Case studies

Ground improvement techniques offer an attractive alternative to piled foundations on brownfield developments

Every developer appreciates that there are unknown risks associated with brownfield developments. As many have learnt to their cost, a site that looks like a bargain may contain risks out of all proportion to the proposed development.

But technical developments, particularly in the production and quality control of ground improvement techniques, means these risks can now, largely, be engineered away.

Low-rise structures, such as houses, can be built directly from ground improved by vibro-stone columns – which as the name implies are closely spaced columns of compacted stone.

Because ground improvement techniques are generally less expensive than foundation techniques, ground improvement of an entire site need not cost more than say normal piling operations. Suddenly, that brownfield plot really is looking like a bargain.

A great case in point is a development of 188 houses on a backfilled gravel pit at Bedfont Road in Feltham, West London.

Although the fill is largely inert, it is very loose and up to 6m deep. Add to this the presence of the vibration-sensitive, 750mm-diameter aviation fuel pipeline, which serves nearby Heathrow Terminal 4, and the site presents what can modestly be called a diverse set of geotechnical challenges.

These extend beyond providing adequate house foundations. The ground is also too loose to support roads and hardstanding areas; even the gardens need treatment, in order to remove the risk of differential movement.

The houses have been built on vibro-stone columns, installed using Keller’s new semi-automatic “vibrocat” which provided new levels of quality control in stone column production.

This is important because there have been a number of projects in recent years, carried out by other contractors, where poor column construction has led to problems. Typically, inadequate amount of stone or insufficient compactive effort, has resulted in badly formed columns - despite the fact that at the surface they may appear to be well constructed.

Such problem can generally be attributed to the fact that, in practice, successful installation is operator-dependent. But with the recent introduction of its latest fifth generation “Vibrocat” rig, Keller has taken site control to a new level, by introducing a number of innovations aimed at improving quality and reliability of the columns.

The new rig semi-automates the column construction process. The operator sets the depth and compaction rate and the machine does the rest, working the stone in each lift until a predetermined density is achieved in the ground, measured by the vibrator's current consumption. The rig is also self-levelling and weighs the quantity of stone placed in each column.

For Keller, it is all about quality control, measurement and risk control. The column logs provide proof that the columns have been constructed to a certain depth and evidence of the degree of compaction achieved along the whole length of the column.

Provided the ground conditions are appropriate, and the column design assumptions are tested and calibrated on site, bearing pressures up to 500kN/m² can be achieved in loose granular soils. In made ground, bearing capacities in excess of 150kN/m² are attainable,

Unsurprisingly then, stone columns can provide an extremely competitive foundation solution for many types of structure up to around six storeys. They are particularly beneficial on brownfield site developments or small projects where mobilisation costs can determine the economic viability of a particular foundation technique.

The Feltham project was also significant because the developer brought in Keller on a partnering basis early in the design stage of the project. This helped Keller to blend a range of solutions to give best value.

It meant that, unusually for a geotechnical contractor, Keller took on responsibility for stripping off topsoil, importing and placing some 10,000 m³ of granular fill, erecting the site perimeter fence and forming a concrete crossing point to protect the fuel pipeline.

Keller’s business development manager responsible for the project believes this approach is particularly attractive to residential developers. “House builders can calculate the above ground costs with incredible accuracy,” he says, “but anything in the ground represents unwanted uncertainty. Keller has the experience and technology to manage that ground risk, and in the process can remove what many house builders will tell you is their biggest nightmare.”