Ground settlement occurs when pipe-jacking machine enters mixed ground with soils and boulders?

The rate of cutting through soils is faster than cutting through boulders for pipe-jacking machine. When pipe-jacking machine enters a region of mixed ground with boulders andsoils, machine has the tendency to move in the direction of soft soils due to the difference of rate of advancement of pipe-jacking machine for boulders andsoils. Therefore migration of soft soils take place which contributes to ground settlement. Degree of settlement is dependent of the depth of soil cover, soil property and size of boulders.