Silt Factor in Bridge Design

Silt factor in bridge design is very important parameter as it is used to determine the scour depth. Scour depth is a function of design discharge and the silt factor.

Interestingly, the formula to determine silt factor comes from Lacey's formula. Lacey investigated the stability conditions of different alluvial channels and came up with Lacey's silt theory for the design of canal. Remember, a canal is a man made structure where bed slope, size of silt and discharge may not vary much. Value of this factor was determined by the experiments and Lacey suggested the empirical formula.

Remember, this was called silt factor as the investigation was done on silt.

We have progressed a lot in engineering design but I have my own doubts on the following;

i. Adoption of silt factor concept for bridge design even though it was for canals given by Lacey.

ii. Changing the concept of determination of silt factor by applying concept of weighted average of size of different type of soils, primarily against the concept of Lacey who considered soil of same grade.

iii. Different terminology and definitions of soils with respect to soil particles as IRC and IS codes.

The size of silt as per IS code is 0.002 to 0.075mm while IRC 078 describes it as coarse silt and its dia as 0.04mm. In IS code, there is no coarse silt.

In IRC code, silt/fine sand is also mentioned having dia as 0.081 to 0.158mm, Medium sand having dia as 0.223 to 0.505mm, coarse sand having dia as 0.725mm, fine bajri and sand having dia as 0.988mm and heavy sand having dia as 1.29 to 2.00mm. As per IS code, fine sand means of dia 0.075-0.425mm, medium sand of dia 0.425-2mm and coarse sand of dia 2-4.75mm. I think, two organizations forming the standards should have same terminology and same defining parameters.

Lacey used silt factor for the design of canals as such we should come up with some other parameter suitable for rivers.

Also, I am of the view that there should be no concept of using weighted average of dia of soil particles for determining scour depth. Suppose, soil investigation is done when sand exists in the bed, will the silt factor change after sand mining? Even, without mining, when scouring of top layer is carried out first, whether value of silt factor should be changed. I feel, we should adopt the silt factor considering the finest soil (of minimum value of the silt factor) for determining the scour depth.

Probably, experts and researchers needs to think over it.

Dr K M Soni

Former ADG, CPWD