The laboratory evidence of phase transformation from landslide to debris flow

The dominant factors initiating the transformation of landslides into catastrophic debris flow are examined. The research found that a threshold pore pressure determined from theoretical and experimental analysis was enough to initiate liquefaction type of failure in sandy materials. Loading tests to failure on sourcearea sandy soils from a catastrophic landslide location show that under definite conditions of loading, a threshold state, characterized by the equality and constancy of pore pressure and shear resistance develops in the sands at a threshold density. Sands at this density clearly define the boundary between contractive and dilative specimens under same effective normal stress. Confirmatory experiments were then conducted using silica sand. Analyses showed that samples in which the threshold pore pressure was exceeded readily liquefied while those in which the pore pressure was below the limit dilated. The concept of threshold pore pressure fills the gap created by classical theories of soils liquefaction that have identified densities at which phase transformation and steady state lines can be defined. The new concept shows there is a density at which both lines merge and it is proposed that sands transiting from dense to loose and vice versa will first pass through the threshold state. While the stability of a slope founded on sandy soils may be breached when the pore pressure exceeds a certain limit, it is possible to make estimates of the limit. Where such estimates are accompanied with adequate field measurements, the effectiveness of landslide prevention projects may be enhanced.