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Finns set new bog standard

GROUND IMPROVEMENT

New equipment is helping engineers stabilise organic clays and peat in Finland and Ireland.

Engineers in Ireland have long had to contend with the problems of peat, silt and clay.

These unstable materials will not support loads and can produce slips if they are present over sloping rock formations. Winter rains after the dry summer of 2003 resulted in several such failures.

The traditional approach is to dig out the soft material and replace it with aggregate. This happened 40 years ago when Northern Ireland's M1 motorway was built across the boglands west of Portadown, where peat extends to depths of 12m.

Where possible it was dug out by draglines and replaced with fill, but in some areas it was necessary to tip thousands of tonnes of stone to displace the semi-liquid peat and form a causeway.

Today the excavated material would be subject to landfill tax and the imported stone to the quarry tax, adding greatly to the cost of the project.

Research has shown that such stone foundations in very soft materials can lead to difficulties because they may act as a drain.

The groundwater flowing along the aggregate layer can result in eventual movement.

In continental Europe, mass stabilisation of soft non-organic soils with lime and cement columns has been carried out for more than 20 years to reduce settlement and improve the stability of embankments and slopes.

But stabilisation of organic soils such as clay and peat is relatively new and has developed rapidly in recent years. This trend is expected to continue, as the method reduces the amount of spoil and the need for imported stone, together with the associated haulage.

Another key driving factor has been the development of machinery to carry out the work. Thorough mixing at all levels is crucial.

Help in this department may be at hand from Finland, where a new system from Finnish manufacturer Allu is making its debut in swampland on a high speed rail link from Helsinki to Lahti in the north of the country.

The area is underwater in places, so stabilisation is needed to allow rigs to drive concrete piles to depths of 25m.

Allu has developed a range of equipment for soil stabilisation and recycling. Its screener crusher processing hydraulic excavator attachment is already widely used in Ireland.

Applications include processing glass and green wastes; crushing and screening excavated material so it can be backfilled as pipeline bedding; and processing gypsum and concrete waste. And Allu's AS38H windrow turner can mix high-density materials (2,300kg/m 3) at up to 4,000m 3/h.

The firm's EPM System, together with Allu SM, Allu VP and Allu OilQuick, is designed for soil stabilisation in pipeline construction.

Mounted on a 35t excavator with quick release that includes automatic hydraulic couplings, the system comprises a fastfill silo for placing the binder agent and a rotating mixer head that is lowered into the soil. The mixer has rotating blades to dig through the peat, evenly distributing the binder agent - typically a mixture of lime and cement - up to 5m down.

This equipment has now been joined by the Allu PF, a trackmounted pressure feeder which continuously feeds binding agent via a hose to the mixer head. Allu says this speeds the processing of large areas as the excavator operator has remote control of the PF unit. A small display and control panel in the excavator cabin shows how much material is going into each area.

The machine is being used for the first time on the high-speed railway contract. Treatment rates are about two linear metres an hour across the 100m wide site, depending on the soil strength. A ratio of 20% binder agent to peat is being used, although Allu says half this amount is usually sufficient for most contracts.

Mass stabilisation using moderate amounts of a binder agent (less than 100kg/m 3) can increase shear strength to at least 50kPa-100kPa and even higher. This form of stabilisation is especially suitable as a ground improvement method for structures requiring reinforcement of large soil volumes as advantageously as possible, but where absolute non-settlement of the ground is not expected and the ground is not subject to excessive single point loads.

Two-component binder mixes are widely used but three-component binders are more versatile and can be more effective for many cases. The most important components are lime, cement, blast furnace slag and gypsum.

Industrial byproducts such as high quality fly ashes can be used for certain cases, especially in peat stabilisation.

Stabilisation causes significant changes to the chemical and physical properties of peat and clay.

The pH value will quickly rise to 11 or 12 and curing starts.

Depending on the binder, some reactions will take place in a few weeks but others may take months. Using only cement, most of the strength develops during the first month after treatment but with lime, gypsum, slag or ash the strength will continue to develop after the first month.

Mechanical interactions are also important. Cement requires very thorough mixing since it does not diffuse as lime does.

While cement increases the strength and brittleness of the stabilised soil, lime increases ductility. However, a too stiffly stabilised material is not necessarily the best solution since it will behave like a pile, Allu says.

The idea is to produce a stabilised soil that mechanically interacts with the surrounding material, so loads are carried by both.

Following treatment on the high speed link, a 400mm layer of aggregate is spread over geotextile across the site and lightly compacted. This provides a clean and stable surface for the piling machinery and provides lateral support for the piles while the ground slab is being built.

Contractor Valto Tikkanen will install vertical and raked piles and then cast the ground slab on top of the piles, providing a solid foundation for the railway which will carry axle loadings of 30t.

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