New research addresses soil stabilisation leaching concerns

New research undertaken by Nottingham Trent University in collaboration with Britpave, the cementitious and concrete infrastructure association, has addressed the misconceptions concerning the leaching potential of soil stabilisation.

The stabilisation of weak natural soils using binders such as lime or cement is a sustainable and proven method to enable the re-use of otherwise unsuitable fill material in civil engineering projects. Notwithstanding the benefits of soil stabilisation, concerns are sometimes raised over the leaching potential of certain chemistries from lime or cement stabilised soils. Although leaching pollution risks are low relative to the volume of soil stabilisation treatment undertaken there have been a small number of specific cases where high pH water has been discharged from sites where soil stabilisation was used.

This research investigated the leaching potential of UK soils stabilised by different dosages of lime and cement following standard leaching test methods in BS EN 15863:2015. Britpave member Balfour Beatty worked with Nottingham Trent University (NTU) to deliver this programme of work. Balfour Beatty obtained, characterised and prepared soil samples for NTU to test which were tested by NTU’s Maudslay Civil Engineering Laboratory with chemical testing of the leachate undertaken by colleagues in the Chemistry Department at the Clifton Campus.

Following analysis of a thorough testing programme the research confirmed that leaching from lime and cement-stabilised soils is minimal, even under aggressive laboratory conditions. The low levels of leachate detected aligned well with industry experience and expectations from previous research. Importantly, there was no evidence of significant chromium leaching. Chromium present within the stabilised soil eluates was well below WHO drinking water standards for total chromium. The very low rate of leaching by diffusion reduced significantly over time further mitigating this risk.

Longer curing times significantly reduced the availability of unreacted lime for leaching. The research examined field samples, which had cured for over three years. They showed negligible lime loss. This highlighted that the risk of leachate progressively reduces for typical applications where prolonged infiltration of water into and through treated is avoided. Even after prolonged water immersion in an unconfined condition, the stabilised soils retained their substantial strength whereas untreated soil disintegrated immediately. This underscored their durability and long-term performance in civil engineering applications.

Commenting on the research Dr Paul Beetham, Associate Professor in Geotechnical Engineering, Nottingham Trent University Civil Engineering department, said: “This collaboration between NTU and Balfour Beatty has delivered a truly impressive scope of work across lab prepared and field recovered, undisturbed blocks to truly link lab to field. The work is resounding to demonstrate that leaching is via slow diffusion mechanisms, resulting in very small amounts from lab samples and negligible from field specimens, even those compacted slightly drier than optimum and with elevated air voids.”

Stephen Phipps, Senior Materials Engineer at Balfour Beatty, added: “The full immersion testing programme represented extreme case scenarios as the tank immersed testing conditions were far more onerous than real-time field conditions. The provides confidence to the conclusions that the potential of leaching pollution are extremely low, especially if good industry soil stabilisation and drainage industry practices are followed.

The tested stabilised soils demonstrated very low permeability, restricting water movement and further minimising the potential for leaching. Good working practices will provide the necessary low permeability that limits leachate. In particular, the research underlined that the rare instances of high pH water discharge identified in past projects were linked to poor drainage design. Ensuring that water does not infiltrate into and or flow through stabilised material is key to preventing leachate issues. Properly designed and executed soil stabilisation presents negligible environmental risk”.