Summary –Solidification and Stabilisation is gaining popularity as a treatment solution for contaminated soils as design and feasibility are better understood, costs of application reduce and regulators become more familiar with the technology. However, relatively few sites have been subject to long term durability testing to verify the performance of the treated material over time. Celtic is able to report on the recent investigation and assessment of the condition of a former tar tank which was stabilised in 2006.
Solidification and Stabilisation (S/S) are two separate processes that are often used together as a remediation technique to reduce the mobility of contaminants in soils and other wastes. Solidification creates a solid mass physically restricting the movement of contaminants, while stabilisation reduces the availability of contaminants by chemical immobilisation. The process is achieved by mixing contaminated media with cementitious/pozzolanic reagents, water and other additives to produce a mixture that then exhibits improved physical and chemical properties. Depending on the site conditions, the mixing can be carried out in-situ or ex-situ and the equipment used varies accordingly. The technology can be used to treat soils, sludge and waste materials contaminated with both organic and inorganic chemicals including; metals and metalloids, asbestos, radionuclides, inorganic corrosives, inorganic cyanides, solid organics, polyaromatic hydrocarbons (PAHs) and dioxins.
The advantages of S/S as a remediation technology include:
- Highly flexible – can be used to treat soils, sludge and waste materials contaminated with a wide range of contaminants to high contaminant concentrations,
- Site constraints can all be addressed through appropriate design,
- Relatively rapid treatment times (subject to material throughput or depths of in-situ mixing),
- Easily adaptable for additional volume, particularly in the case of ex-situ,
- Soils can be re-used on site,
- Geotechnical improvement of materials, often to a site requirement,
- Proven technology in the United States and France, with wide acceptance and rapidly increasing number of case studies in the UK.
One of the major benefits of S/S is that it has the capability to a wide range of materials bearing a range of contaminants, for example Celtic’s 14 contracts to date include: oily sludge, gas works wastes, canal dredging’s, made ground with rubber, lake sediment, chemical waste lagoon and tar contaminated soils (160,000 tonnes total).
S/S Within the UK Remediation Market
The use of S/S for the remediation of contaminated land has increased significantly over the last 10 years and a combination of extensive academic/ industry R&D and an ever increasing number of successful UK applications have changed the perception of S/S to the point now where it is, at least for ex-situ applications, a relatively mainstream remediation technique. The CL:AIRE “State of the Market” report 2012 (1) indicates that in 2010 and 2011 approximately 8% of remediation contracts, reported by its members, for which ex-situ remediation techniques were utilised were undertaken using S/S and only 1% of in-situ remediation contracts.
As with other ex-situ remediation techniques, such as excavation/ off-site treatment and bioremediation, the market is beginning to commoditise S/S and it is now often assessed as a rate item (price per cu.m). Given that in its ex-situ application the physical process of mixing soils/ materials with reagents is similar to many other mechanical soil mixing processes it is an area where the use of existing technologies and machinery (continuous mixing plants), adapted to S/S, has driven down the costs of its application.
Whilst the costs of application have reduced and the design and feasibility phases are now supported by technical guidance from the UK and US (2,3) the technology still has to fully overcome the question of durability and longevity. One of the key early concerns from stakeholders was given that the contaminants were not removed, as they might be by other techniques, could there be confidence that the immobilisation was permanent and how would the stabilised mass behave overtime or when subjected to changes in environmental conditions?
A Demonstration of Durability
The “PASSiFy” International Research Project report “Performance Assessment of Solidified/Stabilised Waste-forms” (4) which was reported in March 2010 provided the results from the investigation and assessment of 10 applications of stabilisation as early as 1989. These sites include a site remediated by Celtic in 2006, although it was only recently completed at the time of the PASSiFy investigation. The findings of the report indicate that the stabilised materials recovered for testing still met their original performance criteria and the conclusion of the authors was that the results affirmed the long term viability of S/S for the remediation of contaminated land.
Celtic has recently had the opportunity to investigate another of its early S/S remediation sites having been commissioned by the landowner to undertake an investigation and testing strategy which was designed to assess the long-term performance of remediated material remaining within an underground gasholder that was stabilised in 2006. The assessment was made through comparison of current performance against original validation criteria, as well as the use of additional testing and analysis to allow further comment on the long term performance to date and anticipated behaviour in the longer term and is detailed below.
Original Remediation Strategy and Works
The objective of the remediation works in 2006 was to reduce the potential for contamination mass flux within the gasholder and its immediate surrounds. The remediation works included the recovery of 52 tonnes of non-aqueous phase liquid (NAPL) by pumping from within and around the perimeter the gasholder followed by S/S the heavily tarred residual soils within the gasholder.
An estimated 633 m3 of material within the gasholder was treated using S/S binders and additives. The S/S materials were allowed to cure for a minimum of 28 days after which cores were recovered from several locations and depths in order to represent the treated gasholder contents. The cores obtained were subject to leachate analysis, permeability determination and strength testing and from these results it was confirmed that in addition to compliance with the chemical remediation criteria, the S/S had resulted in a substantial strength increase and a sustained reduction in permeability compared with untreated materials.
2012 Investigation and Assessment Works
In order to replicate the original validation suite of testing, and also to provide additional information on the stabilised material, the following testing regime was implemented in 2012, 6 years after the original remediation works;
- 4 day tank testing, leachate production and analysis – Assessment of chemical performance against remediation criteria, direct comparison with results achieved during original validation
- 64 day tank testing, leaching over time in accordance with EA NEN 7375 (2004) – Replication of testing undertaken during validation with additional interpretation of results specific to leaching behaviour and stability of material matrix.
- Unconfined Compressive Strength in accordance with BS EN 12504-1 – Direct comparison with results achieved during original validation.
- Unconfined Compressive Strength of Saturated Samples in accordance with BS EN 12504-1 – Samples saturated for a period of 64 days (to mimic the immersion period of the 64 day test) prior to compression to assess durability.
- Permeability, in accordance with BS1377: Part 6: Clause 6: 1990 – Direct comparison with results achieved during original validationIn addition to the above, three samples from the recovered cores were submitted for petrographic analysis to allow a description of the petrography, porosity and alteration characteristics of the material.
Long Term Performance and Durability – Results
Permeability results from both test periods show consistent results, with no discernible change in the achieved permeability over the six year period. This suggests that the performance of the material with respect to permeability is maintained over time.
The UCS test results show a significant increase in strength between 2006 and 2012, which is indicative that the material is durable over time.
The results of the 64 day interpretation show that for samples recovered in 2006 and 2012 there is no evidence for chemical instability, dissolution or breakdown of the produced sample matrix, again supporting the conclusion that a durable matrix has been formed.
Also evident from the 64 day tank test is that the mechanism of contaminant release from the S/S material into pore waters and the wider environment has remained unchanged between 2006 and 2012. This is further evidence of no significant change in the behaviour and performance of the material matrix in the samples tested, and sustained performance over time.
The petrographic analysis highlighted areas of hydrocarbon ‘weeping’ on cut surfaces of the sample, which are comparable to observed localised darker areas noted on the outer surfaces of the cores during core recovery. The presence of oils within the matrix can be expected due to the volume of free product that was present during remediation works, even after recovery of NAPL through pumping prior to S/S commencing.
The absence of a significant volume of observable free product during the core recovery operations and the apparent immediate mobility of these oils when cutting the sampling during preparation for petrographic analysis suggests that while oils are present in the more porous regions of the mass, they are relatively restricted and unlikely to migrate to any significant degree through the S/S material. The potential for migration of NAPL within the mass and to the wider environment will be limited by the permeability of the treated material. It is not considered likely that NAPL release from the mass at levels that will be measurable in the subsurface will occur over time.
While these petrographic studies have shown that residual free product is present in the core samples and thin sections; however this is not contributing significantly to the flux of organic contaminants from the mass, with average leachable levels of organic contaminants still within acceptable levels.
The results of this investigation have shown that the physical and chemical performance of the material between 2006 and 2012 has been maintained, and in the case of UCS and leachability of some contaminants it can be concluded that the performance of the material has improved over the six year time period.
Additional testing not included as part of the original validation testing regime, such as 64 day tank test interpretation and effect of immersion on UCS, has shown that the performance of the material will be maintained into the future and that contaminant retention and strength performance can be expected to improve further with time.
The success of this and all applications of S/S is primarily driven by the detailed, site specific design of the S/S binder mix and thorough quality assurance from design to field application.. Although application of S/S to brownfield remediation in the UK has only been available for around 10 years, durability testing of sites in the UK as well as overseas sites where S/S has been implement for around 50 years, demonstrates that it is a robust and durable treatment solution.
For more information contact; Christine Mardle or Chris Taylor-King.
- CL:AIRE State of The Market Report, 2012.
- Interstate Technology & Regulatory Council (ITRC), Development of Performance Specifications for Solidification/Stabilization, July 2011.
- Environment Agency, Guidance on the use of Stabilisation/Solidification for the Treatment of Contaminated Soil Science Report: SC980003/SR1.
- PASSiFy, Performance Assessment of Solidified/Stabilised Waste-forms An Examination of the Long-term Stability of Cement-treated Soil and Waste, Final Report, March 2010