# What is the modulus of subgrade reaction?
**The modulus of subgrade reaction (k) is a geotechnical parameter that indicates how much force per unit area is required to depress the soil by a given distance.** It is expressed in kN/m³ and represents the stiffness or spring constant of the soil beneath a foundation.
## Meaning
A high modulus means stiff soil with little settlement under load (e.g. sand or gravel). A low modulus indicates soft ground with greater deformation (e.g. clay or peat).
Typical values:
– **Peat:** 2,000–5,000 kN/m³
– **Soft clay:** 5,000–15,000 kN/m³
– **Stiff clay:** 15,000–30,000 kN/m³
– **Sand:** 20,000–80,000 kN/m³
– **Gravel:** 80,000–150,000 kN/m³
## Application
– **Raft foundation design** — to determine deflection and force distribution
– **Ground-bearing slabs** — to calculate required thickness and reinforcement
– **Pavement design** — for dimensioning the foundation beneath roads
– **Pile foundations** — modelling soil reaction along and beneath piles
### Determination
The modulus is determined by plate bearing tests in the field or derived from cone penetration test data and laboratory tests.
## Related terms
– Bedding
– Foundation
– Cone penetration test
– Soil mechanics
– Bearing capacity
Want to learn more about construction terms? Visit our [knowledge base at fredsdiyplans.com](https://fredsdiyplans.com).