Page 1 Page 2 Page 3 Page 4 Page 5 Page 6 Page 7 Page 8 Page 9 Page 10 Page 11 Page 12 Page 13 Page 14 Page 15 Page 16 Page 17 Page 18 Page 19 Page 20 Page 21 Page 22 Page 23 Page 2419 An introduction to pipe jacking and microtunnelling 4 DESIGN AND CONSTRUCTION METHODS SAFETY As stated in Section 2, pipe jacking is an inherently safer method of working than open trench construction or segmental tunnelling. With the advent of the Construction Design and Management (CDM) Regulations clients and designers have significant responsibilities imposed on them and may find that risks may be eliminated or mitigated by opting for the pipejacking or microtunnelling method of construction. Where a pipejack or microtunnel is to be installed using a pressurised slurry system the design control and management of the slurry and the separation equipment that removes the excavated soils from the slurry is critical to the success of the pipejacking or microtunnelling operation. A slurry system uses water based fluid (slurry) to transport excavated soils from the tunnelling machine to the surface where the excavated soil is removed from the slurry enabling the re-use of the slurry for further excavation. There are a number of factors that guide the formulation of the slurry. The main consideration is the geology through which the pipejack or microtunnel is to be constructed but also the length and depth of the drive is important. The EPBM excavation process is similar to that of an STM in that the arisings enter the plenum chamber through the cutterhead. Ground and groundwater pressure at the tunnel face are balanced by compressing these arisings in the plenum. The forward thrust of the shield develops pressure in the plenum whilst at the same time mixing and plasticising the soil. When the face pressure is balanced the soil is evacuated via a screw conveyor. As the soil travels along the auger the pressure dissipates, allowing it to discharge at the end of the screw at atmospheric pressure. EPBMs work best in cohesive soils. Where sand and gravels are encountered in a mixed face, these may be conditioned by injecting additives such as bentonite, water or polymers into the plenum and/or the screw to plasticise them and render them suitable for generating a plug of pressurised soil. The rate of excavation of the pressurised soil through the screw conveyor is balanced to the rate at which excavated soil is entering the plenum, thereby facilitating the maintenance of a plug of pressurised soil in the plenum, which balances and stabilises the face pressure. SLURRY AND EPB TUNNELLING Typical pipe jacking eyes (see note (g) on page 18 opposite) Typical slurry separation plant