# Investigating the Physical Mechanisms of Quicksand Using a Custom-Designed Experimental Apparatus
## 1 Introduction  
Quicksand, as a widespread and hazardous geological phenomenon, significantly impacts foundational structures, superstructures, and infrastructure such as bridges and dams [1,2]. The formation mechanism of quicksand is rooted in hydrodynamic seepage theory [3] and the effective stress principle [4]. This phenomenon occurs when pore water pressure in saturated sandy soils progressively increases under multifactorial triggering conditions. Once the pore water pressure equals or exceeds the overburden pressure exerted by the overlying soil mass, the sand particles become suspended within the pore fluid, resulting in a fluid-like behavioral state. Notably, naturally dry sandy soils cannot exhibit quicksand behavior [5], as its manifestation is governed by the combined physical properties of both granular matrices and interstitial water. Among the numerous contributing factors to quicksand formation in saturated sandy deposits, seismic loading stands as the primary trigger [6]. Since the 1960s, major global earthquakes have frequently induced liquefaction failures and quicksand events [7]. Under seismic excitation, saturated sand particles undergo continuous rearrangement, transitioning into a metastable fluidized state analogous to viscous liquids [8]. [PERSON] et al. [9] demonstrated that seismic loading induces microfabric reorganization in sandy soils, leading to the loss of interparticle contacts and creating preferential pathways for pore water drainage. [PERSON] et al. [10] identified five causal mechanisms for seismic subsidence in saturated sandy soils: seepage drainage, particle deposition, post-earthquake consolidation, partial bearing capacity loss, and soil-structure interaction. Permeability characteristics of sandy soils constitute another critical factor influencing quicksand formation. [PERSON] et al. [11] systematically investigated the impact of fine-grained constituents (clay and silt particles) on quicksand resistance. Their experimental results demonstrate that the anti-quicksand capacity of sandy soils exhibits a non-monotonic variation trend with increasing fines content, a phenomenon primarily governed by the proportional ratio between clay and silt particles [12,13]. Furthermore, particle size and gradation characteristics serve as additional contributing factors to quicksand susceptibility. [PERSON] [14] conducted a series of studies on this phenomenon, revealing that fine particles and their content promote quicksand initiation in sandy soils, while coarse particles and their content inhibit quicksand behavior. In summary, the primary factors inducing quicksand phenomena in sandy soils include hydrological conditions, seismic activity, permeability characteristics, material composition, and particle size gradation. Quicksand formation results from the synergistic effects of multiple contributing factors. While existing studies, both domestic and international, have extensively addressed the static effects of these factors on quicksand phenomena, the dynamic transition from stable seepage to quicksand failure under hydrodynamic conditions remains inadequately understood. Traditional permeameters, such as the classical constant-head and falling-head devices [15,16], are predominantly designed based on [PERSON]'s law (\(Q=k\cdot i\cdot A\)), under the assumption of a static and uniform soil structure. Accordingly, their application has largely been limited to steady-state conditions, focusing on basic permeability parameter measurements [17]. However, these devices often fall short in simulating complex seepage processes encountered in real-world scenarios. Specifically, they lack the capability to accurately control variable hydraulic gradients, making it difficult to perform staged dynamic seepage tests [18]. Furthermore, most are not equipped with real-time, high-precision monitoring systems for pore pressure and seepage flow, resulting in an inability to capture the evolution of hydraulic responses within different soil layers.