Coupling effect of size and strain rate on uniaxial compressive properties of coral reef limestone

Abstract

As the main geomaterials for coral reefs oil or gas extraction and underground infrastructure construction, coral reef limestone demonstrates significantly distinct mechanical responses compared to terrigenous rocks. To investigate the mechanical behaviour of coral reef limestone under the coupling impact of size and strain rate, the uniaxial compression tests were conducted on reef limestone samples with length-to-diameter (L/D) ratio ranging from 0.5 to 2.0 at strain rate ranging from 10−5 s−1 to 10−2 s−1. It is revealed that the uniaxial compressive strength (UCS) and residual compressive strength (RCS) of coral reef limestone exhibits a decreasing trend with L/D ratio increasing. The dynamic increase factor (DIF) of UCS is linearly correlated with the logarithm of strain rate, while increasing the L/D ratio further enhances the DIF. The elastic modulus increases with strain rate or L/D ratio increasing, whereas the Poisson’s ratio approximates to a constant value of 0.24. The failure strain increases with strain rate increasing or L/D ratio decreasing, while the increase in L/D ratio will inhibit the enhancing effect of the strain rate. The high porosity and low mineral strength are the primary factors contributing to a high RCS of 16.7%–64.9% of UCS, a lower brittleness index and multiple irregular fracture planes. The failure pattern of coral reef limestone transits from the shear-dominated to the splitting-dominated failure with strain rate increasing or L/D ratio decreasing, which is mainly governed by the constrained zones induced by end friction and the strain rate-dependent crack propagation. Moreover, a predictive formula incorporating coupling effect of size and strain rate for the UCS of reef limestone was established and verified to effectively capture the trend of UCS.

1. Introduction

Coral reefs cover approximately 348361 km2 of the ocean, predominantly distributed in tropical-to-subtropical latitudes (34°N–32°S) oceanic regions [1,2]. Owing to the favourable reservoir properties, they play a significant role in carbonate hydrocarbon reservoirs [3,4]. The exploration of coral reef oil or gas reservoirs has been a critical measure for achieving green and low-carbon objectives. To further accelerate the utilization of marine oil and gas, the development of underground spaces in coral reefs is a possible measure. In the construction of underground cavern in coral reefs, the method of retaining natural rock pillars can be used as a reference from terrigenous caverns [5–7] to ensure stability during construction and operation (Fig. 1a). In addition, the pillar dimension design must account not only for the static load of overlying strata but also for dynamic loads induced by seismic events, blasting excavation, and impact disturbances.

However, as main geomaterials of coral reefs, the coral reef limestone formations are formed through the complex cementation and diagenesis [8], resulting in dramatically different from terrigenous rocks on physical and mechanical properties [9,10]. Therefore, understanding the strength and deformation properties along with the failure mechanism of coral reef limestone under the coupling effect of size and strain rate are of critical importance.