FLAC3D深基坑的开挖与支护的命令流
一、实例工程
南宁地区地层属于河流阶地二元地层，广泛分布有较厚的圆砾层，国内尚无在类似地层条件下建设地铁基坑的经验，为此，可使用 FLAC3D 对基坑开挖的全过程进行三维数值模拟，在对比实测数据的基础上，总结圆砾层中地铁车站深基坑的地下连续墙水平变形及周围地表沉降变形特征。
该基坑位于大学路与明秀路交叉路口处，沿大学东路东西向布置。车站基坑长465m，标准断面宽度为20.7m，为地下两层式结构，底板埋深为 15.535m（相对地面），顶板覆土厚度大于 3m。本工程主体建筑面积 21163.6m2，主要结构形式为双柱三跨框架箱型结构。 本工程所处的大学路为南宁市东西向的主要交通枢纽，车流量大，人流密集，地面条件复杂。
基坑施工采用明挖顺作法施工，围护结构为 800mm厚地下连续墙+内撑（三道内支撑 加一道换撑）的支护体系。第一道支撑采用钢筋混凝土支撑，尺寸为800×900mm，冠梁同时作为第一道钢筋混凝土支撑的围檩。第二、三道支撑及换撑使用钢支撑并施加预加力，直径为 609mm，壁厚为 t=16mm，斜撑段采用800×1000mm钢筋砼腰梁，其余为 2×I45C钢围檩。
二、模型建立
建模工作由两部分组成，实体模型部分，包括土体和地下连续墙；结构单元部分，包括混凝土支撑和钢支撑。
根据对称性原理，拟选取 1/2 的实际工程尺寸进行分析。考虑到实际的基坑长度将近 500m，根据以往的经验，选取全部长度的一半虽然能够得到满意的结果，但是由于中间部分的基坑基本处于同样的受力状态，这样会使大部分的计算长度变为重复的计算，降低了计算效率。根据初步计算结果和经验，最终确定的基坑尺寸为，宽度取基坑的最大宽度 24m，开挖深度 19m，基坑长度 36m。根据地勘报告，合并相似土层，模型中共划分了 7个土层。
在 FLAC3D 中，围护结构可以用衬砌单元(liner)或实体单元模拟。根据Zdravdovi的研究，在二维平面基坑模拟中，分别采用实体单元和梁单元(相当于三维模型中的衬砌单元)计算所产生的墙体变形差别小于4%，而引起地表沉降的主要原因是围护结构变形造成的地层损失，可见上述两种方法计算结果的差别可忽略不计。相比于衬砌单元(liner)，实体单元物理模型清晰，参数相对较少且容易确定，因此本例围护结构采用实体单元模拟。
模型的范围必须要足够大以覆盖基坑开挖可能影响的区域，同时又要兼顾计算效率。根据现行国家和个地方规范的要求，参考已有的研究成果，影响范围为4 倍开挖深度，整个模型的尺寸为 96×144×39（长×宽×高 m） 。为减少不必要的网格，网格划分原则为基坑附近密集，远处稀疏，模型共有节点 10890个，单元9408个。模型如图所示。
FLAC3D 3.00Step 2 Model Perspective10:08:32 Wed Apr 16 2014Center: X: -1.889e+001 Y: 8.953e+000 Z: -1.123e+001Dist: 3.114e+002Rotation: X: 40.000 Y: 0.000 Z: 290.000Mag.: 0.512Ang.: 22.500Block GroupexcwallnearsoilfarsoilItasca Consulting Group, Inc.Minneapolis, MN USA 图 建立模型 命令流如下： n
;开挖土体
gen zone brick size 12 4 10 p0 0,0,0 p1 36,0,0 p2 0,12,0 p3 0,0,19 ;开挖下部土体
gen zone brick size 12 4 4 p0 0,0,-20 p1 36,0,-20 p2 0,12,-20 p3 0,0,0 group exc ;实体地连墙
gen zone brick size 12 1 10 p0 0,12,0 p1 36,12,0 p2 0,12.8,0 p3 0,12,19 gen zone brick size 12 1 4 p0 0,12,-20 p1 36,12,-20 p2 0,12.8,-20 p3 0,12,0 gen zone brick size 1 4 10 p0 -0.8,0,0 p1 0,0,0 p2 -0.8,12,0 p3 -0.8,0,19 gen zone brick size 1 4 4 p0 -0.8,0,-20 p1 0,0,-20 p2 -0.8,12,-20 p3 -0.8,0,0 gen zone brick size 1 1 10 p0 -0.8,12,0 p1 0,12,0 p2 -0.8,12.8,0 p3 -0.8,12,19 gen zone brick size 1 1 4 p0 -0.8,12,-20 p1 0,12,-20 p2 -0.8,12.8,-20 p3 -0.8,12,0 group wall range group exc not
;-------------------------------------------------------------------------------- ;土体
gen zone brick size 12 5 10 p0 0,12.8,0 p1 36,12.8,0 p2 0,32,0 p3 0,12.8,19
gen zone brick size 1 5 10 p0 -0.8,12.8,0 p1 0,12.8,0 p2 -0.8,32,0 p3 -0.8,12.8,19
gen zone brick size 5 5 10 p0 -20.8,12.8,0 p1 -0.8,12.8,0 p2 -20.8,32,0 p3 -20.8,12.8,19 gen zone brick size 5 1 10 p0 -20.8,12,0 p1 -0.8,12,0 p2 -20.8,12.8,0 p3 -20.8,12,19 gen zone brick size 5 4 10 p0 -20.8,0,0 p1 -0.8,0,0 p2 -20.8,12,0 p3 -20.8,0,19 gen zone brick size 12 5 4 p0 0,12.8,-20 p1 36,12.8,-20 p2 0,32,-20 p3 0,12.8,0
gen zone brick size 1 5 4 p0 -0.8,12.8,-20 p1 0,12.8,-20 p2 -0.8,32,-20 p3 -0.8,12.8,0
gen zone brick size 5 5 4 p0 -20.8,12.8,-20 p1 -0.8,12.8,-20 p2 -20.8,32,-20 p3 -20.8,12.8,0 gen zone brick size 5 1 4 p0 -20.8,12,-20 p1 -0.8,12,-20 p2 -20.8,12.8,-20 p3 -20.8,12,0 gen zone brick size 5 4 4 p0 -20.8,0,-20 p1 -0.8,0,-20 p2 -20.8,12,-20 p3 -20.8,0,0 group nearsoil range group exc not group wall not
;--------------------------------------------------------------------------------- gen zone brick size 12 9 10 p0 0,32,0 p1 36,32,0 p2 0,92,0 p3 0,32,19
gen zone brick size 1 9 10 p0 -0.8,32,0 p1 0,32,0 p2 -0.8,92,0 p3 -0.8,32,19
gen zone brick size 5 9 10 p0 -20.8,32,0 p1 -0.8,32,0 p2 -20.8,92,0 p3 -20.8,32,19 gen zone brick size 3 9 10 p0 -60,32,0 p1 -20.8,32,0 p2 -60,92,0 p3 -60,32,19
gen zone brick size 3 5 10 p0 -60,12.8,0 p1 -20.8,12.8,0 p2 -60,32,0 p3 -60,12.8,19 gen zone brick size 3 1 10 p0 -60,12,0 p1 -20.8,12,0 p2 -60,12.8,0 p3 -60,12,19 gen zone brick size 3 4 10 p0 -60,0,0 p1 -20.8,0,0 p2 -60,12,0 p3 -60,0,19 ;深部土体
gen zone brick size 12 9 4 p0 0,32,-20 p1 36,32,-20 p2 0,92,-20 p3 0,32,0
gen zone brick size 1 9 4 p0 -0.8,32,-20 p1 0,32,-20 p2 -0.8,92,-20 p3 -0.8,32,0
gen zone brick size 5 9 4 p0 -20.8,32,-20 p1 -0.8,32,-20 p2 -20.8,92,-20 p3 -20.8,32,0 gen zone brick size 3 9 4 p0 -60,32,-20 p1 -20.8,32,-20 p2 -60,92,-20 p3 -60,32,0
gen zone brick size 3 5 4 p0 -60,12.8,-20 p1 -20.8,12.8,-20 p2 -60,32,-20 p3 -60,12.8,0 gen zone brick size 3 1 4 p0 -60,12,-20 p1 -20.8,12,-20 p2 -60,12.8,-20 p3 -60,12,0 gen zone brick size 3 4 4 p0 -60,0,-20 p1 -20.8,0,-20 p2 -60,12,-20 p3 -60,0,0 group farsoil range group exc not group wall not group nearsoil not gen merge 0.01 plot block group ;镜像对称
gen zone ref nor 0 -1 0 ;显示坐标轴
plot add axes red
;边界条件------------------------------------------------------------------------- fix x range x -60.1 -59.9 fix x range x 35.9 36.1 fix y range y 91.9 92.1 fix y range y -91.9 -92.1 fix z range z -20.1 -19.9
;重力----------------------------------------------------------------------------- set grav 0 0 -9.81
;土体材料属性---------------------------------------------------------------------- model mohr ;填土
pro bulk 6e6 she 2.22e6 fric 9 coh 20000 dens 1850 range z 16.9 19.1 ;淤泥质土
pro bulk 3e6 she 1.07e6 fric 4 coh 5000 dens 1830 range z 12.9 17.1 ;粘土
pro bulk 12e6 she 4.62e6 fric 15 coh 27500 dens 1970 range z 10.9 13.1
;粉土
pro bulk 10e6 she 3.8e6 fric 18 coh 8000 dens 2000 range z 8.9 11.1 ;中砂,粗砂,砾砂
pro bulk 25e6 she 9.68e6 fric 30 coh 0 dens 2050 range z 2.9 9.1 ;圆砾层
pro bulk 32e6 she 12.8e6 fric 32 coh 0 dens 2200 range z -4.1 3.1 ;泥岩
pro bulk 50e6 she 22.5e6 fric 70 coh 0 dens 2200 range z -21.1 -3.9 ;给地连墙赋本构，属性------------------------------ m e range group wall
ini density 2500 range group wall
pro young 40e9 poisson 0.167 range group wall ;计算初始平衡
plot set rotation 20 0 30 set mech ratio 1e-4 plot con szz Solve
Save inidis0.sav ;保存初始平衡，后面再调用 通过上述命令流可得如下图所示：
FLAC3D 3.00Step 1940 Model Perspective10:11:50 Wed Apr 16 2014Center: X: -9.121e+000 Y: -6.443e+000 Z: -1.123e+001Dist: 3.114e+002Rotation: X: 30.000 Y: 0.000 Z: 100.000Mag.: 0.512Ang.: 22.500Contour of SZZ Magfac = 0.000e+000 Gradient Calculation-8.1280e+005 to -8.0000e+005-8.0000e+005 to -7.0000e+005-7.0000e+005 to -6.0000e+005-6.0000e+005 to -5.0000e+005-5.0000e+005 to -4.0000e+005-4.0000e+005 to -3.0000e+005-3.0000e+005 to -2.0000e+005-2.0000e+005 to -1.0000e+005-1.0000e+005 to 0.0000e+000 0.0000e+000 to 8.7313e+004 Interval = 1.0e+005Itasca Consulting Group, Inc.Minneapolis, MN USA 图 初始平衡应力
三、开挖与支护
该工程分四次开挖，架设三道支撑（第一道为钢筋混凝土支撑，第二、三道为钢支撑），故把工程分成四个工况。