Argillite core samples
When the shafts were being excavated, scientists removed core samples of the formations that the excavations passed through, and created a 'core sample library'. They drew up a detailed geological survey of the entire height of the shaft wall and the geology was confirmed by the addition of data on the sediments and microstructures. In a rock such as clay, where diffusion is extremely slow, the 'harvesting' of interstitial water and the taking of geochemical measurements, including of isotopes, provide crucial information about the rock's history and the prediction of its behaviour.