PC consists of unsaturated thermosetting polyester resin, silica filler, quartz sand and aggregates. All materials are inert and resistant to chemical attack. The polyester resin is the bonding agent between the mineral aggregates. Unlike Portland cement in cementitious concrete products, polyester resin forms a consistently strong bond with the evenly graded aggregates.
It is the high chemical resistance, especially against hydrogen sulfides, that has made PC products favourable in sewer installation and rehabilitation projects. The physical properties including the high stiffness and compressive strength values makes the PC pipes ideal for pipe jacking and microtunnelling operations.
The bonding mechanism of the resin and quartz based aggregates inherent in PC products means that cracks are avoided, whereas in cement based concretes cracks can only be minimised but not eliminated. This bonding mechanism gives PC products their characteristic material behaviour.
Long distance microtunnelling and pipe jackingArticle continues below…
In recent years, trenchless installation methods have become more popular, especially in areas which have high population densities. Owners and engineers value the savings in social costs and the environmentally friendly installation methods compared to open trench, cut and cover methods.
In a pipe jacking operation all jacking forces are generated in the starting pit and have to be transferred from pipe to pipe to the microtunnel boring machine (MTBM) and ahead of the whole pipe string.
The resistance force which needs to be overcome is split into two areas – the machine area and the pipeline area. The machine area contains the tunnel face pressure, operating resistance, edge resistance and the machine skin friction. The tunnel face pressure stabilises the tunnel face and reduces subsidence on the surface, while the operating resistance is mostly controlled by the operator controlling the slurry system and the steering of the MTBM. The pipe area, which is the largest section, only contains skin friction. Skin friction is dependent on the friction factor of the material, the surface area and the nominal forces acting on the pipe OD from the surrounding soil.
The pipe friction represents most of the overall calculation of the required jacking forces and is dependent on the type of soil, burial depth of the pipeline, ring annulus lubrication and material properties of the pipe.
PC pipes have one of the highest jacking capacities compared to other pipe jacking materials like reinforced concrete pipe, centrifugally cast fibreglass reinforced polymer mortar pipe, and clay (Figure 2). This is due to the smooth outer surface, non-water absorbing behaviour and high compressive strength of the PC pipe, which benefits the MTBM operator by allowing jacking length to be maximised and jacking forces to be minimised (Figure 3).
The high jacking capacity can be translated into a real benefit for the contractor and Figure 4 shows the maximum jacking distance without the use of an Intermediate Jacking Station (IJS).
With the use of the appropriate ring annulus lubrication, proper MTBM design and operation, as well as the use of IJS’s, arbitrary jacking drives can be realised (>DN900, man entry).
The role of the IJS is to break the whole jacking length into smaller pipe jacking sections. Generally, only the pipe friction between two IJS points has to be overcome by the following IJS, respectively the first IJS behind the MTBM has to overcome in addition the resistance forces of the MTBM. The last push is concluded by the jacking station in the starting pit, which finishes the time consuming operation.
With the use of PC, the time and cost consuming operation of the IJS can be reduced.
Benefits of PC jacking pipes in curved applications
The rigidness of the PC pipe, together with the high jacking capacity and less point loading sensitivity, makes it ideal for curved pipe jacking applications.
Allowing curved drives in the design process (Figure 5) enables the design engineer to select the most economical route for the tunnel operation. The amount of manholes, total jacking length, volume of excavated material, construction time and costs etc can be reduced significantly.
A large percentage of jacking drives in Europe are curved drives. In fact every jacking operation is a kind of curved drive because of the constant steering of the MTBM operator to stay in line and grade.
Depending on the soil condition the operator has to master different uneven resistance forces during operation.
Steering is undertaken by articulating the first two machine sections where the pipes follow automatically into the machine excavated chamber around the curve.
Figure 6 shows the overall jacking forces being transferred from pipe to pipe along the red line. There is an even load distribution in the straight section (yellow area) and an uneven, eccentric load distribution (red area) in the curved section. This eccentricity results in a reduction of the maximum allowable jacking forces and is taken into account in the calculation according the ATV A 161.
PC has definite benefits in trenchless applications, especially when it comes to installations in a sanitary sewer environment or in corrosive soil conditions. Due to the material properties, new technologies can be used for long distance pipe jacking and curved alignments as well as segment tunnelling operations. The savings involved not only benefit the contractor but also the owner and due to the higher buried asset value, PC pipe benefits every individual who contributes with their tax payments for a cleaner and safer environment.