For the problem that the clay balls formed by dredgers during the construction of certain clay formations are likely to cause blockage of transportation pipeline,we study mechanism of clay pilling.By means of the indoor crash tests and restoring forming conditions,we propose two hypotheses that the clay and sand are bonded into the ball and the block clay is worn into the ball.By studying the characteristics of on-site construction conditions，geological conditions and characteristics of clay balls when clay balls are formed to block the pump，it is found that the hypothesis of wearing into the ball from block clay is more in line with the actual situation on site.In order to reduce the probability of clay balls blockage，we propose a series of construction measures to avoid the blocking of clay balls.