I had never appreciated how clay could offer support but as you say Young's Modulus says that if you push down and squeeze it vertically it will push outwards horizontally .
Even if the overburden is cracked , that doesn't necessarily mean it will subside ; look at bricks in an arch where the mortar is used to keep them apart not glue them together .
The knife edge parallel crip creates a more uniform char-chamber/void than other methods .
I had always thought that the issue of cracking the sealing strata from excessive heat was more an issue of containment of contaminants but maybe I'm mistaken .
The high temperatures are needed to cause the steam to separate into H2 and Oxygen , syngas being a mixture of mainly H2 , CO , CH4 and CO2 .
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