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A hole new approach

News analysis A2

One of London's major road arteries is still blocked by a mysterious hole. Paul Wheeler reports on work in progress to get the traffic flowing again.

It will be November at the earliest before the Blackheath Hill section of the A2 is open to traffic again. After four months of detailed investigation, grouting began last month and is scheduled to last for at least another three months.

Now Transport for London (TfL) is confident it knows why, on Sunday 7 April, a 6m-wide crater suddenly opened up on the main arterial route into London from the south east.

The road was immediately closed, diversions set in place and 37 households evacuated.

Then the investigation started.

'For the first few days we didn't dare get any heavy equipment on site. All we could do was survey up and down the road to detect movements, ' says Trevor Williams, TfL Street Management's south east area roads manager.

'There was total ground loss below a section of the road and adjacent pavement. It was clear the failure was gradually enlarging as the collapsed faces assumed their angle of repose.'

TfL's priority was to stop the hole getting bigger, and so it filled the void with around 100t of pea gravel - in itself quite a challenge, when there was no assurance of the stability of the surrounding ground. Within a week, TfL, term contractor Fitzpatrick and consultant Parkman were sufficiently confident that the collapse had stabilised to allow closer investigation.

Eventually the cause of the collapse was attributed to ground loss, a result of water removing fines from underlying granular fill material (see box).

The source of the water - whether it was natural flow within the ground or from a leaking water main - will probably never be determined.

Two water mains do lie within the collapse zone but, as Williams points out, when water mains rupture soil fines are usually washed up onto roads.

In this case everything went down the hole, suggesting the mains broke during the collapse.

With the engineering effort focused on getting the road re-opened, the assembled team embarked upon a very large site investigation, covering a 600m long section of the A2 where it drops down from Blackheath to Deptford. The main strategy was to use a geophysical technique backed up with boreholes. 'We wanted to use two techniques so that one would be proving the other, ' explains Williams.

For the geophysics, Parkman called upon the services of Keele University-based Microsearch. It then conducted a microgravity survey along the entire section of concern.

The basic principle was to position boreholes where the geophysics identified anomalies.

There was some concern over the weight of drilling equipment and the fact that most site investigation techniques use percussive drilling methods. An early priority was finding a company that could offer lightweight (percussive) window sampling with a rotary coring follow-on.

Bristol-based Structural Soils had the right equipment available for immediate start. It gave the company a foot in the door that has subsequently developed into a 105-borehole investigation, with percussive samples taken down to 13m and rotary cores reaching down as far as 40m.

Investigating the full subsurface extent of the collapse presented some additional problems - principally because of on-going concerns over the area's stability. Williams says they were also keen to maintain the two-technique approach to site investigation.

Parkman therefore enlisted the help of Cementation Skanska to use its '3dT' borehole seismic tomography techniques to produce a 3D image of the ground. Four boreholes were drilled around the perimeter of the collapse, in which was placed seismic source and receiver arrays.

By sending and receiving seismic signals between the four boreholes it was possible to build up a 3D velocity model of the ground. The resulting velocity contour image can be viewed on a PC from many different angles and gives clear definitions of the contrasting densities of different structures in the ground.

From the microgravity and borehole work, TfL identified a 180m section of road that needed to be treated by drilling and grouting before it could be reopened to traffic. Fitzpatrick brought in Keller Ground Engineering to carry out the remedial work.

Drilling and grouting is inherently an empirical process - in that it is not possible to scientifically predict the flow and take of the injected grout. Experience however gives contractors a good indication of what is likely to happen. Established drilling and grouting practice treats early production as a working trial from which the design is refined and then validated. It is a two-phase process and if carried out correctly the injection works are self-checking.

In the first phase Keller is drilling holes on a 3m grid through the affected ground and penetrating the insitu chalk by 2m. Grout is injected into each hole until no more will go in.

After the grout has gone off, a secondary grid of injection holes is drilled in between the primary grid. From the grout take within the secondary injection holes it is possible to check the effectiveness of the grouting and assess the performance of the works.

Keller started work in late July and the hope is always that the spacing of the grout injection holes can be increased to reduce the amount of drilling needed to cover the treatment area.

Provided there are no surprises the road should reopen by the end of November.

Williams remains convinced that TfL's strategy to investigate the cause fully is one that will prove to be the most cost and time effective in the long run.

In the meantime he is highly complimentary of the 'partnership approach' that has seen everyone, from the police, local authorities, local residents associations, Thames Water, the Environment Agency and even the bus companies making the best of a bad situation.

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