Your next project site is an old landfill, which reaches a depth of 30 feet. Below, there are many layers of both competent and caving soils, but none of the competent soil layers capable of giving the needed bearing capacity under the 12’’ diameter and 60 feet deep drilled shafts that you are to use. It is known that the layer at 60 feet provides about 20% of the bearing capacity needed, and all the competent soil layers up to this depth provide an accumulated 50% of the necessary friction.
Understanding that according to design specifications the drilled shaft diameter needs to be kept at 12” diameter and up two consecutive pre-fabricated piles can be used (120 feet). Assuming that the soil layers continue much beyond 60 feet in a similar pattern (obviously no landfill) in terms of friction and bearing capacities, recommend two different solutions that could allow the shafts to transmit their loads by 1) end bearing capacity and by 2) friction. If an underream at 60 feet was to be used, which needs to be its minimum diameter? Write your assumptions.