Connecting engineering properties of chalk to geological logging

Chalk formations have been investigated for many years because of their interest in several engineering applications like oil and gas production, offshore engineering, water resources and environmental issues, foundations and piling, geological hazards and tunnelling. Recently there has been a growing interest for connecting the engineering behaviour of such rocks to their geological history.
The current study focusses on three cored drillholes that were drilled in chalk formations of the Mons Basin (Belgium). This basin is a 300-m-thick pile of Meso-Cenozoic sediments that accumulated in a small but actively subsiding area mainly during the Cretaceous. A geological logging of the cores provides a detailed description of the lithologies, with their specific sedimentary and tectonic features. Distinct lithotypes are identified, including white chalk, cyclic chalk, glauconitic chalk or burrowed chalk.
Simultaneously, a systematic testing program was conducted in laboratory to determine the engineering properties of the chalk: dry and saturated densities, unconfined compression strength (UCS), Young's modulus and Brazilian tensile strength (BTS). The high sampling rate (e.g. almost every 2m for UCS with a total of 200 tests) allowed a statistical analysis of petrophysical and mechanical properties in connection with the geological formations.
The analysed samples are typically soft rocks, with a strength ranging from 2.6 to 14.1 MPa. Some properties tend to increase with the depth (density, UCS). However, the nature of the geological formations, resulting from various depositional environments and diagenetic histories influences the mechanical behaviour of chalk.