Effect of clay with high water adsorption capacity on process of secondary consolidation of peat

Abstract

Introduction. Primary consolidation of peat, underlying an embankment, is completed 3 to 5 years after its filling. It is followed by secondary consolidation, caused by removal of bound moisture from micropores. At this stage, the settlement rate depends on the degree of peat decomposition, temperature, content of various chemicals in the pore water, vibration effects, and other factors. To reduce the embankment deformation, caused by the secondary consolidation, peat is either modified by binding compounds, having filling agents, or other actions are taken to accelerate the settlement process for a short term and to decelerate it thereafter using temporary surcharge, electroosmosis and other actions. Materials and methods. The authors studied the effect of peat dehydration using clayey materials with high water adsorption capacity, such as bentonite and saponite waste generated by the diamond mining industry. 100 days oedometer tests were carried out. In the course of testing, vertical cylindrical elements, made of the above materials, were installed in the specimens. These cylindrical elements occupied about 10 % of the specimen volume. Results. Bentonite inclusions, formed without any peat extraction, had a powerful effect on development of deformations. Intensive dehydration of peat led to a 2–4 times increase in the coefficient of secondary consolidation сαε. Following 25 to 30 days of intensive settlement, the value of the cαε coefficient went down by an average of 1.5 times relative to its initial value. The effect of elements, made of saponite-containing waste, turned out to be different. The value of the cαε coefficient increased by 11 to 15 %, but 20 to 30 days later it went down by an average of 2 times relative to the initial value. Higher resistance of elements to vertical loading served as the factor preventing the development of settlement. Conclusions. Clay has a high water adsorption capacity, and its intrusion into peat results in accelerated water migration from micropores and boosts the rate of secondary consolidation. In addition to peat dehydration, secondary consolidation was accelerated by mechanical compaction in the process of adding clay into specimens followed by clay swelling.