Pipe jacking works – Dynamics in the underground
Pressing until the destination shaft is reached
In the pipe-driving process the jacking machine effects forward boring from the start shaft thus creating space for the sewer pipes that are advanced behind the jacking machine towards the destination shaft by means of hydraulic presses. Once the forward pressing cylinders have reached their end position, they are retracted into their initial position, the next pipe is lowered into the start shaft, installed and pressed forward. The soil material removed is transported to the start shaft, conveyed to above ground and disposed of by trucks.
When the jacking machine has reached the destination shaft, it is recovered and transported to the next start shaft. The distance between the start and destination shafts, depending on the nominal width of the pipes, geology, raw material and number of intermediate jacking stations may be up to 1,200 metres. The daily advance rate during the construction of the Emscher wastewater sewer is approximately fifteen to twenty-five metres.
Balance of forces by means of support pressure
During the advance, a stabilising support pressure is built up at the working face, that is, at the soil in the area of the cutting wheel. The support pressure counteracts the soil and groundwater pressure and prevents uncontrolled ingress of the soil into the jacking machine. For the construction of the Emscher wastewater sewer, closed shield machines with fluid support are predominantly used. The term “shield” designates the external steel jacket of the jacking machine, which protects it against water and soil. The support fluid is a bentonite suspension (mixture consisting of swellable clay minerals and water), it penetrates the close-to-surface pores of the working face and here forms a stabilising mass.
Routing of the pipes accurate to within a centimetre
A navigation system consisting of a gyrocompass and an electronic water level gauge ensures the directional stability of the underground jacking. Comprehensive control measurements are continuously effected; they ensure the accurate position of the wastewater sewer precise to within a centimetre. During the forward pressing process the jacking pipes are also monitored online, in order to detect any overload due to the pressure of the jacking presses in good time and to exclude consequential damage to the pipes by means of suitable countermeasures.
Segmental pipe jacking – inch by inch, ring by ring, pipe by pipe
Boring plus manufacturing at depth
The segmental pipe jacking process for the underground production of tunnel cross-sections is also used during the construction of the Emscher wastewater sewer. This technology is used for working in the area of the dual pipe sewer (DN 2.600) from the mouth of the Berne in Bottrop to the planned pumping station in Oberhausen.
The segments (“tubbings”) are ready-made parts of armoured concrete, which are assembled into a ring, directly following the underground boring process while being protected by a steel pipe, the so-called shield skin. A large number of such rings will then form a wastewater pipe. A remote-controlled crane arm lifts the tubbings either mechanically or by vacuum, moves them and assembles them into a circular profile. Mortar is continuously pressed into the space between the tubbings and the subsoil in order to provide for a stable embedding of the tunnel pipe in the substrate. Following the construction of a complete tubbing ring, the jacking machine is advanced further forward by means of hydraulic presses whilst bracing itself against the most recently manufactured tubbing ring and creates space for the next tubbing ring.
Narrow bends are possible
Therefore, during segmental pipe jacking, it is not the entire pipeline already installed which is pressed forward through the substrate at a high degree of force but only the jacking machine, starting from the most recently installed ring. With the segmental pipe jacking process, in contrast to the pipe-driving process, it is possible to build much longer routes. This method can also be used to implement narrower bend radii. The tubbing rings are designed to be monocoque and watertight. This type of tunnel building requires ready-made armoured concrete parts of a very high dimensional accuracy.