<p>A series of tests involving the infiltration of bentonite slurry into the unsaturated sand were performed by using a dedicated specifically developed apparatus. In these tests, the bentonite slurry was pressurized into a sand column which was placed on a digital recording balance, enabling real-time measurement of the infiltrated slurry. Sensors installed at specific positions along the sand column monitored changes in pore water pressures over time. Results show that rising the saturation of the sand column lowered the mud spurt rate to a minimum value at 60% saturation, followed by an increase in mud spurt rate for higher saturations. Analysis of the pore water pressure data reveals that the flow resistance within the slurry infiltrated zone and within the sand varied with saturation, influencing the mud spurt rate. The permeability of the filter cake formed did not change, but the time before the filter cake formation started (transition time) increased with the degree of saturation, reaching its maximum at 100% saturation. Compared to the drilling of a TBM in saturated soil, drilling in unsaturated soil would trigger much earlier filter cake formation, in the absence of or limited excess pore water pressure in front of the TBM and mitigating potential threats to the stability of the tunnel face.</p>