A 3x3 pile group will be installed in clay. The mid-height of a clay layer is 10 feet below the bottom of the pile group (i. e. 10 feet below the pile points). Given the following information, determine the stress increase at the mid-height of the clay layer (refer to Eq. 12.146 in textbook). Report your answer in pounds per square foot (psf) to the nearest psf (i. e. no decimal places). Total Vertical Load on Pile Group = 450 kips Group Width = 10 feet Group Length = 10 feet Pile Embedded Length = 30 feet