Durability and Mechanical Properties of Deep-Mixed Clays

Abir Osman, Cambridge University, Geotechnical Engineering Group


Cement-based grouts are used in a wide range of geotechnical and geo-environmental engineering applications including the stabilisation of soft soils. However studies have shown that the strength and permeability of such grouts do change considerably when exposed to aggressive environments. The use of zeolite has been suggested by recent research for enhancing the durability of cement-based grouts especially for sulphate attack. Therefore, there is a need to improve the resistance of cement-based grouts to aggressive environments and in particular those that are likely to occur in the long-term over the life span of the material e.g. due to expected climate change conditions or contamination.

The objective of the research work reported here was to investigate the performance of very soft to medium stiff clays stabilised with cement-based grouts concentrating on their performance in aggressive environments and to address improvements in their mechanical and durability behaviour. The binder constituents used herein were cement, zeolite, pulverised fuel ash, calcium bentonite and sodium bentonite. This was conducted using standard mechanical mixing with the aid of a laboratory bench top mixer and compared with auger mixing with the means of a laboratory-scale auger mixing system. Properties investigated were the unconfined compressive strength, permeability, secant modulus and microstructure (using scanning electron microscopy and X-ray diffraction). Extreme climate conditions and aggressive chemicals such as freeze/thaw and wet/dry cycles, strong acid and sulphate solution environments were applied.

It was concluded that increasing the amount of cement-based grout injected in the clay enhanced its mechanical properties and its chemical compatibility in sulphuric acid and sodium sulphate solutions as well as its resistance to freeze/thaw cycling. It was also found that zeolite, incorporated within the cementitious matrix, contributed to the consumption of portlandite, Ca(OH)2, formed during the cement hydration to produce further hydration products. The most effective zeolite content, in terms of strengths development and permeability reduction, was between 10% - 15% at all ages. Zeolite was also found to be more reactive than PFA especially at 28 days. Furthermore, allowing the bentonite to hydrate for 24 hours or using a high shear mixer to mix the bentonite yielded much higher strengths and lower permeabilities at all ages for the two types of bentonites when compared to the bentonite introduction as dry powder.

For the durability of stabilised clays, the replacement of half the cement content with zeolite significantly enhanced the sulphate resistance of the mixes. No signs of cracking appeared on the samples for durations of up to 1.5 years. In addition, it was shown that bentonites in general and sodium bentonite in particular, were resistant to the effects of freeze/thaw cycles especially when prehydrated or mixed using a high shear mixer.

Finally, auger-mixed columns were found to be effectively mixed and to correlate well with mechanically-mixed samples of very soft clay. On the other hand, for the medium stiff clay, the mixing was not effective and the mixing process had to be preceded by the injection and mixing of water with the soil in order to fluidise it and promote its thorough blending with the cementitious binder. It was finally inferred that the mixing procedure using the laboratory-scale auger could simply be utilised to simulate the behaviour of the full-scale in-situ columns in soft clays.