Access to tunnel projects is typically achieved through shafts, which can be either temporary or permanent. As tunnel infrastructure becomes more complex and deeper, traditional shaft geometries such as circular or rectangular often fall short in meeting structural and spatial requirements. The caterpillar or peanut-shaped shaft has emerged as an alternative solution, being utilised in recent projects in Brazil, Hong Kong, and the U.K. Despite its growing use, limited information exists in the public domain regarding its structural behaviour and design methodology. This study investigates the structural response and modelling challenges associated with a caterpillar-shaped shaft using both 2D and 3D finite element analysis (FEA) in DIANA software (versions 10.8 and 10.9).
A representative model of a 3-cell caterpillar shaft, 25m in diameter, with a 52m diaphragm wall (d-wall) depth and a 40m excavation depth was analysed. The Y-panels were supported using cross-walls. The model was reduced to a quarter size using symmetry for computational efficiency. Results revealed that the shaft demonstrated high rigidity. This structural stiffness was attributed to the development of hoop forces in the circular d-walls and the presence of cross-walls. However, this rigidity also resulted in minimal soil displacement, which in turn did not trigger the soil arching effect, keeping the surrounding soil in a neutral state.
Comparative analyses were conducted between the 3D model and the 2D modelling approaches - axisymmetric analysis for circular part and plane-strain analysis for Y-panel joining adjacent cells. The axisymmetric model underestimated hoop forces by 15–20% above excavation level and showed significant discrepancies below it when compared with the 3D model. The 2D plane-strain model overestimated deformations and bending moments at the Y-panel junction by 52% (±17%) and 15–25%, respectively. These discrepancies stemmed from the fundamentally different deformation pattern of the caterpillar geometry, which exhibited deformations like an elliptical shaft — contraction along the long axis and extension along the short axis. The study also explored the utility of adding a buttress support at the Y-panel. Introducing a 1m thick buttress reduced bending moments by up to 30%, though further increases in thickness to 2m or 3m yielded diminishing returns.
Overall, this study highlights the structural advantages of caterpillar shafts, particularly their rigidity and reduced need for heavy strut support. However, it also underscores the limitations of 2D modelling in capturing the complex deformation behaviours of such geometries. Accurate analysis and design of caterpillar shafts require comprehensive 3D modelling, with future work needed to develop reliable 2D approximations validated through field data and back-analysis.