Mudflows in stiff fissured clays

• Main Author: Bhandari, Rajendra Kumar
• Format: Dissertation
• Language: English
• Published: University of London 1971

The work embodied in this thesis deals primarily with the little explored geotechnical aspects of mudflows. Experimental and theoretical work has been carried out in conjunction with field observations on mudflows in the London Clay of the north coast of Sheppey and in the Hamstead Beds and Reading Beds of the north coast of the Isle of Wight. Stability analyses of mudslides have been the foremost objective of the research, not only because these analyses have not been attempted before but also because of the wide occurrence and importance of this type of mass movement. Total and effective stress analyses of some idealised and some actual mudslides are made using limit equilibrium methods. Analyses of idealised mudslides have yielded charts for simple and quick estimates of stability. These are discussed and their usefulness is demonstrated. Analyses of actual mudslides, although not free from assumptions and therefore criticism, are based largely upon measured properties and characteristcs, such as pore water pressures, soil parameters and mudslide geometry. The results of these analyses demonstrate that it is desirable to work in terms of effective stresses particularly because the applicability of Øu = 0 is of questionable validity. The side resistance effects are also shown to be higher than those suggested for rotational landslides. Pore water pressure being a major variable governing strength and therefore stability of mudslides, its measurement was essential. Earlier measurements on mudflows employed Casagrande piezometers which were not very satisfactory. An electrical diaphragm piezometer was, therefore, designed and several of these were made and used for both long and short-term measurements. The pore pressure observations at mudslide M4 in the London Clay of the north coast of Sheppey provided the first direct evidence of strong artesian pressures and consequently the under-consolidated nature of soil mass which may well be characteristic of several other mudslides as well. The mechanism of movement for low-angled mudflows proposed by Hutchinson finds the first experimental supporting evidence in the above results. The incremental pore pressures due to a certain loading on a mudflow are predicted by elastic theory and shown to be in reasonable agreement with field measurements. Shear strengths of mudslide materials in general and of their boundary shears in particular have been measured using conventional techniques both in the field and the laboratory. The strengths on boundary shears have been found to lie considerably below the peak strength of the mudslide material and very nearly equal its residual strength (as determined in the Conventional manner). The need for ring shear tests particularly in the low effective normal pressure range is indicated. Boundary shears have been further studied by examining several thin sections of these at the magnification level of an optical microscope. Microstructures of teal shears, side shears and upthrust shears of a mudflow tongue find their first descriptions in this thesis. Characteristics of boundary shears in mudflows have also been compared with those of shears in landslides. Edge structures associated with a mudslide boundary have been studied in the field by time-lapse photography. The results are discussed in the light of existing published work. Measurements of surface and subsurface movements have also been made using surface markers, flexible tubes and deformable vertical sand columns. Observations of long and short-term surface movement confirms the prevailing idea that mudsliding occurs predominantly by marginal slipping. Theoretical approaches to the computation of velocity distribution in an idealised soil are summarised from earlier published work, particularly in glaciology and rheology, and discussed. Tests on a Weissenberg Rheogoniometer are reported and the usefulness of the shear stress-rate of shear relationship so obtained in calculating velocity distributions in mudflows is discussed.