Relationship between modulus of subgrade reaction & bearing capacity

When i design a RCC box culvert, i confuse for defining value of subgrade modulus. How can i define this value when assign base slab support. I have only value of bearing capacity of soil. May i know correlation between these. Thanks

  • The modulus of subgrade reaction ks is a function of the contact pressure and allowable settlement on an unit area of soil. It is also known as Pressure per unit settlement.

    On the other hand, the safe bearing capacity is the maximum pressure which the soil can carry safely without risk of shear failure. The unit for bearing capacity is KN/m2, whereas the unit for Subgrade modulus is kN/m3. As per Joseph Bowles  the modulus of subgrade reaction is a conceptual relationship between soil pressure and deflection.

    Though, modulus of subgrade reaction and bearing pressure is related to each other, there is no reliable correlation between modulus of subgrade reaction and soil bearing capacity because they are measurements of entirely different characteristics of a soil. The reason, soil and its properties vary from place to place, there is significant effect on the type of soil in layers below etc.

    However, J E Bowles has attempted to give a relationship which is:

    k = 40 x SF x qa, where SF is factor of safety considered in bearing pressure calculation and qa is allowable soil bearing capacity.

    So, you could employ the above equation in determining the Soil Subgrade Modulus.



  • The modulus of subgrade reaction ks is a function of the contact pressure and allowable settlement on an unit area of soil. It is also known as Pressure per unit settlement.

    On the other hand, the safe bearing capacity is the maximum pressure which the soil can carry safely without risk of shear failure. The unit for bearing capacity is KN/m2, whereas the unit for Subgrade modulus is kN/m3. As per Joseph Bowles  the modulus of subgrade reaction is a conceptual relationship between soil pressure and deflection.

    Though, modulus of subgrade reaction and bearing pressure is related to each other, there is no reliable correlation between modulus of subgrade reaction and soil bearing capacity because they are measurements of entirely different characteristics of a soil. The reason, soil and its properties vary from place to place, there is significant effect on the type of soil in layers below etc.

    However, J E Bowles has attempted to give a relationship which is:

    k = 40 x SF x qa, where SF is factor of safety considered in bearing pressure calculation and qa is allowable soil bearing capacity.

    So, you could employ the above equation in determining the Soil Subgrade Modulus.

    In above reply you are using factor of safety while calculating the sub grade modulus. Then SBC shall be check for service load cases or ultimate combinations.

  • In above reply you are using factor of safety while calculating the sub grade modulus. Then SBC shall be check for service load cases or ultimate combinations. Because while calculating the SBC we are using factor of safety. 

  • For example, see Joseph E Bowles' Foundation analysis and design book's Table 9-1

    Range of Ks in  kN/M3

    Loose sand 4800-16000

    Med Dense Sand 9600-80000

    Dense Sand 64000-128000

    Clayey Med Dense Sand 32000-80000

    Silty Medium Dense Sand 24000-48000

    Clayey Soil:

    qa<=200kPa - 12000-24000

    200<Qa<800kPa  - 24000-48000

    Qa>800kPa - >48000

    Hope that helps