The traditional sheet pile shoring system for open-cut trenches, manholes and shafts need the following three main items:
1. Vertically installed trench sheets (overlapping) and/or light and heavy sheet piles (interlocking) which are driven and extracted by a suitable pile driving equipment.
2. The bracing system which includes one or more layers of horizontally placed H-beams (waler beams), cross bracings (mostly trench struts or beams with spindle heads) and safety chains to secure the H-beams in horizontal position.
3. The equipment to drive and extract the trench sheet or sheet piles which can be
a) Hydraulic add-on vibrators which are mounted to an excavator boom instead of the bucket and connected to the hydraulic system of the base machine (hammer line).
b) Telescopic piling rigs type Mobilram or equivalent consisting of an excavator-type carrier, a telescopic mast with automatic verticality control and a hydraulic vibrator.
c) Hydraulic vibrators with power pack suspended from a crawler crane.
If hard layers are encountered (SPT>50) a hydraulic impact hammer may be used to drive the trench sheets or sheet piles through the hard layer and get the piles to the designed level. In some cases it might be necessary and/or feasible to pre-drill hard soil to enable pile driving.
All parts of the sheet pile shoring system have to be designed according to actual site conditions (i.e. pipe length and diameter, excavation depth, soil and groundwater condition, additional side loads, etc) and approved according to international standards (EN, DIN or BS) to withstand the active earth pressure and to provide maximum safety for anybody working inside the trench.
In most cases a suitable ground water control system (submersible pumps, vacuum wellpoints or deep wells) must be foreseen to lower the ground water level temporarily to provide a clean and dry working environment at the trench base and to reduce the water pressure to the shoring system.
1. Make sure that the trench line is carefully and professionally protected on both sides to avoid any accidents.
2. Check the trench line for existing underground utilities (cables and cable ducts, water and gas pipes, sewerage pipes, etc) and remove them before commencing any other work.
3. Remove any pavement (concrete or asphalt) and any other obstacles.
4. Provide trench sheets and/or sheet piles of sufficient length, section modulus and quantity along the trench and drive them by the selected hydraulic vibrator or impact hammer to the designed installation depth.
5. Start excavation with a suitable hydraulic excavator of sufficient operating weight and boom length. Excavation depth in this first step will be done ~300 mm deeper than the first layer of horizontal waler beams.
6. Lift the waler beams of designed section modulus and length into the trench at both sides and fix each beam by two safety chains (which are hooked to the top of the trench sheets/sheet piles) to the correct position and the designed trench depth. Observe that the waler beams (H- beams) are perfectly placed horizontally in longitudinal and crosswise trench direction).
7. Lift the trench struts or other bracings of designed buckling strength into the trench and fix them carefully between the beams. Each pair of beams has to be supported by at least two bracings according to the design. Make sure that the struts or bracings are correctly extended and fixed to the beams.
8. Continue to excavate the trench to the final depth and repeat steps 5, 6 and 7 in case that more than one layer of waler beams and bracings are required according to the approved shoring design.
9. After laying the pipe or culvert the trench has to be filled-back and compacted in layers according to project specifications whereby also the bracing system (waler beams, trench struts and safety chains) will be removed in steps.
10. After reaching the top backfill level extract the trench sheets or sheet piles by a suitable hydraulic vibrator (add-on vibrators, Mobilram piling rig or crane-suspended vibrator).
11. Clean the locks of interlocking sheet piles and store them carefully for next use. If piles are damaged (at the top by the vibrator clamp and at the bottom by stones or bolders during driving) cut the damaged length and make sure, that the locks can interlock again.
This article comes from imeco edit released