$2\mathrm{.}19$ A consolidated$-$drained triaxial test was carried out on a normally consolidated clay specimen, and the following results were recorded: ${}_3^{\text{'}}=150k\frac{N}{m^2}$ and ${}_f=260k\frac{N}{m^2}.$ An identical specimen from the same clay was subjected to a consolidated$-$undrained test with a confining pressure of $150k\frac{N}{m^2},$ and the additional axial stress at failure was $115k\frac{N}{m^2}.$

a$.$ What is the pore water pressure at failure in this second specimen?