Effects of Cavity Size and Number on the Mechanical Behavior of Marl under Uniaxial Compression

Underground cavities significantly alter the load-bearing paths and failure behavior of rock masses. This study experimentally investigates the effects of cavity size and number on the mechanical behavior and fracture evolution of marl under uniaxial compression. Twenty-four rectangular specimens measuring 50 × 50 × 100 mm were prepared, including intact specimens, specimens containing a single circular cavity with diameters of 3, 6, 9, and 12 mm, and specimens containing two to four aligned cavities with a constant diameter of 3 mm. Three replicate specimens were tested under each of the eight conditions. Acoustic emission (AE) and two-dimensional digital image correlation (DIC) were synchronously employed to monitor internal cracking and surface deformation. As the cavity diameter increased from 3 to 12 mm, the mean peak stress decreased from 50.90 to 32.39 MPa. Compared with intact specimens, the maximum reductions in peak stress and elastic modulus were 39.7% and 46.65%, respectively. Increasing the cavity number from one to four reduced the mean peak stress from 50.90 to 32.95 MPa. Cavity-containing specimens also exhibited multiple post-peak stress drops, local stress recovery, and staged residual load-bearing, indicating progressive failure through localized cracking and stress redistribution.