The IBC in Ch 18 Table 1806.2 gives a "presumptive" value of 1500 psf (71.82 kPa) for clay. Also, Table 3-1 in TM 5-818-1 (US Army Corps of Engineers) gives a worst case value of 1400 psf for cohesion for clay soils. By definition, cohesion is 1/2 the unconfined compressive strength (qu), so 1400 psf would be close to qa with a safety factor of 2. I would use 1500 psf (71 kPa). Just my opinion.