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What we are doing
The work we are carrying out across Preston and Penwortham is one of our most complex construction projects to date. To carry out work of this scale we need to utilise a variety of techniques. To find out more about the methods we are using to breath new live into the area's watercourses please check out our jargon buster below.
Shaft- These are large chambers which allow access to our tunnel for construction and future maintenance. They are constructed using a series of piles, followed by concrete rings. Once the shaft is constructed this will enable either the launch or reception of the TBM. See below for an example picture of a shaft.
Segmental Shaft – Pre-cast concrete segmental construction, where excavation of the ground is carried out before the building of permanent shaft rings are constructed and lower into the excavation.
Secant Piling – The method used to strengthen the soil so that the ground is able to support the load of the structure. This is the first part of the construction of the shaft. The piles are drilled into the ground using a ‘screwdriver’ type method. Once the hole is drilled it is backfilled with concrete to provide strength. A piling rig can be seen below.
Diaphragm walls - Diaphragm walls are constructed by using a bentonite powder mixed with water suspension (slurry) to support and stabilise the trench walls during excavation using a grab arrangement (Hydrofraise drilling machines). Diaphragm walls are constructed as permanent walls, which reduces the width of construction and working space required when compared to a solution that has both a temporary ground support and permanent works within.
Tunnel Boring Machine (TBM) - A Tunnel Boring Machine (TBM) also known as a "mole", is a machine used to excavate tunnels through a variety of ground conditions. In the case of the work in Preston City we have been excavating through sand tone rock. TBMs have the advantages of limiting the disturbance to the surrounding ground and producing a smooth tunnel wall. The TBM operates at between 2 and 3 revolutions per minute. This enables it to push its way through the bedrock underground.
Traditionally TBM machines are given female names. In the case of the work in Preston City, the TBM is named Caroline.
TBMs consist of the rotating cutting wheel, called a cutter head, followed by a main bearing, a thrust system and trailing support mechanisms including the conveyors for muck removal, electrical systems, ventilation and mechanisms for transport of pre-cast segments to line the tunnel.
TBM Caroline's cutting head being lowered into shaft C4
Pre-cast segments line the tunnel
Narrow gauge electric train used during construction
Micro tunnelling (MTBM)
Micro tunneling is a digging technique used to construct small tunnels. These small diameter tunnels make it impossible to have an operator in the machine itself. Instead, the micro tunnel boring machine (MTBM) must be operated remotely. In micro tunneling operations the pipe is inserted from the entry and pushed behind the machine. This is a process often called Pipe Jacking.
Micro Tunnel being launched from shaft C4
Weir Wall – This is a structure within the pumping station over which the flow spills during storm conditions. Only once the storm water storage tunnel and associated shafts are full will the flow spill over the weir. Water spilling over the weir is then passed through the screens before discharging into the river.
Terminal Pumping Station (TPS) – this is a structure which houses the pumps, the mechanical screens, the overflow structure and the start of the rising main. The building is within agricultural land and has been designed to look similar to farm buildings.
Pumps – We use pumps to transfer the flow from the Terminal Pumping Station to the next shaft to cross under the river. This is the first place within this project we use pumps. Up to this point the tunnel is operated using gravity.
Rising Main – we are constructing 2 x 800 mm diameter rising mains. They are over 5km in length. They carry the pumped flow from the terminal pumping station site, to the next shaft where the flow crosses beneath the river ribble for the second time.
Mechanical Screen – there are 5 mechanical screens and these act as giant sieves to prevent any particles larger than 6 mm being discharged into the river ribble during storm conditions. Once the storm has subsided the solids are returned to the sewer to be passed forward to the treatment works at Clifton Marsh in the normal way.
Tunnel – the tunnel is 3.05m in diameter and approximately 3.5km long. The tunnel can hold 40,500m3 of storm water. This is equivalent to 16 Olympic sized swimming pools.