Ground Improvement of Organic Soils Using Wet Deep Soil Mixing

Francisco Hernandez-Martinez, Cambridge University
Geotechnical Engineering Group


Organic soils are difficult to deal with due to their particular characteristics such as high compressibility and poor strength and, as a consequence, criteria based on common mineral soils may not generally be applied to them. The Soil Mixing Method (SMM), however, has proved to be an economical and reliable technique, and preferable to traditional solutions, to stabilise organic soils, by virtue of its simple application and adaptation to specific project requirements and site conditions. The dry SMM is usually used to organic soils because have generally a high natural moisture content. However there are various applications where the Wet SMM (WSMM) is far more economical and advantagenous to use, for example in layer soils. The current lack of adequate knowledge of the application of the WSMM to organic soils highlights the importance of conducting laboratory treatability studies on this topic with the aim of providing a better understanding of the performance of the stabilised organic soils and reducing uncertainties in a range of their behavioural aspects

The objective of this research was to assess the use of the WSMM in organic soils, from laboratory experimental work, and to investigate the effectiveness of specific cementitious binders and the effect of temperature, carbon dioxide concentration and relative humidity on the stabilisation of organic soils. This is achieved by conducting a series of unconfined compression tests in wet-soil mixed specimens prepared both mechanically and using a laboratory-scale auger-mixing equipment. Scanning Electron Microscopy (SEM) and X-ray powder diffraction analysis facilitated better understanding of the hydration processes of the cementitious materials when mixed with unstabilised organic soils. Variables studied included: variation of the natural water content of the unstabilised material; density and type of soil; water:grout ratio and quantity and type of binder. Results in terms of strength and stiffness showed that the stabilisation of organic soil can effectively be achieved using the WSMM by choosing the right type and quantity of binder. Cement alone and a mix of cement with blast furnace slag proved to be suitable binders for this type of material. Likewise, it is shown that the strength values obtained from mechanically-mixed specimens were twice and three times as large as those evaluated using the laboratory-scale auger columns in stabilised-peat and stabilised medium-organic clay respectively.

The study of the long-term behaviour of stabilised wet soil-mixed specimens using elevated temperatures and accelerated carbonation as ageing processes was also part of this investigation. The results showed that there is generally a reduction in unconfined compressive strength (UCS) with increased curing temperature from 21ºC to 60ºC, which casts doubt on the well-known ‘fact’ that such elevated temperatures usually speed up the hydration processes of cement leading to an increase in UCS. It constitutes evidence that this effect is caused by the loss of evaporable water, the effect of elevated temperatures on the peat and the fact that elevated temperatures induce carbonation and oxidation in mixes as the X-ray powder analysis showed. In addition, by using high carbon dioxide concentrations it was evident that some initial reduction in strength followed, but a gradual increase occurred thereafter. This initial reduction is caused by the interference of accelerated carbonation with the normal hydration processes consuming some of the portlandite which is needed for hydration.

The conclusions reached in this thesis, therefore, provide valuable information regarding the type of binder and environmental conditions that are suitable to achieving a better, reliable chemical stabilisation of organic soils with the use of the WSMM.