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Essex rocks to Oz ingenuity

A new ground improvement technique that creates natural calcite cement in soil and rock has been tested on a railway embankment in Essex. Max Soudain reports.

Aground improvement technique that recreates millions of years of geological processes in days has undergone a major field trial on a disused railway embankment near Chipping Ongar in Essex.

The Calcite In-situ Precipitation System (CIPS) was developed by two Australian scientists, Edward Kucharski and Graham Price. It involves combining two water-based, non-particulate and non-toxic solutions and injecting or permeating the mixture into a porous material (such as sand, soil or rock).

On mixing, the solutions react to produce calcite or calcium carbonate crystals that precipitate on grains, forming a rind of crystals and creating bridges between adjacent grains. This process cements the material together and increases mechanical strength and stiffness.

Pore spaces and throats essentially remain open, which means porosity and permeability, and hence the material's drainage characteristics, are maintained. This allows repeated applications of CIPS to achieve the required strength.

In effect the technique creates rock from sediment in days rather than aeons. It is now being marketed worldwide by Kucharski, Price and another researcher, Bob Middleton, working as Lithic Technology, .

In Australia, the original research focused on improving ground around offshore foundations, Kucharski explains. Since then, work has been carried out on other applications, including underpinning, ground improvement and the restoration of historic structures.

Recent projects include the 19th century Whalers Tunnel in Perth, where CIPS solution was sprayed on to the crumbling limestone tunnel walls to improve strength and stability by replacing calcite cement (Ground Engineering news July 2001).

'For building restoration, small volumes of CIPS can create a big impact, ' says Professor Peter Vaughan from Imperial College, London. 'For example, in flint and lime mortar walls - used in many UK churches - the solution reverses mortar damage caused by acid rain. '

Vaughan, through Imperial College Consultants (ICON) gave geotechnical advice on the Ongar trial.

He says that while the material is known to work in lime, research is needed into other chemical environments before it can be used more widely.

Kucharski says more research is scheduled for early next year.

The method is novel, unobtrusive, non-toxic and very powerful, Vaughan adds. 'It is not in any way a grout - it does not solidify in place and the ground still has free water. '

Vaughan became involved following Imperial College research into embankment stability for London Underground (LUL) in 1993. The work showed there was often substantial movement in the ash side slopes of these structures, with a significant effect on the tracks. This movement was restricted to the summer months, possibly because the ash became dry as groundwater was absorbed by tree roots, he explains.

'While it was not part of our brief, I was keeping eye out for something that would stabilise the material, ' Vaughan says.

'John Burland [also from Imperial] had heard about a new technique that had been proposed to stabilise the Leaning Tower of Pisa. Through him I contacted Graham Price in Australia and the team offered to come over to the UK at their own expense to run a trial. '

Vaughan admits the first attempt was 'a bit of a disaster' This was mainly due to lack of experience with the UK material which has higher permeability and higher pH than Australian ash. The temperature difference between the UK and Australia also affected the performance of the CIPS solution.

Determined not to be defeated, the team shipped a large quantity of ash to Perth, where further tests were carried out and an appropriate CIPS formulation developed in a cold store.

Between October 1998 and February 1999, the team carried out an indoor trial on a large model embankment at Chattenden in Kent, which proved that ash could be strengthened and stiffened with CIPS.

The Chattenden trial showed that 22 hours after applying CIPS there was a fourfold increase in ash stiffness. Tunnels and overhangs were formed in the stable material to prove the material was well-cemented and had developed substantial cohesive strength.

The latest trial, on the former London Underground Central Line between Epping (now the end of the line) and Chipping Ongar in Essex, aimed to look at ways CIPS could be used to improve track and ash embankment stability in a real environment. The track is now privately owned by the Epping and Ongar Railway.

Geotechnical Consulting Group (GCG) provided geotechnical assistance and organised the field trial for LUL subsidiary Infraco JNP.

GCG associate director Fiona Chow says the research aimed to discover the best way of applying CIPS to compacted and well graded ash, the effects of stratigraphy and permeability and how to verify the results of CIPS application. Finally, the effects of dynamic loading (simulating train movements) on the treated structure were studied.

Scott Wilson Pavement Engineering carried out site investigation and testing and Cementation Foundations Skanska provided plant, technical advice and assistance.

The Ongar site is a typical LUL embankment, with a substantial cover of ash fill below the limestone track ballast and on the slopes.

Before treatment, the embankment structure was assessed using ground penetrating radar, Rayleigh Wave geophysics and an automatic ballast sampler (ABS) which obtained 1m long cores through the ballast, ash and boulder clay embankment material. Laboratory tests of the ash and CIPS solutions were also performed.

A falling weight deflectometer (FWD) was used on every second sleeper to assess trackbed stiffness. FWD uses a large weight dropped in a controlled manner on to a loading beam placed on a sleeper. Deflections are measured using geophones.

Two application techniques were tested at Ongar. The first was a sprayer that could, Kucharski says, be adapted for towing by a track-based trolley. It consisted of pipes at 90º to the track which sprayed the liquid between the sleepers.

The other system was semi-permanent pipework that could be installed beneath a track during possession and used for repeated treatment, even during working hours.

The CIPS solutions were prepared in a small compound 150m from the track. Most chemicals were supplied as powders and one in liquid form. A single pump sent the solutions down two separate lines to be mixed together at a Y-junction beside the track.

Kucharski explains that treatment depth can be controlled to some extent by the placing of the mixing point. This is because the reaction leading to precipitation of calcite crystals begins immediately after mixing.

At Ongar, where up to 1m of ash was being treated, mixing was delayed to ensure that the CIPS solution penetrated to the required depth. If nearsurface treatment is needed, mixing can be done further from the injection point.

A number of formulations were tested on a 20m length of track at Ongar. Some sections were treated with two or three applications of the same mix.

'There is an optimum precipitation rate for a given delivery of calcite, ' Kucharski says. 'We tend to start at a reasonable estimate of what is required [based on site investigation data] and then optimise the application to suit the project.

'There are two main factors: how much strength is required and what the permeability of the material is.

'If the material has low permeability, we use lower concentrations and repeated applications; if it has high permeability we increase the dosage and reduce the number of applications. '

Performance was assessed using Rayleigh Wave geophysics. This surface wave technique was used to detect changes in velocity (and therefore ground stiffness) in the ash fill. A shaker provided the source and the phase difference between surface geophones between sleepers was measured. By varying the frequency of the source, velocity-depth profiles were built up and these were converted to give stiffness-depth profiles.

Repeated FWD tests were carried out on every second sleeper to assess performance of the treated embankment under dynamic loading to simulate loads produced by passing trains.

Results of the trial are now being analysed and the team hopes to carry out further trials on LUL embankments.

Kucharski says the advantages of CIPS over other embankment stabilisation methods is that it is non-intrusive, fast-acting and simple to apply, involving minimal plant, no spoil and very little disturbance to track operations. Nor is there any temporary reduction in the short-term stability of the structure being treated.

Normally LUL stabilises embankments using expensive structural walls. And as track ballast degrades, it develops a substantial fines content which is removed and replaced by new clean ballast.

Vaughan says treating the slopes and trackbed simultaneously can improve embankment performance and reduce maintenance. 'I believe there is a distinct possibility that using CIPS may delay or even remove the need for replacing track ballast, ' he says.

'The aim is to create a stiffer track bed material, ' he adds. 'Getting a uniform material is more important than absolute stiffness. '

While material costs are still relatively high, Kucharski maintains that with larger contracts, economies of scale will come into play.

'The cost will drop over the next 12 months, ' he insists. At the moment, Lithic Technology needs to be on site during application to ensure quality control, he says: 'It is not an off-the-shelf product yet. '

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