THE DEPTH OF THE FOUNDATION IS DETERMINED BY CONSIDERING THE SAFE BEARING CAPACITY OF SOIL AND THE EFFECT OF THE SCOUR.

DEPTH OF THE BRIDGE FOUNDATIONS:-- The depth of foundation is determined by considerations of the safe bearing capacity of the soil after taking into account the effect of scour.The foundations should be taken below the scour level and to the level at which there is minimum variation in the moisture content of the sub Soil .In erodible beds anticipated maximum depth of scour below the designed HFL including that on account of possible concentration of flow ,the minimum depth of the foundation below the HFL should be taken 1.33 D (at site) subject to a minimum depth below the scour line of 2 metres for the arched bridges (for both abutment and Piers) and 1.2metres for other types unless directly resting on rock where rock is met with at a lesser depth.In such cases the foundations shall be securely anchored for about 0.3 metres into the rock and about 0.6 metres where there is other material and if the rock is sloping, the masonry should be anchored to the rock by means of the dowel bars for about 0.30 metres .For small bridges of moderate height on dry land sufficiently solid ,a depth of 1.5 to 2 metres below the bed level in non erodible streams will be found sufficient.Depth of foundations on lands which are not exposed to the erosive action of stream currents may be taken down to a depth on a firm bed as calculated from the Rankine's formula "Depth of Foundations"