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Controlled performance


Keller's new semi-automatic Vibrocat removes driver dependency in stone column construction.

Vibro-stone columns can improve both the bearing capacity and settlement characteristics of poor quality ground, whether it is manmade backfills or natural loose soils.

The method is being used increasingly as an alternative to piled foundations. But its successful use depends on the suitability of the ground, even more so than with piling.

Unlike, for instance, end-bearing piles, the problem is not bypassed with structural elements, but by improving the soil so loads can be transferred through it.

Loads are assumed to be carried by the improved ground, while settlement is governed by the confining action of the surrounding soil. In essence, the ground must laterally contain the loaded column. This means that careful installation is needed, especially in variable soils such as construction backfill.

Provided the ground conditions are appropriate and the column design assumptions are tested and calibrated on site, bearing pressures up to 500kN/m 2can be achieved in loose granular soils.

In made ground, bearing capacities of more than 150kN/m 2are attainable.

This means stone columns can be a very competitive foundation solution for many types of structure. They are particularly beneficial on brownfield developments or small projects where mobilisation costs can determine the economic viability of a particular foundation technique.

However, there have been a number of projects where poor construction has led to problems.

Often these can be attributed to an inadequate quantity of stone or insufficient compactive effort resulting in badly formed columns - despite the fact that at the surface they may appear to be well constructed.

The problem stems from the fact that successful installation depends on the operator.

Stone columns are formed by placing and compacting stone in displacement bores formed using laterally compacting poker vibrators, which can be either electrically or hydraulically driven.

For electrical systems, there is a well-established correlation between the electrical current measured in the poker and the relative density of the column and surrounding ground.

In principle, if an operator compacts the column in small lifts to a predetermined threshold current, a good quality and consistent column should be formed despite any variations in the density of the pre-treated ground.

Addressing this, contractors are beginning to instrument the construction process. Keller Ground Engineering, for example, has for some time produced printouts giving the depth of each stone column, the energy used to compact the stone, lift by lift, and the rate of construction.

But with the recent introduction of its fifth generation Vibrocat rig, Keller has taken site control to a new level, by introducing a number of innovations aimed at improving the 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 consumption. The rig is also self-levelling and weighs the quantity of stone placed in each column.

'It's all about quality control, measurement and risk control, ' says Keller business development manager Derek Taylor. '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.' But success in a vibro contract is not just about ticking off the columns on a site plan, he adds.

'We have to be as transparent as possible in the way we construct the columns and the instrumentation and logs help us achieve that.

It's a tremendous innovation.

People specifying vibro work need to be aware that there are different standards of construction out there and that today a greater level of quality control is available.' Taylor says a rig should be achieving no more than about 70 columns in a day. He suggests that higher productivity often indicates inadequate column construction.

'Probably we consistently use more stone than our competitors, but we believe this leads to more competent columns and so better settlement control, ' he says.

Already used for road embankments and industrial type structures, stone columns are an increasingly common foundation solution for buildings up to five or six storeys. In fact, with greater use of timber-framed buildings, Taylor predicts that stone columns may routinely be used on taller buildings.

'We are treating ever more complex soils, from soft clays to highly variable fills, and for many of our projects settlement control is everything, ' he says.

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