Determination of Modulus of Subgrade Reaction
B-1GENERAL
The modulus of subgrade reaction (k) as applicable to the case of load through a plate of size 300 mm x 300 mm or beams 300 mm wide on the soils is given in Table 6 for cohesionless soils and in Table 7 for cohesive soils. Unless more specific determination of k is done (see B-2 and B-3) these value may be used for design of raft foundation in cases where the depth of the soil affected by the width of the footing may be considered isotropic and the extra-polation of plate load test results is valid.
| Sl No. | Soil Characteristic | Modulus of Subgrade Reaction, kN/m3 | ||
| Relative Density | Standard Penetration Test Value,(N) (Blows per 300 mm) | For Dry or Moist State | For Submerged State | |
| (1) | (2) | (3) | (4) | (5) |
| i) | Loose | < 10 | 15 000 | 9 000 |
| ii) | Medium | 10 to 30 | 15 000 to 47 000 | 9 000 to 29 000 |
| iii) | Dense | 30 and over | 47 000 to180 000 | 29 000 to 108 000 |
The above values apply to a square plate 300 mm x 300 mm or beams 300 mm wide.
B-2 Field Determination
B-2.1 In cases where the depth of the soil affected by the width of the footing may be considered as isotropic, the value of may be determined in accordance with
accepted standard [6-2(35)]. The test shall be carried out with a plate of size not less than .
| SI No. | Soil Characteristic | Modulus of Subgrade Reaction, | |
| Consistency | Unconfined Compressive Strength | ||
| (1) | (2) | (3) | (4) |
| i) | Stiff | 100 to 200 | 27 000 |
| ii) | Very stiff | 200 to 400 | 27 000 to 54 000 |
| iii) | Hard | 400 and over | 54 000 to 108 000 |
The values apply to a square plate 300 mm x 300 mm. The above values are based on the assumption that the average loading intensity does not exceed half the ultimate bearing capacity.
B-2.2 The average value of shall be based on a number of plate load tests carried out over the area, the number and location of the tests depending upon the extent and importance of the structure.
B-3 Laboratory Determination
B-3.1 For stratified deposits or deposits with lenses of different materials, evaluation of k from plate load test will be unrealistic and its determination shall be based on laboratory tests {see good practice [6-2(4)]}.
B-3.2 In carrying out the test, the continuing cell pressure may be so selected as to be representative of the depth of the average stress influence zone (about 0.5 B to B).
B-3.3 The value of shall be determined from the following relationship:
where
modulus of elasticity of soil (see Annex A),
Young's modulus of foundation material,
Poisson's ratio of soil,
moment of inertia of the foundation, and
width of the footing.
B-4 Calculations
When the structure is rigid (see Annex C), the average modulus of subgrade reaction may also be determined as follows: