Ground and Project Consultants have recently completed several projects which involve developments on chalk. We were able to draw upon our considerable experience and specific training in chalk at both an advanced academic training and ‘on-the-job’ experience gained by our staff.
Deriving the correct classification of Chalk is essential for engineering purposes as the grading of Chalk reflects the rocks engineering behaviour. The engineering behaviour of chalk is complex but can be broadly split into two categories: Structured and a Structureless soil.
Structureless chalk refers to rock which has been completely weathered to form a soil and is subdivided into Grade Dc or Dm, dependant on whether the soils matrix is clast dominated (Dc) or matrix dominated (Dm).
Structured chalk is subdivided into grades derived from the CIRIA C574 guidance ranging from C5 to A1. Assessment of the grade is based on three properties: intact dry density, discontinuity aperture, and discontinuity spacing.
The process of identifying the appropriate engineering grade of chalk can be difficult because of the challenges associated with drilling, sampling, and logging the material. Chalk is prone to fracturing or breaking up during drilling and coring, especially in the presence of flint bands, which can lead to significant sample disturbance and complicate the correct grade identification. Dissolution features, flint bands and marl seams are all important features which should be logged as they influence the chalks engineering properties.
The incorrect classification of chalk resulting from disturbance can lead to an over (or under) designed engineering solution. For instance, when designing shallow foundations, CIRIA C574 recommends an allowable bearing pressure of 225kN/m2 for low density Grade Dc chalk, while for Grade Dm chalk, the allowable bearing pressure is recommended to be selected only after in-situ testing.
Experience in logging chalk, using suitable drilling techniques and minimising sample disturbance is therefore essential for the accurate grading of chalk and subsequent engineering design.