DESIGN OF MONTESINHO DAM AND EMBANKMENT MONITORING DURING CONSTRUCTION

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1 DESIGN OF MONTESINHO DAM AND EMBANKMENT MONITORING DURING CONSTRUCTION L. Ribeirinho *, G. Tavares *, M. Romeiro *, M. Samora *, J. Brito *, J. Marcelino, J. Boal Paixão and J. Cordeiro * CENOR Consultores, S.A. Rua das Vigias, 2. Piso Lisboa, Portugal cenor@cenor.pt, webpage: Keywords: Granitic Rockfill, CFRD, Grouting treatment, Waterstops, Instrumentation plan, Embankment monitoring, Ecological flow Abstract. This paper presents the main aspects of the design of Montesinho Dam. One of the main topics is the imperviousness of the dam and its foundation and the treatments and structures used to accomplish it. It also approaches the embankment zoning and the geomechanical characterization of the construction materials, as well as the designed instrumentation plan that allowed monitoring of the embankment deformations and the evaluation of water pressure in the foundation during construction. At last it is also presented the adopted ecological flow system, provided with rather unique characteristics. 1 INTRODUCTION The water supply to Bragança, a city in north-eastern Portugal (region of Trás-os-Montes), is provided by Serra Serrada dam. However, its storage capacity is insufficient for the actual demand for clean water urban supply. So, in the past ten years, many solutions to increase the water supply system were studied. The one that got the Authority's approval was the construction of a new reservoir, created by Montesinho Dam. This dam is located in the Sabor river, approximately 3 km east of Serra Serrada dam, in the Montesinho Natural Park. The design of Montesinho Dam included the embankment and its foundation treatment, the river diversion, to enable the dam construction, the bottom outlet, which uses the same cut-and-cover gallery of the diversion, the water intake and the system that ensures the ecological flow, which was very important for the approval of this project. 2 DAM GENERAL DESCRIPTION Montesinho Dam 1 is a concrete face rockfill dam (CFRD), with a height of 36.5 m and a crest of about 310 m in length and 7 m of width (Figure 1 and Figure 2). The embankment has a total volume of m³, consisting of granitic rockfill obtained from the quarry located upstream of the dam in the reservoir area. Laboratório Nacional de Engenharia Civil (LNEC) Águas de Trás-os-Montes e Alto Douro (ATMAD) 1

L. Ribeirinho, G. Tavares, M. Romeiro, M. Samora, J. Brito, J. Marcelino, J. Boal Paixão, J. Cordeiro.

Photographs during construction The reservoir has a total storage capacity of 3.69 hm³ (net volume of 3.

1 km². The maximum flood level is at 1219.73 m. The freeboard is 1.37 m, thus the crest elevation is 1221.

In the area of the dam and reservoir the outcropping blocks and top layers of the bedrock consists

The rock presents a generalized and mild to medium kaolinization of the feldspars, therefore sometimes

2 L. Ribeirinho, G. Tavares, M. Romeiro, M. Samora, J. Brito, J. Marcelino, J. Boal Paixão, J. Cordeiro. Figure 1: Montesinho Dam plan a) pouring of the face slab b) downstream view of the embankment Figure 2: Photographs during construction The reservoir has a total storage capacity of 3.69 hm³ (net volume of 3.53 hm³) with a flooded area of 35.8 ha for a full supply level of m and a catchment area of 10.1 km². The maximum flood level is at m. The freeboard is 1.37 m, thus the crest elevation is m. The upstream and downstream embankment slopes are 1:1.5 (V:H). In the area of the dam and reservoir the outcropping blocks and top layers of the bedrock consists essentially of two-mica granite with coarse grain (γ). The rock presents a generalized and mild to medium kaolinization of the feldspars, therefore sometimes its mechanical characteristics correspond to a weathered granite (W3 and W4), with low mechanical resistance and a low stiffness. However, a dozen meters below, the quality of the rock increases (W2 to W3). 2

3 Granit massif is exposed everywhere, except along the minor riverbed, where there is alluvium soil (a) that needed to be removed in the foundation area prior to the construction (Figure 3). A few residual blocks were also removed. 3 FOUNDATION PERMEABILITY AND GROUTING TREATMENT The seepage in the foundation occurs through the rock discontinuities by several systems of subhorizontal and subvertical fractures. It appends mainly through the wider and more persistent fractures witch occurs near the surface. From a dozen meters below it, the rock mass becomes almost impervious by the close-up of those fractures. In order to prevent that flow, it was conceived a grouting treatment in the dam and in the spillway. The grouting curtain has about 430 m long and spreads along the following stretches: the left abutment crest wall; the plinths of left and right banks; the diversion gallery, connecting both plinths; the dam axis at the right abutment; the spillway. The foundation treatment has two kinds of grouting: the retaining/consolidation grouting meant for sealing the contact concrete/rock in 5 m long holes, along two rows 2 m apart from each other, creating a rectangular distribution of 2.0 m x 3.0 m in the horizontal plane; the monolinear impervious curtain with holes of variable length, ensuring the penetration of at least 5.0 to 7.0 m in the low permeability massif ( 3 Lu), with a minimum of 10 m. The area of grouting curtain was m 2. The overall of main grouting works were: 352 holes; m of rotary and rotopercussion drilling; 115 ton of cement and 343 water pressure tests for investigations and controls. The sealing and retaining/consolidation grouting is only defined in the stretches with plinth. 4 ROCKFILL EMBANKMENT ZONING The embankment zoning (Figure 3) was established taking into account the construction materials available at the site and the current practice for zoning of impervious face rockfill dams, constructed by sound and free-draining rockfill on strong rock foundations 2. Embankment s cross section comprises 5 different types of rockfill (namely A, B, C, D and E). All rockfill materials were obtained from the excavations executed on the interior of the reservoir. C, D and E materials were obtained directly from the excavations and A and B materials were produced, on site, from the excavation materials, with a rock crushing equipment. A type material is a support rockfill (2 m wide), grading from silt to gravel size, to provide uniform support for the face slab, acting also as a semi-impervious layer to restrict flow through the embankment in the event of concrete face joints anomalies. It was placed in layers of 0.4 m maximum lift and compacted with 8 passes of vibrating cylinder. 3

Figure 3: Design cross-section of the rockfill embankment B type material is a filter transition between A and C materials (2 m wide), grading from silt to cobble size.

C type material consists of a well compacted and free draining rockfill, with high deformability modulus to limit deformations under water load. It was placed in layers of 0.

4 Figure 3: Design cross-section of the rockfill embankment B type material is a filter transition between A and C materials (2 m wide), grading from silt to cobble size. This zone will assure adequate hydraulic behavior of the embankment in the event of concrete face joints anomalies. It was placed with the same specifications of the A material. C type material consists of a well compacted and free draining rockfill, with high deformability modulus to limit deformations under water load. It was placed in layers of 0.8 m lift and compacted with 10 passes of vibrating cylinder, with addition of water during placement (minimum of 15% of rockfill volume). D type material is a well graded rockfill. It was placed in layers of 1.0 m maximum lift and compacted with 10 passes of vibrating cylinder, with addition of water during placement (minimum of 15% of rockfill volume). E type material is a protection layer (Figure 2), 2 m wide, composed of large size blocks (50 to 800 mm). This material was produced from blasting of fresh sound granitic rock. According to the first stage of the geomechanical characterization of the rockfill, used in design and presented in the next chapter, the A, B and C materials were produced from moderate weathered granitic rock with minimum uniaxial compression resistance of 30 MPa. For the D material was allowed the use of more weathered granitic rocks with a uniaxial compression resistance between 20 and 30 MPa. However, the second stage of the characterization revealed that those values were too conservative, namely in regard to compression strength. It also revealed that those better materials were available to fulfil the required construction volume. 5 CONSTRUCTION MATERIALS. GEOMECHANICAL CHARACTERIZATION The construction materials of the embankment were obtained directly from the excavations made on the interior of the reservoir. The geomechanical characterization of the rockfill was performed in two phases: the first phase for the design; the second phase for confirmation of design assumptions, at the beginning of the construction works. 4

5 In both stages the physical and mechanical characteristics (index-properties) of rockfill fragments were determined (Table 1) and triaxial tests were performed on large cells, with 0.30 m diameter cylinder specimens. For the geomechanical parameterization of the rockfill two methodologies were used: (i) directly from triaxial test results; (ii) using the correlations proposed by Wilson and Marsal (1979) 3 based on index-properties of the rockfill fragments. In Figure 4, the variations of shearing resistance angle (φ) with the confining stress (σ 3 ) are presented for the triaxial tests done in both phases. The second triaxial test was executed using a sample of the embankment, retrieved when it has reached half of total volume. These results are based on the fact that, for rockfill materials of good quality, the Mohr- -Coulomb envelope presents a curvature, roughly represented by the following equation 4 : φ = φ 0 φ log (σ 3 /pa) (1) Figure 4: Strength parameters of rockfill from triaxial test The methodology presented by Wilson and Marsal is based on the fact that a correlation exists between the grain breakage (B g ) and the effective principal stress ratio at failure [(σ 1 /σ 3 ) f ]. This methodology takes into account the rockfill fragments characteristics, using a classification based on some index-properties (water absorption, slake durability and L.A. abrasion). From the Wilson and Marsal classification the rockfill is rated as soft well- -graded, with B g values between 3,4% and 6% for octahedral stresses of 120 kpa and 260 kpa (considering γ=21.5 kn/m 3 ), respectively. The shear resistance was obtained using the following equation: sinφ = [(σ 1 /σ 3 ) f 1]/[(σ 1 /σ 3 ) f + 1] (2) The values for the angle of shearing resistance were obtained: between 43º and 45.6º for low confining stresses; and between 39.5º and 41.8º for high confining stresses. 5

6 Characterization Parameter Scatter of results First stage Second stage Granulometry Uniformity coefficient (Cu) Texture Porosity (n) 2.4% 5.1% 0,3% 1,0% Bulk density (G) Compression Uniaxial compression (σ c ) 20.9 MPa MPa MPa strength MPa Crushing strength (Pa50) 361 kgf (air dried) 314 kgf (submerge) kgf (air dried) kgf (submerge) Durability Los Angeles (LA) 59% 73% 24% 47% Slake durability test (Id 2 % 0,95% 0,98% - retained) Water sensitivity Absorption - 0,3% 0,5% Table 1: Index-properties of granitic rockfill fragments Rockfill deformability was also determined using triaxial test results, considering the Young Modulus for an axial extension of 0.1%. Values of 175 MPa (σ 3 =100kPa), 184MPa (σ 3 =300kPa) and 239 MPa (σ 3 =400kPa) were obtained. 6 CONCRETE FACE 6.1 Face slab The face slab is the structure that ensures the imperviousness of the rockfill (Figure 2). It is fully supported by the underlying rockfill and is mostly in compression under the reservoir loadings, except near the dam abutments where tensile stresses can develop. This kind of slab has no structural function and therefore the focus of its design is towards watertightness, instead of strength. That s why the determination of face slab dimensions and reinforcement is usually based on previous experience and construction procedures. Slab thickness is established to facilitate waterstops placement and to ensure proper cover for the reinforcement. Current practice for low or medium height dams is between 0.25 and 0.30 m. In the present case, that value was set to 0.30 m. In order to achieve a good performance of the slab upon the expected deformations in its lifecycle, vertical joints were designed, establishing 10 m width panels. The quantity of reinforcement to be placed in the face slabs of CFRD s is typically specified by the reinforcement ratio, usually 0.3 to 0.4% of concrete cross-sectional area for both longitudinal and transversal reinforcement 5. In Montesinho Dam face slab the prescribed reinforcement was #ϕ10//0.125 applied in two layers ( 0.4%). Reducing the cover, by the use of two layers, and smaller rebar size and spacing were meant to control the crack development, which is the primary concern of face slab design. 6.2 Plinth The plinth is the structure that ensures a watertight connection between the foundation treatment and the face slab. Its width is determined as a function of the hydraulic gradient installed, or, if it is low as in Montesinho, by the space required to construct the three-row grout curtain and the dowels mesh. In the present case the plinth is 5 m wide and 0.5 m thick 6

7 (minimum). The 6.0 m long steel dowels (ϕ32) are sealed in the massif with grouting, in an alternating mesh of 2 or 3 dowels, 3 m apart. The purpose of reinforcement in the plinth is to reduce cracking due to temperature variations and to bending efforts induced by the grouting injections. The reinforcement area in this case is 0.3% of concrete cross-section each way. The plinth is continuous, without structural joints, and the longitudinal reinforcement is also continuous through construction joints. The plinth geometry was also defined to allow the face slab support. A minimum height of 1.0 m of embankment under the face slab was designed to ensure that in the perimeter joint the slab would move normal to its plane, without bending. 6.3 Perimeter and vertical joints and waterstops The perimeter joint connects the concrete face slab and the plinth of the dam to complete the water barrier of the dam. The main function of the perimeter joint is to maintain a watertight seal against full reservoir load while allowing for anticipated movements between the plinth and the face slabs. The watertight seal is achieved through a three-barrier system: bottom, middle and upper water barriers (Figure 5). Figure 5: Perimeter joint detail The bottom waterstop can be in copper, stainless steel or PVC. PVC s can offer advantages over the other water barrier materials for CFRDs lower than 100 m in height 5. Their greater thickness can simplify waterstop placement and reduce the risk of damage during construction. In the present case, the prescribed waterstop was in PVC with an external center rib and 300 mm wide. The height of the center rib should be tall enough to allow the waterstop to deform without rupture and the base width should be large enough to allow for proper placement and consolidation of concrete at the perimeter joint. The middle water barrier consists in a dumbbell shaped PVC waterstop, with a hollow center bulb at the middle. The hollow bulb is preferable to the alternatives due to its ability to undergo greater deformations before tearing or rupturing. The upper water barrier consists of a mastic fill at the top of the perimeter joint. A small diameter neoprene tube is placed in the groove of the joint before the mastic sealant is 7

8 applied. The tube serves to prevent the mastic from flowing into the joint until a sufficiently wide gap has formed. Mastic sealing is covered and protected with a hypalon membrane in order to provide a long lasting upper water barrier. To prevent damage to the perimeter joint during construction, a compressible wood board is placed in the plinth face in order to provide a cushion on which the face slab can rest, without spalling the concrete or damage the waterstops. The imperviousness of vertical joints between face slab panels is achieved through middle and upper water barriers like the ones described for the perimeter joint. The slab faces at the joints are painted with bitumen emulsion. 7 INSTRUMENTATION PLAN AND EMBANKMENT MONITORING To follow the behavior of the dam during construction and during its life cycle, a detailed monitoring system was considered. To monitor the deformation of the dam: vertical and horizontal displacements gauges were installed inside the dam body distributed in three important cross-sections, 1 1, 2 2 and 3 3 (Figure 1), with section 2-2 being the major monitoring cross section of the dam (Figure 3). The concrete slab deformations are monitored by three inclinometers installed in the cushion zone (material A), at the same sections: 1 1, 2 2 and 3 3. Surface settlements at the crest of the dam are measured by 12 monitoring gauges separated from each other about 25 m. To follow the water pressures in the foundation and to evaluate the efficiency of the grout curtain the monitoring system includes 3 sets of 2 piezometers. In each set, the first piezometer is located immediately after the grout curtain while the second is located near the dam axis. A total flow measuring system will be installed near the toe of the dam to evaluate any leakage that might occur. During the construction of the dam regular measurements of the vertical displacements and water pressures in the foundation where made. Figure 6 presents the recordings of the settlement gauges installed in the dam. The maximum settlement recorded so far, is about 20 mm, and occurs in the maximum cross section, near half the height. It is worth to note that to the end of the construction another 2 m of additional rockfill is needed (up to level m), therefore some additional settlement is expected. These settlements are very low and are in agreement to what was expected. Figure 6: Recordings from the settlement gauges (right bank, center, left bank) 8

9 8 ECOLOGICAL FLOW SYSTEM This dam has been provided with a peculiar and rather unique ecological flow system, which is shown in Figure 7. The location of the works, right inside a Natural Park, made environmental concerns especially critical. So, besides the demand for adequate ecological flows all year round, other concerns have been taken into account, one of these being the barrier effect caused by the dam to aquatic wildlife on the Sabor River. It was concluded that, obviously, the barrier effect cannot be fully eliminated, but that, nevertheless, it would be possible to limit it in time. As it can be seen in Figure 7, the spillway will be a structure separated from the dam body, which will discharge onto a tributary of the Sabor River. Computer simulation of the dam reservoir exploitation, balancing natural inflows, outflows and withdrawals for urban water supply, showed that it will remain at full supply level, or above, during 54% of the time on average. Low level periods will be limited, mostly to the dry season. So, the following decisions were made: To abandon the 600 m long reach of Sabor River bed located immediately downstream from the dam toe; To replace it by the bed of the tributary onto which the dam spillway will discharge (see Figure 7), that runs parallel to the Sabor bed and that has about the same length. Figure 7: Ecological flow system plan At the spillway connection point, the tributary has an almost negligible catchment area. So, it s bed hasn t yet the same physical characteristics as the Sabor River has. Nevertheless, since both beds are carved into good quality rock, it is expected that the adaptation of the replacement bed to the new strongly increased flows will happen gradually and naturally over time, without any catastrophic scouring. 9

10 To allow for the barrier effect to be eliminated, the spillway structure was designed as a double bed naturalized channel, with no sudden fall between the reservoir pond and the new river bed - Figure 8. During the periods when the reservoir will be full (54% of the time), there will be a fluvial continuum between the reservoir and the new river bed, allowing wildlife to circulate up and down freely. During the remaining periods, although continuity will be temporarily broken, adequate flow in the new river bed will still be guaranteed by injecting ecological discharges onto the spillway structure. To do so, ecological flow pumps will be installed inside the reservoir s intake tower, which will feed a pumping conduit headed to the spillway (see Figure 7), passing first through a regulation deposit installed over the tower s roof. Minimum ecological flows will vary along the year between 4.1 l/s and 45.5 l/s. Pumps will have redundancy and so will their power supply. But, if all this fails, there will still be an emergency gravity ecological flow outlet at the toe of the dam, feeding into the old abandoned river bed reach. Figure 8: Cross-section downstream the spillway 9 ACKNOWLEDGMENTS The authors are thankful to ATMAD for the permission given to publish this article and for the collaboration in its preparation. REFERENCES [1] CENOR Consulting Engineers, Dam of the Montesinho's Water Supply System. Final Design (in Portuguese), ATMAD, Lisbon, Portugal (2012). [2] R. Fell, P. MacGregor and D. Stapledon, Geotechnical engineering of embankment dams, A. A. Balkema, Rotterdam, Netherlands (1992). [3] S. Wilson and R. Marsal, Current trends in design and construction of embankment dams, ASCE, New York, USA (1979). [4] A.A. Veiga Pinto, Previsão do comportamento estrutural de barragens de enrocamento, LNEC, Lisboa (1983). [5] ICOLD, Concrete Face Rockfill Dams. Concepts for design and construction, International Commission on Large Dams, Bulletin 141 (2010). 10

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