Prokon SUMO Subgrade Modulus – Part 2


In Part 1 we discussed the basics pertaining to the Subgrade Modulus parameter, its’ application and typical values. Understanding how you can determine a more “site-specific” value is important. In Part 2 we will delve into the formula for calculating the Soil Subgrade Modulus.

Section 9-6 of JE Bowles’ “Foundation Analysis and Design” – 5th Edition (McGraw-Hill) approximates an equation for calculating k (Subgrade Modulus) as k = 40 x SF x qa.

  • SF = Safety Factor (typical foundation safety factors apply).
  • qa = Allowable soil bearing capacity.

For the above equation, SF can be omitted as your Load Combination factors will perform the same function.

Consider a set of DCP results, indicating an average bearing capacity/pressure of 712kPa, which will place that soil in the Very Dense category. Calculating the Subgrade Modulus: k = 40 x 712 = 28 480 kN/m³. This can then be used as your Subgrade Modulus value in SUMO for a more accurate model.

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