# What Does Modulus Of Subgrade Reaction Mean In Construction?

The modulus of subgrade reaction (k) is a measure of the stiffness of a soil and an indicator of the unit displacement under a given pressure.

It is used as an input for rigid pavement design to estimate the support of the layers below a rigid pavement surface course (the PCC slab). The k-value can be determined by field tests or by correlation with other tests.

The modulus of subgrade reaction came about due to work done by Westergaard during the 1920s, which developed the k-value as a spring constant to model the support beneath the slab.

It is defined as the pressure per unit deformation of the subgrade at specific pressure or deformation.

Various formulas have been suggested by different authors for evaluation of modulus of subgrade reaction, and typical values range from 0.3 to 1.0 MPa/m.

Factors that affect the modulus include loaded area (size of footing or mat), soil type, and moisture content

## How To Calculate Modulus Of Subgrade Reaction?

The modulus of subgrade reaction (k) is a measure of the pressure per unit deformation of the subgrade at a specific pressure or deformation.

It is expressed as k=p/s, where k is the modulus or coefficient of subgrade reaction, p is the applied pressure, and s is the deformation of soil settlement.

The modulus of subgrade reaction can be determined by performing a field plate load test, which involves applying compressive stress to a soil layer through rigid plates and measuring the deflections.

The modulus of subgrade reaction was developed by Westergaard during the 1920s as a spring constant to model the support beneath a pavement slab.

The reactive pressure to resist a load is proportional to both the spring deflection and k.

The value of k ranges from about 13.5 MPa/m (50 pci) for weak support, to over 270 MPa/m (1000 pci) for strong support.

There is no correlation between soil bearing capacity and modulus of subgrade reaction, so it cannot be used for estimation purposes.

However, it can be used as an input for rigid pavement design in order to estimate the support of layers below a rigid pavement surface course (the PCC slab).

## What Is A Good Modulus Of Subgrade Reaction?

The modulus of subgrade reaction (k) is a measure of the support provided by the layers below a rigid pavement surface course (the PCC slab).

It is expressed as the ratio between the pressure against a flat surface on soil and the settlement at that point.

Typical values of modulus of subgrade reaction range between 100-400 pci for coarse grained soils and 25-150 pci for fine grained soils.

The value of k is in terms of MPa/m (pounds per square inch per inch of deflection, or pounds per cubic inch – pci).

The modulus of subgrade reaction can be determined by field plate load tests, which involve applying compressive stress to the soil layer through rigid plates and measuring the deflections.

The best approach to appropriately include all factors in calculating the modulus is to analyze a full-scale three-dimensional continuum model of the soil layer.

It should be noted that reported values may vary depending on soil type and other factors, so it is recommended to use either field plate load tests or full-scale models for more accurate results.

## Where Is Modulus Of Subgrade Reaction Mainly Used?

The modulus of subgrade reaction (k) is mainly used in the design of rigid pavements and floors-on-grade that are not structural elements in a building.

It is used to estimate the support of the layers below a rigid pavement surface course, such as a PCC slab.

The k-value was developed by Westergaard during the 1920s as a spring constant to model the support beneath the slab.

It is also used for foundation and soil-structure interaction studies.

The modulus of subgrade reaction is determined by field plate load tests, which measure compressive stress applied to soil layers through rigid plates and deflections for various pressures or deformations.

Factors that influence the modulus of subgrade reaction include elastic properties of the soil, loading intensity, size of loaded area, and stiffness of slab or pavement.

Geotechnical consultants need information to provide an accurate modulus of subgrade reaction for use in design.

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