Problems in testing of carbonate sediments

Carbonate sediments are formed in marine environments, in the tropical and sub-tropical climate belts around the world, such as southern Africa, India, Indonesia, Brazil and Australia. These sediments are characterised by their high crushability potential and variability in composition, grain shape, fabric and mineralogy. The design of foundations for offshore structures to be installed in these areas requires engineering parameters, which are generally determined using offshore site investigation data combined with onshore laboratory tests. The reliability and robustness of the design criteria are heavily reliant on the accuracy of the field and laboratory data. The field testing methodologies are generally well understood and can be verified using the recovered samples. However, conventional laboratory testing procedures are generally inadequate for testing carbonate sediments. The results obtained using the standard testing procedures may result in the derivation of a wide range of engineering parameters, which could result in costly design of structures and in some cases may jeopardise the development of the field.

An audit was carried out by Advanced Geomechanics (AG) as part of their QA process. The audit was undertaken in two parts. The first part consisted of testing seven material types, three non-carbonate (Silica Sand, Silica Flour and Kaolin Clay) and four carbonate materials (one terrestrial and three offshore) at four different laboratories. The identity of the tested samples was kept from the laboratories (blind tests). The tests requested included classification, permeability and consolidation. The second part of the audit was to investigate the effect of the operator on the test results, which is currently being carried out at AG’s laboratory (agLAB) and will be reported in a separate paper.