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

Abstract

Excavation of rock materials such as Seafloor Massive Sulfide deposits at great depths is greatly influenced by ambient water pressure, but little is known about the processes involved. Dry rock specimens are known to exhibit a large increase of apparent material properties in high-pressure environments. However, in deep-sea mining processes rock material is fully saturated and it is unknown how the hyperbaric effect changes apparent material strengths and involved physical processes under these conditions. This paper discusses the influence of hyperbaric effect on fully saturated brittle rock specimens during deep-sea excavation using a grab, and the resulting changes in apparent material properties. Computations are described which are used to predict the 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 matrix and reinforces the material. This is based on the low cutting speed of a grab, allowing the material to deform elastically and enabling water ingress into the deformed material, resulting in less cutting energy. Moreover the cutting mechanism of a grab has a very low ratio of cutting energy over excavated rock volume. The theoretical model was developed in collaboration with Delft 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 different hyperbaric conditions with a low rate of loading. The experiments were designed to carry out standard material tests of the American Society for Testing and Materials to determine the compressive and tensile strength, by crushing the material specimens up to their breaking point in different hyperbaric conditions. The experiments were used to validate the theoretical computations that predicted differences in tensile strength between saturated and dried specimen, due to ingress of water during deformation causing not the full increase of the apparent material strength. The results were consistent and a correlation between the environment pressure and the added apparent material strength was found.

Mechanical Tests and Numerical Simulations for Mining Seafloor Massive Sulfides

“…Currently, little data is available on the mechanical properties of SMS samples. Kuiper et al [6] carried out experiments of crushing material samples to the breaking point, aiming to determine the compressive strength and tensile strength of the rocks under different high-pressure conditions. The unconfined compressive strength test and Brazilian test strength (BTS) test were performed to analyze the physical and mineralogical properties of insufficient SMS samples in [7], in which the result revealed that the geotechnical properties of SMS samples were related to the mineralogical characteristics.…”

“…The unconfined compressive strength test and Brazilian test strength (BTS) test were performed to analyze the physical and mineralogical properties of insufficient SMS samples in [7], in which the result revealed that the geotechnical properties of SMS samples were related to the mineralogical characteristics. However, the real SMS samples in [6] were not taken into account, and the SMS samples in [7] are relatively few. Therefore, the further investigation on the mechanical properties of sufficient SMS samples is necessary.…”