DECALCIFICATION AND RESIDUAL SHEAR-STRENGTH REDUCTION IN FULLERS EARTH CLAY

Analyses of borehole samples at 1 m intervals through the Fuller's Earth succession at Bath reveal an inverse correlation between calcite content and Atterberg limits. The calcareous component of the soil is shown to have a major influence on the ultimate residual shear angle with a threshold of 28% calcite content dividing the soils that behave solely in the sliding mode of shear from the remaining soils that shear predominantly in the transitional mode. The threshold can alternatively be defined at 40% I(p). Progressive laboratory decalcification of one sample with 36% calcite content resulted in an increase in liquid limit from 63% to 101% and in plastic limit from 21% to 33%, whereas the percentage clay fraction increased from 52% to 82%, emphasizing the influence of the silt grade calcareous particles. Decalcification is accompanied by a marked reduction in residual shear strength over a range of effective normal stresses, demonstrating the influence of medium and coarse silt grade particles. The distinct relationship between the calcite content and the geotechnical properties is particularly important as the calcite content is prone to change as a consequence of dissolution.