For the conditions descrihed, calculate the soil rigidity index $f_r$ :
(a) Soil modulus of elasticity is $150 \mathrm{ksf}$, Poisson's ratio is 0.40 , soil sait weight is 115 pef, cohesion is $600 \mathrm{pst}$, and angle of inlemal friction is $28^{\circ}$. Find $I_r$ for a point $10 \mathrm{ft}$ below the soil surface.
(b) Modulus of elasticity is $18 \mathrm{ksf}$, Poisson's ratio is 0.45 , woil colassion is 1,200 prf (undrained shear), and soil unil weight is $105 \mathrm{pcf}$. Find $I_f$ for a point 8 ft below the soil surface.
(c) Modulus of elasticity is $200 \mathrm{ksf}$, Poisson's ratio is 0.30 , angle of internal friction is $35^{\circ}$, and soil unit weight is 122 pef. Find $I_r$ for a point $10 \mathrm{ft}$ below the soil sarface.