Chalk putty: specimen preparation and aging properties

Abstract

Chalk putty is a soil-like material that is formed when intact chalk is disturbed. Recently, it has become of special interest due to the development of offshore wind farms in the North and Baltic Seas, where in a number of sites piles are driven into chalk strata. Several pile tests reported in the literature show increased shaft capacity up to 4 months after driving. Although it is a widespread remark in the scientific publications that chalk properties improve over time, laboratory replications of this phenomenon are scarce and the results are inconclusive. The aim of this thesis is to develop a specimen preparation technique yielding representative specimens as well as feasible for commercial purposes. Thereafter, the evolution of the characteristics of chalk putty over time is assessed through laboratory tests. Triaxial with Bender Elements, Direct Simple Shear, Constant Rate of Strain and Thixotropy have been conducted on specimens aged up to 3 months. Finding that the behavior of chalk putty cannot be classified among the classic types of soils. On one hand, Constant Rate of Strain tests have shown that the response is comparable to silty soil, with a gradual transition between re-loading and virgin compression lines, leading to difficulty in the determination of the yield point. On the other hand, in shear, the critical state friction angle is in the range of dense sand. Regarding the volumetric response, there is an initial compressive response, succeeded by a constant volume phase during destructuration and eventual dilation or contraction at larger strains. Furthermore, it was determined that the material possesses thixotropic properties when mixed with de-ionized water. Finally, no increase in the shear strength or was recorded while the initial shear stiffness decreases with aging time -due to unconfined preservation of the samples. However, the ultimate dilatancy of the specimens increases over time, this may be a result of dissolved calcium carbonate during crushing and saturation which re-precipitates or re-cements around the grains.