Support of 26 m high sand backfill using a rock pillar and structural props

Excavation of a deep underground station required support of high and narrow sand backfill that was placed by others during earlier construction of an adjacent 26m deep basement. The sand backfill was supported by a complex retaining system including a rock pillar and two levels of props. This paper describes design of the support system, monitoring results, retaining system operation and dismantling of the props.

The design employed a number of design methodologies developed from first principles including application of three methods for calculation of earth pressure from narrow sand backfill, using beam analogy for preliminary calculations of stresses in the rock pillar and of prop force, design criteria for assessment of stresses in the rock pillar and analyses of potential influence of temperature changes on prop forces and the rock pillar stresses. Construction and operation of the retaining system included systems for movement monitoring and prop force monitoring. The paper describes how monitoring results confirmed adequacy of design methodologies and enabled flexible operation of the retaining system when needed, such as more frequent adjustments of prop forces which was required due to higher than assumed degree of prop restraint. Dismantling of the propping system included its modification to suit construction methodology of the Station while monitoring results during that stage further indicated agreement with design assumptions.