Depth of a Well Foundation (Considerations) | Soil Engineering

The depth of a well foundation is decided based on the following three considerations:

1. Maximum scour depth below the highest flood level (HFL).

2. Minimum grip length below the scour level.

3. Allowable bearing capacity.

The depth of a well foundation is thus equal to the sum of the maximum scour depth and the minimum grip length.

D = d_s + l_g …(21.4)

1. Scour Depth:

The scour depth in a stream should be ascertained by actual soundings at or near the proposed site, during or immediately after a flood, before the scour holes have had time to silt up. The scour depth may also be estimated using Lacey’s silt formula given by Eq. (21.6). As per IRC – 78-2000, the normal depth of scour for regime conditions in a stable channel, for natural streams in alluvial beds, is given by –

where d_s is the scour depth below HFL in m, Q is the design discharge in cumecs (m³/s) per meter width of water­way, f is the silt factor for a representative sample of bed material up to the level of anticipated deepest scour, and d_m is the mean size of the particle in mm.

As per IRC – 78, the scour depth for road bridges should be determined based on a larger discharge over the design discharge, computed as per IRC – 5, as shown in Table 21.2, to provide an adequate margin of safety. For intermediate values of design discharge, linear interpolation may be made.

The mean particle size of a given bed material can be determined from the sieve analysis data using –

where P_ti is the percent weight of the material retained on ith sieve and passing (i – 1)th sieve, d_i–1 is the size of the opening of the (i – 1)th sieve, and d_i is the size of the opening of the ith sieve. Typical values of Lacey’s silt factor are shown in Table 21.3.

Due allowance should be made in scour depth, if the waterway is contracted from regime. The maximum depth of scour should be increased as shown in Table 21.4, depending upon the section.

2. Grip Length:

The depth of the bottom of the well foundation, below the lowest scour level is known as the grip length. Well foun­dations should be provided with adequate grip length such that the required passive resistance of the soil on the rear side of the well is generated to resist lateral loads. The grip length may be taken as one-third of the maximum scour depth. The depth of the well foundation should not be less than 1.33 times the deepest scour depth below HFL. If a non-erodible stratum, such as rock is available at shallow depth, this depth may be reduced.

Further, as per IRC – 45-1972, the foundation should be taken at least 2 m below the maximum scour depth for piers and abut­ments with arches, that is, the minimum grip length in such cases is 2 m. The minimum grip length is 1.2 m for piers and abutments supporting other types of superstructures.

3. Allowable Bearing Capacity:

The depth of a well foundation is also decided based on the condition that the maximum base pressure is less than the allowable bearing pressure of soil or rock at that level. Allowable bearing pressure can be estimated from stand­ard penetration test (SPT) data for wells in cohesionless soils. As per IS – 3955,

q_a = 5.AN²B + 16D(100 + N²) …(21.8)

where q_a is the allowable bearing pressure (kgf/m²), N is the corrected value of SPT N, B is the smaller dimension of the well foundation (m), and D is the depth of the foundation below the scour level (m).

For cohesive soils, the allowable bearing pressure can be determined from the shear strength and consolidation characteristics obtained from undisturbed soil samples tested in the laboratory.