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 24In order to redistribute the total required jacking force on the pipeline, intermediate jacking stations are frequently used between the launch pit jacking rig and the tunnelling machine. A special twin pipe set incorporating an increased length steel collar which slides over a corresponding length spigot detail is introduced into the pipeline. Hydraulic jacks are placed between the two opposing pipes such that when activated they open the gap between the leading and trailing pipes. The interjack station is then moved forward with the pipeline in the normal way until it becomes necessary to supplement the jacking forces available from the shaft. On reaching the design value or when the available thrust force is insufficient to move the pipeline forward, then the pipes behind the intermediate jacking station are held stressed back to the thrust wall in the launch pit. The jacks in the intermediate jacking station are then opened, thus advancing the forward section of the pipeline. 22 An introduction to pipe jacking and microtunnelling 6 JACKING LENGTHS, LOADS AND TOLERANCES INTERMEDIATE JACKING STATIONS (INTERJACKS) At completion of the stroke of the inter-jacks, the main jacks in the thrust pit are actuated, advancing the rear of the pipeline to its original position relative to the leading pipes, and thereby closing the intermediate station jacks. The sequence is then repeated for the duration of the pipe- jack and, on completion, the jacks and fittings are removed and the inter jack closed up. Interjack stations are not only used to increase the jacking lengths achievable, but also to reduce the loads that are transmitted to the shaft structure. This is useful where ground conditions at the drive shaft are poor or of low inherent strength. 6 Jacking lengths, loads and tolerances JACKING LENGTHS Refer to table 4c) and note a) on page 18. The length over which a pipejack can be installed is dependent upon a number of interrelated and variable factors: the stability and friction characteristics of the geology to be tunnelled through, the self-weight and strength of the pipes, the diameter of pipe, the type of excavation method, and the available jacking reaction. The major constraint will be the nature of the ground and the ground water characteristics. However, the distance that can be achieved is optimised by the use of a range of techniques. Intermediate jacking stations