Calculation of buoyancy plays an important role in the anti-floating design of underground structures. However

the determination of the buoyancy in weakly permeable clay is controversial. An apparatus of model test is designed to avoid the effect of the friction in the determination of the buoyancy of underground structure in soft clay. The weight unloading is adopted in a simple and explicit mechanical model. The buoyancy is equal to the surplus weight at floating moment. And the critical state of floating is determined by the real-time monitoring of pore pressure and displacement. Results of experiment show that the varying pore pressure in soft clay is influenced by seepage

temperature

unloading rebound

and soil water interaction

and the quantity is often less than the theoretical water pressure. Measured buoyancy is 0.93 times theoretical value

which presents a small amount of reduction. The real-time curves of water pressure and displacement indicate that the floating of structure is a continuous and dynamic process till to suddenly occur.