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New team takes care of business

GROUND IMPROVEMENT

Skanska Foundation Group's global network carries out a wide variety of ground treatment contracts worldwide.

Ground Engineering takes a look at some of its recent projects.

At the beginning of this year Skanska brought its 11 geotechnical businesses together under one umbrella, forming the Skanska Foundations Group.

The £370M turnover group operates in 14 countries and was set up to enable pooling of skills and resources to allow it to take on large and technically challenging projects (GE March 2002).

Ground treatment is one of the major elements of its work.

Expertise in some techniques, such as soil mixing, is stronger in countries where they have been used more and the idea is that group members help each other out.

For example, Skanska's project team working on the UK Channel Tunnel Rail Link (CTRL) is involved in assessing mixed-inplace foundations and has sought assistance from its Swedish colleagues, who have particular knowledge of the method.

Likewise, UK expertise gained on massive diaphragm walling for CTRL has been passed on to help a Skanska underpinning tender for a New York project.

'The sheer variety of geological conditions here in the UK means we have a wide range of techniques which are exportable to help our overseas partners, ' explains group chair and Skanska UK director Mike Putnam.

The UK's growing need to maintain and repair ageing infrastructure is proving a boon for geotechnical firms such as Cementation Foundations Skanska. Having established a thriving business dedicated to rail embankment stabilisation, the firm is now finding similar demands emerging from the water industry.

One example is a recent contract carried out at Winscar Reservoir in South Yorkshire.

Built in the mid-1970s, the Winscar Dam was giving water operating company Yorkshire Water cause for concern over possible leakage through the original grout curtain.

Extending about 150m beneath the foot of the dam wall, the curtain was built by injecting pressurised cementitious grout into the underlying moorland rock to reduce water seepage through porous fissures.

Although supplementary treatment had been carried out at the end of the 1970s, Cementation was called in to undertake further work in autumn 2001. This involved investigation and treatment of the grout curtain below the upstream toe beam.

The reservoir was drained and site investigation boreholes drilled at the base of the reservoir, with permeability tests using modified Lugeon methods.

This indicated that the original grouting had been washed away over time, leaving large cavities.

Remedial grouting was carried out in two phases.

First, tubes a manchette (TAMs) were installed in drilled boreholes, with each stage isolated by using grout bags. Drilling for the second phase used an excavator base with a rotary percussive drill mast mounted on the main boom. To ensure safe drilling operations on the 20° abutment slope, a winch system was attached to the front of the rig, which was operating close to the limit of its tracking angle.

Micro-fine cement was used for both phases of grouting, with additives used to enhance agitator life and improve pumpability.

Double packers were used for TAM grouting and single packers for the open-hole element of the works. Both phases were carried out in ascending stages.

Cementation's plant division in Doncaster supplied mechanical equipment for the job. This included a containerised pumping system set up on the top crest of the dam. A computerised grouting container, consisting of an agitating tank, grout pumps and computer control system, was used to carry out the individual sleeve/stage injections. The computer was programmed with the required maximum injection pressure, flow-rate and volume.

Work was carried out on time and with minimal disruption to other contractors working on the project. Yorkshire Water was able to start refilling the reservoir during the wet season, increasing the total capacity of fresh water for the district.

In Hong Kong, Skanska's locally based associate company Gammon Skanska recently finished a large ground treatment contract using vibroflotation for a new container terminal.

Demand for land for housing and commercial expansion is still growing on the densely populated island and this usually means reclaiming land from the sea, typically using hydraulic filling with marine sand.

Since 1990, about 220M. m 3ofsand has been used to make room for homes, port facilities and infrastructure. This figure is expected to be exceeded over the next 10 years to meet the pressing need for housing, road and rail developments.

Typically, marine sand is dumped from split barges, and hydraulic filling by pumping is carried out as more and more sand is placed and water depth reduces.

With most of the filling done under water, the sand comes to rest in a relatively loose state.

Generally, untreated marine sand has a relative density of about 40% as placed, and densification is often essential to minimise future settlement, particularly for developments built on a slope. As a result, vibroflotation has been used extensively in Hong Kong for deep compaction of reclaimed sand fill. The ground treatment process can achieve a relative density of typically between 75% and 85%.

At Container Terminal 8 in Hong Kong, Gammon Skanska successfully densified reclaimed ground using the vibroflotation, compacting the fill by inserting a powerful poker vibrator. The HK$7M (£623,000) contract required treatment of 1M. m 3ofreclaimed sand fill up to 25m deep.

Gammon carried out compaction on an equilateral triangular grid at 2m to 3m centres. Grid spacing varied depending on the depth of treatment required.

Specified acceptance criterion for the deep compaction was to achieve a corrected SPT N-value of no less than 40. Acceptance criteria based on cone penetration resistance are also widely used in Hong Kong.

Gammon says that vibroflotation has proved to be a fast and cost-effective way of compacting large quantities of hydraulically placed marine sands.

The firm expects the market for the method to expand with more large areas of reclamation planned in Hong Kong and elsewhere in Asia.

Last year, Skanska Foundations installed lime cement columns using dry deep mixing to stabilise road embankments on a new 3km section on Route 35 between Gamleby and Överum, 40km north of Västervik in southern Sweden.

The ground profile consisted of a thin crust of organic soil overlying a very soft grey clay with a shear strength of about 10kPa, down to a maximum depth of about 11m.

The main aim of the scheme was to improve embankment stability and reduce settlement.

Lime cement stabilisation was chosen over other methods such as embankment piling or dig and replace because of price and environmental considerations.

Work began with installation of many trial columns to optimise the main installation process.

Test column properties were examined to choose the optimal installation parameters, the type of binder and the binder content.

The required shear strength of the stabilised soil was a minimum of 100kPa after 30 days.

Cement and unslaked lime in equal proportions were chosen as binder, the amount varying between 23kg and 28kg per metre of column. In all, 32,000, 600mm diameter columns, averaging 5. 5m long, were installed by two rigs in five months.

Column penetration tests were carried out throughout the project to verify shear strength and check column quality. This also allowed the installation process to be adapted to variations in ground conditions.

Skanska also tested some new quality checking methods, including a modified reversed column penetration test.

Results showed the required stabilisation was achieved and maximum allowable settlement was 500mm, yet after eight months, settlement had almost stopped at just 350mm.

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