Foundation Depth and Arrangement:
There are two types of foundations, viz,
(a) Shallow foundations, also known as open foundations, and
(b) Deep foundations.
Slab culverts and small bridges are generally provided with shallow foundations. Even for large bridges, if good rocky strata are met with at shallow depths, open foundations are adequate. If the strata are erodible, open foundations can be protected by providing impermeable flooring protected by cut-off walls (Figure 3).
Figure: General Arrangement of Floor Protection Works
Deep foundations are generally made of wells for rivers. For land structures, pile foundations are adopted. For erodible beds (such as alluvial streams), the foundations should be taken to a depth below the maximum high flood level equal to 1.33 times the maximum scour depth. The scour depth is calculated from the Lacey's formula:
D = 0.473 Q ^{1/3}/f^{1/ 3}
where D = Scour depth in m,
Q = Discharge in cu m per sec, and
f = silt factor, equal to 1.76 √m, where m is the mean diameter of the particles in mm.