Plate Load Test of Subgrade

Modulus of subgrade reaction

One of the essential requirements of pavement subgrade is incompressibility. Modulus of subgrade reaction is an indicator of this. It is defined as the pressure required to cause per unit deformation by a standard plate of 75 cm diameter in some cases where the stiffness of the soil is very high 30 cm diameter plate is to be used.

The load is applied through a reaction frame as shown in the following Figure 1.

Fig 1. Plate load test setup

Here a stack of stiffening plates of decreasing diameters is placed on top of the main plate. Three or four dial gauges are placed on the periphery of the plate for measuring the average deformation from all sides. The loading jack, on the other hand, is provided with a proving ring assembly for measuring the reaction against the frame. A seating load of 0.07 kg/cm³ is applied and released after a few seconds. A total deflection of 0.175 cm is applied in seven stages each of 0.25 mm. The load for each 0.25 mm settlement is increased until the settlement of the plate is stopped or when the rate of deformation is 0.025 mm per minute. The pressure p to cause each settlement of 0.25 mm is noted down and plotted.

The modulus of subgrade reaction, K is calculated as:
k=p/0.125  kg/cm²/cm where p is the pressure in kg/cm²

Relation between modulus of elasticity and modulus of subgrade reaction

The modulus of elasticity E in kg/cm² and modulus of subgrade reaction K in kg/cm²/cm are related as :

E=1.5Ka for flexible plate

E=1.18Ka for rigid plate

where a indicates the radius of the plate in cm.

Allowance for worst moisture content of subgrade

It is worth noting that the presence of moisture in the subgrade significantly affects the modulus of subgrade reaction and the relation is inversely proportional. On the other hand at the time of conducting this test it is not necessary that the moisture of the subgrade is highest which will be the worst-case scenario. Therefore to negotiate this situation the modulus of subgrade reaction is determined in the prevailing moisture content and the value so obtained is modified for the soaked condition.