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 2423 An introduction to pipe jacking and microtunnelling 6 JACKING LENGTHS, LOADS AND TOLERANCES LUBRICATION The pipe jack shield or machine is designed to produce a small overbreak to the external diameter of the pipeline. By injecting a lubricant, for example bentonite, into this annulus the pipeline can, in theory, be jacked freely through a fluid medium. In practice, however, fluid losses may occur into the surrounding ground. Providing these can be controlled, the technique results in considerable reductions in jacking forces and therefore longer jacking lengths. JACKING LOADS Loads required to jack the pipeline forward are mainly a function of frictional forces built up around the pipeline. These forces depend on the type of ground and, in particular, its arching characteristics, friction angle, the depth of overburden, the depth of the ground water and any surcharge load, the length and diameter of the pipe being jacked and the time taken for the operation. Whilst it is difficult to accurately assess these forces using soil mechanics theory, pipe jacking contractors have, after years of experience, derived empirical values. As a guide, frictional forces fall between 0.5 and 2.5 tonnes per square metre of external circumferential area. The use of sophisticated lubricant injection techniques can reduce frictional forces to as little as 0.1 tonnes per square metre. Frictional forces on the pipeline may be reduced by applying a suitable lubricant, under a nominal pressure above that of the ground water pressure present. If high frictional resistance is anticipated, it is recommended that intermediate jacking stations are placed at regular intervals in the pipeline. These jacking loads must be resisted by a jacking reaction built up within the thrust shaft. This is normally achieved by the construction of a thrust wall at the back of the thrust pit designed to withstand the anticipated jacking load and to suitably transfer such loading through the shaft structure to the surrounding ground. JACKING TOLERANCES In stable, self-supporting, homogenous ground, the typical tolerance for pipe installation is ±50mm of a true line and level at any point in the drive. However, in some ground conditions, particularly unstable ground or where obstructions are present, these tolerances may not be readily attainable. In such circumstances where this tolerance or a finer one must be achieved, larger pipe sizes can be considered. Adjustments to line and level should be gradual to ensure that the pipe manufacturer’s stated permitted angular deflection is not exceeded at any individual joint. Typical lubrication arrangement