Field trials of a soil stabilisation method new to the UK were recently completed in Glasgow. Dave Cooke reports.
Interest continues to grow in contaminated land treatment techniques offering an alternative to bulk removal of soil to landfill.
In the UK there is a general (although not universal) perception that such techniques exist, but that they are still experimental or not cost competitive.
One technique is the Geosta/cement soil stabilisation process. Despite 20 years of proven performance in Europe, Africa, the Far East and South America, there has been a somewhat defensive and sceptical response to its use in the UK.
In an attempt to combat this, a field trial was carried out at Rosebery Park in Glasgow to demonstrate its effectiveness and allow its practicality and cost to be assessed under site conditions.
Rosebery Park is one of a number of sites owned by Glasgow City Council (GCC) known to be contaminated by deposits of chrome waste from industrial processes. GCC has investigated various remediation treatments.
With the help of funding from GCC and the local Scottish Enterprise network (south east Glasgow and south east Lanarkshire), further work was recently completed on a small scale trial looking at the stabilisation/solidification of the waste material using the Geosta system.
The process is a relatively simple technique where soil is mixed with a calculated amount of Ordinary Portland Cement and a proprietary additive called Geosta, along with enough water to bring the mixture to its optimum moisture content.
The stabilised material is initially a loose mix, but goes on to form a solid, concrete-like material that effectively immobilises any chemical contamination and provides significant improvements in the physical strength and flexibility of the treated soils.
Geosta is manufactured in The Netherlands and Switzerland and is distributed, along with other Merkstab products, by Glasgow based Merkfield.
The Rosebery Park trials consisted of an initial batch mixing process conducted in the Weeks Technical Services laboratory in Glasgow, followed by the on-site stabilisation of contaminated chrome waste materials.
The laboratory batch mixing allowed Glasgow Scientific Services to determine the strength and chemical character of the stabilised soil mixture against variations in mixture composition and to compare against a cement-only control.
The field trial involved both insitu surface mixing and exsitu mixing of excavated materials.
Chrome waste and soils were mixed using standard plant, with a measured ratio of cement and Geosta additive to stabilise the soil mass.
In some cases, stabilisation mixtures also had ferrous sulphate added as a reagent aimed at the chemical reduction of hexavalent chromium to the less toxic trivalent form. Stabilised materials from the field trials were then tested for strength and chemical character.
Trials were observed by representatives from GCC, the Scottish Environmental Protection Agency, Edinburgh University on behalf of Scottish Enterprise, Halcrow Crouch and Caledonian University (which has also been carrying out its own independent tests), with additional advice provided by Robin de la Roy of Megatech Engineering Consultants who has been working with the Geosta additive for more than 15 years.
The chrome waste material is soft and friable and the laboratory reported no problems producing the mixes to the specified recipes, other than the maintenance of a satisfactory suspension of the Geosta additive in water. All cement/Geosta mixtures were able to form a competent solid mass and were successfully cast into standard 100mm concrete cube moulds.
The main correlation after three days was observed to be between material density and compressive strength, but as might be expected for a cement based system, all materials increased in strength with time.
The highest strengths were achieved by mixtures containing the highest levels of Geosta and cement. The correlation between density and strength did not persist beyond the seven day tests.
The best results showed cubes with unconfined compressive strengths of up to 3.7N/mm 2(45% higher compared with a cementonly control).
Chemical testing confirmed the highly contaminated nature of the treated materials, with total chromium concentrations of up to 1.5% by weight (15,000mg/kg), with just under half of this present as hexavalent chromium.
The laboratory trial mixes demonstrated a significant reduction in leachable total and hexavalent chromium compared with the untreated soil, with up to 95% reduction in leachable chromium. The best results were obtained from those materials that contained Geosta (up to 50% less leaching compared to a cement only control).
On the basis of the laboratory results it was agreed that the field trials should go ahead using a standard mixture of about 10% cement and 0.1% Geosta by weight of material to be treated.
Field trials were carried out on four areas:
lTwo surface mixed pads, one including a 1% ferrous sulphate addition.
lTwo blocks of material that had been excavated, mixed and replaced, one with 1% w/w (weight for weight comparison) ferrous sulphate added.
Field mixing was relatively straightforward and no significant problems were encountered during the formation of the stabilised materials. All the treated pads quickly formed a hard, concrete-like material able to bear a person's weight within 24 hours.
The larger original soil components were clearly visible within the solidified matrix.
One surface treatment area, on a sloping part of the site, appeared to have suffered significantly due to washout from rainfall and run-off. Core samples taken from the stabilised areas after 14 days showed on average compressive strengths of 4.7N/mm 2(135% of that achievable by only using cement). The maximum strength of the samples from the field trials was more than 200% higher than the cement control.
Chemical testing showed that leachable chromium had been significantly reduced, although the results showed more variation than the laboratory trials.
Average stabilisation performance was a 91% reduction in both leachable total and leachable hexavalent chromium, compared with unstabilised soil. There did not appear to be any significant difference in performance between the insitu and exsitu mixing processes, or in those mixes that included ferrous sulphate.
The trials showed that the chemical and physical stabilisation of unconsolidated chrome waste material was relatively easy to accomplish, even with simple plant. The process works best when uniform mixing, compaction and moisture content can be guaranteed.
While cement stabilisation alone can significantly reduce chrome leaching and increases compressive strength, adding Geosta to the stabilisation mixture results in a further 30%-50% reduction in chromium leachability and increases in compressive strength of up to 235% (provided mixing conditions are good).
Generally, results are in line with those expected from a cement-based stabilisation technique, although that the levels of contamination and the physical character of the treated materials are more extreme than would often be encountered on derelict sites of this type.
The Geosta additive clearly had a beneficial effect on both the physical strength and chemical leaching behaviour of the treated material. Another benefit is that treated material can be considered for reuse on site. The process can be used on material (contaminated or not) to improve ground conditions and does not aim to produce bulk, low grade fill materials.
GCC has indicated that it is pleased with the results of the trial and that it has been helpful in its efforts to manage chrome waste contaminated sites in Glasgow. Other local authorities with similar sites have also expressed interest in the work.
Dave Cooke is an independent consultant geochemist employed by Merkfield to manage the Rosebery Park trials, email: cookegan@ yahoo. co. uk. More information on Geosta can be obtained from Harry Genis at merkfield@aol. com