Experimental Investigation of h-Type Supporting System for Excavation beneath Existing Underground Space

A double-row pile support system combined with existing and additional support piles offers an effective solution for further excavation beneath existing underground space. A large-scale test chamber was therefore built to simulate the whole construction process of underground space extension. Several parallel tests are conducted through observation, data monitoring, and analysis to study the influence of several parameters on an h-type support system containing double-row piles. The relevant parameters include pile row spacing, pile length ratio, pile-head constraint, and in-service foundation pile. The tests reveal that a significant load-transfer effect is generated between the pile rows, and increasing the spacing between pile rows within a certain range can lead to a more reasonable distribution of bending moments and pile force. The displacement of the pile top and its rate of increase are directly proportional to excavation depth, and additional excavation to the bottom of the back-row piles tends to be a critical point, after which the deformation will be significant. The stability of the system varies inversely with the reduction in pile length ratio, but is positively related to the existing pile-head constraint. Furthermore, in-service foundation piles can result in increased bending moments and reduced displacement of the pile top. Finally, the rationality of the model test results was verified according to the numerical simulation and the stability of the double-row piles support system was calculated.