Example $6\mathrm{.}4$: A large excavation was made in a stratum of stiff clay with a saturated unit

weight of $18\mathrm{.}64k\frac{N}{m^3}.$ When the depth of excavation reached $8$ m $,$ the excavation failed as a

mixture of sand and water rushed in$.$ Subsequent borings indicated that the clay was under$-$

lain by a bed of sand with its top surface at a depth of $12\mathrm{.}5$ m $.$ To what height would the water

have risen above the stratum of sand into a drill hole before the excavation was started?

Fig. $6\mathrm{.}27$ Excavation of clay underlain by sand $($Example $6\mathrm{.}4)$