Processes and mechanisms of slow mass movement in a small catchment in wearable, n. England

Abstract

This thesis investigates sediment transport by slow mass movement within a small upland catchment area near Stanhope in Wearable, N. England. In the field, emphasis is placed on measurements of the rate and spatial distribution of mass movement; in the laboratory, a possible mechanism for slow movement has been investigated in several controlled experiments. A review of the literature on slow mass movement processes indicates that there is considerable confusion over terminology. Terms such as creep, slow mass flow and soil slip imply that the mechanism of movement is known, whereas they are usually intended to be descriptive. An alternative classification of movement is suggested which separates description, knowledge of mechanism and knowledge of the domain in which a process operates. The term slow mass movement (S.M.M.) is used throughout this thesis to refer to downslope displacement of soil particles at a rate expressed in mm^/yr but whose mechanism of movement is not necessarily known. A fieldwork programme was established to measure superficial slow mass movement rates in an upland catchment area for a period of two years. Data were recorded from seventy 1m(^2) measurement plots at monthly intervals. Each plot contained four instruments recording mass movement, thus allowing comparison within as well as between plots. The experimental design allows analysis of the effects of slope angle, vegetation type and soil texture on movement patterns and rates. The general aim is to develop an understanding of how S.M.M. relates to physical and ecological variables in an upland catchment area and to assess its importance as an erosional process. The instruments used ai-e techniques, or modifications of techniques which have been described and used successfully by several previous researchers. These include Anderson's Tubes, Anderson's Inclinometer Pegs and Young's Pits. Measurements were also taken of water table levels and of the shear strength of soils at all plots at regular intervals in order to assess how the mechanical properties of soils may change in situ, over short time periods. These changes are related to the temporal patterns of S.M.M. Associated with rapid mass movements such as landslides and mud-flows are slowly deforming soil masses. Several slopes within the catchment area appear to be exhibiting slow deformation. The mechanism of this process has been analysed by simulating the normal and shear forces imposed upon the soil mass in a stress-controlled, undrained, direct shear test. Prom this test the value of the yield stress of the soil can be derived: this is the rnaximum shear stress the soil can withstand without undergoing continuous deformation. By comparing the yield stress with the predicted shear stress occurring in a natural slope it is possible to determine, according to a slope stability equation, whether continuous deformation could occur and at what rate it could proceed. The apparatus was used to determine the contribution that the natural soil structure and plant rootlets make towards strength during slow deformation. Previous analyses have measured either root tensile strength or root permeated soil shear strength. Neither of these techniques is suitable because in the former case the frictional and apparent cohesive strengths of the soil are ignored and in the latter case the forces imposed during testing far exceed those actually encountered in the field. The creep-shear tests allows the tensile and shear components of strength to be combined with realistic shear stresses because the slow rates of strain which occur during testing allow roots to stretch along the zone of failure causing a tensile stress to build up in the roots, thus increasing the apparent cohesion of the soil. The research described in this thesis concentrates on substantiating and amending previous ideas on the rate of slow mass movement, its temporal persistence and on variables which control its action. The investigation is based upon both empirical and theoretical methods with field observations being compared with the results from laboratory experiments and also with theoretical ideas being analysed using data collected from a field experiment.