Creep and Collapse Behaviour of Mechanically and Biologically Pre-treated Solid Waste in Oedomester Tests

The paper deals with the compression behaviour of mixed fractions of a mechanically treated and biologically dried waste material with methanogenic fraction appropriate for landfilling in a bioreactor landfill. To this end oedometric compression tests were carried out with specimens taken from the Waste Management Center Marišćina, in Croatia. The compression behaviour up to a maximum vertical stress of 119.5 kPa was investigated in a sequence of seven load steps with an adapted oedometer device under three different drained test conditions: the dry state, the wet state, and wetting of the initially dry and pre-compressed material. The interpretation of experimental data focuses on the immediate compression and mechanical creep behaviour obtained within each load step. As the settlements are time dependent from the beginning of loading, no clear distinction between immediate and secondary compression can be gathered from the settlement-time curves. To this end a fictitious time where the immediate settlement ends is introduced based on common methods and a new method proposed in this paper. The latter, is named the strain-rate method, which defines the time where a given strain rate is relevant for the transition from the immediate compression to mechanical creep. Within the stress range considered, the approximation of mechanical creep using the so-called modified secondary compression index showed that the value of this index decreases for the moist waste material but it is almost constant for the dry material. Particular attention is also paid to so-called collapse settlements under constant load which can take place as a result of an increase of the moisture content. For the mathematical description of the mechanical behaviour enhanced approximation functions are proposed, which are also easy to handle for practical application. The experimental results are comprehensively discussed and compared with data from the literature. Graphical Abstract