In Problem 15.5, if the maximum allowable total swell is 25 mm, determine the undercut beneath the foundation that has to be replaced by nonswelling soil.

Problem 15.5

The expansive soil at the top of a soil profile consists of a 4.5 m thick active zone that is subjected to seasonal moisture variations. A shallow foundation is to be placed at 1.5 m depth. Constant volume swell pressure tests were carried out on specimens from different depths of this soil where the swell percentage s_{w (1)} under the overburden and foundation pressures (σ'_o + σ'_s) were measured as follows.

Depth below ground level (m)	Swell percent under overburden + foundation pressure, sw(1) (%)
1.5	5.2
2.5	3.7
3.5	2.1
4.5	0.0

Estimate the total likely swell beneath the foundation.