Performance of double-row piles retaining excavation beneath existing underground space

Abstract

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.

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

“…Therefore, cooperation of the double-row piles support system can be reasonably applied through the transfer path of force of pile-row-pile in the test, such that the double-row pile support system can continue to work until the additional excavation reaches 80 cm after the failure of the back-row piles, rather than the overall failure of the system when the additional excavation reaches just 20 cm, as in group T8. These characteristics are quite consistent with the phenomenon of the double-row pile test carried out by Yu et al, and verify the reliability of the conclusions of this test [44].…”

“…In addition, 'h' in Table 2 means the height difference of pile top in the adjacent row. The process of making model piles is taken from Yu et al [44]. First, the PVC piles were split as a whole.…”

Numerical Simulation of Double-Row Retaining System with Different Piers

“…The bending moment directions are precisely opposite between the upper and lower sections. The bending moment borne by retaining walls 4 ⃝ and 5 ⃝ in absolute value is greater than the sum of bending moments borne by the corresponding three piles. Additionally, compared to the piles near the pier, the bending moment of the piles closer to the center of model gradually increases in absolute value.…”

“…For the retaining structure containing T-shaped piers, the overall trend of bending moment variation follows this: with an increase in depth, the piles and walls initially experience a negative bending moment, followed by a positive bending moment, and finally For the retaining structure containing T-shaped piers, as shown in the horizontal displacement diagrams of the piles and walls, the horizontal displacements at different selected pile tops are all near 14.7 cm, while those at different selected wall tops are around 14 cm, and the horizontal displacements at the bottom of the piles and walls are close to zero. However, it can be observed from the figures that, compared to retaining walls 1 ⃝, 2 ⃝, and 3 ⃝, the horizontal displacement curves of retaining walls 4 ⃝ and 5 ⃝ exhibit rebound at the top, with the maximum horizontal displacement occurring not at the top but near the top position. Due to the connection of the cover plate, the interconnection of the piles and walls at the top reduces the top displacement, and the difference in horizontal displacement at the top of both is determined by the tensile deformation caused by the stretching stiffness of the cover plate.…”

“…When subjected to loading from soil mass, the structure can generate a force couple acting along the opposite direction to resist the active soil pressure, thereby significantly reducing the displacement and deformation of the double-row retaining structure as compared with a single-row support configuration [4]. In some projects where the use of anchor rods is restricted by the environment or policies, and the layout of internal supports is limited by spans, spatial combined double-row retaining structures have received attention from design and construction units and have good application precedents [5][6][7].…”