COMPARISON OF COALESCENCE BEHAVIOR BETWEEN NATURAL OUTCROPS AND LABORATORY TESTING SPECIMENS

One of the key concepts used to explain the origin of fault initiation and propagation is the initiation and coalescence of tensile cracks from tip regions of pre-existing flaws (joints or faults) which are subject to a shear stress. The coalescence processes achieved by these cracks allows the slipping flaws to lengthen and to facilitate the development of a fault zone. Structures of pull-apart zones or damage zones are also commonly identified in the bridge region between the flaw tips. All these features are often observed in the field.

Laboratory uniaxial compression tests were conducted on prismatic molded gypsum and Carrara marble specimens containing pre-existing artificial straight open flaws. Using a high speed camera (Phantom V5), the observation of crack initiation and the sequence of the development of multiple cracks was made possible. Besides confirming the role of newly initiated tensile cracks in leading to coalescence and subsequent creation of pull-apart zones as observed in the field, the tests also showed that shear cracks and cracks consisting of both shear and tensile segments took part in coalescence. In addition, the overall strength of the specimens was found to be dependent on the coalescence types. In short, the coalescence behavior and specimen strength were found to be related to the geometrical relationship between the pre-existing flaws.

The present study thus not only highlights the significance of crack coalescence in the engineering behavior of brittle materials, but also offers a new perspective for interpreting faulting mechanisms operating in the earth crust.