Explanation: When water flows through the soil, it exerts drag forces called seepage forces on the individual grains of the soil. The presence of seepage forces, which act in the direction of flow, will cause a change in the pore water pressure and effective stress in the soil. In upward flow condition effective stress at a depth Z from the top, effective stress = Zγ' - iZγw. Thus, in this case, effective stress is reduced. In upward flow condition, we can have a head difference for which the effective stress becomes zero, i.e. Hγ' - iHγw = 0 ⇒ i = γ'/γw = icr on putting value of γ', icr = (G -1)/(1 + e) icr is called the critical hydraulic gradient When water flows in a soil mass, a seepage force acts on soil particles in the direction of the flow of water. If this flow is in an upward direction, then seepage force will act in an upward direction, due to there is a reduction in effective stress of soil. As the upward flow of water progress continuously in soil, a time comes, where effective stress in the soil becomes zero, this called a quick condition. At the critical hydraulic gradient, a soil losses all its shearing strength and can not support any load. The soil becomes "quick" or 'alive' and boiling occurs. The popular name for this phenomenon is quick condition. It is usually observed in coarse silt or fine sand.