Study of parameters that controls competing action between segregation and disaggregation on an analogue jigsaw-fit block inside a dense granular flow

Abstract

Jigsaw-fit blocks are highly fractured boulders of up to tens of meters that stand out in volcanic debris avalanches. Despite the avalanche long runout and agitated motion, jigsawfit blocks are found on the avalanche deposit with no apparent disaggregation or with no relative displacement between fragments. The mechanisms behind the fragments frustrated disaggregation remain unclear and are limited to field observations. The rheology of granular flows down inclined planes suggests that segregation mechanisms prevail and high fragmentation rates are directly associated to internal shearing and intense inter-granular collisions, challenging the theorized kinematics associated to jigsaw-fit blocks. This work aims for better understanding the jigsaw-fit blocks motion inside a dense granular flow, and to study the parameters that controls segregation and disaggregation processes of jigsaw-fit blocks. With this in mind, an experimental model was designed to test analogue jigsaw-fit blocks called intruders. The chosen setup correspond to a planar Couette cell, that can apply a defined shear strain over a granular material at a desired shear rate, and it is instrumented to describe the granular flow kinematics. The intruders were laser cut from thin plates of two different materials, in solid disk of different diameters, and fractured disk with special fracture patterns. The main discovery is that disaggregation takes place regardless of the intruder fragmentation pattern. However, disaggregation start is conditioned by the fragment density and complexity of cracks configuration of the analogue jigsaw-fit blocks. Besides, in the competition between segregation and disaggregation process, it was clear that the disaggregation process inhibits segregation process in fractured intruders. This results led to a simplified level of disaggregation model, that may be applicable to understand jigsaw-fit blocks found in the field. Solid intruders results show that segregation occurs for all solid intruders that exhibits a size difference with the surrounding granular material. Segregation direction depends on the size ratio, and how fast this process develops depends on the shear rate. We predict that this work sets a starting point for reviewing the interpretation of jigsawfit blocks in avalanche deposits, allowing the inference of kinematic features from the fragments configuration and its level of disaggregation.