根据《桩规》第 5.3.12、5.8.4 条

桩顶铰接，桩底非岩石

$$ l _ {0} + h = 3 + 2 + 4 + 8 = 1 7 \mathrm{m} $$

$$ h ^ {\prime} = h - \left(1 - \varphi_ {l}\right) d _ {l} = 1 4 - (1 - 0) \times 2 = 1 2 \mathrm{m}, l _ {\mathrm{o}} ^ {\prime} = 1 7 - 1 2 = 5 \mathrm{m} $$

$$ \alpha \times h ^ {\prime} = 0. 5 4 6 \times 1 2 = 6. 5 5 > 4 $$

$$ l _ {\mathrm{c}} = 0. 7 \times \left(l _ {0} ^ {\prime} + \frac {4 . 0}{\alpha}\right) = 0. 7 \times \left(5 + \frac {4 . 0}{0 . 5 4 6}\right) = 8. 6 3 \mathrm{m} $$

因此选（D）