On site investigations are conducted in a deep (180 to 280 m) clay layer in Belgium since 1980 to study its potential as a host formation for a disposal facility for high-level radioactive waste. The clay environment requires all underground openings (shafts and galleries) being lined. A proper understanding of the porewater pressure and total stress evolution is essential for the safe and economical design of these linings. Porewater pressure measurements have been made since more than 20 year, mostly through multifilter piezometers. They give a detailed record of the short- and long-term evolutions due to construction activities (mainly excavations). Total stress observations, which are less straightforward to perform, have been made through a large range of techniques, varying from visual observations at the excavation front over pressuremeter tests up to long-term stressmeter measurements. These observations have largely improved our understanding of the phenomena in this particular medium, which behaves under some circumstances as a rock (e.g. fracturing), and under other circumstances as a soil (porosity and two-phase flow). This paper focuses on the measurement techniques applied, their performance in the field, and the associated measurement results that have been obtained. Further we show how these data have improved our knowledge of this clay medium. Feedback of the measurements into the hydro- mechanical models also has improved their reliability and versatility, so that they are being used to obtain safe and economical designs.