（2d）隧道模型，应力释放，开挖后运算出现错误

clxzz1994

求助大神是怎么回事？
同样的模型如果不考虑流固耦合，不会出现错误。命令流如下。
config gwflow
grid 50,50
model elastic
gen circle 24.024822,25.097517 6.0256114
group 'User:new2' notnull
model mohr notnull group 'User:new2'
prop density=2000.0 bulk=3.7E7 shear=1E6 cohesion=6000.0 friction=38.0 dilation=0.0 tension=10000.0 notnull group 'User:new2'
prop por=0.5 perm=1.0E-7 notnull
mark i 19 j 26
mark i 31 j 26
mark i 25 j 20
mark i 25 j 32
fix  x y j 1
fix  x i 51 j 2 51
fix  x i 1 j 2 51
set gravity=9.81
water bulk=2.0E8
water density=1000.0
apply nstress -290000.0 from 1,51 to 51,51
initial pp 294000.0 j 51
history 999 unbalanced
solve
call 'ms.txt'
initial xdisp 0 ydisp 0
initial xvel 0 yvel 0
initial xdisp 0 ydisp 0
initial xvel 0 yvel 0
model null region 26 24
apply xvelocity 0.0 long from 25,20 to 25,20
apply yvelocity 0.0 long from 25,20 to 25,20
step 1
set ninc = 1000 rstart = 1.0 rstop = 0.7
relax_ini
set ib_ap = 19 ie_ap = 31 jb_ap =  20 je_ap = 32
apply_rf
hist relax_hist
set=large
solve