根据《地规》第 5.2.4、5.2.7 条

$$ p _ {\mathrm{k}} = \frac {F _ {\mathrm{k}}}{A} + \gamma_ {G} \mathrm{d} = \frac {2 3 5}{\mathrm{b}} + 2 0 \times 2. 5 = \frac {2 3 5}{\mathrm{b}} + 5 0 $$

$$ p _ {\mathrm{c}} = 1 8 \times 2. 5 = 4 5 \mathrm{kPa} $$

$$ \frac {E _ {\mathrm{s} 1}}{E _ {\mathrm{s} 2}} = \frac {7 . 5}{2 . 5} = 3, \quad \mathrm{z} = 2 \mathrm{m}, \quad \mathrm{b} \leq 3 \mathrm{m}, \quad \theta = 2 3 ^ {\circ} $$

$$ p _ {\mathrm{z}} = \frac {b \left(p _ {\mathrm{k}} - p _ {\mathrm{c}}\right)}{b + 2 z \tan \theta} = \frac {b (2 3 5 / b + 5 0 - 4 5)}{b + 2 \times 2 \times \tan 2 3 ^ {o}} = \frac {2 3 5 + 5 b}{b + 1 . 7} $$

$$ p _ {c z} = 1 8 \times 4. 5 = 8 1 \mathrm{kPa} $$

$$ f _ {\mathrm{az}} = f _ {\mathrm{ak}} + \eta_ {\mathrm{d}} \gamma_ {\mathrm{m}} (d + z - 0. 5) = 6 5 + 1 \times 1 8 \times (4. 5 - 0. 5) = 1 3 7 \mathrm{kPa} $$

$$ p _ {\mathrm{z}} + p _ {\mathrm{cz}} \leq f _ {\mathrm{az}} $$

$\frac{235+5b}{b+1.7}+81\leq137$ ，解得 $b\geq2.74m$

因此选（C）