New methods for box culvert pipeline construction
Jun 17, 2010
Circular pipes have been used for over a century to construct infrastructure in urban areas. Rectangular pipes, on the other hand, and in particular pipes installed by pipe jacking, are a fairly recent development. In this article the authors describe the development and assessment of a rectangular pipe jacking system.
The advantages of using a rectangular pipe over a circular pipe include a larger effective cross-section and reduced maintenance. A disadvantage of rectangular pipe has been the difficulty in mechanised installation. This has led to the development of rectangular pipe jacking.
The rectangular pipe jacking machine has a face structure consisting of three rotating shafts each equipped with cutters at varying positions. This arrangement improves ground mixing and mud slurry interaction compared with other rectangular excavating machines. As a result the machine has the ability to construct a rectangular pipe line faster than other construction methods, with improved safety measures and economic benefits.
The machine to be tested includes a three-shaft rotation/revolution type cutter, which forms the cutter face structure. This structure improves ground and mud slurry mixing at the cutter face, which results in higher cutter torque. As a result of various investigations, it was found that the excavation of the variant shape is enabled by changing the rotation and revolution ratio of the bits. The locus of each cutter bit is affected by changing the rotation/revolution ratio.
- The cutter torque is higher than other excavating machines, therefore several soils – from clay to gravel or weak rock – can be excavated.
- Soil and injected mud slurry are well mixed at the cutter face, so fluidity of the discharged material is improved.
- The whole rectangular shape is excavated, so resistant force and thrust force is decreased.
The field test was held in order to confirm the safety of the construction method. The ground conditions comprised of muck and gravel, with 400 mm of maximum particle form. By the test of grain size accumulation a rate of 2 mm over of grain was 89 per cent.
The result of the field tests concluded that the subsidence caused by rectangular pipe jacking was quite low, from -5 mm to +1 mm, with an average of -2 mm. Permeability of the ground was measured before and after pipe jacking. The variation ratio shows from 65 to 117 per cent. Big cobbles caused the ratio to reach 117 per cent.
Although a bigger subsidence was expected from the rectangular pipe jacking, the results show it was less than with circular excavators. This is due to the faster cutter bits. However, it was necessary to make mud films and tail void out of pipe with the workable high-density mud slurry in order to jack pipes with low thrust force, necessitating the mixing of soil and mud slurry at the cutter face. These tests supported the viability of the technology.
- Pipe size: 2,400 X 2,000 mm PC box culvert
- Pipe jacking length: 36 metres
- Cover depth: 1.92 metres at the starting shaft and 2.23 metres at the arrival shaft
- Soil condition: silt layer N value is 0-8.
A noteworthy part of the project was that the depth of cover was less than the box culvert height. Furthermore, the supervisor of the road sets the allowable settlement of the road surface to 5 mm, which was severe for low cover depth construction. Maximum settlement was considered with prior analysis.
The analysis results for surface subsidence at the subject crossing indicate the maximum subsidence would be 16 mm at the top of the box culvert. The analysis indicated that ground cover may rise because of the face pressure during pipe jacking.
The analysis shows that surface settlement is affected by tailvoid subsidence behind the machine. In addition, more subsidence should be assumed because of repeated traffic loading which was not well reflected in the analysis. As a result, pre-construction stabilisation of the ground cover was considered.
The stabilisation method adopted at the site was the CCP-L method, involving horizontal high pressure jet grouting. This method has three primary advantages; narrow onsite mobility, working space security, and good stabilisation of the soil body.
The following considerations were taken into account during construction;
- arching of the ground to support loads is unlikely
- The relatively large nature of the rectangular section compared to the ground cover thickness
- The problematic nature of an alignment in soft soils when working with low cover.
- The monitoring system
- Real-time management of cutting face pressure
- Quantity of discharged soil management that does not generate ground settlement Pipe jacking accuracy management.
The rectangular pipe jacking machine can be controlled remotely to enable the viewer to review excavation conditions. In addition, the cutting face pressure can be calculated by computer from the cover depth in real-time, facilitating management of the operation. A horizontal clinometer was inserted in the CCP-L upper-centre aperture, and the road surface form management carried out automatic measurement management separately from hand-operated level techniques.
Cutting face pressure management
The face pressure was managed by gauges at four locations on the face. The pipe jacking length was short, but because the excavating soil and cover depth can change, the pressure management was set with a different value at each point.
Discharge soil management
The machine discharging system consisted of a screw conveyer and an air valve. To secure the fluidity of discharged soil, a screw was set in the stock tank in the machine and discharged using a vacuum unit set on the surface.
Laser surveying was used to control pipe jacking accuracy and vertical accuracy was controlled by a water pressure sensor. Ground penetrating radar (GPR) was also used. The results did not confirm clear movement of the ground.
The maximum settlement of the surrounding soil was measured automatically by a horizontal clinometer set in the CCP-L injecting hole. The maximum settlement is -2 mm, meaning that the pipe jacking finished within the allowable settlement under the national highway.
Thrust force during construction almost equalled the planned force, excluding the improved area around the jacking and reception shafts because of the increase in cutting face force.
The maximum thrust force was 3,457 kN; 16 per cent of the load bearing capacity of the box culvert. The machine has the ability to excavate soil and construct a better cutting area, which leads to low frictional force between the box culvert and the over-cutting area.
Pipe jacking speed
The average pipe jacking speed was 32 mm per minute, exceeding the preconstruction estimate of 15 mm per minute. This is a result of optimised soil mixing and injected mud slurry by the three shaft rotation/revolution type cutter implemented.
Pipe jacking accuracy
Pipe jacking accuracy at the reception shaft was 20 mm right and 26 mm above for a management standard value, 50 mm horizontally and 30 mm vertically. In addition, the rolling of the box culvert (the difference on right and left of both ends) was from 1 mm to 9 mm.
The specifications of GPR used in the investigation:
- Radar system: SIR 2000A
- Antenna: 200MHz
The rectangular pipe jacking machine that has been developed has shown good results. The machine has the ability to construct a box culvert pipeline safely and quickly even if the cross section of the cutting face becomes large and the cover is limited. The impact on the neighbouring environment can be significantly reduced.
The use of this machine for infrastructure construction in Japan is slowly but surely increasing. The authors believe this machine is well equipped to cope with rectangular pipe sections and it is hoped that it will be another beneficial tool for the underground industry.
>> This article is presented with permission of Trenchless International magazine <<
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