On the Measurement of the Tensile Strength of Soft Rocks

Summary. This paper reports on a comparative study of various types of experimental tests for measuring the tensile strength of rocks and rock-like materials. A critical assessment is presented of some widely used laboratory techniques on the basis of experimental data from the literature and from the laboratory investigation performed in this study.Tests were carried out using a triaxial apparatus recently set up at Milan University of Technology. This was designed to reduce random misalignments between specimen and loading frame, which are typical of conventional triaxial cells with external tie bars. The apparatus was then modified to perform various types of tests for determining the tensile strength of materials. An artificial building stone and a natural calcarenite of the Gravina di Puglia geological formation, sampled at Montescaglioso (Matera-Italy) were tested in this research. The experimental investigation included various types of tests, namely the uni-axial, the Brazilian, the ring, the three and four points bending and the Luong test. Specimens of both materials were also compressed to failure in unconfined conditions and loaded cyclically in unconfined tension and compression, Young’s moduli being measured by means of local instrumentation.     

Seismic characterization of hyperconcentrated flows in a volcanic environment

We present direct observations and monitoring data of a hyperconcentrated flow that occurred along La Lumbre ravine, one of the most active channels of Volcán de Colima in Mexico. Flow properties were inferred from video images and seismic data recorded by a geophone installed outside the channel. We collected flow samples 400 m upstream from the monitoring station and analyzed the variation of sediment concentration and grain‐size distribution over time. A joint analysis of hydrological (i.e. flow velocity, wetted perimeter) and rheological (i.e. yield stress τ_y and dynamic viscosity μ_m ) parameters was performed to characterize the flow. Different flow regimes and sediment transport processes were identified and analyzed in comparison with both the amplitude and spectral features of the seismic signal. We observed differing sediment concentrations at the same discharge, suggesting a decoupling between sediment transport processes and discharge for low‐magnitude flows. A straightforward correlation was found between the amplitude of the seismic signal and the sediment concentration, and a value of 1.8 × 10^?3 mm/s was identified that can be used as a threshold to recognize the hyperconcentrated phase of the flow. This information was tested on the complete seismic dataset gathered at La Lumbre ravine during the 2015 rainy season. We identified the transition from streamflow to hyperconcentrated flow (and/or vice versa) in 16 low‐magnitude events and we validated this result using the video recordings. The correlation between seismic amplitude and sediment concentration is valid at La Lumbre ravine but would need to be tested in other locations for the development of automatic flow classification methods. This work contributes to standardized seismic methods for characterizing flow processes in volcanic environments, also for the development of lahar early warning systems. Copyright © 2018 John Wiley & Sons, Ltd.