Behaviour of Poorly Indurated Clay/Concrete Interface Due to Lateral Stress: Application for the Disposal of Radioactive Waste

Abstract

In the framework of studies for the disposal of radioactive waste, ONDRAF/NIRAS, the Belgian National Agency for Radioactive Waste and enriched Fissile Materials investigates the option of a geological repository. In a geological repository, the waste is sidelined within a deep and stable geological layer, named host rock, behind a whole series of artificial barriers. Together, natural and artificial barriers ensure the isolation of waste, their confinement and the delay and spread of release of radioactive substances. The interaction of the host rock and the concrete support of disposal galleries is investigated to analyse the long-term sustainability of the structure. The host rock considered in this study is a poorly indurated clay present in the northern part of the Belgium, the Boom Clay. The behaviour of the interface between the Boom Clay and the concrete of the galleries is studied experimentally from both mechanical and gas transfer perspectives. Samples have been prepared consisting of two half cylinders, one made of concrete and the other one of Boom Clay to obtain an interface. A dedicated device has been designed using an HOEK type cell, which allow applying confining pressure to simulate lithostatic loading, injecting gas to measure transfer properties and generating relative displacement of both sides of the interface. Mechanical behaviour of the interface under short loading has first been studied. One originality of this study consists in using gas injection and Poiseuille’s law to monitor and quantify the opening and closure of the interface during mechanical loading. A partial irreversible closure of the interface and effects of long term loading have been observed. By increasing the confinement, the opening of the interface (clay-concrete) gradually decreases (initially 38 μm for 1.2 MPa of confining pressure) until it reaches 5 μm for 4.5 MPa. There is moderate expected irreversibility during unloading (19 μm at 1.7 MPa). The objective is to identify the main parameters allowing predicting the properties of the interface, such as initial roughness of the interface, water content, applied stress and time. The second part of the work consists in studying transfer properties at the interface. The aim is to identify paths available for gas produced during waste storage.