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Flexible solution

PILING & FOUNDATIONS

Simon Lebon - the Bachy Soletanche geotechnical engineer rather than the ageing front man for pop group Duran Duran - can claim to be one of the few in his profession to know of, let alone worry about, soil erosion coefficients and jetcrete strengths.

Tthe former is a function of the surrounding soil density, grading and cohesion and the latter the precise breakdown of water, grout and soil in the jetgrouted column. Lebon is a keen advocate of the technique's versatility.

'Jet grouting is a highly flexible, temporary and permanent works solution offering waterproofing, underpinning, retaining walls and cofferdams especially in congested awkward shaped sites, ' he claims.

'At Reading we knew immediately it was the right answer, resolving all the problems without any drawbacks.'

Of the several versions available, Bachy Soletanche prefers the 'doublejet' technique with air and grout dispersed through a concentrically arranged twin aperture nozzle located 500mm up from the drill's tricone rock roller cutters.

Grout is pumped out at pressures up to 406 bar through the central, 4.3mm diameter nozzle while a shroud of air, escaping from the 16mm annulus around it, helps focus and increase the power of the grout jet.

Initially the drill forms a 140mm diameter full depth bore. Then, while the drill is being rotated and withdrawn at a predetermined speed, the air and grout jet cuts, churns and mixes the surrounding ground to create the maximum 1.6m diameter column.

Further air, flowing upwards through the outer annulus between the drill string and bore walls, helps transport displaced soil up to the surface.

To form the cofferdam's full height perimeter columns, the high pressure grout flow is continued virtually to the surface. For the internal network of short columns forming the cofferdam base, the flow is stopped some 1.2m up from the column toe.

Lebon concedes that use of high grout flows and pressures carries an inherent risk of triggering damaging surface heave, and much is owed to the skill of the rig operator. Continuous air flow back to the surface is essential and any blockages in the drill bore are bad news.

Precise information on soil consistency, plus grout, air and drill rig parameters is vital and the operator has in front of him a rig-mounted computer screen displaying up to a dozen real time readings.

Crucial to his control is the total energy imparted in mixing the soil.

Bachy Soletanche prefers to keep energy output - grout flow and pressure - constant, varying only drill rotation and withdrawal rates to maintain a constant column diameter up through the varying ground conditions.

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