Method of determination of modulus of subgrade reaction (K-value) of soils in the field
1979 Edition

Per IS 9214, the standard bearing plate is a circular mild steel plate with a diameter of 75 cm and thickness of 25 mm. For fine-grained soils, smaller plates of 30 cm, 40 cm, or 45 cm diameter may be used to ensure better contact. The plate’s lower surface should be oiled and rotated on the soil surface to achieve full contact, with irregularities trimmed as needed. The test area must be prepared by stripping soil to twice the plate area and removing the top 25 cm layer. Additionally, stiffening plates can be placed atop the bearing plate to enhance rigidity if required.

Load is applied incrementally using a hydraulic jack that reacts against a beam positioned at least 210 cm from the bearing plate. The plate, typically 75 cm in diameter, rests on a prepared and oiled soil surface. Load increments continue until a total settlement of at least 1.75 mm is achieved. After each increment, readings are taken once settlement rate drops below 0.025 mm per minute for clayey soils or stabilizes for other soils. Load is measured via a pressure gauge or a calibrated proving ring with 150 kN capacity and ±0.5% accuracy. Deflections are recorded using dial gauges with precision up to 0.002 mm.

When the uncorrected modulus of subgrade reaction (Ku) is equal to or exceeds 0.555 MPa/cm, the load-deflection curve exhibits non-linearity. The test involves applying loads up to 93 kN in 15.5 kN increments to plot the curve. A straight-line portion between 31 kN (0.07 MPa) and 93 kN (0.21 MPa) is identified, and a correction line parallel to this segment is drawn through the origin. The deflection at 31 kN on this corrected line is used to calculate the corrected modulus Ka. If no clear straight portion exists, three points with minimal curvature are selected to draw the correction line. Additionally, saturation effects are corrected by adjusting K-values based on deformation measurements under saturated conditions.

Saturation typically decreases the modulus of subgrade reaction because increased moisture content softens the soil, resulting in greater deformation under load. Since direct saturation during field plate load tests is impractical, IS 9214 recommends applying a correction derived from consolidation tests. The corrected modulus Kc is calculated by multiplying the uncorrected modulus Ku by the ratio of deformation at natural moisture (d) to deformation at saturation (ds). This adjustment accounts for the reduction in subgrade stiffness due to moisture, ensuring safer design considerations.

Yes, IS 9214 provides guidance for converting K-values from smaller diameter plates to the equivalent value for the standard 75 cm plate. Using graphical relationships (Fig. 3 in the standard) between the modulus of subgrade reaction and plate diameter, the ratio of K for the smaller plate to the 75 cm plate is determined. The measured K from the smaller plate is divided by this percentage to estimate the equivalent K for the 75 cm plate. Additional corrections for plate bending and load-deflection non-linearity should also be applied as necessary to ensure accurate results.