What is Secondary Settlement of Spread Foundation?
The secondary settlement takes place under constant effective stress. When consolidation is completed, the adsorbed water is drained out and plastic readjustments of soil particles occur. Consequently the soil mass is settled down. Secondary  settlement is also negligible as compared to consolidation settlement.
Settlement of Footing in Clay
The total settlement is given by:
Where s is the total settlement, si is the immediate settlement, sc is the consolidation settlement and ss is the secondary settlement.
The secondary settlement is given by:
![s_s = H_f C_s log [\dfrac{t_1 + \Delta t}{t_1}]](https://s0.wp.com/latex.php?latex=s_s+%3D+H_f+C_s+log+%5B%5Cdfrac%7Bt_1+%2B+%5CDelta+t%7D%7Bt_1%7D%5D&bg=ffffff&fg=000&s=0&c=20201002)
  ———- (1)
Where, ss is the secondary settlement in time 
after completion of consolidation settlement and t1 is the time of completion of consolidation settlement. Hf is the thickness of compressible stratum. Cs is the slope of secondary branch of the time compression curve.