# Investigating the Physical Mechanisms of Quicksand Using a Custom-Designed Experimental Apparatus
## 3 Result
### Quicksand Mechanism Analysis  
The physical state transition of sandy soils during quicksand formation, specifically, the gradual transformation from a solid to a fluidized state, remains insufficiently characterized. Quicksand behavior is governed by the combined effects of particle gradation and hydrodynamic velocity, with each factor contributing distinct mechanistic influences. Through repeated hydrodynamic seepage experiments grounded in hydrodynamic seepage theory and the effective stress principle, this study analyzes pressure variations recorded by micro-sensors at different elevations under varying flow velocities. Consistent pressure response patterns were observed across all specimen groups. Quicksand initiation in quartz sand specimens exhibits distinct patterns based on median particle diameter (d50) as a gradation indicator. At the top layer, the specimen with d50 = 2.010 mm (1#) shows fluctuating pressure values over time (Figure 3(a)), primarily due to particle spatial reorganization induced by hydrodynamic forces acting on particles of varying sizes. These fluctuations stabilize after prolonged constant flow, as particle reorganization ceases and a steady seepage network forms, resulting in stable pressure readings. The specimen with d50 = 1.590 mm (2#) experiences quicksand initiation at 3000 s, lasting approximately 1000 s before stabilizing (Figure 3(b)). This transient behavior is attributed to ongoing particle reorganization, which eventually establishes a final stable seepage network. For the specimen with d50 = 1.150 mm (3#), quicksand initiates more rapidly under lower flow velocities (Figure 3(c)). The increased proportion of fine particles reduces quicksand resistance, yet a stable seepage network ultimately forms. The specimen with d50 = 0.710 mm (4#) also exhibits rapid quicksand initiation under low flow rates  
Figure 3: Infiltration characteristics of quartz sand with different particle gradation. (Figure 4d). However, its higher fines content results in only a metastable seepage network, which fails completely under elevated flow velocities. The specimen with d\({}_{50}\) = 0.430 mm (5#) forms a briefly metastable network (Figure 4e), with early-stage quicksand failure due to its limited stability. Experimental observations demonstrate that particle gradation governs the susceptibility of quartz sand specimens to quicksand initiation, while hydrodynamic velocity determines the timing and rate of quicksand occurrence. Due to the occurrence of complete quicksand failure at the top layer, the pressure variation trends during specimen loading and permeation were further analyzed, as shown in Figure 4f. The maximum pressure values consistently occur during the loading and initial saturation phases, independent of gradation variations, thereby validating the accuracy of the specimen preparation protocol described in Section 2.3. The average pressure values decrease progressively with reductions in median particle diameter (d\({}_{50}\)), indirectly indicating a critical average pressure of 1.77 kPa for complete quicksand initiation in quartz sand. The minimum pressure values remain near 0 kPa, confirming the proper functioning of the pressure sensors and further verifying the reliability of the newly developed experimental apparatus. Further investigation of quicksand evolution at the middle layer reveals behavioral patterns. The specimen with d\({}_{50}\) = 2.010 mm (1#) exhibits only particle spatial reorganization during permeation, forming a steady seepage network without quicksand initiation (Figure 5a). In contrast, the specimen with d\({}_{50}\) = 1.590 mm (2#) experiences quicksand at 1500 s (Figure 5b), occurring 300 s earlier than at the top layer due to temporal lag effects caused by vertical position differences, followed by stabilization into a consolidated seepage network. Similar lag behavior is observed in the d\({}_{50}\) = 1.150 mm specimen (3#, Figure 5c).