Mechanical behavior of tire rubber–reinforced expansive soils

Abstract

Expansive soils are amongst the most significant, widespread, costly, and least publicized geologic hazards. Where exposed to seasonal environments, such soils exhibit significant volume changes as well as desiccation–induced cracking, thereby bringing forth instability concerns to the overlying structures and hence incurring large amounts of maintenance costs. Consequently, expansive soils demand engineering solutions to alleviate the associated socio–economic impacts on human life. Common solutions to counteract the adversities associated with problematic soils include soil replacement and/or soil stabilization. The latter refers to any chemical, mechanical or combined chemical–mechanical practice of altering the soil fabric to meet the intended engineering criteria. Though proven effective, conventional stabilization schemes often suffer from sustainability issues related to high manufacturing and/or transportation costs, and environmental concerns due to greenhouse gas emissions. The transition towards sustainable stabilization necessitates reusing solid wastes and/or industrial by–products as part of the infrastructure system, and more specifically as replacements for conventional stabilization agents such as cement, lime, geogrids and synthetic fibers. Among others, discarded tires constitute for one of the largest volumes of disposals throughout the world, and as such, demand further attention. Given the high–volume generation (and disposal) of waste tire rubbers every year throughout the world, a major concern hitherto has been the space required for storing and transporting such waste materials, and the resulting health hazards and costs. Those characteristics which make waste tire rubbers such a problem while being landfilled, make them one of the most reusable waste materials for engineering applications such as soil stabilization, as the rubber is resilient, lightweight and skin–resistive. Beneficial reuse of recycled tire rubbers for stabilization of expansive soils would not only address the geotechnical–related issue, but would also encourage recycling, mitigate the burden on the environment and assist with waste management. The present study intends to examine the rubber’s capacity of ameliorating the inferior engineering characteristics of expansive soils, thereby solving two widespread hazards with one solution. Two rubber types of fine and coarse categories, i.e. rubber crumbs/powder and rubber buffings, were each examined at various rubber contents (by weight). The experimental program consisted of consistency limits, standard Proctor compaction, oedometer swell– shrink/consolidation, soil reactivity (or shrink–swell index), cyclic wetting–drying, cracking intensity, unconfined compression (UC), split tensile (ST), direct shear (DS) and scanning electron microscopy (SEM) tests. Improvement in the swell–shrink/consolidation capacity, cracking intensity and shear strength (DS test) were all in favor of both a higher rubber content and a larger rubber size. However, rubber contents greater than 10% (by weight) often raised failure concerns when subjected to compression (UC test) and/or tension (ST test), which was attributed to the clustering of rubber particles under non–confinement testing conditions. Although the rubber of coarser category slightly outperformed the finer rubber, the effect of larger rubber size was mainly translated into higher ductility, lower stiffness and higher energy adsorption capacity rather than peak strength improvements. The volume change properties were cross–checked with the strength–related characteristics to arrive at the optimum rubber content. A rubber inclusion of 10%, preferably the rubber of coarser category, satisfied a notable decrease in the swell–shrink/consolidation capacity as well as improving the strength–related features, and thus was deemed as the optimum choice. Based on the experimental results, along with the SEM findings, the soil–rubber amending mechanisms were discussed in three aspects: i) increase in non–expansive fraction; ii) frictional resistance generated as a result of soil–rubber contact; and iii) mechanical interlocking of rubber particles and soil grains. A series of empirical models were suggested to quantify the compaction characteristics of soil–rubber mixtures as a function of their consistency limits. Moreover, the dimensional analysis concept was extended to the soil–rubber shear strength problem, thereby leading to the development of a series of practical dimensional models capable of simulating the shear stress–horizontal displacement response as a function of normal stress (or confinement) and the composite’s basic index properties, i.e. rubber content, specific surface area and initial placement (or compaction) condition. The predictive capacity of the proposed empirical and dimensional models was examined and further validated by statistical techniques. The proposed empirical and dimensional models contain a limited number of fitting/model parameters, which can be calibrated by minimal experimental effort as well as simple explicit calculations, and thus implemented for preliminary assessments (or predictive purposes). To justify the use of higher rubber contents in practice, a sustainable polymer agent, namely Polyacrylamide (PAM) of anionic character, was introduced as the binder. A series of additional tests were then carried out to examine the combined capacity of rubber inclusion and PAM treatment in solving the swelling problem of South Australian expansive soils. As a result of PAM treatment, the connection interface between the rubber particles and the clay matrices were markedly improved, which in turn led to lower swelling/shrinkage properties, higher resistance to cyclic wetting–drying, and reduced tendency for cracking compared to that of the conventional soil–rubber blend. A rubber inclusion of 20%, paired with 0.2 g/l PAM, was suggested to effectively stabilize South Australian expansive soils.