The School of Civil Engineering and Surveying
Research: Pore Water Pressures and Chalk Cliff Failures
Chalk Cliff Failures
For chalk cliffs, tension cracks are often the first indicator of future collapse. The mechanism of collapse varies between shear failure and topple, with a genera of variables between. Although in part discontinuity controlled, the role of small scale matrix failure is little understood in the environs of coastal cliffs.
For many years, it has been known that Chalk cliff failure is prevalent after periods of high rainfall. Chalk cliff hydrology indicates that the water tables are often at sea level by the time the coast is reached but rising hinterland. The rainfall that effects Chalk cliff stability, therefore is transient with rapidly different flow rates depending on whether fracture flow or matrix flow. Marly beds and slight facies changes in the paleo Chalk seas also giving rise to perched water tables. Such complex hydrology translates to partial saturated phases and cyclic pore water pressures and saturated phases.
Chalk deposition in total spanned some 30 million years, in which time many hundred of metres were laid down. The deposits seen today are only the remnants of what was deposited, much being weathered away in the last ice age. Formed by a multitude of calcium carbonate (Ca CO3) plantonic organisms called cocolith, deposition was estimated to be 0.5mm per year. The Cretaceous Chalk Sea in which the Chalk of Southern England was deposited was remote from any landmasses giving rise to Chalk’s well-known purity. Treatment of Portsdown Chalk samples with acetic acid (5%) has shown impurities in some of the upper Chalks to be as low as 0.66%.
Pore Pressure Parameters
The stress state of a Chalk element within a Chalk cliff is governed by the height of the cliff, the extent of lateral relief and the influence of transient water flow through the cliff. Any change in the total stress system will induce a change in the initial excess pore water pressure. The relationship between the magnitude of initial excess pore water pressure produced at a given point by a change in the total stress system is governed by a pore pressure parameter.
A number of pore pressure parameters exist dependent on the type of stress loading. The A parameter, most widely accepted to represent a stress state path of three dimensional uniform loading followed by one-dimensional loading could be used to approximate to a Chalk cliff prior to significant lateral relief.
It is important to remember that the A parameter is not a constant soil or soft rock property and as such is infinitely variable. The A parameter is known to be influenced by strain, initial stress system, stress history and type of stress change.
The stress path cell system established at the University of Portsmouth comprises a London Imperial College cell operated by the University of Durham computer control system, TRIAX. TRIAX 5 enables a number of paths to be followed using a range of stress state variables. Digital signals are converted by stepper motor controlled pneumatic Manostat into subtle pressure changes within the cell.