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Further excavation beneath constructed underground space offers a material-saving solution to supplement vertically utilisable space in developed urban regions. Under such circumstances, the existing and newly added retaining piles are combined to form a double-row retaining system with shorter piles facing excavation. A large-scale test chamber was established to simulate the working performance of this particular retaining system during supplementary excavation. A series of tests were grouped and compared with respect to a few key arguments on pile behaviour. Involved parameters included the pile embedment, the row spacing and the excavation depth. The studies reveal that considerable bending moment is generated in the existing piles during further excavation; and its magnitude varies inversely with the embedment of the new piles but positively with the row spacing. Presence of the existing piles gives an advantage to the new piles; thus, their absence results in an abrupt increase in bending moment and a dangerous situation of the new piles. Furthermore, the plane position of the individual retaining pile puts a significant effect on the pile's bending moment. The presented research and related implications are expected to support rational design of retaining structures towards further excavation for underground space extension.

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