A cohesion of 0.5 Kg/cm^2 is not so bad, just am wondering if that value could be qu, the unconfined compressive strength. Where qu=2c. Either case, a low blow count, N, is a given in soft to medium soft clays. In such a case, we would place a geofabric first to separate the stone from the clay, increase the bedding thickness to a minimum of 30 cm, and limit the excavated trench width to slightly larger than the pipe diameter.
My experience was in watermain pipe project. We had soft clayey silt, and I asked for a geaofabric and doubled the 6" bedding to 12". That was about 6 yrs ago, and our C value was close to 0.7 Kg/cm^2. Hope this link helsp: