Geotechnical Engineering

DHARM

780 GEOTECHNICAL ENGINEERING

These values are set out in Table 19.2:

Table 19.2 Values of Lacey’s silt factor (IS:3955-1967)

S.No. Type of bed soil Size of particles mm Lacey’s silt factor f

1. Coarse Silt 0.04 0.35


2. Fine Sand 0.08 – 0.15 0.50 – 0.68


3. Medium Sand 0.30 – 0.50 0.96 – 1.24


4. Coarse Sand 0.7 1.47
   1.0 1.76
   2.0 2.49


5. Gravel 5 3.89
   10 5.56
   20 7.88


6. Boulders 50 12.30
   75 15.20
   90 24.30

Values of maximum scour depth as recommended by IRC (1966)* and IS:3955-1967 are

given in Table 19.3:

Table 19.3 Maximum scour depth–[IRC (1966) and IS:3955-1967]

S.No. River Section Maximum Scour Depth

1. Straight reach 1.27 d′


2. Moderate Bend 1.50 d′


3. Severe Bend 1.75 d′


4. Right-angled bend or at nose of Pier 2.00 d′


5. Upstream nose of guide banks 2.75 d′


6. Severe Swirls (IS:3955-1967 only) 2.50 d′

*IRC:6-1966 “Standard specifications and code of Practice for Road Bridges”, Sec. II-Loads and
Stresses, IRC, 1966.

The grip length for wells of railway bridges is taken as 50% of maximum scour depth,
generally, while for road bridges 30% of maximum scour depth is considered adequate. The
base of the well is usually taken to a depth of 2.67 d′ below the HFL.
According to IS:3955-1967, the depth should not be less than 1.33 times the maximum
scour depth. The depth of the base of the well below the scour level is kept not less than 2 m for
piers and abutments with arches, and 1.2 m for piers and abutments supporting other types of
structures.