One dimensional compression behaviour of unsaturated granular soils at low stress level.

Poor performance of trench reinstatements affects the quality and safety of highways. As materials in shallow trenches are unsaturated and under low stresses, the factors affecting their behaviour are not understood clearly. This thesis reports on an experimental investigation into the one dimensional compression behaviour of typical coarse granular trenchfill materials.
Literature from the fields of rockfills and unsaturated soils are reviewed, and some experimental difficulties faced when working with such materials are noted. The limestone test material is classified using standard specification tests for aggregates.
Four series of tests were performed to investigate the behaviour of three gradings over a range of water contents. Monotonic, constant strain and repeated load conditions were used, and investigations made into the influence of inundation on strain development, the influence of water content on Ko and shear strength for a given density, and the influence of compactive effort.
A new consolidation cell was developed to perform two of the test series. This cell allowed the measurement and control of suctions within the test specimens, and allowed inundation under controlled conditions. Air diffusion effects were allowed for in the design.
The experimental work shows measurable suctions exist within coarse granular soils. Compressibility and repeated load behaviour are both shown to be a function of water content and dry density for a given compactive effort, and soil suction is shown to be an important influence. Collapse settlements on inundation were recorded and were shown to be directly influenced by soil suction. Shear behaviour was shown to be affected by the suctions also, but the Ko tests were inconclusive.
A qualitative model to explain the influence of suction in coarse materials is presented, and the current test results compared with those of previous workers. The practical implications of the research are discussed.
The research reported in this thesis was conducted within the Department of Civil and Structural Engineering of the University of Sheffield under the supervision of Dr W F Anderson and Dr I C Pyrah, and in conjunction with the British Gas Engineering Research Station. Liaison staff at the Research Station were Dr Rowen, Mr D Boyes and Mr G Leach. The author wishes to express his sincere gratitude to them all for their time and efforts, but in particular wishes to thank Dr Anderson and Dr Pyrah for their guidance, suggestions and patience both during the experimental work and the prolonged period over which the thesis was written. Thanks also go to Professor T Hanna, the Head of the Department of Civil and Structural Engineering, for the use of the departmental facilities throughout the work.
The design and development of the apparatus was helped greatly by the enthusiasm and competence shown by all technical staff at the University, but the greatest endeavours were provided by Mr P L Osborne and Mr M Foster. Their input does not go unrecognised, and neither does the cooperation received from Mrs S Rowe and her staff when the soils laboratory at the British Gas Engineering Research Station was extensively used for several months.