$(4$ pts$)$ For a SPT blow count $($N$60)$ of $15$ bpf

a$.(2$ pts$)$ Estimate the friction angle of the clean sand at a depth of $9$ ft in Figure $1.$

b$.(2$ pts$)$ Estimate the undrained shear strength $($cu $)$ of the medium plasticity clay at a depth

of $30$ ft in Figure $1$