Formation of box canyons by mass failure in limestone: A modelling study of the role of groundwater seepage

<p>Groundwater seepage has been shown to unambiguously lead to channel formation inunconsolidated sand to gravel sized sediments. However, its role in the evolution of bedrocklandscapes remains controversial. In this study, we use the coastline of the Maltese Islands as a case study to establish if and how groundwater seepage can form box canyons in limestones. The study area comprises up to 40 m high coralline limestone cliffs, with a mean fracturedensity of 1 in 5 m, overlying a ductile marl layer. The permeability contrast promotes the development of a perched aquifer and groundwater seepage at the cliff face. We  ran numerical simulations using a 3D distinct element model based on geological, geotechnical and hydrological baseline information from the study state, and explored three potential mechanisms: (i) fracture widening by fluid pressure and dissolution associated to groundwater flow and seepage, (ii) fracture widening by loss of support at the base due tomarl displacement resulting from increased water content, and (iii) a combination of (i) and (ii). We also took into consideration two scenarios: (a) uniform groundwater seepage, and (b)focused groundwater seepage. Our results suggest that the combination of mechanisms (iii) and the scenario with focused groundwater seepage (b) give rise to the box canyonmorphology observed at the site. Box canyons thus initiate and grow via detachment of limestone blocks and their toppling, which is more concentrated at the head where groundwater seepage occurs.</p>

Mechanical Properties and Failure Behavior of Composite Samples

In underground coal mining systems, the occurrences of coal burst hazards and pillar failures relate not only to the condition of stress distribution but also the geometry of roof-coal-floor structures. To study the failure response of these structures, the rock-coal-rock (RCR) sample, in which a coal component is sandwiched between rocks, is always employed as the experimental subject. In this study, the effect of height ratio (a ratio represents the height percentage of coal component in an RCR sample) on the mechanical properties and deformation behavior of RCR samples was numerically investigated by using the distinct element model (DEM). The results reveal the following. (1) The uniaxial compression strength (UCS) of the RCR sample decreases with increasing height ratio as an inverse proportional function. (2) With increasing height ratio, the elastic modulus of the RCR sample decreases exponentially, while the postpeak modulus is strengthened in an inverse proportional manner. (3) Microcracking activity of the RCR sample is different from that of the pure sample during loading. Specifically, a reactive period always occurs after the quiet and active periods in the RCR sample. (4) The RCR sample fails in a progressive manner, in which cracking bands develop preferentially in coal and then extend to rocks. Expectably, the mechanical properties and failure behavior of RCR samples are height ratio dependent, which may contribute to predicting the hazard of coal bursts and estimating the failure of rock-coal-floor structures.