The measurement of mode I fracture toughness (K_IC) , which represents the resistance to the propagation of pre-existing defects under tensile stress, is crucial for various engineering applications involving rocks, such as tunnel boring, rock drilling, hydraulic fracturing, and oil exploration. Recently, the pseudo-compact tension (pCT) test has been proposed as a reliable method to measure K_IC in rocks under pure tension conditions, yielding consistent results for both fragile and ductile rocks. Although the pCT method offers several advantages, such as simple sample preparation, small sample requirement, and controlled fracture propagation beyond the peak load, its original approach requirements the use of a specially designed, large-size testing device. This limitation may restrict the broader adoption of this testing methodology. To overcome this drawback, we present in this work a simplified pCT test apparatus that can be easily installed in any conventional compression frame. The proposed device, thanks to its mechanical configuration, allows for application of true tension to the notch of the sample while the axial actuator of the frame operates in compression over the loading piston. The mechanical behavior of the prototype was assessed numerically with a finite element method model in ABAQUS, and experimentally using aluminum specimens. Additionally, to further evaluate its stiffness and performance, digital image correlation (DIC) was employed to obtain full-field strain characteristics of the critical, most stressed parts of the apparatus. To validate the new configuration, polymethyl methacrylate (PMMA) specimens with different notch lengths were tested. The results demonstrate that the new simplified pCT test system exhibits sufficient stiffness and provides comparable K_IC values with those previously obtained with the reference pCT testing device.

Shear stiffness is an important parameter governing deformation behavior and stress dis-tribution. However, its current use is inadequate for detailed models, particularly in sedi-mentary rocks. Most published shear stiffness data for sedimentary rocks are for joints at low normal stresses, with limited available at normal stresses above 1 MPa. This paper ad-dresses this deficiency by presenting results from direct shear testing for bedding parallel fractures under high normal stress and implementing a recently proposed method to isolate fracture deformations, this method is then validated using several measurement methods. The shear stiffness of 18 samples from the Bowen Basin (QLD, Australia) was obtained for different values of applied normal stresses (1 to 8 MPa). The results show a strong positive correlation between applied normal stress and shear stiffness. The findings high-light opportunities for improvement in current guidelines and emphasize the need for them to better reflect results and data processing protocols.

Solving engineering tasks and assessing the suitability of rocks as building raw materials requires determining the physical properties. Ultrasonic testing as nondestructive testing is useful for the initial assessment of elastic properties. Adapting seismic tomography technique for application at the laboratory scale using ultrasonic frequency waves allowed the characterisation of variations in ultrasonic propagation velocity inside the study granite specimen. The P- and S-wave velocities were measured using 54kHz and 250kHz transducers. The dynamic modules and anisotropy ratio were calculated based on obtained seismic wave velocities. The ultrasonic tomography method allowed for an initial assessment of the homogeneity of the rock medium, which allows an optimal selection of its lithological variety for a given engineering purpose.

Although the determination of joint shear strength of layered heterogeneous rock masses constitutes a challenge in engineering, since they are widely present in natural rock masses, the available scientific-technical publications are relatively scarce. Studies have mainly been focused on homogeneous rocks. Thus, the objective of this research is to evaluate the effect of heterogeneity on the shear behaviour of different types of rock joints and provide the basis for evaluating the stability of the heterogeneous rock masses, as well as to know the differences and similarities with respect to the behaviour of homogeneous rock joints. In this research, various direct shear tests have been conducted on artificial joints of heterogeneous rocks with different joint roughness coefficients, as well as tests on artificial joints of homogeneous rock in order to allow comparisons between them. The experiment consisted of performing 18 direct shear tests on rock joints of homogeneous and heterogeneous materials with three different roughness profiles. These rock joints were shaped in the laboratory with artificial stone materials, manufactured using different dosages of lime, cement and sand, obtaining samples of low, medium and high strength. The test results show that in heterogeneous materials there is a great influence of the block of rock with lower strength. In this sense, it was shown that the strength of the weak rock is dominant in the joint shear strength when there are wide strength differences between the joint faces. On the other hand, if the rock of the lower face of discontinuity is of low or medium strength and the upper face is of medium or high strength, the joint shear strength is lower than the case in which both faces are of low or medium resistance. This situation occurs for all roughness profiles, although especially in the profiles with the highest JCR. In other words, if the results were transposed to a natural slope, a homogeneous shale-type slope would be more stable than a heterogeneous one with alternating shale and limestone. The shear behaviour is also heterogeneous in the damage suffered by both faces of the joint. That is, when the heterogeneous rock blocks that, assemble the joint, exhibit a great difference in the strength, the degradation of the asperities only occurs on the side of the rock with lower strength, while the one with greater resistance remains intact.

Field-scale rock masses are discontinuous and heterogeneous in that they are composed of different sections of intact rock intersected by discontinuities. Joints are common discontinuities in rock masses that have formed previously in the rock mass's geological history. Joints can be grouped based on their orientations and other properties. When subjected to stresses, deformations occur in these rock masses. These strains either accumulate in the intact rock blocks or generate movements along discontinuities. Reproducing the stress-strain behaviour of rock masses at laboratory scale or by means of physical models is a complex task. To attempt to replicate rock mass behaviour at the laboratory scale, jointed laboratory-scale cylindrical granite samples were prepared with two different configurations of jointing. The specimens contain two smooth joint sets, forming intact rock blocks of similar sizes. Compressive laboratory tests were conducted at various confinement levels on jointed granite samples, where global deformations were measured using LVDTs (Linear Variable Differential Transformer) and corrected using energy approaches to estimate the total deformation produced in the entire jointed specimen. Furthermore, some strain gauges were fixed on the intact rock blocks to compute the localized strains in these blocks. The results of the deformations on these laboratory tests can be compared to previously conducted tests on intact rock. Based on the strain measurements of both the intact and jointed samples, it is possible to compute a preliminary estimate of the stiffness of the joints, particularly that of the sub-horizontal contacts. Moreover, the indirectly obtained local deformation values are compared to those obtained using strain gauges in order to understand the heterogeneous nature of strain in these rock mass analogue samples, which can help to better understand deformation processes of field-scale rock masses. In conclusion, this study presents the results and a first interpretation with preliminary conclusions of a set of triaxial tests conducted on jointed samples, where an estimate of the stiffness parameters was obtained based on the observed deformations on the different constituent blocks of the samples.

Rocks are natural materials and their microstructure is usually complex, involving for example voids, cracks, planes of weakness, different sizes of grains or different minerals. The micro-structural properties of the rocks influence fracture propagation, for instance, under opening fracture modes (Mode I). This paper presents fracture propagation analysis of notched rock prismatic samples tested under four-point bending conditions using a petrographic microscope of transmitted and polarized light. The two analyzed rocks are a Floresta sandstone and a Mol-eanos limestone. After testing, the sample is reconstituted and thin sections of the area sur-rounding the main fracture are obtained. The initial type of fracture (i.e. transgranular or in-tergranular), the initial deviation of the main crack and its overall sinuosity are studied to try to gain a deeper understanding of fracture processes in rocks.