Table 2 Two scenarios for waste avalanche initiation

(1) The infiltration of rainfall decreased the shear strength of the dump, leading to its slide;

(1) Heavy rainfall on the dump site led to increased water percolation through the waste material.

(2) Once in motion, the disturbance of the mass structure allowed large volumes of methane to contact smouldering waste, leading to the explosions.

(2) The percolating water reached the top of the combustion zone deeply inside the waste mass.

(3) The vaporization of the water produced an increase of gas pressure at the base of the dump that probably lifted up a part of it, resulting in a sound-like explosion during the liberation of the accumulated pressured gas.

(3) The material experienced a long runout, because of the intrinsically low internal friction of the saturated waste material, aided by steam production. The ‘still-hot’ material would have then also burnt the victims of the slide.

(1) The vaporization and the lift up processes combined their effects to bring air to the combustion zone. The second and probably the third explosion then occurred.

(2) As the destabilized mass began to slide, the fire propagated to the whole moving mass, helped by the significant amount of CH₄ trapped into the waste material (numerous swirling flames were reported by eyewitnesses and were probably due to the gas trapped in the plastic bags).

(3) The combustion of each plastic bag within the moving mass produced gas that allow an expansion of the whole mass to occur. Once started, this mechanism, as well as water vaporization, triggered a self maintaining expansion during the transport. This self maintaining expansion, responsible of a friction reduction within the waste material, determined the unusually high mobility of the moving mass and can also explain why the whole material was deeply burned.