Validating Bearing Capacity Equation using Plaxis Model

Hello Plaxis team, 

I did a simple exercise to validate the conventional bearing capacity equation of a footing on cohesive soil. I considered a cohesive soil with Cu=50kPa, unit weight of 20kN/m3, and Mohr-Coulomb model. I also considered a footing size of 3mx3m. 

The ultimate bearing capacity of a footing on ground surface was 2,895kN (322kPa), and a footing at 1m depth was 4,880kN (542kPa). The ultimate bearing capacity of a footing at 1m depth without a backfill was 4,100kN (455kPa).

As you see in the above results, the difference in bearing capacity due to 1m depth was 133 kPa (455 - 322), which suppose to be only 20kPa as a result of qNq term. Also the difference of bearing capacity due to footing backfilling was 87kPa (542 - 455), which suppose to be only 20kPa (unit weight times depth).

Do you have any explanation on the above results? I can send the Plaxis model files if it is required. 

Regards,