When a site investigation is carried out, cylindrical rock cores are collected from boreholes for their identification, laboratory tests and classification. What we learn from these rock cores is only part of the story; the situation can be very different in the larger rock mass in situ, thanks to the discontinuities present in the rock mass in the form of joints, faults and bedding planes. These are the planes of weakness, which become the weakest links and can cause instability.