Differential settlement allowed, clay vs sand

More differential settlement is allowed in clay than in sand because the nature of the settlement is different.

• Time: settlement in clay occurs slowly through consolidation, over months or years. The structure can gradually adjust, creep and redistribute stresses, so cracking is reduced. In sand, settlement is rapid and mostly happens during construction, so the structure cannot adjust.
• Pattern: settlement in clay is smooth and bowl-shaped, and the angular distortion between adjacent footings is small even if the total settlement is large. In sand, local variations in density cause an erratic pattern, and the differential settlement may be as high as 75% of the total.
• Magnitude: clay settles by more in total, so the same ratio of differential to total settlement gives larger values. Terzaghi and Peck suggest total settlement limits of about 40 mm in clay against 25 mm in sand.

So the allowable values are higher in clay without a greater risk of damage.