The undrained mechanical behaviour of a fibre-reinforced heavily over-consolidated clay

The benefits of micro-reinforcement in soils have attracted a lot of research in the last couple of decades. However, not enough is known about the effects of the compaction procedure used to prepare samples in the laboratory. This paper presents data from laboratory undrained triaxial tests on reinforced and unreinforced samples of heavily over-consolidated clay from the Lambeth Group. The samples were prepared by chopping the in-situ natural soil into small peds (typically 10 to 20mm) so that some of the in-situ characteristics of the natural soil would be retained. The soil peds were then dynamically compacted with and without the addition of fibres. Undrained triaxial tests were carried out using 100mm and 38mm diameter triaxial samples. The results presented have shown that the addition of reinforcement appear to cause a shift of the One Dimensional Normal Compression Line (1-D NCL) of the reinforced material. The undrained shearing behaviour of the reinforced and non-reinforced soil is also analysed and conclusions drawn. Fibre alignment studies were also conducted, indicating that the majority of the fibres are aligned within ±20o with the horizontal plane while more than 50% are oriented within ±10o. This preferred orientation is believed to be caused by the compaction method used to prepare the samples.