根据《地规》5.3.5条，O点某一深度处的平均附加应力系数为：$\bar{\alpha}i=\bar{\alpha}}-\bar{\alpha{i,bghd}+\bar{\alpha}}-\bar{\alpha{i,dhjf}=2(\bar{\alpha}}-\bar{\alpha{i,bghd})$。$z_n=20\mathrm{m}$，对矩形aghc和矩形bghd，$l}=20\mathrm{m}$，$b_{ac}=10\mathrm{m}$，$l_{bg}=10\mathrm{m}$，$b_{bd}=10\mathrm{m}$。$z_1/b_{ac}=20/10=2$，$l_{ag}/b_{ac}=20/10=2$，查表K.0.1-2，$\bar{\alpha{11}=0.1958$。$z_1/b}=20/10=2$，$l_{bg}/b_{bd}=10/10=1$，查表K.0.1-2，$\bar{\alpha{12}=0.1746$。$\bar{\alpha}_1=2\times(\bar{\alpha}}-\bar{\alpha{12})=2\times(0.1958-0.1746)=0.0424$。$s=\beta}\psi_s\sum\frac{p_0}{E_{si}}(\bar{\alphai z_i-\bar{\alpha}$。选B。} z_{i-1})=1.0\times0.7\times100\times\frac{0.0424\times20}{10000}=5.936\mathrm{mm