A review of selected engineering geological characteristics of English Chalk

Chalk is a variable material, the properties of which are dependent upon it s composition, textural features and diagenetic history. With the exception of certain horizons in the Lower Chalk that contain appreciable amounts of clayey material, the English Chalk is a remarkably pure micritic carbonate rock that generally can be divided into coarse and fine fractions. The lat ter comprises 70-80% of chalk. Cementation took place more or less contempo raneously with deposition so that the sediment was able to support relative ly high overburden pressures. Hence, high values of porosity were retained. Chalk varies appreciably in density and hardness. The harder chalks are th e result of diagenetic processes and bioturbation that brought about densif ication. In soft chalks the grains are only bound together at the points of contact by thin films of calcite. The latest classification of chalk is based on an assessment of intact dry density, discontinuity aperture and discontinuity spacing. Chalk tends to v ary from moderately weak to moderately strong and its strength is significa ntly reduced on saturation. Under triaxial loading conditions diagonal shea r failure tends to occur at lower confining pressures but at higher confini ng pressures barrel-shaped failure occurs indicating plastic deformation an d textural disaggregation. Similarly, at low loading, chalk exhibits low vo lume compressibility but much more significant consolidation occurs if the yield stress is exceeded. Chalk undergoes dissolution and so solution features are found throughout i ts outcrop. Mineworkings in the Chalk extend back into the distant past, the most ancie nt being those excavated in the Neolithic Age for flint. Several types of w orkings exist. Collapse of old mineworkings, most of which are unrecorded, is difficult to predict. The potential for subsidence, caused by the collap se of both mineworkings and dissolution features, affects development and i ts occurrence can lead to the abandonment of property or, worse, the loss o f lives. (C) 1999 Elsevier Science B.V. All rights reserved.