To enable optimal design of shallow foundations, anchors and pipelines, it is common practice to test seabed samples at the anticipated consolidation stresses. The direct simple shear (DSS) apparatus has proved to be a favoured means by which the response of a calcareous sediments can be characterised Previously unpublished DSS data for a fine-grained calcareous sediment, presented herein, has helped reinforce the theory that some calcareous sediments exhibit cyclic resilience at low consolidation stress levels that are disproportionately high when compared to the cyclic resilience shown when tested under elevated consolidation stress levels. When analysing DSS test results, a direct proportionality between cyclic strength and consolidation stress has traditionally been adopted. Data presented in this paper helps confirm and extend the relevance of previous work that suggested that the cyclic resilience of certain (in that case, coarse-grained) sediments more closely reflects their behaviour under monotonic loading, with the strength being related to the consolidation stress by a power function, i.e. square-root. Up to now, some difficulty has been experienced obtaining reliable and repeatable test results at low consolidation stress levels that conform with the theoretical expectations. It is suggested in this paper that the reason may be associated with previously unnoticed preferential slip at the platen interfaces in the DSS apparatus Simple modifications to the apparatus have been implemented successfully to minimise this effect and allow more reliable definition of the cyclic resilience of these sediments at low consolidation stress levels It is considered that the information contained with the paper may prove especially useful to designers of shallow foundations in calcareous sediments.
The direct simple shear (DSS) apparatus (eg. Airey & Wood, 1987) has become a favoured method to establish the undrained cyclic strength of uncemented seabed sediments, which often forms the primary basis for the design of offshore shallow foundations, anchors and pipelines, including seismic liquefaction studies.
In their state of the art presentation of the laboratory testing of calcareous soils, Carter et al. (2000) reproduce convenient design charts for calcareous sand and silts that represented a reasonable average of DSS data from many offshore projects. Charts like these suggest that cyclic strength is directly proportional to the consolidation stress Airey and Fahey (1991) used this assumption when summarising the cyclic strength of silty sand from the North West Shelf in Australia Finnie et al. (1999) presented data from DSS tests performed on calcareous sand from an offshore project in the Philippines and modified the assumption of direct proportionality to consolidation stress, indicating that cyclic resilience tended to reflect the monotonic strength. This concept was originally suggested by Andersen et al (1994) for non-calcareous soils. The term cyclic resilience (rather than strength) was used to define the resistance a sediment has to accumulating strain under particular cyclic load levels, given that the sediments tested were always able to sustain the demanded load, without collapsing This paper presents some data from DSS and triaxial tests performed on calcareous silt, sand and clay sized soils recovered from the seabed in the in the Timor Sea, supported by data from the Malampaya development in the Philippines