An understanding of the vertical acceleration response of geosynthetic-reinforced slopes subjected to horizontal input ground excitations is essential to robust seismic design. A series of stepwise intensified shaking tests was performed using simple sinusoidal waves with various horizontal peak ground accelerations (HPGA = 0.2-1.0g where g = gravitational acceleration) and wave frequencies (f = 3, 6 and 15 Hz) to investigate vertical acceleration responses at the crest of a reinforced model slope subjected to horizontal input ground excitations. Vertical accelerations at the edge of the slope crest (av1) occurred under relatively small HPGA associated with a negligibly small displacement of the slope. A large vertical acceleration of the unreinforced zone (av2) occurred under relatively large HPGA associated with a large displacement of the slope. Measured horizontal peak acceleration (ahp) against vertical peak acceleration (avp) at the crest of the slope indicate that values of avp under a specific HPGA increase with increasing input wave frequency. At the verge of the ultimate displacement state, values of avp at the crest of the unreinforced zone were consistently greater than those for the crest of the reinforced zone, suggesting that a downward slump of the failure wedge behind the reinforced zone occurred at the ultimate displacement state of the slope. Pseudo-static stability analyses were performed for the tested slopes based on the observed range of vertical-to-horizontal acceleration ratio (o) at the crest of slopes loaded with f = 3 and 6 Hz. It was shown that the influence of o on the critical seismic coefficient (khc) of the slope increases with increasing reinforcement force. Values of khc decreased by 8-30% when o increased from 0 to +1.5, indicating that a vertical acceleration induced by the slump of the soil mass plays a role in destabilising the slope and/or in increasing the plastic displacement of the slope.
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