structure will carry arch, or cross-canyon loads. However, post-hydration heat dissipation temperature drop and associated shrinkage compromise arch structural action, which must often subsequently be restored through joint grouting. As a consequence of a high level of structural redundancy in such arch types, only the parts of the structure that incur arch action require grouting and strategic, rather than comprehensive grouting will generally produce satisfactory results (SHAW 2015). Depending on the relative stiffness of the cantilevers, the applicable closure temperature relative to the climate in which the dam is constructed and the extent of stress relaxation creep, the associated mass gradient thermal effects will not always necessitate grouting of the induced joints.
2. Layout
An arch-gravity dam will typically indicate a prismatic cross section and a simple, single radius, circular curvature with a vertical upstream face and a downstream face inclined at a constant slope of between 0.35H:1V and 0.6H:1V.
A curved gravity dam will generally indicate a simple cross section and a circular curvature, with a vertical, or steeply inclined upstream face and a downstream face typically with a constant slope of between 0.7H:1V and 0.85H:1V, depending on the applicable loading, the foundation conditions and the valley topography. The applied curvature is usually included to increase the safety factor under a particular extreme loading case, to provide additional foundation sliding resistance, or to reduce total concrete volume. In combination with a crest tension belt e.g. CERVETTI (2015) & YZIQUEL, NDRIAN
& MATHIEU (2015), such a configuration represents an effective solution to accommodate high seismic design loading with a lower consequential increase in concrete volume than would be required to achieve the necessary stability in a straight gravity dam.
9.3.9. Arch and arch-gravity dams using low stress-relaxation creep RCC
As discussed in Chapters 1 and 2, prototype research has identified a low early stress relaxation creep behaviour particularly in flyash-rich RCC and this has been used to benefit efficient arch-gravity dam design. Reduced stress-relaxation creep implies that the maximum temperature drop load to be accommodated in an arch dam is decreased, which can eliminate the need for joint grouting in a temperate climate.
It is, however, important to note that the most significant thermal load condition in an arch dam constructed with a low stress-relaxation creep RCC is not necessarily the long-term case, when the heat of hydration is fully dissipated and temperatures in the dam structure are determined only by seasonal variations. Particularly with an arch-gravity configuration, the broader base of the dam structure will require a substantially longer time to achieve equilibrium temperatures than the dam crest. As the dam structure cools, the shrinkage particularly in the base of the structure will cause the cantilevers to displace downstream, closing the joints at the crest and enhancing arch action. When the crest cools more rapidly than the base, however, the induced joints open in the upper part of the structure, but the tilting of the cantilever does not occur to mitigate this effect, with the consequence that vertical tensions
gives rise to potentially beneficial opportunities. Allowing crest cooling before impoundment, or applying strategic post-cooling in the crest while the base still retains significant heat will allow transverse joint grouting at maximum joint opening, which in turn will ensure that arch compressions progressively increase as the dam structure continues to dissipate hydration heat.
It is considered particularly significant to note that Changuinola 1 Dam plots close to the upper envelope line on the graph of average monthly production against total RCC volume, as illustrated in Figure 5.1. Being one of the fastest-constructed RCC dams of its size confirms that intelligent design can allow the full simplicity of RCC construction to be effectively maintained for an arch-gravity dam configuration.
While no joint grouting was necessary in the Changuinola 1 Dam due to the temperate climate of Panama, a different approach was required at the Kotanli and Köroğlu dams in north-eastern Turkey due to significantly more extreme conditions. As the smaller Kotanli dam will only incur significant arch action under extreme loads, it was possible to design for natural cooling and minimal joint grouting in the crest. With an increased arch action in the case of Köroğlu Dam on the other hand, strategic post cooling in the arch zone and associated joint grouting prior to impoundment was included in the design.
Fig. 9.3
Changuinola 1 Dam, approaching completion (Panama) (Photo: Lose, 2011)
9.4. RCC MATERIALS & MIXES FOR ARCH DAMS