how to define mohr coulomb theory in Abaqus

Dear all 

i am a new abaqus user, now i use this software to modeling a few layers of soil layers (in geotechnical engineering), here is the information of the ground profile

[0-4m] fill sand layer; Density = 18kN/m2, firction angle=30, cohesion=0, void ratio=0.5  

[4-12m] marine deposit; Density = 16kN/m2, firction angle=10, cohesion=30, void ratio=0.5   

[12-22m]  Alluvium; Density = 18kN/m2, firction angle=30, cohesion=5, void ratio=0.5   

 [22-42m] C.D.G completely decomposit granite; Density = 19kN/m2, firction angle=33, cohesion=10, void ratio=0.5  and Ground water is located on 2m below the ground  

now i meet some problem, on the output stress results (S11, S22, S33), that S11 & S22 represent the stress in horizontal (in x & y direction) and S33 for vertical stress (in Z-direction).

According to the above infomation   if we calculate the stress on a point on 0.5m below the ground, the vertical stress = 18 x 0.5 = 9 kN/m2                                                     

  the horizontal stress = ko x vertical stress = 0.5 x 9 = 4.5 kN/m2  

 However the output result comes out is total different.  And i  use mohr coulomb theory in the model. If anyone who know how to solve this problem, please provide your kindly help, Big Big Thanks  Regards Paul