Influence of water content on the basic friction angle of porous limestones—experimental study using an automated tilting table

Abstract

An accurate evaluation of the shear strength of discontinuities is frequently a key aspect for determining the safety of mining and civil engineering works and for solving instability issues at rock mass scale. This is usually done by using empirical shear strength criteria in which the basic friction angle (φb) is a relevant input parameter. Tilt testing is probably the most widespread method to obtain the φb due to its simplicity and low cost, but previous research has demonstrated that the results are strongly affected by several factors (e.g. surface finishing, cutting speed, specimen geometry, wear, time and rock type). In this connection, despite it is well known that water significantly reduces the mechanical properties of sedimentary rocks, very scarce research has focused on assessing the impact of the variations in water content on tilt test results. With the aim to fill this gap, saw-cut slabs of three limestone lithotypes were tilt tested in dry state, wet condition (fully water saturated, non-submerged samples) and under exposure to an environmental relative humidity (RH) of 90%. The results revealed that full water saturation caused moderate φb reductions in two lithotypes and a φb increase in one lithotype. This can be explained by their different microstructure and mineralogy, which makes that lubrication effect prevails over suction effect or vice versa. However, the exposure to a high RH environment did not cause significant φb variations. In addition, some important considerations related to tilt testing are provided and discussed, such as the intrinsic variability of the sliding angle (β) and the impact of multiple sliding on the same rock surfaces on β.

Experimental study on the influence of water saturation and temperature condition on the basic friction angle

“…As Rabat et al [41] found, the water distribution on rock surfaces is unavoidably slightly different for each test during the test process. Therefore, in this paper, to control the moisture condition of rock surface, the saturated samples were placed in the air to lose water naturally, and φ b was tested during the water loss process.…”

“…The basic friction angle (φb) is an essential parameter in the shear strength model of rock joints, which is an inherent material property related to the rock type, mineral composition and particle size [7,18,41,49]. It has been summarized that the φb is approximately 25°~30° for sedimentary rocks, and 30°~35° for igneous and metamorphic rocks [1].…”

“…The suction effect and lubrication effect are two simultaneous actions for the wet surface of samples, however, they have opposite influence on the basic friction angle, in which one plays the domination role dependent on the combined action of water saturation, mineral composition and microstructure [41,49]. If the suction effect is greater than the lubrication effect, φb will increase, otherwise it decreases [30,46,49].…”

“…For the same material, the moisture conditions on the rock surface significantly affected φb [31]. Rabat et al [41] found that the moisture content adsorbed on the surface of limestone was relatively low in a 90% humidity environment, and it caused minor changes in φb. Therefore, the change rule of φb against the water content is not entirely clear yet, and more related research needs to be conducted.…”