In order to classify the fine-grained soils within one group and for judging their suitability as subgrade material, an indexing system has been introduced in HRB classification which is termed as group index. Soils are thus assigned arbitrary numerical numbers known as group index (GI). Group index is function of percentage and is given by the equation G.I = 0.2a + 0.005ac + 0.01bd
Where, WL = Liquid limit, IP = Plasticity Index.
P = Percentage fines passing from 0.074 mm sieve.
0 ≤ G . I ≤ 20 Lower the group → best quality.
k = Modulus of subgrade reaction
P = Pressure Corresponding to settlement of 0.125 cm.
ks = Modulus of subgrade reaction for soaked condition.
Ps = Pressure required in the soaked condition to produce same deformation as deformation Produce by pressure ‘P’ in consolidated condition.
k = Modulus of subgrade reaction for the consolidated stage.
Δ = Deformation in ‘cm’.
a = Radius of rigid plate in ‘cm’.
k = P / Δ = E / 1.18a
E = Modulus of elasticity of soil subgrade in kg/cm2.
k.a = constant
a1 = Radius of smaller plate (other plate)
k1 = Modulus of subgrade reaction of other plate of radius ‘a1’ cm.
(i) Aggregate crushing test
Aggregate crushing value
Where, w1 = Weight of the test sample in ‘gm’
w2 = weight of the crushed material in ‘gm’ passed through 2.36 mm sieve.
(ii) Shape Tests
Where, Ga = specific gravity of aggregate
W = mass of mould containing aggregate
C = mass of mould containing water
(iii) Abrasion Test
(a) Los Angeles Abrasion Test
HveeM Method of Bituminous mix Design Stabilimeter Value, (s)
Where, Pv = Vertical pressure at 28 kg/cm2 or at a total load of 2268 kg.
Ph = Horizontal pressure corresponding to Pv = 28 kg/cm2.
D2 = Displacement on specimen represented as number of turns of pump handle to raise Ph from 0.35 to 7 kg/cm2.
Where, L = Weight of shots in cm.
w = Diameter of width of specimen in cm
H = Height of specimen in cm
where, Pv = Vertical pressure applied (11.2 kg/cm2)
Ph = Horizontal pressure transmitted at Pv = 11.2 kg/cm2.
D2 = Displacement of stabilometer fluid necessary to increase the horizontal pressure from 0.35 to 7kg/cm2, measured in number of revolutions of the calibrated pump handle.