根据《桩规》第5.3.8条
$$
h _ {\mathrm{b}} / d _ {1} = \frac {1 0 0 0}{2 1 0} = 4. 7 6 < 5
$$
$$
\lambda_ {\mathrm{p}} = 0. 1 6 h _ {\mathrm{b}} / d _ {1} = 0. 1 6 \times 4. 7 6 = 0. 7 6 2
$$
管桩： $A_{\mathrm{j}} = \frac{\pi}{4}\left(d^{2} - d_{1}^{2}\right) = 0.25\times 3.14\times \left(0.4^{2} - 0.21^{2}\right) = 0.09098\mathrm{m}^{2}$
$$
A _ {\mathrm{p} 1} = \frac {\pi}{4} d _ {1} ^ {2} = 0. 2 5 \times 3. 1 4 \times 0. 2 1 ^ {2} = 0. 0 3 4 6 2 \mathrm{m} ^ {2}
$$
$$
Q _ {\mathrm{uk}} = Q _ {\mathrm{sk}} + Q _ {\mathrm{pk}} = u \sum q _ {\mathrm{sik}} l _ {\mathrm{i}} + q _ {\mathrm{pk}} \left(A _ {\mathrm{j}} + \lambda_ {\mathrm{p}} A _ {\mathrm{p1}}\right)
$$
$$
= 1. 2 5 6 \times (2 5 \times 1 + 2 2 \times 6 + 5 0 \times 4 + 1 0 0 \times 1) + 9 0 0 0 \times (0. 0 9 0 9 8 + 0. 7 6 2 \times 0. 0 3 4 6 2) = 1 6 2 9 \mathrm{kN}
$$
根据《桩规》第5.2.2条
$$
R _ {\mathrm{a}} = \frac {1}{K} Q _ {\mathrm{uk}} = 1 6 2 9 / 2 = 8 1 4. 5 \mathrm{kN}
$$
因此选（C）
