Instability of Back-Rotated Piles with Near Singularity Stiffness

Abstract

The two-layer model previously established by the author is expanded to well capture the response of back-rotated and shape-distorted piles set with P−Δo effect (P is the vertical load on piles and Δo is the initial lateral displacement) and subjected to lateral spreading. It is underpinned by sets of parameters of modulus (ks), modulus ratio (m), rotational stiffness (kθ), and limiting force per unit length (pb), and calibrated using back-rotating four-pile groups (1g model tests). The study shows that the imposed PΔo values reduce the normalized stiffness k¯θ to −(0.14–2.54) or (0.114–0.494). This k¯θ together with pb at m=1.5–8 incur the diverse response of (1) stable I2 and I5 piles for sufficient forward-rotating capacity; (2) stable I9 piles for a high k¯θ; (3) distorted I14 piles at stiffness singularity (SS); (4) initially stable through to later distorted (at SS) I4 piles due to pb reduction; and (5) hinged I6 piles despite the largest PΔo value and back-rotation. The flexible two-layer model should be adopted to design restrained piles subjected to lateral spreading.