Pore Water Pressure (u = γw zw)

Below a static water table the water in the pores carries its own hydrostatic pressure, quite independently of the grains around it. Four metres below the table, u = 9.81 × 4 = 39.2 kPa; fifteen feet below it, u = 62.4 × 15 = 936 lbf/ft² = 6.5 psi. Fresh water is 9.81 kN/m³ or 62.4 pcf, sea water about 10.05 kN/m³ or 64 pcf, and drilling mud can be half again as heavy — which is why $\gamma_w$ is a field here rather than a constant buried in the code.

The trap is assuming hydrostatic conditions when they do not exist. This formula describes still water. Where there is seepage, an artesian layer, or a clay that has not finished consolidating, the pore pressure is set by the flow net or by the excess pressure still draining away, and it can be far above or below $\gamma_w z_w$. The 1976 Teton Dam failure and countless excavation blowouts came from exactly this: an underlying pervious layer under artesian head that nobody had piezometers in. Measure it if it matters — a standpipe piezometer costs less than a lawsuit.