Soil liquefaction is a major cause of foundation settlement and structural damage during earthquakes. Currently, there have been numerous studies on the use of gravel piles and overlying gravel layers separately for site liquefaction treatment, but relatively little research has been conducted on the mechanism and effect of combining the two to improve the liquefaction resistance of foundation soil layers. This study combines physical model experiments with FLAC3D numerical simulations to investigate the anti-liquefaction effects of using gravel piles in combination with overlying gravel layers in sandy soils.

For the physical model testing part, different combinations of physical models involving overlying gravel layers and gravel piles were constructed in a model box. Using a small shaking table, the liquefaction process of the soil layer under seismic action was simulated, and the excess pore water pressure under different working conditions was measured.

In the numerical simulation part, based on the corresponding physical experimental models, the responses of gravel piles, overlying gravel layers of different thicknesses, and the combination of gravel piles with overlying gravel layers of different thicknesses under earthquake action were simulated. Emphasis was placed on analyzing their impact on the excess pore water pressure ratio of the soil layer and the changes in the final surface settlement.

The results show that the overlying gravel layer can effectively increase the effective stress of the liquefied sand, significantly reduce the peak value of the excess pore water pressure ratio, and thus is unfavorable to sand liquefaction. As the thickness of the overlying gravel layer increases, the anti-sand liquefaction effect in shallow areas becomes more pronounced.

Gravel piles provide a rapid drainage path in the liquefied soil layer, reducing the accumulation of excess pore water pressure. The closer to the pile, the better the drainage effect. Their influence range shows a linear growth trend with the increase of burial depth. The cloud plot of the excess pore water pressure ratio in the numerical simulation also exhibits a trapezoidal distribution characteristic, being narrow at the top and wide at the bottom.

The overlying gravel layer significantly reduces the excess pore water pressure ratio in the shallow layer of the foundation, while gravel piles mainly function in the deep layer. The combination of the two can improve the overall liquefaction resistance of the foundation.

The significant difference in modulus between gravel piles and the surrounding soil can lead to uneven surface settlement of the site, especially in the initial stage of gravel pile application. Increasing the overlying gravel layer can effectively improve the overall compressive strength of the engineering site and reduce the uneven surface settlement caused by the modulus difference between gravel piles and the surrounding soil.

The installation of overlying gravel layers and gravel piles at the construction site can significantly reduce the pore water pressure of the soil layer and increase the effective stress of the soil layer, thereby reducing the excess pore water pressure ratio and effectively preventing liquefaction. Especially in the prevention and control of liquefaction in shallow and deep soil layers, the combination of overlying gravel layers and gravel piles has shown a good synergistic effect. Reasonable design of the thickness of the overlying gravel layer and the spacing of gravel piles can help improve the liquefaction resistance of the site and reduce the risk of earthquake disasters.

Although gravel piles can effectively enhance liquefaction resistance, the uneven settlement they cause needs to be fully considered in the design. By appropriately increasing the thickness of the overlying gravel layer, this impact can be effectively avoided.

Overall, this paper systematically studied the anti-liquefaction effect and interaction mechanism of overlying gravel layers and gravel piles on liquefied soil layers through a combination of physical model experiments and numerical simulations. A reasonable design optimization scheme was proposed, providing valuable references for the liquefaction treatment of actual engineering sites.