Clogging occurs during mechanical tunneling with a Tunnel Boring Machine (TBM) because of adhesion of clayey soils to the cutter head and conveyor system. The present study examined the effects of water and sand contents on clogging in Montmorillonite clayey soil. Testing was carried out using an adhesion test device on 28 samples with different water and sand contents to determine adhesion stress and degree of clogging. The results indicate that the consistency index (Ic) of the samples decreases as the sand and water content increases. The results for variation of adhesion stress versus water content at different sand contents formed similar bell- shaped curves. In all graphs, an increase in sand content decreased adhesion stress. Adhesion stress increased until the water content increased to 138%, at which point it began to decrease. The results show that adhesion of the soil to the surface of the metal piston did not occur in samples having a sand content of >40% and in samples with >133% water content having a sand content of <40%, Ic>0.5 adhesion occurred.

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