The stability problem of the bored pile hole wall in saturated soft clay is taken as the unloading shrinkage problem of a cylinder in the semi-infinite space. According to the SMP criterion-based Cam-clay model

the elastoplastic solutions of the cylindrical unload shrinkage problem are derived using the stress transformed method

and the analytic expressions for the critical plastic pressure and the hole-wall shrinkage are obtained. On this basis

the methods are applied to calculate the minimum slurry density and the borehole safety factor

and the factors influencing borehole stability are analyzed in details. The results show that

after the bored pile hole-wall yield

the soil element around the wall is in the elastoplastic state

with the hole-wall shrinkage increasing and the borehole safety factor decreasing. Under the mud supporting condition

the stability of the bored pile hole-wall is dependent on the soil density

over consolidation ratio

internal friction angle and the slurry density

etc.

and independent of the hole diameter and the hole depth. The larger the over consolidation ratio and the friction angle

the smaller the critical support pressure and the minimum slurry density for maintaining bored pile hole-wall stable. In practice

the slurry density must be properly chosen so that the minimum slurry density is satisfied.