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plate loading test

Updated on October 16, 2012

Plate loading test

the plate loading test is a field test used to determine the supporting power of subgrade, subbases, bases and a complete pavement. the test results are applicable for the design of flexible and rigid pavements.

the plate loading test is done by applying a compressive force on a stack of circular steel plates rested on the surface of the tested layer by means of hydraulic jack. the resulting surface deflection is read from at least three dial gauges near the plate outer edge.

Modulus of subgrade reaction

from the test data the modulus of subgrade reaction is determined by the following equation:

K= P/Δ


k = modulus of subgrade reaction

P = unit load on the plate (stress)

Δ = deflection of the plate

for design use a stress P = 10 psi (68.95 KN/M² )

the modulus of subgrade reaction (K) is used as a primary input for rigid pavement design. it estimates the support of the layers below a rigid pavement PCC slab.

Effect of plate size

the stress that a plate can carry depends upon the perimeter over area ratio as well as the soil strength. the following formula may be used to compute the unit load which a plate can sustain at a given deflection:

P = n + m (P / A )


P = Unit load

n,m = regression coefficients obtained from testing

P = perimeter of the plate

A = Area of the plate

the values of n and m can be determined by running plate loading tests with at least two different plate sizes at the same deflection.

Effect of saturation

If the subgrade soil may be exposed to saturation at some point of time, it is then desired to determine the modulus of subgrade reaction at the saturated condition. since it is inpractical to run field tests at different moisture and density conditions, it is necessary to adjust the field value of the K to reflect the changes in moisture and density. this is done by performing a consolidation or simple compression tests in the lap on soils at density and moisture conditions similar to those in the field. then the deformation corresponding to a 10 psi stress is determined. another sample of the same soil is saturated in water an exposed to the same laboratory test to determine the deformation corresponding to a 10 psi stress.

the modified K for the design is then calculated by the following formula

Ks = (Δu / Δs) * Ku


Ks = modulus of subgrade reaction corrected for saturation

Ku = Field modulus of subgrade reaction

Δu / Δs = ratio of the deflection in the unsaturated and saturated tests

various procedures can be used to run the plate loading test based on the information needed from the test. in all test types, when a load is applied to the plate, the plate deflects. If it is required to determine the relative amounts of plastic and elastic deformation, a cyclic load is applied and relationships can be obtained


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