Roller-compacted-concrete (RCC) dams are characterized by fast and economical construction, which requires the fast and cost-effective transport of RCC mix. Horizontal transport of RCC mix is fairly straightforward, and dump trucks and/or conveyer belts are predominantly used to build: RCC dams in valleys with gentle abutment slopes, low and middle height RCC dams, and the lower part of large RCC dams in steep-sided valleys.1
Vertical transport of RCC mix in deep gorges with steep abutments is a difficult task, and it often becomes a bottleneck in the construction of large RCC dams. For an RCC dam in a deep valley with steep abutments, say with a slope exceeding 40 degrees, the construction of temporary roads for trucks or installation of conveyer belts on the steep abutments become difficult and expensive, or even not viable. Other transporting equipment and methods shall be considered.
Requirements for vertical transport of RCC mix
Equipment for the vertical transport of the RCC mix should be determined considering the local topographical conditions and the construction characteristics and schedule. Below are important criteria:
Topography at the dam site is a critical factor influencing the selection of the delivery methods. The contractor’s experience will considerably influence the selection as well.
Technologies for vertical transport of RCC mix
Although cable cranes and tower cranes can be used for the vertical transport of RCC mix, the transport capacity of such devices is limited and cannot meet the fast construction requirement. To tackle the hindrance of the steep abutments and meet the fast construction requirement, various technologies have been developed and applied in the construction of large RCC dams. Of the special technologies, the vacuum chute, full-tube chute, M-Y boxes and M-Y box-pipe system, cable bucket-car system and slow drop elephant trunk should be highlighted. These methods reflect the current knowledge and practice in this field.
In principle, these vertical transporting systems convey concrete mix primarily relying on gravity, resulting in energy saving and cost reduction. All of these methods depend on local conditions and must be combined with the horizontal transporting devices.
A vacuum chute is a sealed semi-circular conduit system developed by Chinese engineers.8 Principally, it uses the vacuum created as the RCC mix moves in the closed chute and friction induced by the chute body and flexible cover to adjust the mix moving speed. A vacuum chute consists of the following primary elements:
The vacuum chute system is supported on rigid frame columns of steel (see Figure 1).
Figure 1: The vacuum chute system is a viable option for moving RCC mix vertically.
The RCC mix is dumped into the hopper. By opening the valve, the mix slides into the transit section by gravity, where the mix movement is accelerated. As the mix enters the chute sealed with the flexible cover, its speed is further increased by gravity. Meanwhile, the pressure within the sealed chute is decreased, forming a vacuum. The pressure differential between the chute interior and exterior (atmosphere) results in deformation of the flexible cover, which restricts the mix movement, decreasing the falling speed. As the speed slows, the internal pressure increases again, leading to recovery of the flexible cover deformation, so that the mix falling speed is accelerated. As the RCC mix slides down the chute, this process is repeated, resulting in a wave-like movement of the cover, so that the falling speed of the RCC mix is controlled. The magnitude of the vacuum and the speed of the mix movement can be regulated by adjusting the valve opening in a desirable range of 10 to 15 m/s at the outlet.
Using the vacuum chute, segregation of the RCC mix while transporting is prevented, and the change in VeBe time is minor. The vacuum chute has a high efficiency to vertically deliver the RCC mix, with a transport capacity of 200 to 550 m³/h. Moreover, its manufacturing costs are low and it is convenient in operation and maintenance. The vacuum chute system is suitable for an abutment slope of 40 degrees to 70 degrees, with the optimal slope of 45 degrees to 55 degrees. For a slope below 45 degrees, the chute may get choked. When the slope exceeds 55 degrees, the friction of the RCC mix to the chute body and impact on the outlet elbow will be intensive, requiring frequent repair. The maximum descent height of a single chute should not exceed 70 m.
The full-tube chute (also called full-bin system) transports RCC mix with its full section.7 The full-tube chute does not create the vacuum during the RCC mix moving. Instead, the mix moves in the full-tube chute in a controlled speed under pressure. Unlike a drop chute, the mix movement in the full-tube chute is not in free fall.
Similar to the vacuum chute, the full-tube chute contains: a collection hopper at the inlet, the full-tube chute body (mostly in standardized section lengths and diameters), a radial valve and an outlet elbow. The hopper has a typical capacity of 12 to 20 m³. The full-tube chute system is supported on the rigid steel structure on the slope (see Figure 2). As the dam raises, the lower part of the tube can be successively removed to allow the placement to continue. Sometimes, an additional radial valve can be installed at the inlet to control the hopper and tube filling at the starting time.
Figure 2: This schematic shows the full-tube chute system, used to deliver RCC mix vertically.
The full-tube chute body of steel has a quadratic cross section of 40 x 40 cm to 80 x 80 cm side length or a circular cross section of 40 to 80 cm diameter. The wall thickness of the steel tubes is 10 mm or more. For convenience of construction, the steel tubes are manufactured in 1.5 to 2.0 m long pieces, which are connected with bolts. In the upper part of the steel tube, vent holes are drilled in the steel pipe. The radial valve is installed near the outlet at the lower end of the tube chute to control the mix movement. To avoid choking, a vibration device is frequently installed on the outside of the radial valve. In addition, the vibration devices can be installed along the tube if the slope is less than 55 degrees. Sufficient space should be maintained below the outlet area to allow movement of the horizontal delivery devices at the dam.
To start anew, the steel pipe must be entirely filled with RCC mix. During routine delivery, the steel tube shall be kept fully filled with RCC mix, requiring that the hopper be kept with RCC mix at least 1/4 of its height. Through regulation of the opening of the radial valve at the outlet, the speed of the concrete mix is controlled, so that the mix moves in the full tube under pressure to prevent free fall and segregation. In optimum, the RCC mix should move at a speed of 0.5 m/s and should be retained in the tube no longer than one hour. The steel tube shall be emptied when a placement interruption longer than one hour is predicted. Delivery of the RCC mix should be kept at a uniform speed, and the inlet and outlet should be balanced to avoid segregation of the RCC mix.
The full-tube chute is suitable for an abutment slope between 45 degrees and 90 degrees, with a maximum descent height of 10 to 100 m. A too-small height difference is not economic, while a too-large height difference will considerably increase the manufacturing costs of the chute. The transport speed of the RCC mix can easily be controlled during the normal delivery operation. However, the RCC mix may be segregated or even the coarse aggregate may be crushed during the first filling of the steel tube. This first batch of mix often cannot meet the requirement of the RCC dam.
M-Y boxes and M-Y box-pipe system
An M-Y box, also called M-Y mixer, is a vertical mixing and fall-damping box that was developed and applied in Japan as a concrete mixer and as a vertical transporting device.3 It features a series of box-shaped units comprising two twisted aligned boxes separated by steel plates. Each unit of the M-Y box has two parallel rectangular inlets and outlets. The width of the rectangle shall be at least three times the maximum size of aggregate to prevent blockage. The cross-sectional area is kept the same all the way. From an inlet, the rectangular cross section is gradually changed, using the steel plates, into a square in the middle and then gradually changed again into a rectangle at the outlet but perpendicular to the inlet in direction. The inner structure of the M-Y box causes the material to be kneaded by gravity while passing through each of the box units, allowing continuous mixing while transporting on very steep or even vertical slopes.3,4 Meanwhile, the falling speed of the concrete mix is damped.
In principle, the M-Y box system is more adequate as a mixer. Although it can be used to vertically transport concrete mix, the transporting capacity is limited, and the equipment is costly when the descent height is large. Nevertheless, material mixing and speed decreasing while transporting prevents the concrete mix segregation. This principle of the M-Y boxes can be used for the vertical transport of RCC mix (also conventional concrete) by combining the M-Y box units with steel pipes, so that the so-called M-Y box-pipe system is developed. In this system, a series of the M-Y boxes and steel pipes of 6 to 15 m length (depending on the slope steepness) are alternately connected in tandem (see Figure 3). The speed of the concrete mix is accelerated as it passes through the steel pipe section, while it is slowed and re-mixed in the M-Y box, reducing the falling speed and preventing segregation.4 Thereafter, it passes through the next steel pipe section and M-Y box unit. This process is repeated as the concrete mix is transported, until the mix drops out of the outlet.
Figure 3: An MY-box can be used to vertical transport RCC mix but is better used as a mixer.
To keep over-sized aggregate from entering the system, a screen can be installed at the inlet. The M-Y box-pipe system is especially suitable for the steep abutments with a slope of 60 degrees up to vertical (90 degrees), with the optimal slope of 70 degrees to 90 degrees. The M-Y box-pipe system is used in vertical transport of RCC mix, conventional concrete mix and hardfill material, including cemented sand and gravel in an increasing trend. The equipment is cost-effective and can be repeatedly used. Owing to the re-mixing, the uniformity of mix is improved. The M-Y box-pipe systems have been applied in the construction of several RCC dams with very steep abutments, especially in the construction of RCC arch dams.
Cable bucket-car system
In Japan, a cable bucket-car system (also called inclined railway system or incline) was developed and applied in the construction of several large RCC dams.5 The inclined system consists of two bucket-cars and two sets of rails installed on a slope abutment. RCC mix is loaded into the bucket-car that runs on the rails. To overcome the steepness of the grade, the bucket-car grasps a continuously moving cable for propulsion, hauling the bucket-car up and down the track. The maximum speed of the bucket-cars is 2.5 m/s. The maximum transport rate can reach 250 to 300 m³/h.5
It is worth mentioning that, instead of the bucket cars, an end dump truck can be directly placed on the platform and hauled down to the dam construction surface. This will save the mix loading and discharging, but the truck must be hauled down and up.
After the completion of dam construction, the bucket-cars can be modified into a cable railway for tourism, as was done at the Migagase and Tamagawa dams in Japan. However, the cable bucket-car system is difficult to use on very steep slopes, say 60 degrees or more. Moreover, placing the rails on steep abutments is demanding work.
Slow drop elephant trunk
The slow drop elephant trunk or hose is a flexible and abrasion-resistant rubber hosepipe and can be attached on the outlet end of a conveyer belt or other transporting equipment to vertically deliver the RCC mix. Steel collars are used to support the elephant truck and connect it to hopper at the end of a conveyer belt or a vacuum chute by chain linkage. The elephant trunk is not new but is a common device used in delivery of conventional concrete and pumping concrete. It uses the restriction of rubber hosepipe deformation caused by the mix falling in it. Using an elephant trunk for placing concrete can significantly minimize aggregate separation, reduce the amount of splatter and trapped air. The elephant trunk will allow the worker to move the hose short distances quickly if required.
In the construction of RCC dams, the elephant trunk is frequently deployed. However, its transport height is limited. Therefore, it usually is not used as a standalone device for vertical transport. Rather it is frequently used as an auxiliary device attached at the end of conveyer belts or vacuum chute system.
An application example
The Gomal Zam RCC arch-gravity dam in Pakistan is 133 m high from its low foundation level at El. 630.0 m, with a crest length of 231 m at El. 763.0 m.2 The maximum width at the base is 78 m. A four-bay spillway of 17.5 m length each is integrated in the central part of the dam, abutting non-overflow sections on both sides. A bottom outlet of 3.0 m diameter with an invert at El. 680.0 m is constructed in the center of the dam to flush sediment. The dam is located in a V-shaped gorge with a bottom width of 25 to 40 m. The narrow canyon is slightly asymmetrical and has steep abutments with an average slope of 75 degrees for the left and 65 degrees for the right flank, respectively, while its upper flanks tend to flatten with an average slope of 40 degrees to 45 degrees. The total RCC volume of the dam is 409,000 m³ with additional conventional concrete of 84,000 m³.
Gomal Zam dam in Pakistan was built using some of the vertical transport technologies discussed in this article.
For construction of the arch-gravity dam, various conveying methods were used:
The transport rate of the vacuum chute was 1,100 to 2,000 m³/d in an average of 1,600 m³/d and a maximum of 2,200 m³/d.
With these construction technologies, the problem of the vertical transport of RCC mix on steep abutments is solved with low costs of manufacture and maintenance. The applicability and efficiency of the delivery methods are demonstrated in the construction of large RCC dams. Using these delivery methods, aggregate segregation is avoided. The VeBe time increase of the RCC mix is minor. The heat gain is limited and the temperature rebound of the RCC mix is less than 1 degree Celsius even in warm summer seasons. It is reasonable to expect that use of this delivery technology with the transporting equipment will boost the further development and construction of RCC dams.
Chongjiang Du, PhD, is senior civil engineer with Tractebel Engineering GmbH.