Influence of the Hyperbaric Effect on Apparent Material Strength of Fully Saturated Porous Rock for Low Strain Rates

摘要

Excavation of rock materials such as Seafloor Massive Sulfide deposits at great depths is greatly influenced by ambient waterpressure, but little is known about the processes involved. Dry rock specimens are known to exhibit a large increase ofapparent material properties in high-pressure environments. However, in deep-sea mining processes rock material is fullysaturated and it is unknown how the hyperbaric effect changes apparent material strengths and involved physical processesunder these conditions.This paper discusses the influence of hyperbaric effect on fully saturated brittle rock specimens during deep-sea excavationusing a grab, and the resulting changes in apparent material properties. Computations are described which are used to predictthe outcome of this effect, and the validating experiments that were carried out.A new theorem is stated based on elastic deformation of the grain matrix, which causes a pressure difference in the matrixand reinforces the material. This is based on the low cutting speed of a grab, allowing the material to deform elastically andenabling water ingress into the deformed material, resulting in less cutting energy. Moreover the cutting mechanism of a grabhas a very low ratio of cutting energy over excavated rock volume. The theoretical model was developed in collaboration withDelft University of Technology and experiments were carried out with Seatools BV.Experiments were conducted to investigate the phenomenon and to validate the stated theorem for saturated rock material.An experimental setup was developed in collaboration with Seatools BV, to test rock material properties at differenthyperbaric conditions with a low rate of loading. The experiments were designed to carry out standard material tests of theAmerican Society for Testing and Materials to determine the compressive and tensile strength, by crushing the materialspecimens up to their breaking point in different hyperbaric conditions.The experiments were used to validate the theoretical computations that predicted differences in tensile strength betweensaturated and dried specimen, due to ingress of water during deformation causing not the full increase of the apparent materialstrength. The results were consistent and a correlation between the environment pressure and the added apparent materialstrength was found.