A sand box or sand jack is a device used for removing the
centering of an arch. Each
prop
A prop, formally known as (theatrical) property, is an object used on stage or screen by actors during a performance or screen production. In practical terms, a prop is considered to be anything movable or portable on a stage or a set, distinc ...
is mounted on a sand box. After the plug is removed, the sand pours from the box, causing the centering to move downwards, diminishing the pressure from the arch, and enabling to ultimately remove it.
[Ashok Kumar Jain, B.C. Punmia, ''Building Construction'', Firewall Media, 2005, p. 435.]
References
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Arches and vaults
Sand
Tools