Mud volcanoes and the behaviour of overpressured clays and silts
Mud volcanoes are a poorly understood phenomenon whereby overpressured, fine-grained sediments extrude to the surface from depths of up to several kilometres. They are observed worldwide, usually in areas of tectonic compression and thick, rapidly-deposited sedimentary sequences. This research is aimed at achieving a better understanding of the type of sediment behaviour leading to mud volcanoes. The study combines a geological investigation of the origin of mud volcanoes with a geotechnical investigation of their behaviour. As part of the geological study, the mud volcano examples around the world are described, and possible mechanisms of overpressuring and extrusion are investigated. Particular attention is paid to the field description and the subsurface geology of the mud volcano areas visited in South Trinidad and Southwest Taiwan. The results of laboratory analyses on the muds sampled in the field, mineralogy, particle size and shapes, Atterberg limits and, where possible, micropalaeontology, are presented. These are aimed at understanding the origin of the clay and establishing any characteristics unique to mud volcanoes. The geotechnical investigation concentrates on the undrained shear behaviour of mud volcano clays, which originate in an environment where thick, low permeability sedimentary sequences are subjected to tectonic compression. The experiments were conducted in a high pressure triaxial cell, using confining pressures of 5 to 50 MPa. Particular attention is paid to the effects of consolidation path, stress magnitude and material properties on the behaviour of mud volcano clays. The results of the experiments are discussed in the light of the geological study, and equivalent testing results from low pressure soil mechanics studies. The discussion concentrates on the role of tectonic activity in the formation of mud volcanoes.