Submerged tunnel piercing has been a regular practice for more than 90 years. Up to 600 piercings have been performed in Norway, mostly to create draw down reservoirs for hydroelectric power plants (figure 1). Currently, the deepest breakthrough registered in Norway is at a depth of 120 metres (1987).
Sediments are common in lakes and at the seabed, but do not necessarily create problems. Small amounts of stable sediment, not apt to slide, can be an advantage filling cracks in the rock after breakthrough, and reducing water leakage into the tunnel. An overburden of 1 to 3m of sediment appears to be ideal. Large amounts of loose rock and sediment can be a problem. If breakthrough takes place in an underwater scree or stone heap the opening can be blocked or the tunnel can be partly filled with loose stone masses.
Planning an underwater breakthrough needs several phases of careful inspection. A geological survey including aerial photographs will give information on rock structure, cracks and fissures and information about the landscape shape and the location of loose rock and sediment. This with a general survey of the area, will provide data for alternative breakthroughs and make a basis for further investigation.
For complicated areas, a more detailed investigation requires a seismic survey. Profiles from seismic refraction are preferable, as they differentiate zones of different acoustic velocity and locate boundaries between loose masses and solid rock. The acoustic velocity of the rock also gives information about the rock quality. Visual inspections are then carried out by divers or remote controlled submarines.
Tunnel driving
Driving a sub sea tunnel towards the breakthrough area is usually done by drill and blast, allowing flexibility and space at the face. Extensive test drilling and grouting are often required. The tunnel is normally driven to 30-100m of the breakthrough area with burden above the tunnel ideally at least twice the tunnel height. The face is carefully blasted in subsequent steps by back ripping from a small previously driven pilot tunnel with rounds gradually reduced to 2 – 1.5m.
Test drilling should be carried out regularly within the last 30-100m. It is normal to test drill with 4 to 5 holes every second round. The test holes should be drilled to at least twice the length of the round so that the holes overlap each other. Alternatively, the test holes should cover a rock zone ahead of the face equal to twice the tunnel height or diameter.
The object is to locate weak zones and pockets of running water, so that necessary precautions can be taken such as rock bolting and grouting; and also to increase safety by controlling the amount of overburden above and in front of the face. Generally speaking, at least one test hole should be drilled for each round above sea level and 4-5 holes for each round sub sea. The extent of drilling is supplemented in zones of poor rock quality or excessive water leakage.
Procedures to measure water leakage, the amount of clay in flushing water and reporting abnormal conditions must be decided beforehand to maximise test drilling. Workers at the face must be instructed to stop work on indication of excessive zones of poor rock quality/clay or water leakage. A breakthrough into such zones can have catastrophic consequences.
Conventional test drilling using drill rigs registers, primarily, water leakage. Zones with crushed rock and clay can, to a certain extent, be registered by the advance of the drill steel and the flushing water. Observations tend to be less accurate from deep holes. The most dependable information about rock quality, crack and clay zones is from core drilling.
The use of rock bolting is the most effective and commonly used technique to improve stability, usually in formations with cracks and fissions, and schistose rock. Systematic bolting to stabilise the rock surface, or if needed, more scattered bolting is used to stabilise blocks and fissions. This can be supplemented by straps and wire netting in badly cracked structures.
Sprayed concrete is an effective method of stabilising badly cracked structures, areas with clay deposits and impaired zones. Primarily used as a temporary measure, it allows permanent stabilisation to be performed later. The permanent alternative is in most cases a cast concrete lining. Running seawater will eventually break down concrete, therefore water must be drained off before any concrete is sprayed on or cast. Under extreme conditions, the pouring of a complete lining can be the only solution, often in conjunction with rock bolting and sprayed concrete. In zones with running water, there is a risk that the concrete lining may cause water pressure to build to an unacceptable level. In such cases, the water can be piped out through tubes through the scaffolding.
Grouting is considered to be the only safe method of stopping water leakage through the rock and will, in most cases, have a stabilising effect. There are two main types of grouting material: Concrete based grout is most common and is well suited to fill open cracks; Chemical grouts have better intrusion and hardening properties, but also cost limits. The extent of grouting is decided in each separate case as work proceeds.
Extensive water leakage in unstable rock may require a double concrete lining. This design consists of two linings with a watertight plastic membrane or asphalt impregnated material in-between.
Methods of breakthrough
The method of breakthrough should be decided during the planning stage. It may be necessary to adjust the alignment to secure success. There is a differentiation between the two main methods of breakthrough, the open and the closed system, although preliminary investigations are the same for both systems.
With the open system, a direct connection exists between the tunnel face at the plug and the atmosphere through the gate shaft. The tunnel is closed either by a gate or by a concrete plug downstream of the gate shaft to prevent uncontrolled flow of water or rock through the tunnel. The concrete plug is fired after the breakthrough has been made.
The open system principle is shown in figures 3 and 4, either with a closed main gate or with a temporary concrete plug, respectively, both with a water filled tunnel and air pocket below the breakthrough plug. The advantages are that the hydrodynamic conditions are clearly set out; The concentration of rock from the breakthrough plug and sediments above, if any, are controllable; The pressure rise against the gate can be calculated fairly accurately. The disadvantages are the complicated procedure/arrangement for the filling of water, air, measurements etc. and the considerable time lag between loading and shooting of the plug.
The principal of the closed system is illustrated on figures 5 and 6, with the shooting of the plug into a partly water filled tunnel and a dry tunnel, respectively, both with the gate closed. A breakthrough into a dry tunnel, figure 6, is easy to perform, and the shooting of the plug can take place shortly after loading. However, water of high velocity can carry the rock of the plug deep into the tunnel. Further, unacceptable pressure rise at the gate can be generated if the distance between the plug and the gate is too short. A closed system with a partly water filled tunnel (figure 5) will keep the rock from being blown deep into the tunnel. However, water filling will delay loading and shooting. The advantages of the closed system are the simple design and shorter time between loading and shooting. Disadvantages are the unclear hydrodynamic conditions; Rock masses from the plug and sediments above are carried deep into the tunnel and can damage to the gate; A considerable distance is required between the plug and gate.
The plug
The thickness of the breakthrough plug is between 1m in solid, non-leaking rock and shallow water, and more than 10m in poor quality rock and large cross sections with leakage. The point of breakthrough should be localised to good rock. The plug should be rectangular in shape with rounded-off corners. The rounded corners will reduce the hold. The rectangular shape will simplify drilling and positioning of the boreholes. The thickness of the tap can, under normal conditions, with good quality rock and water depth in the region of 10 to 80 metres, be determined by the following rule of thumb:
Thickness of the plug = 1.2 x shortest side of the plug (rectangular); Thickness of the plug = 1.0 x diameter of the plug
(circular). For a good result it is essential that the plug is closely examined by accurate test drilling.
Equipment to plug holes, which are drilled directly into the water, must be to hand and tested beforehand. The breakthrough should be as close to 90o to the seabed as possible, simplifying drilling.
The level of the tunnel should be placed so low that the breakthrough leaves a shaft of limited length (5 to 10m) which will hold sufficient cushioning air under the plug when filling the tunnel with water. The borehole diameter for the loaded holes is usually 45mm-64mm, while the large diameter holes (unloaded) have a diameter of 102mm. Drilling should be through a guiding sleeve for precision and to avoid contact between adjacent holes. All holes should be drilled within 0.5m of the solid rock of the magazine. It is a precondition that the contour of the rock surface and the plug length across the cross section are known (figure 8). The cut is placed where the quality of the rock is best and the hold is the least. The least hold is where the plug length is at a minimum.
Explosives and dets
Explosives and detonators must be used that are water/pressure resistant for the time period between loading and shooting and for the water pressure. Quantity of explosives depends upon the cross section, plug length, type of rock, sediment amount and water pressure, in addition to the configuration of the plug and the burden. To find the quantity of explosives required for plug shooting, the following applies: Twice the normal use in the tunnel up to the plug, plus 0.01kg/m3 in addition for each metre of water pressure. Additional quantities in cases of large amounts of deposits must be considered. Finally, 10% should be added for plugged/lost holes. Two detonators of the same number should be used in each hole with water and pressure resistant electric millisecond detonators recommended.
The explosives and detonators should be tested before use, to make sure that they fulfil the specifications. Explosives should undergo the following functional tests: Crushing of a lead block (Hess’ test) before storage under water; Crushing of a lead block (Hess’ test) after storage under water at the actual depth and length of time; Detonating velocity after storage under water at the actual depth and length of time; Gap/sensitivity tests after storage under water at the actual depth and length of time.
Two detonators of each interval ordered should be tested for insulation resistance after storage under water at the actual depth and length of time. None should show less than 30M ohm.
Stemming
Polystyrene plugs, 50-100mm long, are used as stemming in the unloaded parts of the borehole. Solid wooden plugs with a notch for the detonator leads are used to keep the explosives and stemming in place. The hook up of detonators must be done accurately and in equal series which are coupled in parallel. All joints should be insulated in a watertight manner, and the series tested with an approved ohm-meter. The resistance of each series should be as equal as possible and the deviation not more than +/- 5%.
The shotfiring cable must be new, of high quality and the right dimensions for the shot and the voltage of the exploder. The place of firing is usually at the top of the gate shaft. The cable should have a dimension of at least 2 x 2.5mm2. The joining of the cables should be done carefully, with a distance between the joints of at least 2m. The joints are insulated and waterproofed by crimping the sleeves with silicone and tape.
The open system requires a certain level of water to reduce the flush of stone masses towards the gate and to reduce the upsurge into the gate shaft following the plug shot. The degree of water filling will have a decisive effect on the pressure. The design pressure i.e. the explosion gas pressure, is the pressure generated when the gas of the explosion expands into the air pocket. This gas pressure would, without the air pocket, be propagated undampened through the water filled tunnel causing great damage to the gate.
In a closed system, a high degree of water filling will give the same design pressure as the open system. Low water filling means that the tunnel is less than half full of water, 20% full is normal. In this case the design pressure is generated by the water flowing through the tap opening and compressing the air in the tunnel. The increasing pressure will decrease the inflow of water. The pressure will reach a maximum when the inflow of water comes to a standstill. At this point the pressure of the entrapped air is higher than the water pressure, and the water starts flowing out of the tunnel.
The explosives in the plug will, on detonation, create a gas pressure in the air pocket which is dependent on the quantity, pressure and volume of the air pocket. 1kg of explosives detonated will give approximately 0.8Nm3 of gas (1Nm3 is the quantity of gas which will give a volume of 1m3 at 1 atmosphere of pressure, at 0oC). The explosion gas will expand into the air pocket and the pressure in the pocket will increase rapidly. The pressure rise will propagate through the water towards the closing device (gate, concrete plug) and be reflected. The rise of pressure at the closing device will be twice the amplitude of the pressure wave.
Closing remarks
The driving of a tunnel underwater and the preparation for a breakthrough with plug thickness of 4-8m are difficult yet exciting operations. It requires careful preliminary inspection and accuracy in every detail of the planning and shooting. Superficial inspection, poor planning and workmanship can have dramatic and costly consequences. Uncontrolled water leakage into the tunnel or land-slides of loose masses blocking the breakthrough exit are examples. Deviation from the original plan may be required, regardless of the planning accuracy. In such cases, it is vital to have experienced people on site, so critical situations can be evaluated and appropriate corrective action taken.
Preparation for the breakthrough shooting, loading, water filling etc. is a busy and hectic period. There may be considerable water seepage, and it can be a wet, cold and unpleasant job. Willingness and endurance are essential qualities for everyone engaged in the operation. It is important the crew is well prepared and familiar with all operations and safety procedures. Procedures for loading, hook-up and air-filling etc. must be prepared and fully discussed in advance. This is followed up by the use of control points ‘along the road’ which are checked off by signature. T&T
Related Files
Figure 8: diagram showing the holes to be drilled within 0.5m of breakthrough
Figure 3: Open system of piercing with gate
figure 7: Diagram of an idea plug breakthrough arrangement
Figure 2: Exploratory drilling procedure
Figure 6: Closed systemof piercing in dry tunnel
Figure 4: Open system of piercing with bulkhead
figure 1: Typical hydro-electric pwoer plant section
Figure 5: Closed system of piercing with partial water fill
