Stress-strain curves and fracture distribution characteristics indicate that gravel content influences the mechanical properties of rocks. In this study, uniaxial compression tests were conducted on the conglomerate containing gravels having diameters between 2–26 mm. In these tests, we found that many micro fractures were generated around evenly distributed gravels. Additionally, we found that as the gravel content increased, the uniaxial compressive strength and elastic modulus of conglomerate decreased; however, the plasticity characteristics of conglomerate increased. The results of our analysis imply that when the gravel content is less than  (14.61–31.72 %), the macro mechanical properties are mainly influenced by the cementing material, and between  and  (78.50 %), failure is determined by the local Orowan additional stress, which is related to the mechanical properties of the cement and the cementing strength,and higher than f_ch, failure is determined by the mutual Hertz stress among the gravels.

The samples in our study are from Mahu 1, which is located in the middle of the Mahu depression, with a sampling depth of 3242–3587 m. Limited by the size of bottomhole coring tools, samples of size Φ 50 × 100 mm were drilled and polished on both ends until the parallelism error was less than 0.05 mm. To get the distribution of gravel contained in samples, gravel images were extracted from the outer cylinder images of the samples. Then, the ImageJ software was used to calculate the content and particle size of gravels having diameters larger than 1 mm. In the six samples, the area percentages of gravel larger than 1 mm in diameter were found to be 0.03 %, 2.46 %, 7.27 %, 14.61 %, 31.72 %, and 68.47 %, and the maximum particle sizes of gravel were 2.50, 3.50, 5.49, 16.67, 21.30, and 26.13 mm, respectively. The gravel is mainly garnet, amphibole, potassium feldspar, and albite; the former two minerals belong to igneous rocks. Small minerals are mainly quartz, potassium feldspar, albite, and calcite. The rocks are cemented by calcite, with uniform cementation.

An RTR-1500 [Geotechnical Consulting & Test Systems (GCTS), USA], which is a high temperature and high pressure triaxial rock testing system, was used in this experiment. The system, which is controlled by an electro-hydraulic closed-loop digital servo, can provide 1500 kN axial pressure. The rigidity of the loading frame is 10000 kN/mm. The confining pressure is provided by the silicon-oil injected into the chamber and can reach 70 MPa, with a precision of 0.01 MPa. The strain data acquisition was realized using two axial sensors and one radial sensor. Through the LVDT (linear variable differential transformer) operation, the radial sensor uses a non-extendable chain to precisely measure the mean diameter change caused by radial strain, which is more accurate than the point-based strain gauge method because the local deformation caused by gravel varies in different parts of conglomerate.