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% Ip. Progressive laboratory décalcification 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. L'analyse des échantillons pris tous les I m dans des sondages traversant les couches de sols de Fuller à Bath a montré une corrélation inverse entre la teneur en calcite et les limites d'Atterberg. On montre que la fraction calcaire du sol a une grande influence sur l'angle de cisaillement résiduel avec un seuil de 28% de teneur en calcite séparant les sols qui se comportent uniquement dans le mode de cisaillement par glissement des autres sols qui se cisaillent principalement dans le mode transitoire. Le seuil peut être également défini à 40% de Ip. La décalcification progressive en laboratoire d'un échantillon contenant 36% de calcite a entruiné un accroissement de la limite de liquidité de 63% à 101% et de la limite de plasticité de 21% à 33%; la teneur en argile s'accrut de 52% à 82% soulignant l'influence des particules calcaires de taille limoneuse. La décalcification s'accompagne d'une réduction marquée de la résistance au cisaillement résiduel pour des variations des contraintes effectives normales, démontrant l'influence des particules de taille limoneuses moyennes et grosses. La relation claire entre la teneur en calcite et les propriétés géotechniques est particulièrement importante car la teneur en calcite est sujette à des changements en raison des phénomènes de dissolution.
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Decalcification is proposed as a cause of the continued movement observed in many natural and engineered slopes in calcareous mudrock formations. It is deemed that such movement may potentially occur as granular calcareous particles, which otherwise maintain high residual shear strengths, are leached from shear surfaces within slipped masses, changing the mode of residual shear from transitional to sliding. Measurements of the residual shear strengths of 42 natural samples taken through the Fuller's Earth succession near Bath over a range of effective normal stresses emphasize the curved nature of many failure envelopes. The results demonstrate that the threshold between the two modes of shear occurs at approximately 25% calcite content. Fuller's Earth Clay with more than 25% calcite content is thus prone to significant decreases in residual shear strength upon partial or total decalcification.
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