Unlike previous research on soft and clay-bearing rocks, this study seeks to investigate the effect of wetting and drying (W-D) cycles on the engineering properties of two hard rocks. A series of W-D cycles were applied to fresh specimens of greywacke and basalt, and their strength, slake durability and density were progressively assessed as the number of cycles increased. It was found that unlike the basalt, the strength of greywacke rapidly decreased with increasing W-D cycles while the rock density and slake durability index remained almost the same. Analysis of the crack propagation in both rocks revealed that, compared to soft and clay-bearing material, the process of strength degradation in hard rocks was rather different, with a more pronounced effect of crack development. In addition, comparisons with the data obtained for the same type of rocks yet naturally weathered was performed to establish similarities between the properties of laboratory-deteriorated and naturally weathered rocks. The obtained results are expected to provide more practical values for such laboratory studies where the rate of rock deterioration is relatively high (several weeks) in comparison to what is commonly observed in the field (several years).

The effect of high temperature on engineering properties of rocks has recently become an important topic for geotechnical engineering applications due to the development in geothermal energy extraction, underground coal gasification, disposal of nuclear waste and deep mining applications. This study aims to investigate the engineering properties of four rocks heated at high temperatures and to identify the corresponding mineralogical changes. Two types of sandstone (Beaudesert sandstone and Jimboomba sandstone), basalt and argillite were subjected to a range of temperatures from 25 o C to 800 o C and their properties such as porosity and point load index (PLI) were studied through a series of laboratory experiments. The physical and mineralogical alteration of the heated rock specimens were also investigated to better understand the thermally induced processes that occurred in each rock type using the X-Ray Diffraction (XRD), Thermogravimetry (TG), Differential Scanning Calorimetry (DSC) and Derivative Thermogravimetry (DTG) techniques. The obtained data indicated the important role of mineral composition in the thermal behaviour of the studied rocks. Noticeable changes in the mineral compositions at the threshold temperature were observed for each rock type. This behaviour was confirmed with TG/ DSC/ DTG analysis. This temperature-related process resulted in the formation of cracks, an increase in porosity and a decrease in strength. However, the effects of high temperatures on engineering properties were found to be different in the two sandstone types.While Beaudesert sandstone underwent significant weakening at high temperatures, only a slight change in the strength of Jimboomba sandstone was recorded. The threshold temperatures were identified as 400 o C, 400 o C, 500 o C and 300 o C, for the tested Beaudesert sandstone, Jimboomba sandstone, basalt and argillite samples, respectively. The critical temperatures (obtained using a damage coefficient) were found as 570 o C for both Beaudesert sandstone and basalt, and 440 o C for 2 argillite. Analysis of the laboratory data revealed the difference between the threshold temperatures and the critical temperatures. It is recommended to use both threshold temperature and critical temperature parameters to provide more accurate estimations of the temperature range at which rock undergoes significant changes. Article Highlights• Engineering properties of four heated rocks were investigated to identify factors affecting their mechanical and mineralogical behaviour.• At threshold temperature, noticeable changes in the mineral compositions occur, resulting in the formation of cracks, increases in porosity and a drop-in strength.• The thermal damage coefficient can lead to better estimation of the critical temperatures.

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This study is an investigation of the effect of high temperatures on the engineering properties of sandstone. The rock was first subjected to a range of temperatures such as 25 o C, 200 o C, 400 o C, 600 o C and 800 o C, and a series of porosity, unconfined compression, X-ray diffraction and scanning electron microscope tests were then performed on the heated specimens. In addition, sandstone specimens were also subjected to different numbers (i.e., 1, 2, 4, 6 and 10) of thermal cycles to better understand the effect of high temperature variations on rock behaviour.It was observed that high temperature in the range of 200 o C -400 o C had some influence on the rock properties; that is, the porosity slightly decreased while the strength of the rock increased.However, for T≥600 o C, there was a significant increase in rock porosity that correlated with a decrease in rock strength. The observed changes in engineering properties were attributed to the changes in rock mineralogy and microstructure that occurred at T>600 o C where the major minerals underwent the process of phase transformation and a significant increase in rock cracking. Regardless of the temperature, almost all specimens failed in tension during unconfined compression. The effect of thermal cycles was seen in a progressive increase in rock porosity and a corresponding decrease in the elastic modulus of the rock.

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