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Construction Methods in Siah Bishe CFRDS

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The Siah Bihe project with 1000 MW capacity is the first pumped Storage project in Iran. Two CFRDs with 104 and 84 meters height are serving as upper and lower dams of this project. These two dams are first CFRDs of the country. The project is located in north of Iran in a mountain region with high seismicity. The project has some complicated aspects due to complex geology, several slopes prone to instability, steep slopes and adjacent public main road above the excavation area. Complex geology of the region with tectonic activities poses high uncertainty to planning and execution of the project. This paper represents our experiences during design and construction of Siah Bishe CFRDs. The construction method for foundation preparation, excavation, dam body, rockfill and concrete face are presented in this paper and different aspects of them are discussed.

Now the upper dam is almost completed and construction of lower dam concrete face has already been completed also. Method of construction of concrete face, tools, techniques and influencing parameters are presented in detail in the paper. Different parameters affecting method of execution and rates of progress are explained also. Special issues confronted during these years like difficult excavation in such complex geology and their effects on the designing and construction of this project are mentioned.

The Siah Bishe project consists of two concrete face rockfill dams (CFRDs), serving as upper and lower dams for the 1000 MV pumped storage scheme. This pumped storage project is located about 150 km north of Tehran, Iran.
The Siah Bishe project is the first pumped storage project in the country and the dam type is also different from the other projects in Iran. The upper and the lower CFRDs are the first concrete face rockfill dams which are designed and constructed in Iran. The lower dam has 104m and the upper dam has 84m height. The project is now near complete with 94 percent physical progress and would be impounded in the near future.

Layout of the project is shown in Figure 1. Project is divided to two contracts. Contract A including the Upper and Lower CFRDs, spillways, bottom outlets and headrace tunnels; Contract B includes the power plant, surge tank, pressure shafts, tailrace tunnels and outlet facilities.

Both upper and lower dams are located in a mountain region with very complex geology and high seismic risk. Special topography with steep slopes and potentially unstable regions in the dam site required many engineering challenges.

Several special features of the project including the foundation treatment of lower dam for the fault crossing it’s plinth and relevant dam design measures, coping with the potentially unstable huge mass on the downstream of the upper CFRD axis were discussed by authors in several papers [1] , [2].

In this paper a short review of the project is presented. Dam cross section and parts of dam body are shown and shortly discussed. The main part of this paper is dedicated to foundation excavation and slope stabilization and also construction of concrete face of dam. The slope instabilities and measures undertaken to stabilize the overburden soil under a public main road for dam abutment excavations are the challenge of this project which are discussed in detail in this paper.

Dam Cross Section

Most of the dam bodies in Concrete Face Rockfill dams (CFRDs) are made of rockfill materials. The dam body of both upper and lower concrete faces rockfill dams have different zones of rockfill similar to cross section shown in Figure 2. Both dam bodies has been designed and constructed according to international engineering standards. Typical cross section of lower dam showing rockfill zoning and material gradation curves are shown in Figures 2 and 3.

In CFRDs the rockfill should bear the whole impounded water loads. Therefore rockfill is considered as main material which should be controlled for quality both in laboratory and field. Main dam bodies of both CFRDs are constructed from rockfill with different qualities and gradations for different zones. Specification of each zone is explained in following paragraphs.

3A Material) The 3A material consists of good quality well compacted rockfill material, able to carry the main part of the water load with maximum grain size of 900 mm. The materials are delivered to dam site by dump trucks, spread by bulldozer to lift thickness 1.0 meter and compacted with 15 tons vibratory rollers with at least 4 to 6 passes. This method was checked by field tests in trial embankments for each quarry material.

3B Material) Zone 3B material is used in the downstream part of the dam up to some elevation. The upper zone of dam body is again from better quality rockfill of 3A. According to the technical specifications, the grain size distribution of 3B material shall be the same as for 3A but it is allowed to be placed in greater lift thickness of 1.5 meters to accommodate the oversize rock from the quarry.

3C Material) Zone 3C or random rockfill zone is located at the downstream part of the dam, between zones 3A and 3B. It consists either of rockfill 3A or 3B, depending on the actual quarry operation and results. Larger amount of fines is permitted in this zone if blasting of quarry would produce more fines. The layer thickness may vary between 1.0 to 1.5 m. Temporary haul roads within the rockfill embankment can be built with slopes of maximum gradient of 13%. No ramps were built in the transition zones 2A and 2B, which was constructed in horizontal layers from abutment to abutment. When it was possible to perform all 3A, 3B and 3C material together, the efficiency of operation with 2 bulldozers for spreading of rockfill material and one 15 tones vibrating roller for compaction and enough dump trucks for transportation, was about 3500 m3 per shift.

But as discussed in the following sections, the inherent instability of slopes in this region postponed excavation in abutments and ramping in dam body, was employed to minimize the risk of delays.

2AA Material) The fine transition zone 2AA were placed just downstream of the plinth and perimeter joint to provide continuous support for the lower part of the concrete face and to reduce seepage through the dam when leakage develops due to defects in the joint.

This material was produced in the crushing plant. The selected, processed and tested material should be stockpiled as much as possible for future usage. It should be placed in layers of 0.2m and compacted with mechanic rammers or vibrating plate compactors. Particular attention and care must be taken to prevent damage of the water stops embedded in the concrete. The upstream slope of zone 2AA will be compacted and protected as for zone 2A.

2A Material) The semi pervious transition zone 2A were placed beneath the upstream concrete face to provide a continuous support for the concrete face slabs and to reduce the seepage through the dam when leakage develops due to defects in the joint water stops or slabs. The constant horizontal width of this zone is 4.0 meters according to the technical specifications. This material was produced in the crushing plant. The semi pervious transition material was selected from moderately weathered to fresh rock for processing. The suitability of the material was tested in accordance with the relevant standards. This material was placed in layers of 0.4 m at the specific moisture content and compacted by at least 4 passes of 10 tons vibratory roller in accordance with the results of the field compaction tests. The surface of each layer was wetted during compaction by water spray. The main procedure for regulation and protection of surface of embankment is construction of a lean concrete curb along the upstream edge of the embankment before placing the transition material. The 0.40 m high curb section has the external face inclination of slope of upstream face, i.e.1:1.6 (v:h) and a sub vertical internal face to act as a lateral support for the 2AA and 2A materials during compaction. A 12 cm wide crest allows for a minimum overlapping of the curb at successive layers.

2B Material) The coarse transition zone 2B with a constant horizontal width of 4.0 meters is located beneath the semi pervious transition zone 2A and act as a filter layer and prevent movement of the fine transition material into the main rockfill. This layer was selected from moderately weathered to fresh sound rock. This material was obtained from grizzly. As for zone 2A, the 2B type material was placed in 0.40 m thick layers to an approximately horizontal surface in such a way that prevents segregation of the particles and was subsequently compacted with at least 4 passes of a 10 tons vibratory roller and according to results of the field compaction tests. The different rockfill materials had different thicknesses. The thickness of 2AA layer had 20 cm, while the thickness of 2A and 2B layer was 40 cm, and 3A layer was 100 cm. In order to avoid great differences between the elevation of the surface 3A and 2A/2B layers, it is better to execute one layer of 3A material and then 2 layers of 2A/2B material and try to follow this sequence in the placement of each layer of 3A, as much as possible. In this way the surface of rockfill layers will be as smooth as possible. One other advantages of giving priority to the execution of coarser material is that the coarser material gives lateral support and confinement to the finer material which results in better compaction for finer material. More details on rockfill construction method and also plinth are presented by authors in references [1] and [3].

Foundation Preparation

Stabilizing of a Public Road Passing Near Dam Excavation Zone

The general criterion for the foundation of CFRDs is that it shall consist of non erodible material with adequate shear strength and deformation characteristics [1]. In the upstream side of the dam foundation, where most of water load is applied to, the excavation could be continued up to the bedrock. For the foundation of the plinth and the upstream part of dam competent foundation rock which is groutable and erosion resistant should be reached. In this portion, if blasting is required, only controlled blasting procedures should be applied close to the plinth foundation. This criteria is to minimize disturbances of the rock foundation and to minimize over break, therefore in this region pre-splitting method could be employed.

The foundation of the central and downstream part of the dam does not require such stringent requirements. However in both CFRDs of Siah Bishe project, the foundation was stripped to rock surface.

Foundation preparation is predecessor of execution of plinth and rockfill placement. The Siah Bishe CFRDs are located in mountain region with a very complex geology and high seismicity. Due to high seismicity of this region, there have been many small faults detected during excavation. But one of them with 15m width was detected during excavation of lower dam. Fortunately this fault was inactive and treatment measures were applied for this fault. The details of modifications in design and construction are covered in reference [1] and [2] by authors.

This region is flagged by geological instabilities. Due to geological and geotechnical investigations, several minor and major landslides were discovered. One of the most important ones, called Duna landslide, is resting at the downstream side of upper CFRD. To counterbalance forces of this old huge landslide, a big mass of embankment material was placed downstream on upper dam. This mass is called stabilizing fill and details of its design in presented in reference no. [2].

As mentioned before in this region, slopes are usually steep and unstable and there are an almost thin topsoil layer covering the under laying rock, Figures 4 and 5. In most regions near to both dams, these soil layers are extended up to several hundred meters high. These slopes are marginally stable in nature which any disturbance in their in-Situ situation, can lead to landslides. Therefore any incautious excavation at the elevation of dam foundation can trigger tons of soil to come down.

In this way before foundation preparation in most problematic zones, stabilization of slopes should be performed. These time consuming slope stabilizations were necessary due to two reasons:

a) Construction activities of dam body could be started after excavation and foundation preparation. Before any excavation, the slope should be stabilized to prevent landslides.

b) Slopes in the fluctuating zones of water level are more prone to instabilities and should be stabilized prior to dam impounding. In other words, pumped storage action will cause about 30 meters water level fluctuation every day. Lowering of water level can cause slope instabilities similar to rapid draw down in clay core dams where water will inflow from the soil mass out and reduce factor of the safety of slope.

This situation in Siah Bishe project made foundation excavations, the most problematic and time consuming task in design and construction.

Both dams are located near to main public road named Chalus road. This road is one of the main roads which connects the capital, Tehran to Caspian seaside. This road passes through Alborz mountain chain as is a heavily loaded road especially during holidays. Due to its importance, the highway ministry did not allow closure of this road for any construction activities. Therefore all construction activities must be planned with no interference with traffic flow of Chalus road. However the geological properties of this region are inherently near to instability. So that in winter many avalanches occur in this area and in spring, during melting of snows, there are numerous rockfalls.

In the lower dam, the chalus road level is about 40 m above dam crest level and only about 60 m far from the end of dam crest, as shown in figure 6.

The limits of excavation in dam reservoir were extended up to the slope and in some places undercut the foundations of the road. The Chalus Road is founded on overburden and instabilities below the road may also affect the stability of the road itself. There are several gullies filled with thick layer of soil material where before any excavation in these zones (zone 1 to 3), stabilizing measure should be performed.

Geological conditions

The road is founded on the rock except in three gullies where the road is located on overburden soil. The gullies have been named zone 1, 2 & 3 from dam axis toward upstream, figure 6.

The bedrock contains mainly sandstone, shale and some tuffs which belong to Upper Doroud formations. The bedding is gently (15-20°) dipping toward south which is favorable.

Thickness of overburden was about 15m and composed of mainly gravel with silt. The material has angular fragments, and was moderately pervious and compacted. Several cracks have been observed on pavement and shoulders of the road and used as a guideline for back analysis to find the material parameters. In this regard, safety factor was assumed to be equal to one in static condition. Results of in-situ and laboratory tests on soil were utilized to verify the assumptions.

Measures to stabilize road before excavations

Many different measures were evaluated including diversion of traffic to a new tunnel and stripping of all top soil, solpe stabiliziation by different measures including soil nail, anchors, piles and so on. Finally post tensioned anchors were foreseen to be installed to stabilize the slope. These anchors are installed parallel to each other and sloping downwards at 12 degrees. The anchors extended through the weathered rock and bonded in the sound rock. The capacity of anchors was derived from anchor investigation tests at the site.

Excavation and support of right abutment, below chalus road were done through staged excavation and support of the slopes with shotcrete, wire mesh and installation of prestressed anchors in each stage were performed.

Each stage of slope stabilisation includes:
–    Removal of overburden
–    Applying reinforced shotcrete on the excavated face immediately after excavation
–    Construction of concrete anchor pads,
–    Drilling the holes to the specified depth
–    Grouting the hole in case of air loss and re-drilling the hole after enough setting time,
–    Installation of tendon and grouting,
–    Allow 7 days or more, for grout to gain required strength,
–    Stressing the tendons to the designated load and perform acceptance (Proof) test,
–    Performing lift-off test after 24 hours,
–    Excavation of the slope down to the next step.

Excavation Design

The geometry of excavation, especially slope of excavation was mainly enforced by the reservoir stripping plan, so that in the areas where stripping of the overburden material to the bed rock is foreseen, the slope was designed so that it do not undercut the Chalus road. Another criteria for excavation plan was minimum required number of tendons.

Considering these factors and slope stability analysis, a slope of 2V:1H in zone II and 1.5V:1H in zone III with 10m high benches and 3m wide berms, has been designed and constructed for permanent slope stability.
Typical geological section of slopes with thin layer of top soil covering the underlying rock is shown in Figure 4. Cutting toe of the slope and starting excavation can trigger sople stability issues in dam site.

Anchor installation

The long term stability of the slope excavated below chalus road was foreseen to be provided by installation of pre-stressed tendons. The installation method of anchors were as follows:

a) Preparation of the anchor pads

At first anchor pads with specified dimensions were constructed. To have suitable bedding for pads, the soil surface was treated by shotcrete firstly, Figure 8. Concrete with Maximum aggregate size of 25 mm and minimum 28 days strength of 25MPa (250 Kg/cm2) were used for construction of the pads, Figure 9.

b) Drilling

Drilling of the holes were done by rotary-percussion. The hole diameter for 90mm double corrosion protected anchors was 147 mm and 117 mm in overburden and rock, respectively. The hole diameter provided a minimum grout cover of 12mm for the tendons. To prevent clogging of the hole due to any possible material falling, an extra drilling length of 500mm were provided.

After drilling, the hole was cleaned by using compressed air. If there was water seepage / flow in the area of the bond length of the anchor, the hole was grouted by a pressure of 2 bar excess of the hydrostatic head measured at the top of the hole. Then after approximately 12 hours it was re-drilled and prepared for anchor installation.
Drain holes might be required below such anchors to drain the water before anchor installation.

c) Anchor Preparation

The anchors were prepared and assembled at site. This stage includes corrosion protection of the anchors. Anchor preparation includes cutting strand by required length; removal of plastic sheet and degreasing; fabrication of tendons by means of steel wire and plastic spacer; fixing of spacers; covering of corrugated sheet over bond length, fixing centralizers on corrugated sheet and fixing end cap on bond length end.

d) Installation of anchors and Grouting

Installation of anchors could be done by a special pushing device, which guide the anchor into the hole. After putting anchor in the hole, grouting was performed. Grout material for anchores were cement type II or V with W/C = 0.38 to 0.45.

Admixtures were used to reduce w/c ratio and minimize shrinkage and also to improve workability of the grout mix. Workability of the grout were be checked by marsh cone and the outflow time of 11 and 25 seconds for 1 litre cement grout through the 10 mm marsh funnel were considered acceptable. Compressive strength of the grout mix after 28 days was around 30 MPa. Grout mix design were defined and optimized by laboratory tests. The installed achors in gully number 2 is shown in Figure 10.

e) Tensioning and Testing

Stressing and testing are required for every anchor, to fulfill the following two functions:

–    To stress and lock-off the tendon at its specified load
–    To ascertain that the anchor meets the acceptance criteria

During stressing, safety precautions are essential. Operators and observers must stand to the side of the stressing equipment and never pass behind when it is under load. No tendon were stressed beyond 80% of the specified minimum tendon strength (Fpu). Tensioning the anchors to the specified load was done after grout reaches the required strength. Tensioning was done after setting of gout and least 7 days after installation and grouting. Before using the stressing, the equipment was calibrated within an acceptable accuracy using relevant graphs.The anchors were tensioned to 1.33 times of design load and locked at design load.The design load for each tendon was specified based on slope stability.

f) Monitoring System

Monitoring is an important stage of tendon installation. Monitoring systems for excavated slopes consist of geodetic points, inclinometer, load cells and convergency measurements. Installation of convergence sections were done when excavation of relevant part will be started. Zero readings were taken immediately after installation. After that, daily readings were taken for at least 15 days or till the excavation reach the lower berm, and after that, weekly reading were continued.

Concrete Face Slab

The Siah Bishe project was the first CFRDs designed and constructed in Iran. Therefore the first face slip forming with concrete placement was certainly on the “learning curve” of those involved in the face slab placement, and the construction of small slabs at the beginning of the works can be used as trials for the larger slabs of the main face slab work, as has occurred in similar projects.

The upper and lower dam concrete face slab is 84 and 104 m in height respectively. The thickness varies with its elevation according to the formula T=0.30+0.002H for both dams. Construction duration of the concrete face slab has been determined as the construction time of starter slabs with temporary formworks plus the time for construction of the main face slab with slip forming. The upper and lower dam consist of 35 and 27 stips of slabs respectively. The total construction area of face slab is 33,700 m2 for the upper dam and 35700 2 for the lower dam (excluding the starter slabs).The total amount of concrete for upper dam was 14,300 m3 for the upper dam and 15,900 m3 for the lower one (excluding the starter slabs). Concrete face slab construction was conducted in four main activities: Cleaning, Preparatory works, construction of starter slabs and main slabs.

Before start of this stage of construction, some predecessors including: Access road, water, air and power supply, lighting were prepared. Also concrete curbs and gaurd rails were installed to prevent accidents due to falling objects.
For construction of face slabs, several equipments and facilities should be provided on dam crest or nearby. Some areas in construction layout were dedicated to followings: Store house for concrete tools and joints material, rebar processing and pile, Side formworks, rails, supports parts, and fixed bolts piling, wood workshop, concrete delivery chutes, slope ladders, cabling, piping and lighting facilities, power, sand asphalt mixture preparation, slip forms and trolley car for reinforcement, multipurpose trolley cars for mortar, transporting materials and tools.

Firstly, even concrete face strips were poured alternately from central part towards right abutment and then towards left, and the remaining intermediate slabs were poured with a minimum of eleven days interval between adjacent strips.

Treatment of the Bedding of Concrete Face

Cleaning of the concrete face bedding

For temporary protection of the fill surface, extruded curb method was employed. Therefore preparation of rockfill surface for pouring concrete face did not need trimming and cleaning of the bedding was limited to using brooms and compressed air. A fine cleaning with compressed air must be carried out before the protection with the bond breaker. The final and comprehensive cleaning was carried out after the completion of preparatory works just before concrete placement. The material retained in the joints and edges have been manually removed. The most important area to be prepared is the perimeter joint, close to the plinth. This is, also the most difficult region to be cleaned due to the concentration of fine and coarse material and dust, and due to the presence of plinth waterstops, special attention and care must be dedicated to this area.

Any damage to waterstops will arise many difficult and time-consuming tasks. Therefore before start of any cleaning activity, the joints were covered securely by wooden planks.

Checking for cavities beneath the face bedding (Curbs)

The bedding will be carefully checked for existence of possible holes, in order to realize probable cavities beneath the curbs. Holes with maximum diameter of 20 cm and minimum depth of 30cm were dug through the extruded curbs, to reach the boundary between curb and 2A material. If necessary, grouting with mixture of pozzolanic cement with water-cement ratio of 0.8-.85 should be done. But fortunately no gaps were observed in Siah Bishe project.

Checking for cracks in curb-wall

During the cleaning of the curbs, visual inspection was done to check the existence of cracks or defects in curb-wall. Repairs should be done if necessary. No cracks were observed in upper dam, but some cracks were detected on curbs almost along the perimeter joint in lower dam. It was believed that the cracks were due to the settlement of dam body in higher elevations, which caused lateral movements of rockfill towards outside of dam body in lower elevations. As the plinth is a fixed point, some cracks appear in the inflection points of the dam body. The cracks were monitored for about 4 months after finishing of rockfill of body of dam, as well as the settlement of dam body and dam crest. The length and width of the cracks on the curb were stable times before than these 4 months, but however, exploratory holes were drilled to ensure that no gaps were beneath the curb in whole length of these cracks. Hence, the creep settlement of dam body came to less than 8 millimeters per months (2 mm per week) after 4 months, and the concreting of concrete face started afterwards.

Final Survey of the Bedding

Due to settlement of rockfill and dam body during construction, the surface of curbs could be deformed. The deformation would be greater in higher dams or those constructed with weaker rockfill. Therefore, final surface of the bedding should be determined before construction of face slab. The surveying with a grid of 3m x 3m were conducted. For more confirmation, another surveying were also done in the alignment of mortar pad and in the middle of slab. The last surveying was when the side formwork was installed before start of concreting of each slab. The comparison between these surveying in different stages with the surveying of the curb during the placement of rockfill can show the history of settlements.

Peparatory Works

Mortar Pads And Vertical Joints

Before construction of the starter slabs and main slabs, the bottom embedded part of the vertical joints should be completed. The first stage would be placement of the mortar pads. Details of the vertical joints are illustrated in Figure 11.

Placement of the Mortar Pads

The main purpose of this pad is to ensure the perfect laying of the PVC band and copper waterstop over the slope. The dimensions of mortar pad are 0.80m wide by 3cm thick. The pads have another very important function, being the surface where the side formworks are positioned and consequently the support to the slip form, Figure 12. The center to center distance between the mortar pads is defined by the width of the face slab strip.

The mortar mix was composed of cement/sand with the ratio of 1:3. The mortar was transferred from the dam crest to the point by using half-barrel chutes, or multipurpose trolley cars, and was shaped by hand trowels. The mortar pad surface must be flat, and checked by survey and comply to design specification.

Bond Breaker- Emulsified Bitumen Coating

The emulsified bitumen specified was the Bitumen MC-250 with water content of 0.2% in volume, and the mix design as used in starter slab.

Installation of PVC band

After placement of the mortar pad a PVC band of 6 mm thickness and 50 cm width was placed. This would be the final base for the copper waterstop, Figure 14.

Copper Water stop

The copper water stop for both vertical and perimeter joints were made from copper sheet of 1 mm thickness and 475 mm width. The copper class was DHP-O (UNS 12200).

Before fixing the copper water stop on the PVC band, the middle bulb should be filled with neoprene, in order to avoid its distortion or crumpling during construction, under the fresh concrete hydrostatic pressure, or as a cause of probable differential movements between adjacent slabs, Figure 13 and 15. Therefore, a neoprene rod, 20 mm in diameter, was placed at top of bulb at first, and then the remaining part of the bulb was filled with neoprene foam filler from the bottom.

A non-standard simple testing system, whose idea was obtained from Kannaviou dam, has been prepared at the site. A vacuum chamber is fixed over the joint and the perimeter of the chamber was sealed completely to get a fully isolated space inside the chamber. Then the chamber was connected to a suction pump and a negative pressure equal to 0.8 bar applies inside the chamber and then the valves close the air connection hosts. In this way we have a fully isolated chamber, vacuum inside, over the joint. It is expected that if the connection is fully sealed, then the pressure gage will be on a constant pressure but if the brazed joint has any void area or discontinuities, then air suction will happen only through the joint (because the perimeter of the joint is sealed) and the pressure gage will drop during the time. This method is not a standard test, but can be a simple and useful instrument to check the continuity of the joint and the void area inside the brazing joint.

The design of the perimeter joint is shown on Figure 16. Construction of the concrete face can be started by completion of the perimeter joint. The top hypalon waterstop and GB filler would be placed after completion of the concrete face slabs.

Cleaning of the F-Copper Waterstop and Plinth Face

Profile of the copper water stop is shown on Figure17. After construction of plinth, half of the waterstop is previously placed within the plinth face, and the other half (protected by the wood plank) is waiting to be placed at the concrete face. Therefore, the protective cover of the F-copper water stop was removed to be exposed to poured concrete. Then the outer face of plinth and waterstop were cleaned carefully by hand.

Placement of DFOT System (Distributed Fiber Optic Temperature Measurement)

This system was used for seepage measurements along perimeter joint. The fiber optic cables were already installed below the sand asphalt mixture in 2AA material. During the construction of perimeter joint, special care should be considered to avoid any damage to this system.

Equipping the Copper Waterstop with Neoprene

After removing the protective wood plank, surface of the 2AA material was dug immediately downstream of the plinth by careful hand excavation, in order to place the sandasphalt mixture later. This was done in a trapezoidal shape parallel to the 2AA surface, with the depth of 15 cm, and the top and bottom lengths of 45 cm and 25 cm, respectively.

PVC band

After filling the center bulb with neoprene and foam filler, a PVC band of 6 mm thickness and 20 cm width is fixed to the bottom of the copper water stop, immediately downstream of the plinth reference line.
Placement of Sand-Asphalt mixture

Next stage was filling the sand-asphalt mixture beneath the copper waterstop. The sandasphalt mixture is made by 5-7 percent (by weight) of steam refined asphalt.

Reinforcement

There are two alternatives to place the reinforcement on the face slab. The first alternative is placing all the reinforcements “in-situ” by workers. The second alternative is using prefabricated rebar. In order to improve the production rates, pre-fabricated reinforcement were implemented using a trolley car to deliver the pre-fabricated mesh. The reinforcement will be placed at the middle of the slab. Within 1.0m of the perimeter joint and the vertical joints, 2 layers of additional reinforcement were placed around the central main reinforcement layer, as anti-spalling reinforcement. The vertical rebar were continuous through the construction joints at the top of the starter slabs. No horizontal reinforcement passed through the vertical joints.

“In-situ” Reinforcement Assembly

In the areas where using the pre-fabricated reinforcement is not possible, the reinforcement works will be carried out “in-situ”, on the slope surface. These areas includes starter slabs; Anti- spalling reinforcement; slabs where can be worked simultaneously with the second alternative; Following Figures 18 and 19 show an example of “in-situ” reinforcement assembly.

Main Slab Formwork

Side formwork

The even slabs will be the leader slabs and cast in sequence from the left to right bank. In this way, the side form will be assembled at the both sides, only for the even slabs and two sets of the side form will be utilized.

Location and leveling

The location and leveling of the mortar pads guided the position of the side forms. The exact location of each panel of the side form must be accurate since, due to variable thickness of the designed face slab, each panel has a height different from the others. As the thickness of the slabs varies per equation T= 0.30+0.002H, the differences between the neighbor panel side forms will be about 1mm, which is difficult to pre-fabricate. All steel side formworks of 2 m length therefore had the same height, also slab thickness varies from 0.30 m to 0.42 m.
Compensation of slab thickness differences were treated by using timber shims installed under the side forms.

Labor Access Ladders

Due to gradient of face slab slope and the height of dam, it is necessary to construct and permanently maintain the ladders with handrail, not only to make access easier to the work sites but also to prevent accidents.

Fixing the Side Forms and Rails

Placement and fixing the side forms would be started, when the copper water stop is fixed and reinforcement is installed. Side forms also serve as supports for the slip form rails, hence top edges of these forms should coincide the top surface of the concrete face.

Side Formwork Installation

The installation of the side forms began after the completion of the reinforcement. The first task is the assemblage of the rail support structure at crest of the dam and the concrete counter weight blocks. Then the first rail stretch with the upper panel could be positioned, Figures 20 and 21.

Rails Installation

The rails (I beams) were assembled over the side form panels through the special designed clamps bolted to the panel frame. The rail were joined by means of splice plates and bolted to the clamps. The alignment of the rails is mandatory, mainly at the splices, since the clearance of the safety nuts are very tight.

Slip form

The face slab was constructed by railed slip form after installation of the side formworks, reinforcing steel and other embedded parts. The slab strips were 12 m wide, and numbered from 1 to 35, left to right in the upper dam. The slip form system consists of:

a)     Slipform and work platform with accessories platform
b)     Finishing platform
c)     Concrete distributor
d)     Cars with steel wheels to ride on rails on both ends of the System
e)     Jacking and climbing devices also on both ends
f)     Hydraulic power pack for the jacking
g)     Side forms
h)     Rails
i)     System anchor at crest with counterweights on both ends, which is also for System removal at the end of each slab.

Adjustment of the Slip form and Placement of Concrete

After placement of the rails, the slip form was lowered by a crane on the rails to the bottom of the first main slab to be constructed, using a 15 tons winch.

Concrete Placement

Chutes Installation

At least two lines of chutes were installed to deliver concrete from the dam crest to the slip form. Fresh concrete was transferred to the crest platform by truck mixers, then poured to the two chutes using simple half-barrel chutes and delivered to the front of the slip form, 1.0 to 1.5 m away from the work front, by using chutes and distributor, Figure 22. Chutes laid over the reinforcement and strengthened with welding to steel in order to avoid deformation during concrete placement. To keep the concrete from water loss/rainfall in course of sliding down, the top of chute were covered with rainproof clothing. Covering was fixed properly to scaffold to avoid damage due to hard windstorm. Two concrete distributors were set at the top opening of slip form and were connected with main chutes.

Concreting of the Face Slabs

The concrete for the face slab would be a workable mix with pozzolanic cement and of C25 class, with the f’c of 25 MPa at 28 days and the maximum size of aggregates equal to 25 mm. The fresh concrete had the suitable slump between 8 to 10, and micro air entrained between 4% to 6%. The team worked about 6 months on proper mix design to ensure the workability and durability and water tightness of the concrete. The construction of the face slab was accompanied by continuous monitoring of the rate of placement, excess volume of concrete, amount of admixtures, i.e. superplastisizers and micro air entrained to provide suitable slump and percentage of micro air entrained, permeability, and expected strength to register a statistical analysis of each pour.

Weather conditions during concrete operations

The of concrete using slip forms was continuously placed within each strip of face slab. Second batching plant was always ready to support the main batching plant in the case of any unexpected damages that might interrupt the continuity of concrete. The weather forecasts were monitored closely to determine the condition during the placement of concrete and to make arrangements to do this job with high quality and good progress without any interruption. However the action plan for hot or rainy weather was determined before start of face slab concreting and all provisions were provided for such situations. The speed of slipform which is related to the initial setting time of concrete was adjusted with temperature

Dealing with interruptions to the concreting operations

Suitable measures were taken to reduce risk of interruption during concrete placement. However interruptions due to mechanical breakdowns, weather, people, etc., might occur and have an adverse effect on the concreting operation. For this case, the following Instructions and procedure were followed:

–    Enough expected maintenance spare parts were be kept on site
–    The shift engineer at the concrete batching plant would inform the shift engineer at the concrete face slab if there are going to be any delays in the concrete supply. In this situation the concrete placing operation would be slowed to prevent long delays later and possible cold joints
–    If a mechanical breakdown occurs on the Slipform platform, the shift engineer at the batching plant should stop mixing. This is to prevent concrete waiting to be placed for excessive durations. Should a mechanical breakdown occur in the batching plant, the stand by batching plant must be immediately mobilized
–    If the last placed concrete achieved initial set due to the duration of the breakdown being long enough, the concreting operation should be stopped, the breakdown repaired and a construction joint prepared with green cutting before placement of a new concrete.

Surface finishing and curing of the concrete face

During the slipforming, the surface finishing was done from the lower platform of the slip form, using hand trowels, Figure 23.

The curing was started with curing compound when the concrete was still fresh to avoid shrinkage cracks, and continued by water pipe until 28 days and covering the face slab by sack-type covers, Figure 24. Usually a 12m wide by 9m long PVC or plastic sheet (0.8 kg/m²) or longer was fixed to the handrail of the finishing platform. It is suggested that both working platforms are roofed with the same type of plastic to protect also the workers against sun and rain. The PVC or plastic sheet spanned the last 7 m of the finished concrete, allowing the concrete to achieve initial set as well as protecting the surface against possible rain damage. Groove or chamfer on face slab concrete along vertical joint was made by square timber being processed with its respective size according to design, and every section of the mold was 3m long.

After completion of the slipforming of a main slab at the top, the slip form was transferred to another slab, using a crane. The slip form was first pulled over the rails, then it is again lowered on the new slab rails using one 15 ton winch, until reaching to the new slab start point. The slabs were constructed in an alternate order from left bank, in order to use the adjacent constructed slabs as the side forms for the new slab. The contact element between adjacent slabs was applied by soft wood filler with 20mm thickness.

Conclusion

In this paper Siah Bishe pumped storage project and its lower and upper concrete face rockfill dams are introduced. The different parts of dam body, its cross section, specification of each zone and the method of construction are presented in second part. Both dams are located near a public road. The geology of the region is so that any excavation from lower parts can trigger landslides and rockfalls to any size. The challenges among construction of dams regarding dam foundation excavation are discussed in third part of this paper. Details of excavation and post tensioned anchors are mentioned in this section. It was concluded that when CFRDs are located in specific regions with geological issues, the foundation excavation can be a difficult task which consumes so much time and cost of the project. At the fourth part of paper, gained experiences during execution of concrete faces of dams are explained. This section is dedicated to different stages of construction of face slab using slip form technology including preparatory works, reinforcement, concreting and curing.

References

–    L. Modarres & Z. Ghannad , Construction of Siah Bishe CFRDs ,7th Dam Engineering Conference, Lisbon, Portugal, (2007).
–    Z. Ghannad & L.Modarres, Siah Bishe Concrete Face Rockfill Dams; Challenges with Geology and Climate, HYDRO 2008 Conference, Slovenjia (2008).
–    Z. Ghannad, Stability of Plinth Blocks in Concrete Face Rockfill Dams , 2nd International Conference of Long Term Behavior of Dams ; Graz, Austria (2009).

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