The Seismic Response of Ultra-Deep Strata with a Weak Interlayer and Targeted Reinforcement Using VRJPs: A Large-Scale Shaking Table Study

Abstract

Deep saturated layered foundations are common in coastal and reclaimed areas, where weak interlayers can significantly influence seismic response and settlement. To investigate the seismic behavior of ultra-deep strata and the effectiveness of targeted reinforcement, a large-scale shaking table test was conducted at a geometric similitude ratio of 1:200, representing a 640 m thick prototype. The shallow soil was densified, while a deep silty clay weak interlayer was selectively reinforced using vertical rotary jetting piles (VRJPs). Horizontal and vertical accelerations, Fourier spectra, excess pore water pressure ratio, and residual settlement were evaluated under 0.1 g, 0.2 g, and 0.4 g excitations. Under 0.4 g excitation, horizontal acceleration generally attenuated in the middle and lower strata, whereas vertical acceleration showed pronounced shallow amplification, reaching 0.87 g in the untreated zone. VRJP reinforcement reduced pore pressure buildup near the weak interlayer and decreased residual settlement from 1.12–1.20 mm to 0.94–1.02 mm. However, acceleration and pore pressure responses were redistributed in some overlying strata. The results support a hierarchical strategy combining shallow liquefaction mitigation with targeted deep deformation control for coastal and marine foundations.