The device used for Stormwater Management Systems can also be adopted below buildings either by using foundations on plinths designed to transmit high contact pressures or by digging several dm in the clay and filling with granular fill. This fill, combined with the pressures of the foundations, contains the bulge.
The swelling of expansive soils is a difficult effect to estimate as the extent depends on the mineralogy of the clay, the orientation of the particles, the geostatic confinement pressure and the instantaneous water content in situ at the time of reference.
Estimates of the swelling can be obtained from consolidation tests in which the sample is subjected to a very low confinement pressure (5 ÷ 8 kPa) and is allowed to absorb water and swell. By measuring the volume change under these conditions a free swelling test is obtained.
By trying to prevent the expansion of the sample, it is possible to measure the pressure required to maintain a zero volume change. The data thus obtained can be extrapolated to the expected swelling or to the pressure of the foundation or overload pressure to be applied to eliminate the swelling. Estimates obtained are as accurate as the quality of the sample and the instantaneous value wN if the degree of saturation for the volume change in the laboratory is representative of the long-term in situ value.
This consideration is of considerable importance since the laboratory sample is thin and has access to sufficient quantity of water to reach S = 100% in a short time; this never happens in situ. Correlations, using index properties, can provide useful estimates of the change in volume. An estimate of the overload pressure necessary to contain the expansion within a tolerable amount is obtained from the Komornik & David equation:
The swelling% is often of some importance in determining differential settlements. The Johnson & Snethen equation provides very comparable results with the measured swelling. Both equations are based on statistical analysis and water content must be used in Stormwater Management Systems.
The free swelling obtained by these equations can be related to the confinement pressure σv by adopting an equation obtained from the interpretation of the swelling curves% as a function of the confinement pressure of Cogoli. From the equations, results emerge with an error of the order of ± 50%, not very different from that obtainable with a consolidation test.
Foundations on clays and silts
The consistency of clay and silt deposits can vary from very soft to normally consolidated up to very hard and highly over-consolidated. The most controversial problems are always associated with very soft and fluffy deposits, referring both to bearing capacity and consolidation failures.
