Sodium silicate stabilization of collapsible clayey calcareous soils

Abstract

Collapsible soils have caused infrastructural damages resulting in several economic losses and loss of lives in certain cases. The prerequisite for collapse occurring is an open metastable structure; capable of developing in any soil type given the right placement or aging conditions. Natural and anthropogenic (engineered & non-engineered) collapsible soils exist in many regions of the world. In their unsaturated state, these soils exhibit high enough strength (provided by inter-particle bonds of either suction, clay, calcium carbonate or other salts) but upon wetting and/or loading they undergo repacking due to bond softening/weakening. Despite the large body of research in this subject, there is still poor understanding of the process of softening/weakening and the collapse mechanism of certain bond elements, particularly CaCO3, considering its low solubility in water. Because CaCO3 is common in natural soils, reaching 1-30 % contents in the most commonly known natural collapsible soil (loess), understanding its influence on the collapse phenomenon is crucial for geotechnical characterization of soils in order to design safe and economic earthworks with their long-term serviceability also in focus. In this thesis, an innovative method of simulating an open structure supported by calcite and/or clay bonding has been developed to study mainly the effect of calcite and clay content on the collapse potential (CP) and rate of collapse of calcareous silty-clay soils analogous to marls. A relationship between unconfined compressive strength (UCS) and CP was also examined. Collapsibility and UCS were examined using single and double oedometer techniques and unconfined compressive test (UCT), respectively. Additionally, the potential for a single-step application of sodium silicate solution (silicatization) to improve the soils was investigated. Silicatization was carried out by both impregnation of pre-formed specimens and mixing & kneading prior to specimen formation. Treated oedometer specimens were simply heat-cured (45 – 50 0C) for 24 hours whereas, some UCT specimens were heat-cured, and others cured under room ambience for 7, 14, and 28 day to assess the effect of curing method on UCS. CP was estimated by the percentage decrease in height of an oedometer specimen due to wetting. Different wetting fluids (distilled water and a 5% acid solution (AS)) were used in order to investigate the influence of pore fluid pH on CP. Several wetting stresses were adopted but 300 kPa (roughly equivalent to the self-weight of a 15m high embankment) was the main stress studied. The research found that UCS and stiffness increase with increasing calcite content and clay content, which is possibly due to an increasing in inter-particle bonding. It also reveals that the magnitude and rate of collapse are controlled mainly by the calcite content, pH of wetting fluid, clay content, dry density, and wetting stress. Both magnitude and rate of collapse decrease with increasing CaCO3 content and dry density but CP increases with increasing wetting stress: each sample reaching a maximum defined by their initial strength, beyond which CP decreases. Increasing clay content resulted in higher CP for non-calcareous samples but resulted in lower CP for calcareous samples. Wetting with acidic solution demonstrated higher CP and tends to prolong time to reach complete collapse resulting in long-term collapse known as subsidence settlement. Silicatization was found to reduce stiffness at early curing with minimal increase in UCS, but at later ages both UCS and stiffness progressively increased with curing time and drying. Silicatization was more effective at increasing strength and stiffness of specimens with higher clay and calcite contents. For each curing period, mixing & kneading produced higher UCS than impregnation by between 140% - 356%, 63% - 193%, and 16% - 71% at 7, 14, and 28 days curing, respectively. At 300kPa wetting stress, silicatization and heat-curing reduced CP by between 59% – 73% under distilled water wetting and 70% – 78% under AS wetting. Key Words: Calcareous, Collapsibility, Unconfined compressive strength, silicatization.