Abstract: To investigate how varying foundation conditions affect the shear performance of prefabricated utility tunnel structures and their joints, a case study is conducted on an underground utility tunnel project in the Baotou New Urban District. Shear model tests and three-dimensional refined numerical simulations are conducted on socket joints under weakened foundation bed conditions. By analyzing the failure patterns and shear stiffness differences of these joints across four scenarios (CD-1 to CD-4), the influence of weakened foundation bed coefficients on the mechanical performance of prefabricated utility tunnels is uncovered. Key insights are as follows: (1) Lower foundation bed coefficients expand joint failure zones and exacerbate damage, particularly at the chamfered regions of the socket mouths, which are prone to stress concentrations. (2) Weaker foundation beds reduce resistance to segment dislocations, with dislocations increasing by 0.7% in CD-2 (3/9 stiffness loss), 3.4% in CD-3 (5/9 loss), and 13.6% in CD-4 (9/9 stiffness loss) compared to the baseline CD-1 (gravel sand). (3) All joints exhibit a three-stage shear-dislocation behavior, including partial slip, global slip, and progressive failure. The weakened foundation beds lower the foundation counterforce at equal dislocation levels, reducing the overall shear stiffness. (4) Weakened foundation bed coefficients boost contact pressure and area between socket interfaces, peaking at chamfered regions. This drives convergent deformation: the roof/floor slabs bend inward under tension, while the sidewalls bulge outward across all tunnel compartments.

Key words: prefabricated utility tunnel, joints, weakened foundation bed, model test, failure characteristics, shear stiffness