Earthblog

PIPE JACKING

Pipe Jacking is a laser guided trenchless method for installation of pipe sections under obstacles. The bore is formed from a launch pit (usually 8m x 3m) by several sets of hydraulic jacks pushing specially designed Reinforced Concrete Jacking Pipes (RCJP) or Steel Pipes behind a jacking shield (2). The jacking shield will usually house a mechanical digging arm (1) or rotating rock head, conveyor and hydraulic steering jacks (3). The operator is based in the shield which provides full safety and protection at all times and also supports the ground above. From within the shield the operator excavates the face in front of him and also controls the steering jacks to keep the laser target central around the laser beam as the pipes are advanced. The laser (9) is mounted safely at the back of the launch pit and is checked regularly. The mechanical arm is usually used when softer ground conditions are present and excavation is usually in 200 – 500 mm increments, depending on the ground specifications. This is also known as open face excavation method, which provides for excavation face access and the ability to remove man made obstacles and boulders. The spoil is removed by the conveyor and spoil trolleys which are winched out. The hydraulic jacks pushing the pipes are held by the main jacking frame and are part of the jacking rig (7). The jacking frame transfers the horizontal loads from the jacks back to the thrust wall. The size of the thrust wall is determined by TBA and it’s dimensions are mainly based on the geotechnical conditions, frictional jacking records and past experience.

The jacking rig, jacking shield & other hydraulic equipment is powered by portable diesel hydraulic power-pack (8) located outside of the launch pit. Service lines (6), including the air supply to the tunnel face are in suitable modular lengths with easy connections for quick extensions. The pipe joints (4) are usually steel bands into the external rebate with timber plywood rings to prevent concrete to concrete contact.

Pipe jacked tunnels are with diameters from 1200mm to 3000mm and up to 300m in length. In ground conditions where the tunnel can be exposed to additional pressures due to the plasticity of the ground, intermediate jacking stations are introduced to reduce the pressures.

The intermediate jacking stations provides for sections of the pipeline to be independently moved which reduce the force on the main jacks. In cases of high plasticity ground conditions, reducing of the pressure is also achieved by lubricating the outer annulus of the pipelines, usually with bentonite. The retrieval pit is usually with dimensions 4m x 3m. The advantages of this system are that it causes no surface disruption, spoil is removed by skips which is a dry method.

Jacking Pit

Shield Head

Jacking Head from inside pipe showing skip