Question

In three dimensional consolidation of sand drain, having radial symmetry, the governing consolidation equation is _______

(a) \(\frac{∂\overline{u}}{∂t}=C_{vr} (\frac{\overline{u}}{∂r^2}+\frac{1}{r}\frac{∂\overline{u}}{∂r})-C_{vz}\frac{∂^2 \overline{u}}{∂z^2}\)

(b) \(\frac{∂\overline{u}}{∂t}=C_{vr} (\frac{\overline{u}}{∂r^2}+\frac{1}{r}\frac{∂\overline{u}}{∂r})+C_{vz}\frac{∂^2 \overline{u}}{∂z^2}\)

(c) \(\frac{∂\overline{u}}{∂t}=C_{vr} (\frac{\overline{u}}{∂r^2}-\frac{1}{r}\frac{∂\overline{u}}{∂r})+C_{vz}\frac{∂^2 \overline{u}}{∂z^2}\)

(d) \(\frac{∂\overline{u}}{∂t}=C_{vz} (\frac{\overline{u}}{∂r^2}+\frac{1}{r}\frac{∂\overline{u}}{∂r})+C_{vz}\frac{∂^2 \overline{u}}{∂z^2}\)

The question was asked during an interview.

I need to ask this question from Consolidation Equation in Polar Coordinates in chapter One Dimensional Consolidation of Geotechnical Engineering I

Answer 1

Right option is (b) \(\frac{∂\overline{u}}{∂t}=C_{vr} (\frac{\overline{u}}{∂r^2}+\frac{1}{r}\frac{∂\overline{u}}{∂r})+C_{vz}\frac{∂^2 \overline{u}}{∂z^2}\)

To explain I would say: In three dimensional consolidation of sand drain, for the case of radial symmetry,

Cvx=Cvy =Cvr

∴ the governing consolidation equation is,

\(\frac{∂\overline{u}}{∂t}=C_{vr} (\frac{\overline{u}}{∂r^2}+\frac{1}{r}\frac{∂\overline{u}}{∂r})+C_{vz}\frac{∂^2 \overline{u}}{∂z^2}.\)