Soil creep: an assessment of certain controlling factors with special reference to upper Weardale England

Abstract

Soil creep is important as a process affecting landform development, as a factor influencing slope stability and as an element of land wastage. Pew definitive studies have been made of the process and work on variables controlling rates of soil creep is very limited. Therefore, in the present study it was decided to monitor creep with a number of devices over different lengths of time so that variability over space and time could be determined. This involved investigations on three distinct scales: those of the laboratory, the plot and the drainage basin. To measure rates of creep and to monitor probable controlling variables, new instruments were designed and established procedures were modified. All new instruments were validated and essential features at each stage of the work were replication and the measurement of local variability. After initial tests on experimental plots a drainage basin was selected and instrumented for the pilot study, in which both creep rates and a number of basin variables were investigated. The experimental. design for the main study was then constructed, using six main instruments: Anderson's Inclinometer, Anderson's Tubes, aluminium pillars, dowelling pillars, Young's Pits, and Cassidy's Tubes to monitor rates of creep on each of twenty measuring plots. The basin factors considered as probable controls were categorised, as:(a) external or meteorological(b) surface, including slope angle and vegetation(c) internal, comprising the main soil variables Annual rates of creep, shown to vary from'0.3mm to 2.4mm, were correlated with eighteen major basin factors. The results of graphical and statistical analysis showed that rates of creep are controlled by a number of 'force' factors, chiefly soil moisture, field capacity and plasticity index, together with 'resistance' factors, particularly bulk density and a number of shear stress measures. The importance of moisture and its related variables was reinforced by principal component analysis and confirmed in a model produced by stepwise multiple regression. Laboratory experiments with soil troughs demonstrated the relationship between soil movement and meteorological cycles. The daily monitoring of creep for extended periods on an experimental plot allowed short terra movement to be assessed. Further developments indicated by these results include long term monitoring and the mapping of spatial variation of creep over a 'landscape' using the key variables identified.