Engineering geological characteristics of boulders in underground: A case study from the Yu-Wan Section of Beijing Metro Line 16.

The present study investigates the distribution and geological characteristics of cobbles and boulders in the southwestern area in Beijing to provide a basis for the subway engineering design and construction process. An abundance of boulders with large particle sizes in the underground leads to a challenge to the construction of metro tunnels. Based on drilling exploration, the exploration shaft method is used to investigate the distribution of large-size boulders in the soil. The spatial distribution of boulders was determined, and it was found that large-size boulders were mainly distributed at burial depths of 17–19 m, with a maximum particle size of 67 cm, and the total boulder content reached a maximum of 48%. The physical and mechanical properties of the cobbles and boulders were tested in the laboratory. The results show that the size distribution of boulders at the same depth exhibits a large dispersion, and boulder strength is positively correlated with size. Furthermore, the mineral composition analysis indicates that the equivalent quartz content (EQC) is the main factor in determining the geological characteristics of the pebbles. Additionally, it is found that the EQC of the pebbles is strongly correlated with the uniaxial compressive strength and Cerchar abrasivity index value, and larger-size pebbles result in higher EQC values. [ABSTRACT FROM AUTHOR]