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Tunnelling


Microtunnelling and pipe jacking are essentially from the same family of pipeline installation techniques and can be used for installations from about 150 mm diameter upwards.

The term microtunnelling applies to remotely controlled, steerable, evacuation tunnelling methods for pipelines. Microtunnelling is well suited to situations where a pipeline has to conform to rigid line and level criteria. Microtunnellers often use a laser guidance system to maintain the line and level of the installation, though, as with larger pipe jacking installations, both laser guidance and normal survey techniques can also be utilised. Microtunnelling machines are controlled from the surface, with location and operation of the machine being continuously monitored, usually by means of a laser guidance system. Most microtunnelling drives are straight between shafts, although increasingly in recent years various companies have developed guidance systems that enable curved drives to be completed, particularly on longer length, larger diameter bores.

The most critical factor in any microtunnelling project is the geology. Extensive ground investigation should carried out to determine the soil characteristics along the proposed alignment. Modern technology has enabled this method to be applied to a wide range of ground conditions from waterlogged sands and gravels, through soft or stiff, dry or waterlogged clays and mudstones, to solid rock.

Microtunnelling was first introduced into Australia in the mid-late 1980s. Since this time, microtunnelling has been extensively used for the installation of new pipeline infrastructure, particularly for the water industry. The capability to be able to bore at the flatter grades has rendered this technology extremely suitable for the installation of sewerage and storm water drains.

A pipe jack is defined as a system of directly installing pipes behind a shield machine by hydraulic jacking from a drive shaft, such that the pipes form a continuous string in the ground. The pipes, which are specially designed to withstand the jacking forces likely to be encountered during installation, form the final pipeline once the excavation operation is completed.

One of the most common pipe jacking applications is for gravity sewers, where not only is the line and level critical but the depth is such that the techniques tend to become more cost-effective when compared with open-cut installation.

Pipe jacking systems are more often than not supplied with jacking frames, which are designed to provide the level of jacking pressure likely to be required by the shield being used. The requirements for the jacking frame on any project are determined by the ground conditions, length of drive and the type of shield being used.

Probably the most important aspects of design in respect of pipes for a pipe jack project are the allowable degree of joint deflection and the joint face geometry. In general, the deflection at the pipe joint face should not exceed 0.5°. To ensure squareness, the joint face should be manufactured to the recognised standards, or the local equivalent, and in the case of rigid jacking pipe (eg concrete or vitreous clay) must also be fitted with a suitable packer material to ensure the even distribution of the jacking force across the joint. If CC-GRP Jacking pipe is being used, no packer rings are needed.

An essential feature of pipes for both microtunnelling and pipe jacking is that the entire joint is contained within the normal pipe wall thickness.

Pipe length varies according to the microtunnelling system used, the pipe diameter and constraints of space. Typical pipe segment lengths usually range from 1 to 2.5 metres, although lengths of 0.75 metres are available for small diameters and longer pipes are sometimes available.

Pipe jacking and microtunnelling are both commonly used for main line or trunk pipelines.

Pipe ramming

Pipe ramming is a non-steerable system of forming a bore by driving a steel casing, usually open-ended, using a percussive hammer from a drive pit. Soil is then removed by augering, jetting with water or compressed air. In appropriate ground conditions a closed casing can be used.

Pipe ramming is most often used to install new pipelines or casings into which new utilities will be installed. Installation distances have increased dramatically in recent years from about 50 metres to up to 100 metres today. Steel pipe is used for the casing, and once installed, it can be used as a pipeline or as ducting for most types of pipe or cable.

Typically installs diameters of 100-1,500 mm, however, bores up to 3,700 mm diameter have been installed in suitable ground conditions, using impact ramming hammers of up to 800 mm diameter generating ramming forces of up to 40,500 Nm.

A typical ramming operation requires the establishment of a solid base at the launch of the installation. The first length of steel pipe is positioned on guide rails set to the line of the bore, a cutting edge is formed or fitted to the lead end of the pipe, and the ramming hammer is attached to the rear of the pipe. The ramming hammer forces the steel pipe into the ground along the dictated line and when one pipe has been driven the hammer is stopped and removed, and the next length of steel pipe is welded in place. The cycle is repeated until the leading edge of the first pipe arrives at the reception end or shaft.

The main issue associated with pipe ramming is the fact that there is usually no means of monitoring the direction of the pipe during a bore - so it is vital to establish a clear bore path. Overall, the principles of pipe ramming are relatively simple, and the technique can offer cost-effective solutions to relatively short length installation projects.

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