公路桥梁桩基穿越超大型溶洞的荷载传递机制试验研究

Experimental study on load transfer mechanism of highway bridge pile foundation passing through huge karst cave

为探明公路桥梁桩基穿越超大型溶洞的荷载传递机制,开展回填法施工的25.5 m溶洞的桩基受力现场试验;结合数值仿真方法,研究采用回填法处治溶洞时,穿越不同高度溶洞的桩基竖向承载特性和荷载传递机制,重点探明回填材料固结引起的负摩阻力最大值及范围占比变化规律。结果表明:采用回填法处治溶洞后,荷载较小时,穿越超大型溶洞桩基的轴力在一定范围内增大,桩侧产生负摩阻力;桩基的竖向极限承载力随洞高增加呈减小趋势;洞高由5 m增至30 m时,极限荷载作用下负摩阻力最大值增幅达102.7%,其范围占比由0%增加至24%,洞高超过25 m后,负摩阻力范围占比超过20%。若采用回填法处治溶洞,建议当洞高为5~10 m时,可忽略负摩阻力的影响;为超大型溶洞时,需考虑负摩阻力对桩基竖向承载特性的不利影响;洞高为15,20,25,30 m时,溶洞顶面下0.092 H,0.145 H,0.213 H,0.240 H(洞高)范围内按负摩阻力计算桩基承载力。

The field test of bridge pile foundation passing through the cave 25.5 m in height were carried out to explore the load transfer mechanism of the pile foundation of highway bridge passing through the super-large cave.The vertical bearing characteristics and load transfer mechanism of the pile foundations passing through the caves with different height were studied when the backfilling method was used to treat the karst cave.The maximum value of negative friction caused by consolidation settlement of the backfill materials and the proportion of its distribution range were proposed.The results show that:when the load is small,the axial force of the pile foundation passing through the super-large cave increases in a certain range,and the negative friction resistance of pile foundation is generated after the cave is backfilled.The vertical ultimate bearing capacity of the pile foundation decreases with the increasing height of karst cave.When the height of the cave increases from 5 m to 30 m,the maximum negative friction resistance under ultimate load increases by 102.7%,and the proportion of its range increases from 0%to 24%.When the height of the cave exceeds 25 m,the negative friction range accounts for more than 20%.If the backfill method is used to treat the cave,it was recommended that when the height of the cave was 5 m to 10 m,the influence of negative friction resistance could be ignored.When the cavern is very large,the adverse effect of negative friction resistance on vertical bearing characteristics of pile foundation should be considered.When the cavern height is 15,20,25,30 m,it is recommended to calculate the bearing capacity of the piles according to negative friction resistance within 0.092 H,0.145 H,0.213 H,0.240 H(cavern height)from the cavern top.