Study On Engineering Geology And Acoustic Emission Characteristics Of Coral Reef Limestone

In terms of petrology,coral reefs are collectively referred to as coral reef limestone.According to different rock structures,they can be divided into four structural types:coral skeleton limestone,coral gravel block limestone,coral gravel clastic limestone and coral sand clastic limestone.According to the lithological characteristics of multiple boreholes and RQD index of a coral reef body in nansha islands of China,the inner structure of the coral reef body presents the vertical zonation phenomenon of alternating hard and soft layers.The physical and mechanical properties of Marine biogenic coral reef limestone are different from those of terrestrial geological rocks.In consideration of the needs of coral reef engineering,the basic engineering geological characteristics are taken as the core,and acoustic emission technology is preliminarily used in the study of mechanical properties of coral reef limestone to provide scientific and reliable design basis and key technical support for the feasibility construction and stability evaluation of coral reef engineering.The main research results are as follows:1.According to the related engineering rock mass classification standard,based on coral reef limestone p-wave velocity,the relationship between dry density and porosity,combined with a coral reef body multiple borehole core rebound value the evolution law of depth as well as the core chip optical detection results,a coral reef formation of springback value classification method,put forward a coral reef limestone quality level of the longitudinal wave velocity based on the standard.Based on the principle of rock two-phase model,the modified time average formula is established.2.Strength characteristics of coral reef limestone of different structural types all conform to mohr-coulomb criteria,and the confining pressure effect of strength characteristics of coral reef limestone is more sensitive than that of other three types of coral reef limestone.Different structure types of coral reef limestone under uniaxial compression is showed significant differences in the macroscopic failure characteristics,coral skeleton limestone and coral calcarenite of tensile damage,coral debris piece of limestone and coral calcirudite show the local extensional fractures,and along with the hole collapse type damage caused by stress concentration.Under the action of triaxial compression,the macroscopic failure characteristics of coral reef limestone show obvious confining pressure effect,that is,uniaxial compression presents as a split-t)ype tensile fracture surface.With the increase of confining pressure,the macroscopic fracture of "two-stage stepped"gradually transforms into a shear zone with a single inclined plane running through.3.The evolution law of acoustic emission time series of coral reef limestone under triaxial compression is basically consistent with the characteristics of its stress-strain curve,which is mainly manifested as four-stage distribution characteristics.The concept of"equivalent failure timel"was proposed to reveal the"skeleton structure effect"of the mechanical properties of coral reef limestone."Equivalent failure time"and its corresponding accumulative ringing count and accumulative energy count increase with the increase of confining pressure in a good logarithmic function law,which is consistent with the confining pressure effect shown by the macroscopic failure form of coral reef limestone.4.The pre-peak energy evolution law of coral reef limestone of difLferent structure types is consistent with the characteristics of its pre-peak stress-strain curve,and the rock capacity expansion location determined by the energy conservation principle is consistent with the application of acoustic emission technology.The peak-to-peak dissipation energy,cumulative ringing count,and cumulative energy count showed a steady and gentle phenomenon before the expansion point,which was in line with the linear elastic deformation characteristics of the rock,and showed a sharp increase at about 90%,which was the beginning of the rapid and unstable expansion of micro-cracks in the rock.