Geotechnical Study of a Trial Embankment on Alluvial Deposits at King's Lynn.

A geotechnical study of a trial embankment on the alluvial deposits of the fenlands is presented. The study considers the problems associated with the construction of highway embankments over the low strength, highly compressible deposits encountered in the Fens. The trial embankment was designed to furnish data on the behaviour of the alluviums underload and to test some of the solutions to the engineering problems. The geological history of the deposits is given and the parameters and characteristics of the materials revealed by the site investigation are discussed. The aims of the trial, its construction and behaviour, particularly of the designed failure of one section are described. Measurements of pore water pressures, lateral subsurface movements, settlement, and movements of the ground surface outside the toes of the batter were amongst those taken. The study considered and discusses in detail the pore water pressure data, lateral subsurface movements, settlement and stability of the embankment. The main features of the sections dealing with these various facets are the problems associated with prediction of pore water pressures and the drawbacks in the use of such predictions for the control of the construction of earthworks on these alluvial deposits, the considerable subsurface lateral movements and settlements measured, and also that total stress atability analysis gives a reasonable assessment of the factor of safety. The solution of some of the engineering problems posed by design considerations was achieved by this study. Some of the problems originally envisaged were found to be of little consequence, for example the zone of influence beyond the loaded area and the movements of the ground surface within this zone. Also the movements of the ends or noses of the embankment were insignificant compared with the movements predicted by the site investigation. The trial examined the use of sand drains and found that in engineering terms these were of little benefit. The trial also revealed some problems that were not originally anticipated, such as the magnitude of and time period over which settlement occurs and the large subsurface movements encountered which have a significant effect on the design of piled foundations. The study of insitu permeability measurements has led to the development of a method of determining the insitu value of Ko using the constant head permeability test. A method of using strain measurements to monitor filling operations has been devised and shown to reveal the onset of instability of an embankment. This method gives rapid results and enables filling to be stopped or other measures taken before failure actually occurs. The prediction of settlements using observational methods was shown to be possible with acceptable accuracy over a period of up to approximately five years. Two curve fitting methods are described and discussed to enable these predictions to be made