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 2415 An introduction to pipe jacking and microtunnelling 4 DESIGN AND CONSTRUCTION METHODS 4a) Design of working shafts in dry ground DRY GROUND: Dry Cohesive, Dry Non-Cohesive and Dry Mixed and Fill Conditions Type Size and shape Depth Ground treatment Remarks Segmental 2.4m dia and above Unlimited Not required Diameter dependent on pipe jacking method, rig and depth considerations Sheet piled Any Generally Up to 15m Not required Secant piled Any Generally Up to 20m Not required Needs large working area Trench sheeted Any Up to 6m Not required Not recommended Pre-cast caisson 2.4 to 4m dia Generally Up to 15m Not required Suitable for microtunnelling Cast insitu caisson Any Unlimited Not required Can be water retaining and flotation resistant Battered excavation Any Shallow Needed if angle of repose of soil exceeded Ground anchorages Any Surface level Not required For shallow work only 4b) Design of working shafts in wet ground WET GROUND: Wet Cohesive, Wet Non-Cohesive and Wet Mixed and Fill Conditions Type Size and shape Depth Ground treatment Remarks Segmental 2.4m dia and above Limited by ground treatment Wet caisson method Grab excavation below water, generally 50m depth Well pointing Up to 7 metres depth Deep well dewatering Compressed air Suspension grout Chemical stabilisation Ground freezing Depth dependant on ground conditions and water level Sheet piled Any Generally Up to 15m Well pointing Up to 7 metres depth Deep well dewatering Depth dependent on water table draw down and pile cut-off level Secant piled Any Generally Up to 20m May be required for base stability Needs large working area Trench sheeted - - - Not recommended Pre-cast caisson 2.4 to 4m dia Generally Up to 15m May be required for base stability Suitable for microtunnelling Cast insitu caisson Any Generally Up to 15m May be required for base stability Self weight can overcome skin friction and flotation Battered excavation - - - Not applicable Ground anchorages Any Surface level Ground treatment and/or piling may be required to provide jacking reaction Generally used for drives through embankments Notes (a) Working shafts are designed to suit ground conditions and their intended use. They can be converted to permanent works, ie segmental or caissons to manholes or piled shafts to manhole rings. (b) The type of construction is not generally governed by the ground treatment method, which should be considered as an assistance to the construction method. (c) In rock, the shaft design will depend on the characteristics of the material. (d) Most shaft construction methods require external concrete collars. Consideration should be given during design to the overall plan area and depth. (e) For very deep drives in water bearing ground, consideration should be given to pressure limitations of available equipment.