根据《地规》第 5.3.5 条
$$
p _ {0} = 8 0 \mathrm{kPa}, E _ {\mathrm{s}} = 2. 5 \mathrm{MPa}, E _ {\mathrm{s}} = 2. 5 \mathrm{MPa}, b = 3 \mathrm{m}
$$
$$
\frac {z _ {1}}{b} = \frac {2}{1 . 5} = 1. 3, \quad \frac {l}{b} = \frac {1 5}{1 . 5} = 1 0, \quad \overline {{\alpha}} _ {1} = (0. 2 2 8 9 + 0. 2 2 2 1) / 2 = 0. 2 2 5 5
$$
$$
\frac {z _ {2}}{b} = \frac {8}{1 . 5} = 5. 3, \quad \frac {l}{b} = \frac {1 5}{1 . 5} = 1 0, \quad \overline {{\alpha}} _ {2} = (0. 1 3 2 0 + 0. 1 2 9 2) / 2 = 0. 1 3 0 6
$$
$$
s = \psi_ {\mathrm{s}} s ^ {\prime} = \psi_ {\mathrm{s}} \sum_ {\mathrm{i} = 1} ^ {n} \frac {p _ {0}}{E _ {\mathrm{si}}} \left(z _ {\mathrm{i}} \overline {{\alpha}} _ {\mathrm{i}} - z _ {\mathrm{i} - 1} \overline {{\alpha}} _ {\mathrm{i} - 1}\right) = \frac {4 \times 1 . 0 \times 8 0 \times (8 \times 0 . 1 3 0 6 - 2 \times 0 . 2 2 5 2)}{2 . 5} = 7 6 \mathrm{mm}
$$
因此选（B）
