根据《桩规》第 5.9.8 条

$$ f _ {\mathrm{t}} = 1. 4 3 \mathrm{MPa}, h _ {0} = 7 3 0 \mathrm{mm}, h = 8 0 0 \mathrm{mm}, \beta_ {\mathrm{hp}} = 1. 0 $$

$$ b = 0. 8 \times 0. 5 = 0. 4 \mathrm{m} $$

$$ a _ {1 \mathrm{x}} = 2 0 0 0 - 5 0 0 - 2 0 0 = 1 3 0 0 \mathrm{mm} > 7 3 0 \mathrm{mm} $$

取 $a_{1\mathrm{x}} = 730\mathrm{mm}$

$$ a _ {1 \mathrm{y}} = 1 2 5 0 - 5 0 0 - 2 0 0 = 5 5 0 \mathrm{mm} < 7 3 0 \mathrm{mm} $$

$$ \lambda_ {1 \mathrm{x}} = 1. 0, \quad \lambda_ {1 \mathrm{y}} = a _ {1 \mathrm{y}} / h _ {0} = 5 5 0 / 7 3 0 = 0. 7 5, \quad 0. 2 5 \leq \lambda_ {1 \mathrm{y}} \leq 1. 0 $$

$$ \beta_ {1 \mathrm{x}} = \frac {0 . 5 6}{\lambda_ {1 \mathrm{x}} + 0 . 2} = \frac {0 . 5 6}{1 + 0 . 2} = 0. 4 6 7, \quad \beta_ {1 \mathrm{y}} = \frac {0 . 5 6}{\lambda_ {1 \mathrm{y}} + 0 . 2} = \frac {0 . 5 6}{0 . 7 5 + 0 . 2} = 0. 5 9 $$

$$ c _ {1} = c _ {2} = 5 0 0 + 2 0 0 = 7 0 0 \mathrm{mm} $$

$$ \begin{array}{l} N _ {l} \leq \left[ \beta_ {\mathrm{1x}} \left(c _ {2} + a _ {\mathrm{1y}} / 2\right) + \beta_ {\mathrm{1y}} \left(c _ {1} + a _ {\mathrm{1x}} / 2\right) \right] \beta_ {\mathrm{hp}} f _ {\mathrm{t}} h _ {0} \\ = \left[ 0. 4 6 7 \times (7 0 0 + 5 5 0 / 2) + 0. 5 9 \times (7 0 0 + 7 3 0 / 2) \right] \times 1 \times 1. 4 3 \times 7 3 0 \times 1 0 ^ {- 3} = 1 1 3 1 \mathrm{kN} \end{array} $$

因此选（C）

补充争议说明：

$m = \frac{d^2}{d_{\mathrm{e}}^2} = \frac{d^2}{(1.13s)^2} = \frac{0.5^2}{(1.13\times 1.5)^2} = 0.087$ ，取 $f_{\mathrm{spk}} = \lambda m\frac{R_{\mathrm{a}}}{\mathrm{A}_{\mathrm{p}}} +\beta (1 - m)f_{\mathrm{sk}}$

$\beta=\frac{f_{\mathrm{spk}}-\lambda m\frac{R_{\mathrm{a}}}{\mathrm{A}_{\mathrm{p}}}}{(1-m)f_{\mathrm{sk}}}=\frac{100-1.0\times0.087\times\frac{150}{0.25\times3.14\times0.5^{2}}}{(1-0.087)\times70}=0.52$ （承台变阶处并不在角桩 $45^{\circ}$ 的冲切锥体

范围内，故不必取到承台变阶边）

$$ R _ {a} = \eta f _ {\mathrm{cu}} A _ {\mathrm{p}}, f _ {\mathrm{cu}} = \frac {R _ {a}}{\eta A _ {\mathrm{p}}} = \frac {1 4 0}{0 . 2 5 \times 0 . 2 5 \times 3 . 1 4 \times 0 . 5 ^ {2}} = 8 5 3. 5 \mathrm{kPa} $$

$$ d _ {\min} = z _ {\mathrm{d}} - h _ {\max} = 2 - 0. 9 5 = 1. 0 5 \mathrm{m} $$

$$ d \geq 1. 0 5 \mathrm{m} $$

$$ a \geq 2. 5 \mathrm{m} $$

因此选（B）。