Abstract
The formation of ettringite has been defined as a major failure mechanism of lime stabilised cohesive soils. It can result in both disruptive volumetric changes and loss of mechanical strength. The mechanisms of its formation and the role it plays in deleterious processes are complex. This paper reports the dimensional and strength changes of a range of artificial lime stabilised cohesive soils subject to two swell test procedures: the UK linear California Bearing Ratio swell test (BS1924-2) and the European accelerated swell test (EN13286-49). The resulting microstructural composition was analysed using a combination of scanning electron microscopy and electron dispersive X-ray spectroscopy. The results are explained in terms of established theories of crystal formation and subsequent expansion mechanisms.
Highlights
Soil stabilisation can be defined as the enhancement of the engineering properties of a soil by blending in a chemical or granular additive [McNally, 1998]
The rate of swell at 28 days of soil M6L3S remained constant indicating that expansive reactions were continuing, whereas the results indicate that the medium sulfate clay (M6L1.5S) was starting to plateau at around 20 days, as seen by by a flattening of the swell curve and a lower total swell at the end of the immersion (7.3 mm compared to 8.1 mm)
Lime stabilised cohesive soils with increasing amounts of sulfate, exhibit a proportional degree of dimensional instability and loss of strength, when subject to swell testing. This can be attributed to the formation of the hydrous calciumsulfoaluminate species ettringite (AFt);
Summary
Soil stabilisation can be defined as the enhancement of the engineering properties of a soil by blending in a chemical or granular additive [McNally, 1998]. Under UK specifications (Specifications for Highway Works, Highways Agency., 2009), if a pavement foundation is to be constructed on a weak subgrade (CBR < 5 %), a stabilised capping layer (with improved engineering performance) can be constructed, so that sufficient support is provided to the overlaying pavement This incorporates the insitu material into the final works and allows a reduction in the thickness of the overlaying layers [McNally, 1998]. A reduction in water content (more so when CaO is used over Ca(OH)2), plasticity and density These are the result of the drying effect of the lime and of cation exchange processes that reduce interparticle repulsion, causing the clay particles to flocculate together [Sherwood, 1962]. This in itself improves the workability of the soil and is marked for ‘heavy clays’ such as London and Gault clay (West et al, 1997)
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