Time and stress dependent strength and stiffness of reconstituted chalk

Chalk is generally a competent soft rock when found in its intact state. However, engineering working operations - such as pile driving, tunnelling, and earthworks - and weathering can destroy its structure creating a very weak silt and clay-size material, generally known as putty chalk. Nearly all occurrences of putty chalk can be problematic for geotechnical engineers, but laboratory studies of its mechanical properties remain infrequent. This paper presents the results of an extensive laboratory campaign aiming to provide an advanced mechanical characterization, from small strain stiffness to large strain strength and deformation properties, of reconstituted samples from crushed chalk (i.e. putty chalk). Using non-destructive wave velocity propagation measurements by bender elements, this paper provides a full characterization of the small strain shear stiffness (Go) of this material for a range of stress levels; including over-consolidation ratios. Attention is also directed to understand the effect of ageing on both strength and stiffness properties. A number of fully saturated reconstituted chalk samples were aged for different time periods up to 72 days under different isotropic stresses and subsequently sheared to failure. While age-related strength gains were found to be negligible, a consistent increase of the soil shear stiffness with time was observed by use of the wave propagation measuring method. Such increase in stiffness appears to be mostly related with secondary (creep) deformations, although some further increase was also observed when measurable deformations had ceased. Modelling of the shear behaviour of the material within a critical state framework is also attempted. The results of this experimental campaign may aid the design of geotechnical structures in contact with remoulded or putty chalk, especially when small deformation (serviceability) assessments are required.