$($a$)$ Discuss factors affecting the stress$-$strain modulus and approximations. $(5$ marks$)$

$($b$)$ Why is shear modulus a soil property of interest in Soil dynamics?

$(5$ marks$)$

c$)$ A seismic refraction study of an area has given the following data:

Distance from impact

Point to geophone $($m$)$

$15$

$30$

$60$

$90$

$120$

Time to receive wave$($s$)$

$0\mathrm{.}025,0\mathrm{.}05$

$0\mathrm{.}1$

$0\mathrm{.}11$

$0\mathrm{.}12$

Plot the time travel data and determine the seismic velocity for the surface layer and underlying layer.

II$.$ Determine the thickness of the upper layer.