Hydropower Water Conveyance System (Part - 3) | Additional Documents & Tests for Civil Engineering (CE) PDF Download

Tunnels 

Tunnels need to be designed and constructed in an efficient manner for the best performance. The Bureau of Indian Standards code IS: 4880-1976 “Code of practice for design of tunnels conveying water” (Parts 1 to 4) provide guidelines for design of a tunnel under various situations. The following paragraphs provide the salient points from these codes. 

Tunnel layout 

The first aspect that needs to be decided for a tunnel is the alignment, that is, the route layout of the tunnel in plan. Figure 18 shows the possible alignment for the tunnel water conveyance system for a hydropower system using tunnel.

Figure 18. Typical layout of a tunnel alignment & other structures

The layout is usually governed by the geological features of the surrounding hills. Complicated geological conditions and extraordinary geological occurrences such as intra-thrust zones, very wide shear zones, geothermal zones of high temperature, cold/hot water springs, water charged rock masses, intrusions, fault planes, etc. should preferably be avoided. Sound, homogeneous isotropic and solid rock formations are the most suitable for tunneling work. However, in the Himalayan region, such conditions are rather rare compared to the hills of peninsular India. This is because the Himalayan geological formations are mostly sedimentary in nature whereas the peninsular upland of the country is of igneous nature. Hence, geological investigations have to be carried out in detail before a tunnel alignment is finalized. 

Tunnel section 

The second aspect requires the determination of the size and shape of the tunnel. The size or cross sectional area can be determined from the amount of water that is to be conveyed under the given head difference. Regarding shape, the following types are generally provided for hydropower tunnels: 

1. Circular Section (Figure 19): The circular section is most suitable from structural considerations. However, it is difficult for excavation, particularly where crosssectional area is small. For tunnels which are likely to resist heavy inward or outward radial pressures, it is desirable to adopt a circular section. In case where the tunnel is subjected to high internal pressure, but does not have good quality of rock and/or adequate rock cover around it, circular section is considered to be the most suitable.

Figure 19. Circular shaped section of tunnel

2. D section (Figure 20): This type of section would be found suitable in tunnels located in massive igneous, hard, compacted, metamorphic and good quality sedimentary rocks where the external pressures due to water or unsound strata upon the lining is slight and also where the lining is not required to be designed against internal pressure. The principal advantages of this section over horse-shoe section (discussed in next paragraph) are the added width of the invert which gives more working floor space in the heading during driving and the flatter invert which helps to eliminate the tendency of wet concrete to slump and draw away from the tunnel sides after it has been cast. 

D : Width of tunnel

Figure 20. D-shaped section of tunnel

3. Horse-Shoe and Modified Horse-Shoe Sections (Figure 21 a and b): These sections are a compromise between circular and D sections. These sections are strong in their resistance to external pressures. Quality of rock and adequate rock cover in terms of the internal pressure to which the tunnel is subjected govern the use of these sections. Modified horse-shoe section offers the advantage of flat base for constructional ease and change over to circular section with minimum additional expenditure in reaches of inadequate rock cover and poor rock formations. 

Figure 21. (a) Horse shoe section ; (b) Modified horse shoe section of tunnels

4. Egg Shaped and Egglipse Sections (Figure 22 a and b): Where the rock is stratified, soft and very closely laminated (as laminated sand stones, slates, micaceous schists, etc) and where the external pressures and tensile forces in the crown are likely to be high so as to cause serious rock falls, egg shaped and egglipse sections should be considered. In the case of these sections there is not much velocity reduction with reduction in discharge. Therefore, these sections afford advantage in cases of sewage tunnels and tunnels carrying sediments. Egglipse has advantage over egg shaped section as it has a smoother curvature and is hydraulically more efficient.  

Figure 22. (a) Egg shaped section ; (b) Egg lipse section of tunnels

In addition to the sections mentioned above there may be other composite geometrical sections which may be adopted particularly for tunnels which are free flowing and often only partly lined. If the characteristics of a rock formation are fairly well known it may be possible to evolve a section which is likely to fit the shape in which the rock will break naturally. Thus, while a horse-shoe or D section is fairly easy to obtain in some formations there are others where the tunnel crown tends to break into a form more nearly square, and if there is no risk of heavy external pressure upon the lining or if the tunnel is to be unlined there is no reason why the designed cross section should not be made to suit the characteristics of the rock. 

Tunnel entrance and exits

It is also essential to design the entry and exit points of the tunnel very carefully. Where the tunnel emerges out of the hill slope, a structure in the form of an arch is usually provided, which is called the portal (see Figure 18). Since at these points the water enters or leaves the tunnel, they are prone to hydraulic head loss and proper transition shape has to be provided to keep the loss minimum and to avoid cavitation. The length and slope of the transition depends upon the velocity and flow conditions prevailing in the tunnel, economics, construction limitations, etc. It is generally preferred that a hydraulic model study is conducted to arrive at an efficient but economic transition. 

Where a tunnel meets a surge tank, some head loss may be expected because of the expansion. Similarly, head losses have also to be taken into account for any contraction as well in the shape of the tunnel. As seen from Figure15, there could be a possible change of alignment in plan of a tunnel and this may also lead to a loss in a pressure tunnel.