降雨入渗对故宫城墙稳定性的影响

Influence of rainfall infiltration on stability of Forbidden City wall

故宫城墙是由内芯土和外包砖砌体组成,是具有代表性的中国古城墙。降雨入渗是影响城墙稳定性,导致城墙外包砌体开裂和失稳坍塌的直接原因。为研究降雨入渗对城墙稳定性的影响,以故宫城墙为研究对象,采用ABAQUS软件建立其非线性有限元分析模型;通过考虑墙体材料的渗透系数、吸湿曲线和初始孔隙率,模拟8种不同降雨历时和降雨强度工况,对城墙外包砌体的位移、应力和应变沿路径的变化曲线进行对比分析;利用位移量判断滑动面的位置和趋势,采用强度折减法得到城墙的最小安全系数,从而分析城墙的力学性能和稳定性,为故宫城墙的安全评估和保护修缮提供依据。分析结果表明:降雨作用下,故宫城墙外包砌体两侧外边缘沿高度中部4~8 m处、两侧底部0~1 m处和顶部沿轴向左右偏离1.5~2.5 m处的塑性应变和位移值较大,属于危险区域,应予以加固;降雨强度相同时,城墙水平位移随降雨时间的增长而增大;降雨历时相同时,城墙水平位移随降雨强度的增大而增大;相同降雨量下,短时强降雨容易因为排水不畅而导致内芯土吸水膨胀变形增大,从而造成城墙失稳破坏,城墙的安全系数更低。

The Forbidden City wall is composed of inner core soil and outer brick masonry,which is a representative ancient city wall in China. Rain infiltration is the direct reason affecting the stability of the city wall,which causes the instability and collapse of outer masonry after cracking. In order to study the influence of rainfall infiltration on the stability of city walls,the Forbidden City wall was selected as the research object,and the nonlinear finite element analysis model was established with the software ABAQUS. By considering the permeability coefficient,hygroscopic curve and initial porosity of the wall material,eight different conditions of rainfall duration and intensity were simulated. The curves of displacement,stress and strain along the path of outer masonry were compared and analyzed.The incremental displacement was used to judge the position and trend of sliding surface,and the minimum safety factor of the city wall was obtained by strength reduction method. Therefore,the mechanical property and stability of the city wall can be analyzed more comprehensively,which provides a basis for the safety assessment,protection and repair of the Forbidden City wall. The analysis results show that,under the action of rainfall,the plastic strain and displacement values of the outer masonry of the Forbidden City wall are relatively large at 4-8 m in the middle of both sides along the height,0-1 m at the bottom of both sides and 1. 5-2. 5 m in the axial left and right deviation from the top,which belong to dangerous areas and should be strengthened. With the same rainfall intensity,the horizontal displacement of the wall increases with the increase of rainfall time. With the same rainfall duration,the horizontal displacement of the wall increases with the increase of rainfall intensity. Under the same rainfall,the short-term heavy rainfall is easy to cause the swelling deformation of inner core soil due to poor drainage,which leads to the instability and damage of the wall,and the safety factor of the wall is lower.