It is the industry's dilemma that perimeter walls used to support excavations deflect laterally during excavation and before they can be held in place, ' says Applied Geotechnical Engineering (AGE) director John Hislam.
'Propping at ground level does assist, but as soon as the excavation reaches an appreciable level, the wall deflects inwards under the earth pressures acting on the reverse side of the wall, ' he explains.
Deep basements in particularly sensitive locations have in the past used props installed in advance of excavation.
In other projects, attempts have been made to stiffen the internal ground zone.
In some cases slurry walling technology has been used to advantage but these systems normally only allow excavation within berms left to uphold the weak, essentially non-structural wall material.
Recent extensions of this technology have incorporated steel reinforcement.
This strengthening has the advantage of holding back groundwater as well as providing earthworks support. Slurry walls have also been used with contiguous bored pile walls to achieve the same result for deeper excavations.
AGE's concept is to pre-place props to support the perimeter wall before internal excavation is commenced.
Hislam explains: Short lengths of wall would be installed as panels with a prop attached.
The prop would be connected to the wall by means of a hinged connection and held vertically, initially, flush with the panel.
Subsequently the prop would be rotated about its hinge, enabling it to be connected to a restraint at ground level prior to bulk excavation.
'The system is envisaged to be used either as a pre-trenching or driven method and could in the former method be used in unsupported or slurry filled trenches, ' continues Hislam.
'Essentially short sections of trench would be excavated to the designed length and depth.
Pre-made wall panel units would be lowered and positioned into the trench. The wall panels would have a prop fixed via a hinge at a predetermined level.
'The trench would be backfilled or slurry would be allowed to set. After completion of the other wall panels, discrete secondary excavations would be made at the prop locations, allowing them to be swung away from the wall and connected to a restraining system.' He adds: 'For driven panels, a pre-bored hole could be made to allow entry of the hinged prop.
The secondary excavations could also be made at the same time as the panel excavations. Guide walls could be employed to assist positional accuracy.' Hislam claims it is possible to achieve significantly shorter penetration depths for the perimeter wall as it is always propped, not in cantilever.