高速公路拓宽工程沉降控制复合地基优化设计

Optimum design of composite foundation according to settlement control for extension project of expressway

按沉降控制设计理论,结合有关公路设计规范、路面功能性和结构性的要求以及有限元法分析路基差异沉降引起路面附加应力的计算结果,提出了采用工后路基路拱横坡度变化小于等于0.45%作为软土地基上高速公路拓宽工程的容许新老路基差异沉降控制标准。采用有限元法对佛山—开平高速公路拓宽工程标准断面进行分析,采用不同参数计算出水泥土搅拌桩复合地基的新老路基差异沉降。根据沉降控制标准,优化出既满足新老路基差异沉降控制要求,又效益最优的高速公路拓宽工程软土路基水泥土搅拌桩处理的设计参数,即工程水泥土搅拌桩采用面积置换率为8.9%(对应桩间距1.6m),桩长8m,桩径为0.5m,水泥掺入比为15%;实际使用时,考虑桩土存在一定的不协调,可适当提高桩体面积置换率达10.1%(对应桩间距1.5m)。

Finite element method was used to calculate the additional tensile stress of different layers of the pavement and of steady broken stone-cement layer, which are both caused by the differential settlement between new embankment and old embankment for expressway＇s extension project. Based on the results obtained by finite element method, and the requirement of differential settlement of subgrade in code and that of the function and structure of pavement, this paper put forward a control standard of variation range of cross-slope gradient of road bed＇s crown after completion, less than or equal to 0.45 %, as the allowable differential settlement for expressway＇s extension project on soft soil foundation according to design theory of control settlement. Finite element method was also used to analyze some cross-sections of Foshan-kaiping expressway extension project in China. The extension part of subgrade in this project would be treated by composite foundation of deep mixing pile. Through analyzing composite foundation of deep mixing pile with different construction parameters by FEM, the differential settlement of subgrade were respectively obtained. Combining with the control standard of differential settlement drawn in this paper and the design theory on control settlement, the optimum design parameters of deep mixing piles were obtained, i.e. pile spacing equal to 1.6m, length of pile equal to 8m, diameter of pile equal to 0.5 m. In practice, pile spacing would be 1.5 m, 0.1 m less than the calculation＇s result because certain non-concordance happens between piles and soil when bearing load.