Five years ago, when building an extension on the rear of my house with deep strip foundations, I started to think hard about spoil removal.
When the spoil filled the drive to a height of 1.5m and the grab lorry had to come twice when the budget had only allowed for one visit, it concentrated my wallet.
Six months later I attended a meeting for a project in Liverpool in north west England where a precast concrete frame was to be produced in Italy and then transported by road. Hardly sustainable construction. These two events gave pause for thought.
First, spoil should be minimised (the landfill tax some years ago gave a lesson in that) and second, the industry needs to transport its construction products as little as possible. Within geotechnics, this surely means the trend of lateral thinking engineers opting for ground treatment will continue. Undoubtedly far more sustainable.
Probably the most sustainable of all solutions on a brownfield site is to leave everything as it is (including old foundations) and use dynamic compaction or a similar technique.
If this is not applicable then vibro techniques (preferably using spent rail ballasts) should be an early choice.
Such systems generate no spoil and often the stone travels by rail to a nearby rail head or, better still, is crushed and graded on site. If prepared to a suitable quality, high integrity stone columns can be formed. The only spoil generated is when casting subsequent strip foundations, and normally these would be shallow and lightly reinforced.
Taking treatment a stage further, the Swedes have brought dry-soil mixing to the UK. Introducing lime, or lime and cement, can mean rapid improvement of soft and very soft clays.
Considering the environmental impact of piling systems, the true green would consider the concept of "pile miles" [kilometres to mainland Europeans, but it does not have the same ring – ed]. Having accepted that a structure needs piles, surely the first choice should be a displacement pile and preferably not one that is transported significant distances from a casting factory.
Better still, a driven cast insitu pile could be one of the greenest solutions around, particularly if the concrete comes from a nearby plant that uses granulated furnace slag in the mix. Surely, a near-perfect sustainable product, except for timber piles of course.
All this assumes that there are not piles on the site that can be reused.
If there are, see the handbook produced following the RUFUS (Reuse of foundations for urban sites) project.
If they are there, then reuse them.
Having opted for a driven piling system, the question may arise "What about the neighbours"? The answer to this might be to contact the local environmental health officer and explain that the reason for driven piles is to minimise vehicle movements (muck-away lorries). In the current climate they are generally sympathetic and will usually agree an appropriate Leq (noise threshold) value for any piling contractor to work within.
Another advantage with some driven systems is the opportunity to form enlarged monolithic pile heads. These, beneath the floor slab
of warehouses and distribution centres, avoid breaking down piles and lead to thinner floors – meaning less work and less concrete.
Obviously, the greenest solution of all is to do neither piling nor ground treatment, and for the most flexible of structures, that can be an option. However, when performance expectations are high or structures are heavy, this is not possible. Then it is time to think laterally about possible solutions and the impact of those choices.
After all, nobody wants that grab lorry coming twice, or to be stuck behind a lorry load of precast products. Far better to tell a client you have found a treatment solution using recycled material – that has also saved them money.
Derek Taylor is sales director at Keller Ground Engineering