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Backfill battle


Construction of a complex motorway junction near the German city of Leipzig called for stone column construction on a mammoth scale.

Leipzig's new southern motorway ring road, the A38, will provide a much-needed link between the A9 and A14 motorways. The route is not so easy, however, for Bauer Spezialtiefbau. On its way the motorway crosses the former Espenhain opencast lignite pit, and the contractor has to deal with the considerable geotechnical demands this poses.

Up to 70m thick backfill comprises up to 50m of slag tipped by a conveying bridge system between 1974 and 1979, overlain by 20m of truck-tipped slag dumped during the mid-1980s.

'Former lignite opencast mining areas are extremely difficult for road construction, ' explains Bauer Spezialtiefbau technical director Manfred Stocker. Not only is the slag weak and very inhomogeneous, but 'because it was backfilled in wavelike patterns, we calculated surface settlement could vary from 0.5m to 2m.

'Preventing undulations at the surface, which would transmit into the road, required an immense ground improvement effort.'

Bauer proposed soil improvement by vibro-displacement stone columns to 'limit total settlements and eliminate large differential settlements by attaining improved and more uniform soil parameters' - particularly in the upper truck-tipped slag.

Requiring an especially dense concentration was the junction where the A38 crosses over the B2 trunk road and the proposed A72 motorway. Bauer installed 711,000 linear metres of columns to depths between 5m and 25m.

To achieve this mammoth task - around 43,000 stone columns in a programme of just 27 weeks - Bauer had up to 11 large piling rigs working day and night, putting, as Stocker says, 'extreme demands on site personnel as well as plant and equipment'.

Working with earthworks joint venture partner Josef Radlinger, under a E12M contract, there were up to 144 operatives working on the contract at any one time, 64 from Bauer.

At peak production, Bauer needed a daily supply of more than 7,000t of sandy gravel, provided by up to 280 articulated trucks each carrying 26t.

The contract was carried out in four phases.

First, the topsoil was removed and the site covered with a blanket of gravel.

The second task involved construction of the stone columns, set out using a GPS-based surveying system. Column construction took place between April and October last year and made use of a novel top-vibratory bottom gate system (see box).

This involved using piling rigs to vibrate up to 25m long, 0.5m diameter steel tubes into the ground to predetermined depths.

Tubes were then filled with gravel and extracted and repeatedly lowered by a top-mounted vibrator to densify the stone.

Electronic instrumentation and monitoring provided a control over construction quality.

'The result is a compacted gravel column with a diameter of 700mm, ' explains Stocker.

'Columns were installed on a triangular grid spaced from 2m to 2.5m apart, creating a subsoil formation with a relatively uniform load-bearing capacity. An important secondary effect is that the gravel columns act as vertical drains, accelerating pore water dissipation.'

Third phase was installing a horizontal drainage blanket and constructing an oversized surcharge embankment to speed up the ground's consolidation in a controlled way.

'This excessively high embankment simulates the load applied by the future motorway and permits the total anticipated consolidation settlement to occur during a comparatively quick six month preloading period in advance of final road construction, ' says Stocker.

Measurements to date suggest consolidation settlement is on target, meaning that later this spring, in the fourth phase, Josef Radlinger will reduce the embankment back to road formation level, leaving the underlying backfilled mine fully prepared for contractors to complete the project.

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