As the concentration of particles in a suspension is increased, a point is reached where particles are so close together that they no longer settle independently of one another and the velocity fields of the fluid displaced by adjacent particles, overlap.  There is also a net upward flow of liquid displaced by the settling particles.  This results in a reduced particle-settling velocity and the effect is known as hindered settling.
There is a common case for hindered settling occurs. the whole suspension tends to settle as a ‘blanket’ due to its extremely high particle concentration. This is known as zone settling, because it is easy to make a distinction between several different zones which separated by concentration discontinuities.  Fig. 3 represents a typical batch-settling column tests on a suspension exhibiting zone-settling characteristics. There is a clear interface near the top of the column would be formed to separating the settling sludge mass from the clarified supernatant as long as leaving such a suspension to stand in a settling column. As the suspension settles, this interface will move down at the same speed.  At the same time, there is an interface near the bottom between that settled suspension and the suspended blanket.  After settling of suspension is complete, the bottom interface would move upwards and meet the top interface which moves downwards.