Sulphate increase in laboratory prepared samples

Abstract

Introduction In February 1990 a large number of acid soluble sulphate analyses of Lias Clay samples were undertaken to determine the possible significance of sulphate in causing ground heave. Sub-samples of approximately 100 g were dried and powdered and representative 2g aliquots removed for analysis as recommended in BS 1377: 1975. The remaining powdered material was retained in glass bottles and stored at room temperature. In July 1990 a further analysis of the material was undertaken. In all but two of the twenty-eight powdered samples there was an increase in the acid soluble sulphate content. Background The samples were taken from an almost flat Lias Clay site where it was believed disseminated pyrite would. be present in the dark mudstone/silty clays. In 1987 Hawkins and Pinches drew attention to the increase in acid soluble sulphates over the seventeen-month period in which dark mudstone samples had been stored in the laboratory. In view of this, prepared Lias samples which had been taken at 200 mm vertical spacings from two of the trial pits were re-analysed after a five-month period of storage. Analytical results The SO3 percentages measured in February and July 1990 on the powdered samples from TPs A and B are given in Table 1. In the case of TP A low SO3 percentages were obtained on both occasions to a depth of 2.2 m, below which the acid soluble sulphates rose rapidly to about 5%. An increase was noted at each depth, a maximum of 0.50eing recorded at 2.85