Abstract: Lime and cement are widely used in engineering projects to modify the special engineering properties of expansive soil such as water swelling and dehydration shrinkage; but such traditional soil solidification materials are difficult to be well mixed, and also pollute the environment. The physical, mechanical, chemical and micro-structural properties of the typical strong expansive soil of Hanzhong stabilized by calcium bicarbonate from pseudo-karstification are studied by means of free swelling rate test, unloaded expansion rate test, shrinkage test, direct shear test, pH test, carbonic acid content determination test, particle size analysis test and scanning electron microscope test. Test results show that calcium bicarbonate from pseudo-karstification could reduce the swell-shrinking property of expensive soil and improve the soil's shear strength. The optimal mass ratio of calcium bicarbonate solution from pseudo-karstification to expansive soil is 6∶1. The content of exchangeable calcium ions, fine particles and interior pore decreased, while the content of calcium carbonate and coarse particles increased. The mechanism of solidifying expansive soil by calcium bicarbonate from pseudo-karstification can be explained as follows: given a large number of H⁺, Ca^{2 +}, HCO₃^-, CO₃² and excess carbon dioxide dissolved in solution, the Ca²⁺ adsorbed by soil particle are replaced by H⁺ through ion exchange reaction, hence reducing the thickness of electronic double layer. The newly formed calcium carbonate from karstification has effects of filling and reinforcing so that the connection between soil particles is enhanced and the swell-shrinking potential is reduced. In conclusion, calcium bicarbonate from pseudo-karstification has a good effect on reducing the swell-shrinking potential of expansive soil.

Key words: soil modification, expansive soil, karstification, calcium bicarbonate from pseudo-karstification, action mechanism