## 1 Introduction  
Creep deformation is commonly observed in geological materials, in particular in clayey rocks. Description of time-dependent behavior of these materials is an essential issue for long-term stability analysis of related structures. Creep tests are widely performed to characterize time-dependent deformation. Such tests are generally realized for a relative long period and sometimes under complex environmental conditions, for instance under controlled temperature and moisture content. On the other hand, most geological materials contain different kinds of heterogeneities such as mineral inclusions and pores. As a consequence, strain fields in these materials can be strongly non-uniform even at sample scale. Classical measuring methods such as strain gauges are not suitable for long duration tests and not able to provide non-uniform full strain fields. During the last decades, different kinds of non-destructive techniques have been rapidly developed for the characterization of non-uniform deformation and micro-structural evolution. For instance, the optical microscopy, scanning electron microscopy and other techniques have been used for studying micro-structural evolutions at selected two-dimensional sections [1; 2; 3; 4; 5]. In order to get three-dimensional evolutions of micro-structure, X-ray micro-tomography has been increasingly used in a very large range of materials from biological tissues to geological formations [6; 7; 8; 9; 10]. On the other hand, in order to compute quantitatively local strain fields from images obtained from the different kinds of observation techniques, Digital Image Correlation (DIC) method has been developed for two-dimensional images [11]. DIC has further been extended to Digital Volume Correlation (DVC) for dealing with three-dimensional deformation processes [6; 12]. In the present study, we shall apply the combination of DVC and X-ray micro-tomography to investigate creep deformation of the Callovo-Oxfordian (COx) claystone. This clayey rock is selected as a potential geological barrier for underground disposal of radioactive waste in France. In this context, the time-dependent behavior of claystone is a crucial feature, affecting the long-term thermo-hydromechanical responses of storage facilities [13; 14; 15; 16; 17]. On the other hand, the claystone has also complex mineralogical compositions and its macroscopic mechanical properties can significantly vary with the mineralogy [17; 18]. Due to the diagenetic history, most sedimentary rocks contain sub-horizontal bedding planes. This generally leads to a transversely isotropy on both mechanical and transport properties at the macroscopic scale [19; 20; 21; 22; 23; 24; 25]. In addition to macroscopic studies mentioned above, a number of works have also been conducted to studying deformation and cracking processes in clayey rocks, by using different types of imaging methods and the DIC or DVC. For instance, some studies have been devoted to the three-dimensional strain characterization of the COx claystone in uniaxial or triaxial compression tests [10; 26; 27]. By considering heterogeneities of different scales, it was found that the deformation of the COx claystone was dominated by the clay matrix [9]. Interface debonding and crack growth inside the clay matrix have been highlighted in [22; 28] during drying and hydration processes. Some authors have used a combination of Scanning Electron Microscopy (SEM), Broad Ion Beam (BIB) polishing and DIC to investigate the evolution of microstructure in deformed samples of the COx claystone [5]. Recently, the dynamics of water absorption in the COx claystone was revealed by using multi-modal X-ray and neutron tomography [29]. However, few studies have so far been devoted to the characterization of non-uniform local strain fields during creep tests in the COx claystone. In this study, a series of new In situ creep tests are performed by using a specially designed device.