Doctor of Philosophy
School of Civil, Mining and Environmental Engineering
Alazigha, Dennis Pere, The efficacy of lignosulfonate in controlling the swell potential of expansive soil and its stabilization mechanisms, Doctor of Philosophy thesis, School of Civil, Mining and Environmental Engineering, University of Wollongong, 2015. http://ro.uow.edu.au/theses/4483
Many techniques have been developed and applied to prevent and/or remediate infrastructural damage caused by expansive soils throughout the world. Of these techniques, traditional chemical (lime and cement) stabilization has gained world attention because of a good understanding of the underlying mechanisms, availability of technical guidelines, and years of demonstrated field experiences. However, despite the global acceptance of traditional additives for treating expansive soil, other environmentally benign alternatives have been an important subject of research due to the inherent health and safety concerns for traditional admixtures. One such alternative is from the paper industry that manufactures pulp from wood and in the process produces over 50 million tons annually of a waste substance known as lignosulfonate (LS). This substance has been disposed of as a waste product resulting in colossal disposal cost; however, it does have a potential application in geotechnical engineering under the concept of sustainable development.
This investigation into LS admixture consists of experimental and theoretical studies. The experimental investigation involved a laboratory evaluation of the efficacy of LS admixture in controlling the swell potential of a remoulded expansive soil. The swell potential was examined in terms of percent swell and swell pressure of the soil. In addition to these engineering properties, the Atterberg limits, unconfined compressive strength, durability (wet/dry and freeze/thaw), compaction characteristics, permeability, consolidation characteristics, and shrinkage behaviours were also investigated. Furthermore, the mechanism by which the remoulded soil was modified or altered by the LS admixture was probed and identified.
The optimum content of LS admixture was found to be about 2% by dry weight of the soil. Standard geotechnical laboratory tests performed on untreated and treated compacted soil specimens showed significant and consistent changes in the swell potential and other engineering properties such that the percent swell decreased by 22% while maintaining the soil’s pH. In some instances, identical specimens treated with 2% cement were prepared and tested for comparison. Although the specimens treated with cement recorded a 33% reduction in the percent swell, the ductile characteristics were replaced by brittleness and a significant increase in pH. Further analysis of the laboratory test data also suggested that LS admixture is a resourceful alternative for “low” swelling soils. This finding led to the formation of a “LS application chart” that will help geotechnical practitioners on admixture choice for a particular expansive soil deposits.
The physical-chemical analyses of untreated and 2% LS treated specimens were studied microstructurally after 7 days of curing. When LS was added into expansive soil, the stabilization mechanisms consisted of an insignificant exchange of interlayer cations due to the “cover-up-effect”, basal/peripheral adsorption on mineral surfaces through hydrogen bonding (water bridging), direct bonding to dehydrated cations with the subsequent formation of flocculation-aggregates, initial expansion of diffuse double layer and water entrapment, and a waterproofing effect. An elemental analysis of untreated and treated specimens suggested inter-molecular interactions between soil minerals and the LS admixture as opposed to major chemical reactions. Thus, LS summarily altered the crystallographic characteristics of the soil minerals, and helped to reduce shrink-swell behaviour of the otherwise expansive soil.
The theoretical aspect of this research work involved the development of a robust mathematical model to predict the swell behaviour of expansive soil treated with LS. Relationships were proposed to estimate the suction behaviour of treated soil using laboratory data obtained experimentally. Suction behaviour was governed by a single constant (β), which depends on an input variable; the degree of saturation (Sd). A reasonable correlation was found between the percent swell determined experimentally and the predicted values.
A non-traditional admixture such as LS has the potential to become a technically and economically competitive alternative in the stabilization of expansive soils. With over 50 million tons being produced annually, the successful use of LS admixture as a new stabilization material for expansive soil appears to be one of many viable solutions to the sustainable use of a waste by-product, green construction, and as well as saving the disposal problems inherent in the paper manufacturing industry.