The expansive soil in Luwangfen of Xinxiang in Henan is from the Tertiary marlite weathering

whose main clay mineral is kaolinite

and that owns weak expansive characteristic which is affected largely by the pore structure. In order to study the expansive characteristic of weak expansive soil in testing section

it studies of the expansion with loads

swelling stress and expansion-contraction cycle

and gets the different conclusions from other expansive soil before: (1) when the initial dry density is smaller and overlying loads are larger than expansive stress

the remolded weak expansive soil doesn’t occur expansion

but collapsible phenomenon after immersing water; when the compaction degree is more than 96%

even when the overlying loads are larger than expansive stress

the remolded weak expansive soil doesn’t occur collapsible phenomenon

and the swelling deformation tends to a constant value with the increasing of overlying loads; (2) The swelling stress of the weak expansive soil in testing section first increases then decreases with the increasing of initial water content

and occurs the expansive stress peak; (3) With the expansion-contraction recurring series increasing

the expansion-contraction characteristics of the weak expansive soil decrease rapidly

and the expansion-contraction deformation tends to be stabilized when the cycle progression reaches to 4th. The three expansive characteristics above show fully that the swelling characteristics of the weak expansive soil are related closely to the pore structure.