桩顶位移过大怎么回事？

simulin_k

finish$/clear$/prep7
apile=1.0
hpile1=2.0
hpile2=8.0
asoil=7.0
hsoil=24.0
et,1,solid45
et,2,170
et,3,173
keyopt,3,9,0
keyopt,3,12,2
mp,ex,1,2.6e10
mp,prxy,1,0.167
mp,dens,1,2500
mp,ex,2,2.6e8
mp,prxy,2,0.42
mp,dens,2,1900
tb,dp,2$tbdata,1,19000,31,29
mp,mu,3,0.2
r,1$r,2$r,3

blc4,,,asoil,asoil,-hsoil
wpoff,apile$wprota,,,90$vsbw,all$wpcsys,-1
wpoff,,apile$wprota,,90$vsbw,all$wpcsys,-1
wpoff,,,-hpile2$vsbw,all$wpcsys,-1
numcmp,all
vatt,2,2,1$esize,0.5$mshkey,1$vmesh,all

vsel,none
!blc4,,,apile,apile,hpile1+hpile2
block,0,1,0,1,-8,2
vatt,1,1,1$vmesh,all
asel,s,area,,37,41,2
nsla,s,1
real,3$type,2$mat,3$esurf,all$allsel,all
asel,s,,,7,9$nsla,s,1$real,3$type,3$mat,3$esurf,all
asel,s,loc,x,0$asel,a,loc,y,0$da,all,symm                   !选择对称面并施加对称约束
asel,s,loc,x,asoil$asel,a,loc,y,asoil$da,all,all            !选择边界面并施加全约束
asel,s,loc,z,-hsoil$da,all,uz                               !选择底面并施加约束

asel,s,loc,z,hpile1$sfa,all,1,pres,50e4$allsel,all          !施加桩顶面荷载

/solu
antype,static
nlgeom,on
autots,on
acel,,,9.8
time,1
nsubst,100
outres,all,all
solve
finish

simulin_k

说明:此段代码是王新敏老师P496的，里面有些错误已经改过来了，可计算时出现桩顶面上的节点位移过大，不知道问题出在哪，求教