Dewatering a big tunnelling project demands both reliable dewatering pumps and a well planned sedimentation system. This case study from the Swedish railway project Ådalsbanan shows a good example of this.
Ådalsbanan is one of the biggest railway projects in Sweden and comprises investments of 6600 MSEK. It stretches as far as 130 kilometres, from the city of Sundsvall to the connection to the Bothnia Line. The project is scheduled to be completed during 2011.
Lemminkäinen at work
Ådalsbanan will have eight new tunnels with a total length of 14 km, straight through the Swedish bedrock. One of the contractors is Lemminkäinen Infra; they are contracted for the construction of Bjässholmstunneln, Kroksbergstunneln and Utansjötunneln (among others). The two longest tunnels will be 4.5 kilometres (Kroksbergstunneln) and 3.6 kilometres (Bjässholmstunneln), which takes them to the top-ten-list of long tunnels in Sweden.
Before each blast, Lemminkäinen are drilling some 160 holes for the explosives. Each hole is 6 metres deep and holds a diameter of 64 mm. This tells us that some 3.1 m3 (7.7 metric tonnes) rock is pulverised before each blast. It will never turn to dust as the drilling water will flush it out to the tunnel floor.
A lot of water is used when drilling - Lemminkäinen estimates the amount of water to 60 m3 between each blast. When all 14 kilometres of tunnels are drilled for blasting, a total of some 140.000 m3 water has been used. All this water needs to be pumped to the drill rigs and then dewatered, an operation that demands a reliable system with endurable pumps.
Solids in the water
Let's say that only10 % of these suspended drill cuttings will reach the dewatering pumps, it still leaves us with more than 310 litres (770 kg) drill cuttings that will be pumped away with the water between the blasts. This means that when all 14 kilometres of tunnels are completed, a total of some 720 m3 (some 1800 metric tonnes) pulverised rock will run through the dewatering system, if it's not being dealt with.
Minimizing the amount of solids
Naturally, you want to minimize the amount of solids in the dewatering system as it may cause heavy wear on the pumps and piping system. There is also a risk of sedimentation in the system. A typical symptom of this is that pipes and/or hoses will be filled with sediment, resulting in capacity losses. In the end, the whole system may get clogged and unable to operate.
One way to prevent this problem is to use sedimentation tanks, where the solids are allowed to settle and only water is pumped. The sedimentation tank needs to be placed as early as possible in the dewatering system. In order to be efficient, it takes a large tank area where the drill cuttings may settle in peace. The more solids present in the water, the more careful the sedimentation system design needs to be.
Pumps at work
Lemminkäinen is using between 35 and 45 pumps in each tunnel work, a total of 160 Grindex pumps are used. With delivery heads of up to 75 metres from the tunnels, it takes both strong and reliable pumps. Grindex pumps in sizes from Minor (3.7 kW) to Maxi (37 kW) are used.
The drill water is collected both from the municipal water line and from a nearby river. Before the water is pumped back to the river, samples are taken on regular basis to ensure that the city regulation of water quality (with regards of chemicals and amount of solids) is not violated.
The calculation explained
The formula for calculating the volume of a cylinder is pi x radius2 x length, this gives us 3.14 x 0.032 x 0.032 x 6 x160 = 0.0193 m3 stone per hole, or 3.1 m3 for each drilled front. We assume that the specific weight for stone is 2.5 kg/dm3.