Q. What do you mean by Laying of apron in design of bridge?

Width of apron laid on river bed should be 1.5 D1 where D₁ is estimated scour depth in metres below river bed. Thickness of apron as laid can be worked out as quantity of apron stone/ width of apron laid and works out to 1.5 T1 where T1 is the thickness of apron after launching. The thickness at the head should be increased to 2.25 T1 and checked up to verify whether the quantity is adequate for fanning out. If not, the thickness should be increased further as required.