Precast concrete pipes – in-the-wall joint jacking pipes
Concrete pipe jacking is a method of installing pipelines without disrupting existing surface facilities or activities. With the increasing growth in South African cities and towns and the consequent need to refurbish, renew and upgrade existing facilities, pipe jacking is viewed as a growing industry.
Although the basic methods of pipe jacking are similar for all pipeline applications, South African contractors have developed their own individual techniques to achieve accuracy during jacking, to lengthen the jacking distance and to control varying ground conditions.
Concrete pipes are jacked from a jacking pit to a receiver pit, usually located at a manhole or access chamber. The main jacks are installed in the jacking pit and are used to push the concrete pipes into the cavity excavated ahead of the progressing pipeline. The main jack capacity depends on the size of the pipe, length of the line and they type of ground in which it is being jacked. There are typically two or four jacks with capacity of up to 300 tonnes.
The jacking forces are resisted by the thrust block in the jacking pit and applied to the pipeline end through the steel thrust ring. The concrete pipe end is protected against spalling by soft wood or woodchip joint packers between the thrust ring and the concrete surface. The steel cutting shield fitted to the lead pipe gives protection to workers and allows steering and alignment adjustments. Intermediate jacking stations may be required in long lengths and hard ground conditions. Alternatively, bentonite injections may be used to reduce the friction between the pipes and the soil.
Pipe line alignment is continually monitored and adjusted during jacking by laser from the jacking pit to a target fixed to the inside crown of the lead pipe or shield. Gradual adjustments are continuously made to avoid over stressing the concrete pipe joints.
Trenchless technology, as the name implies, stands for subsurface construction works where fewer trenches or no continuous trenches are required to be dug. It is a rapidly growing sector of the construction and civil engineering industry. It can be defined as "A family of methods, materials, and equipment capable of being used for the installation of new or replacement or rehabilitation of existing underground infrastructure with minimal disruption to surface traffic, business, and other activities."