Rethinking an 'impossible' precast brings wide benefits Winner of last year's awards, the BAA/Amec Pavement Team, tackled a new problem this year, and once again demonstrated a wide range of benefits across the board, from reduced pollution and noise to waste reduction, improved worker safety and security, as well as overall costs savings.
The task was relatively straightforward: namely how to provide service pits, so called P-pits. These relatively large chambers contain primarily high tension power equipment, usually beneath taxiways and aircraft stands.
'They are awkward to build, take up resources and demand a lot of in and out movement of materials from the airport, ' says Andy Delchar head of performance improvement. A standard solution would be to make them as single precast units, but at 2.9m by 2.9m and at least 2.4m deep, they are too large for that.
'They must also fit tight tolerances of just ¦3mm at pavement level and sustain high loads, possibly from a passing jumbo jet, ' he adds. 'And they are complex, with various cable entry points. Some 55 components are embedded in the walls.'
As a consequence, and because there are only a few on each project, the P-pits have usually been constructed insitu, he says, taking around three weeks to complete.
But the team decided to take another look and eventually devised a way to precast the chamber using modules. The base is infilled with a small amount of poured concrete. Rectangular 'ring' sections above lock together with grooves and tongues before being grouted. Cast in jacks enable the tolerances to be achieved.
The team worked with supplier partners CV Buchan on the precast design and Elkington Gatic to design the cast iron top covers.
With this system a pit takes just 15 man hours on site, excluding initial excavation, against the 300 hours previously needed.
'Most of the work has already been done in the factory, ' says Delchar 'where quality control is better anyway.'
That means many fewer operatives passing to and fro through airside security checks and truck journeys are cut from nine HGVs to five. Modules are designed with lifting points and are kept within weight limits for a standard truck mounted crane, keeping plant requirements low and manual handling to a minimum.
The excavation is reduced in size since less working space is required, and confined space working is almost eliminated.
Site presence of around one day instead of weeks, all makes the work much safer.
'It is also noticeably cleaner, ' says Delchar 'because there are none of the little bits and pieces around that you have with insitu work.' With no offcuts, loose components, and spare concrete wastage is reduced too.
Crucially, hot works like spark-causing steel cutting are eliminated, and there is no messy, noisy scabbling.
'The end result can also be modified easily, ' he says, 'by adding on another precast ring for example. This will extend the benefits to future projects and maintenance.'
The modular solution could now find a much wider use for similar pits such as drainage manholes, extending the social, safety and environmental benefits further.