During slurry shield tunnelling in interbedded clay-gravel strata, soil-rock agglomerates are readily formed, which increases the risk of deposition and blockage of agglomerates in the slurry chamber and circulation system. A slurry circulation system discharge test apparatus was independently designed to analyse the motion patterns of single agglomerates and agglomerate particle groups. The effects of agglomerate size, quantity and slurry density on the critical velocity in horizontal, inclined and vertical pipelines were investigated, and the transport characteristics of soil-rock agglomerates under different slurry velocities were revealed. The main conclusions are as follows. In horizontal pipes, agglomerates are mainly transported in the form of rolling and saltation. After entering inclined and vertical pipe sections, the critical velocity of agglomerates is higher than that in horizontal pipes, and deposition or stagnation is prone to occur at elbows. The critical velocity of agglomerates is inversely proportional to slurry density and exhibits a trend of first increasing and then decreasing with the increase in agglomerate size. As the number of agglomerates increases, the critical velocity gradually increases. The largest increment occurs in horizontal pipes, while the increments in inclined and vertical pipes are relatively smaller. As slurry velocity increases, the transit time of agglomerates through the pipe section becomes shorter, the fluctuation amplitude of agglomerate velocity increases, and the periodicity of the motion.