The swelling-shrinking mechanism of expansive soils was expounded; and the swelling force concept was analyzed. It was pointed out that the swelling force defined by nomenclature standard GB/T50279-98 is the maximum welling force or limiting swelling force. Then a new concept of physical swelling force was put forward

as its test method. It was the first time that the study was carried out about the variation regulation of physical swelling force changing with moistening degree. And it was performed that the trial research on the rules of swelling force with moistening degree

primary dry density

primary moisture content and so on

as well as the rule of swelling force with identical soil sample after various degree dehumidifying. Accordingly

some valuable conclusions were drawn

which made it more deeply to catch on the inotion of swelling force and more practically to be utilized.