Clay-Lime Stabilization: Application of Fly Ash to Minimize the Risk of Heave

Clay soil stabilization with lime is a well-established, low cost and effective way of enhance physical properties, particularly strength or bearing capacity. However, many clays can contain soluble sulfates that can also react with lime to form ettringite with a corresponding increase in volume, referred to as heave. Indeed, there have been some spectacular problems in recent times, eg M40 motorway in the UK and the Stewart Avenue in Las Vegas. The reactions are similar to sulfate attack in concrete and thus the protection that fly ash brings to that application could potentially help reduce ettringite-induced heave. Work at Dundee has indeed demonstrated that this does indeed work but the mechanism but which heave is suppressed is different and unexpected. In this paper, the authors will present key data arising from several large scale projects, for a range of fly ashes and clay soil types. The rate dynamics of ettringite formation will be discussed, together with the role fly ash plays in moderating or suppressing these reactions. Based on physical (SEM and MIPS) and compositional (XRD) analyses the authors will show that, contrary to expectation, the incorporation of coarser fly ashes are more effective than finer materials. Guidance will be provide for adoption of a fly ash-based solution for minimizing ettringite heave in lime stablised.