QUESTION $2$

$($a$)$ Give two reasons why it is necessary to determine the coefficient of permeability in the field?

$(2$ marks$)$

$($b$)$ A layer of sand $6m$ thick lies beneath a clay stratum $5m$ thick and above a bed of thick shale. In order to determine the permeability of the sand a well was driven to the top of the shale and water pumped out at a rate of $10{10}^{-3}\frac{m^3}{s}.$ Two observation boreholes were driven through the clay at $15m$ and $30m$ from the pump well and the water was found to rise to levels of $3m$ and $2,5m$ below the ground surface respectively. Calculate from first principles the coefficient of permeability of the soil assuming Darcy's law.

$(15$ marks$)$