Chalk physical properties and cliff instability

Abstract

Physical properties such as porosity and intact dry density (IDD) are compared with strength testing in relation to the Chalk formations in the cliffs of the English Channel. Natural moisture contents are close to saturation moisture contents for chalks with intact dry densities above 1.70 Mg/m3. Below this IDD, the natural moisture contents show a much greater range and greater divergence from the saturation line. There is also an indication that certain types of chalk retain water at saturation level while others gain and lose water more readily. Strength tests (Point Load Index, Brazilian Crushing Strength and Uniaxial Compressive Strengths) show up to four times reductions in strength between dry (higher strength) and saturated (lower strength) samples. Absence of a strong correlation between density and strength is interpreted as resulting from either mineralogical differences in the samples and/or textural differences between different chalks. The variation in physical properties and strength in the different chalks forming the cliffs indicates the strong stratigraphical and sedimentological controls on mechanical performance of the material and mass in cliff failures.

Using mechanical models to investigate the controls on fracture geometry and distribution in chalk

“…There is an extensive body of literature detailing the results of laboratory measurements of different mechanical properties on samples of chalk taken from outcrop or boreholes, including Carter & Mallard (1974), Bell (1977), Mortimore (1979), Jones et al (1984), Clayton (1990), Clayton & Saffari-Shooshtari (1990), Flexer et al (1990), Kronieger (1990), Mimram & Michaeli (1990), Mortimore & Fielding (1990), Bell et al (1999), Katz et al (2000), Gommesen & Fabricius (2001), Bowden et al (2002) and Mortimore et al (2004). These studies all show that the key control on mechanical properties of chalk is the porosity: as this decreases with burial and compaction, the tensile strength, UCS and Young's modulus of chalk will all increase (the relationship between porosity and Poisson's ratio is less clear so we have used a constant value for Poisson's ratio in this study).…”

Combined rock slope stability and shallow landslide susceptibility assessment of the Jasmund cliff area (Rügen Island, Germany)

“…The Pleistocene glacial sediment succession is composed of till, clay and sands, whereas the undisturbed sequence consists of two tills, covered by glacial sands, and one clayey till that discordantly covers the entire deformed sequence (Panzig, 1995;Müller and Obst, 2006). However, due to intense sub-glacial tectonic deformations (Steinich, 1972) the original layering is barely preserved throughout the study area.…”

Coastal Cliff Instability, a Case Study From the Uk

“…Generally, they determine a reduction of strength (angle of friction and cohesion) as the degree of saturation increases. The critical role of the water contained either in the pores or the fissures in the stability of chalk cliffs was confirmed (Mortimore et al, 2004).…”

“…Chalk is considered as a homogeneous material but in reality is a very complex sedimentary formation with various physical properties. Furthermore, the complex nature of the material is derived from processes like bioturbation, diagenesis, sendimentation and the region's structural history (Mortimore et al, 2004;Duperret et al, 2005). Mortimore et al (2004) conducted a series of different experiments in order to establish a reliable classification of chalk.…”

“…Furthermore, the complex nature of the material is derived from processes like bioturbation, diagenesis, sendimentation and the region's structural history (Mortimore et al, 2004;Duperret et al, 2005). Mortimore et al (2004) conducted a series of different experiments in order to establish a reliable classification of chalk. Generally, they determine a reduction of strength (angle of friction and cohesion) as the degree of saturation increases.…”