Hydropower Water Conveyance System (Part - 4) | Additional Documents & Tests for Civil Engineering (CE) PDF Download

Tunnel flow problems

The presence of air in a pressure tunnel can be a source of grave nuisance as discussed below:

• The localization of an air pocket at the high point in a tunnel or at a change in slope which occasions a marked loss of head and diminution of discharge.
• The slipping of a pocket of air in a tunnel and its rapid elimination by an air vent can provoke a water hammer by reason of the impact between two water columns.
• The supply of emulsified water to a turbine affects its operation by a drop in output and efficiency thus adversely affecting the operation of generator. The presence of air in a Pelton nozzle can be the cause of water hammer shocks. Admission of air to a pump may occasion loss of priming.
• If the velocity exceeds a certain limit air would be entrained causing bulking. 

Source of Air

Air may enter and accumulate in a tunnel by the following means:

• During filling, air may be trapped along the crown at high points or at changes in cross-sectional size or shape;
• Air may be entrained at intake either by vortex action or by means of hydraulic jump associated with a partial gate opening; and  
• Air dissolved in the flowing water may come out of solution as a result of decreases in pressure along the tunnel.

Remedial Measures

The following steps are recommended to prevent the entry of air in a tunnel:

• Shallow intakes are likely to induce air being sucked in. Throughout the tunnel the velocity should either remain constant or increase towards the outlet end. It should be checked that at no point on the tunnel section negative pressures are developed.
• Vortices that threaten to supply air to a tunnel should be avoided, however, if inevitable they should be suppressed by floating baffles, hoods or similar devices.
• Partial gate openings that result in hydraulic jumps should be avoided.
• Traps or pockets along the crown should be avoided.

Tunnel structural design

The geometric and hydraulic design of a tunnel is followed by the structural design, which investigates the loads that are expected on the tunnel opening from the surrounding rockmass and whether a support is required to hold it in place or a lining is necessary to resist the pressure of the rock and water pressure from the saturated joints and cracks of the surrounding rocks. 

Only some limited geological formations are so perfectly intact that they require no external support for their stability. In general, most of the tunnels are driven through rocks with certain defects requiring provision of some form of support until a lining can be completed. Thus, the basic philosophy of design of an underground excavation (tunnelling, surge tanks, power houses etc.) is such as to utilise the rock mass itself as the principal structural material, creating as little disturbance as possible during the excavation process and adding as little as possible in the way of steel supports or shotcrete (which is a wire mesh fixed to the tunnel wall by nails and sprayed with cement slurry with or without steel fibre is used to form a layer, as explained further on).     The type of rock support that has to be provided for a tunnel depends upon the type of rock quality, which is classified according to its behaviour when an opening is made in the rock. The Bureau of Indian Standards code IS: 15026-2002 “Tunnelling methods in rock masses-guidelines” indicates the features of the various types of rocks that are generally encountered. It also recommends the type of excavation method that is to be adopted and the type of support that would be appropriate.  

The methods for providing temporary or permanent supports to the tunnels are as described the following paragraphs: 

Steel supports 

These are built of steel sections, usually I-sections, either shaped or welded in pieces in the form of a curve or a straight section as shown in Figure 23.

Figure 23. Steel support for tunnel and finished in the form of (a) Horse shoe section & (b) Circular section

IS: 15026-2002 recommends various types of steel sections, also called steel ribs, as follows: 

• Continuous rib (Figure 24a)
• Rib and post (Figure 24b)
• Rib and wall plate (Figure 24c)
• Rib, wall plate and post (Figure 24d)
• Full circle rib (Figure 24e)
• Invert strut with continuous rib (Figure 24f) 

Figure 24. Types of steel support syste

Grouting 

This is a cement mortar with proportion of cement, sand and water in the ratio 1:1:1 by weight usually, though it may be modified suitably according to site conditions. 

Grouting is carried out to fill discontinuities in the rock by a suitable material so as to improve the stability of the tunnel roof or to reduce its permeability or to improve the properties of the rock. Grouting is also necessary to ensure proper contact of rock face of the roof with the lining. In such cases grouting may be done directly between the two surfaces. All the different types of grouting may not be required in each case. The grouting procedures should aim at satisfying the design requirements economically and in conformity with the construction schedules. The basic design requirement generally involve the following: 

• Filling the voids, cavities, between the concrete lining and rock and /or between the concrete and steel liner;
• Strengthening the rocks around the bore by filling up the joints in the rock system;
• Strengthening the rock shattered around the bore;
• Strengthening the rock, prior to excavation by filling the joints with cementing material and thus improving its stability; and
• Closing water bearing passages to prevent the flow of water into the tunnel and/or to concentrate the area of seepage into a channel from where it can be easily drained out. 

Rock/roof bolts 

Roof bolts are the active type of support that improve the inherent strength of the rock mass which acts as the reinforced rock arch whereas, the conventional steel rib supports are the passive supports and supports the loosened rock mass externally. All rock bolts should be grouted very carefully in its full length.  

Figure 25 Different types of rock/roof bolts
 (a)    Wedge & slot bolt
 (b)    Wedge & sleeve bolt
 (c)    Perfo bolts

There are many types of rock bolts and anchors which may also be used on the basis of past experience and economy. The common types of rock bolts used in practice are the following: 

Wedge and Slot bolt 

These consist of mild-steel rod, threaded at one end, the other end being split into two halves for about 125 mm length. A wedge made from 20 mm square steel and about 150mm long shall be inserted into the slot and then the bolt with wedge driven with a hammer into the hole which will force the split end to expand and grip the rock inside the hole forming the anchorage. Thereafter, a 10 mm plate of size 200×200 mm shall be placed over which a tapered washer is placed and the nut tightened (see Figure 25a). The efficiency of the spiliting of the bolt by the wedge depends on the strata at the end of the hole being strong enough to prevent penetration by the wedge end and on the accuracy of the hole drilled for the bolt. The diameter of such bolt may be 25mm or 30mm. Wedge and slot bolts are not effective in soft rocks. 

Wedge and Sleeve bolts 

This consists of a 20 mm diameter rod, one end of which is cold-rolled threaded portion while other end is shaped to form a solid wedge forged integrally with the bolt and over this wedge a loose split sleeve of 33 mm external diameter is fitted (see Figure 25b). The anchorage is provided in this case by placing the bolt in the hole and pulling it downwards while holding the sleeve by a thrust tube. Split by the wedge head of the bolt, the sleeve expands until it grips the sides of the tube. Special hydraulic equipment is needed to pull the bolts. 

Perfo bolts 

This method of bolting consists of inserting into a bore hole a perforated cylindrical metal tube which is previously filled with cement mortar and then pushing a plain or ribbed bolt. This forces part of the mortar to ooze out through the perforations in the tube and come into intimate contact with the sides of the bore hole thus cementing the bolt, the tube and the rock into one homogeneous whole (see Figure 25c).  

Steel fibre reinforced shotcrete (SFRS)

Steel fibre reinforced shotcrete either alone or in combination with rock bolts (specially in large openings) provides a good and fast solution for both initial and permanent rock support. Being ductile, it can absorb considerable deformation before failure. 

Controlled blasting should be used preferably. The advantage of fibre reinforced shotcrete is that smaller thickness of shotcrete is needed, in comparison to that of conventional shotcrete. Fibre reinforced shotcrete along with resin anchors is also recommended for controlling rock burst conditions because of high fracture toughness of shotcrete due to specially long steel fibres. This can also be used effectively in highly squeezing ground conditions. It ensures better bond with rock surface. With mesh, voids and pockets might from behind the mesh thus causing poor bond and formation of water seepage channels as indicated in Figure 26. 

Figure 26, Differences in shotcrete consumption when wire mesh (a) or steel fibres (b) are used

The major draw-back of normal shotcrete is that it is rather weak in tensile, flexural and impact resistance strength. These mechanical properties are improved by the addition of steel fibres. Steel fibres are commonly made into various shapes to increase their bonding intimacy with the shotcrete (see Figure 27). It is found that hooked ends types of steel fibres behave more favourably than other types of steel fibres in flexural strength and toughness. Accelerators play a key role to meet the requirement of early strength. 

Figure 27. Typical fibre used in shotcrete work.

Steel fibres make up between 0.5 to 2 percent of the total volume of the mix (1.5 to 6 percent by weight). Shotcrete mixes with fibre contents greater than 2 percent are difficult to prepare and shot.   

Concrete lining 

This is a protective layer within the tunnel made of plain or reinforced concrete. Tunnels may be completely lined, partially lined, or even unlined. Tunnels in good sound rock may be kept unlined. However, lining is recommended when:

• The internal water pressure exerted by water conveyed by the tunnel is high, say above 100m of water head. For very good competent rock, tunnels may be kept unlined for pressures even up to 200m water head.
• The rock strata through which the tunnel passes has low strength and where the rock is anisotropic. 

Lining a tunnel increases the cost of a project and should be adopted considering the advantages expected as given below: 

• Lining transmits part of the internal water pressure to the surrounding rock which, to some extent, is balanced by the external rock pressure. In tunnel empty condition, it helps to resist the external rock load together with the support system.  
• Lining may be carried together with the tunnel excavation work and hence minimizes the danger of accidental rock falls within the tunnel.
• Lining helps to reduce water loss through joints in rocks by seepage. d) Lining is invariably provided at the inlet and outlet portals of a tunnel, even if located within competent rock.  

Tunnels conveying water under free flow conditions may be un-reinforced. The external rock load is expected to be carried by the steel supports. Usually, a tunnel lining has to be reinforced when the depth of rock cover (from the tunnel soffit up to the free surface of the hill) is less than the internal water pressure.

The design of concrete linings for tunnels may be done according to the recommendations of the following Bureau of Indian Standards code IS: 4880(Part IV)-1971 “Code of practice for design of tunnels conveying water (structural design of concrete lining in soft strata and soils”.

The construction of tunnels could be by manual methods like drilling holes, placement of explosive, blasting, and then removal of the muck from the head-face or by competent rocks well. As soon as the tunnel face is excavated to a certain depth, the temporary supports are provided to prevent any rock fall or squeezing. At the same time, or later, permanent supports are also put in place.