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Concrete and cement-based pavings are traditional linings materials for a wide range of spillway channel construction and repair applications. Most of the cement-based mitigation techniques tend to eliminate or decrease the possibility of rock plucking occurring within unlined spillway channel. In many cases, the cement-based approach could be the best choice to mitigate erosion impacts on unlined spillways (Cameron et al., 1988). On the other hand, in some cases the application of these mitigation techniques is cost-prohibitive or inappropriate. Commonly used as a reinforced lining blanket, cement-based tech- nique comprises of grouting, lean concrete, high-strength concrete, rollcrete, reinforced concrete and dental concrete.

More details on cement-based techniques for scour mitigation of unlined spillways are given in the following paragraphs.

2.9.4.1 Grouting

Unlike most of cement-based mitigation techniques that are applied by workers and/or machines to the area of interest, grout is a cement or chemical-based mixture generally placed by borehole injection in voids and open rock discontinuities that are not accessible by workers or equipment. There are two types of grout namely; chemical grout which is used for soil consolidation and Portland cement which

is generally used to consolidate a mass of rock. Grouting techniques have many advantages which are:

• to consolidate or strengthen the whole rock mass by grout filling the open discontinuities

• to strengthen the entire treated rock mass in order to increase the overall strength of the rock mass

• after consolidation the grouted rock mass could likely resist uplift forces even during the maximum flood discharge event.

2.9.4.2 Lean Concrete

Lean concrete is defined as a low cement content mixture comprising of fine sand, aggregate and cement. Its main purpose is to fill voids and open discontinuities in order to prevent plucking and increase erosion resistance. According to Cameron et al. (1988) lean concrete is assumed to be a low-cost material and is typically used where high structural loads are absent, for instance it can be used to fill discontinuities located in the channel bottom .

2.9.4.3 Shotcrete

Shotcrete is a specific mixing and placement method. The shotcrete mitigation technique consists of spraying concrete on the area of attention through a nozzle equipped with a perforated manifold. Due to its low water-cement ratio, shotcrete can reach a high compressive strength, as pointed out by Merritt (Cameron et al., 1988) and therefore can play an erosion resisting and rock mass reinforcing role in unlined spillways. This mitigation method is recommended for exposed inclined surfaces.

2.9.4.4 High-strength Concrete

High-strength reinforced or unreinforced concrete consists of varying mixtures of concrete so that abrasive resistance and compressive strength are provided. These mixtures comprise of a basic concrete mixture, silica fume and a high-range water reducer providing a low water-cement ratio and a hard aggregate. Although high-strength concrete is especially effective in resisting abrasive-erosion at unlined spillways, it remains the most expensive of concrete-based treatments when applied over large areas (Cameron et al., 1988). In addition, its application requires drainage facilities to withstand uplift pressures.

Section 2.9. Mitigation Measures to Limit Unlined Spillway Scour Page 32 2.9.4.5 Soil Cement and Rollcrete

Rollcrete is a combination of a small fraction of cement mixed with soil before compaction and it is used to fill in relatively large voids. This technology is widely and successfully used as a remedial measure to erosion on some unlined spillways (Cameron et al., 1988). Nevertheless, it is generally not recommended for primary resistance to erosion and has relatively high shear strength and very low compressibility when placed in thick bodies (Cameron et al., 1988).

2.9.4.6 Reinforced Concrete

Reinforced concrete is the cement-based mitigation technique that has commonly been used for a long time in hydraulic structure. Its particularity lies in the use of large amounts of reinforced steel which must be installed before placement of the concrete. This makes reinforced concrete cost-prohibitive when it has to be applied over a large area of unlined spillways. Therefore this remedial technique is recommended only for use in a small portions of the spillway channel. Special care should be taken not to allow flood waters to flow beneath a reinforced concrete section in order to avoid the removal of the subgrade material that can be caused by turbulence which could possibly lead to the failure of the remedial measures. Although reinforced concrete often proves to be very expensive and susceptible to undercutting erosion, it is believed that it offers highest assurance of erosion resistance for unlined spillways, under conditions of proper placement.

2.9.4.7 Dental Concrete

Dental concrete mitigation technique is based on the use of high-strength concrete to fill in irregularities on excavated bedrock. This technique is generally applied directly to joint-bounded surfaces in order to yield a high-integrity bond between rock and concrete. As a result, the dental concrete technique is basically suitable to remediation at unlined spillways in which the high-strength concrete is used to fill in joint-bounded surfaces on dipping and jointed bedrock. One great advantage of dental concrete is that it does not only reduce the entrance of water into the joints but also smooths the channel surface which contributes to a decrease in the flow turbulence and, therefore, keeps overall erosive forces to a minimun. To be more effective, dental concrete can be used in conjunction with rock bolts.

surfaces. This helps to withstand sliding or to bring individual discontinuity-bounded rock-mass blocks closer together in order to resist the erosive capacity of the water (Annandale, 2006).

Dental concrete combined with rock bolts is illustrated in Figure 2.10.

Figure 2.10: Representation of rock bolting at spillway (Cameron et al., 1988)