Properties Of Soil as a Highway Material

Properties Of Soil as a Highway Material

Properties Of Soil as a Highway Material- Following are the materials to deal with in highway materials

  • Soil Characteristics
  • Stone Characteristics
  • Bituminous Characteristics


Soil is considered as one of the principal highways materials, compacted soil, and stabilized soil are often used in subgrade or base course of highway pavement

Desirable Properties:- a) Stability b) Incompressibility c) Strength d)Good drainage etc

Important terms
a) Field moisture equivalent- It is the moisture content at which the demands for adsorbed water are fully satisfied
b) Group Index Method (G.I Index) – Group index is a function of percentage material passing through 200 mesh sieve (0.074mm), liquid limit, and plasticity index of soil and is given by equation.


a= portion of material passing 0.074mm sieve greater than 35% but less than 75% (expressed as whole number 0 to 40)
b = portion of material passing 0.074mm sieve greater than 15% but less than 55% (expressed as
whole number 0 to 40)
c= The value of LL in excess of 40 and less than 60 (expressed as whole number 0 to 20)
d= The value of Plasticity index exceeding 10 but less than 30 (expressed as whole number 0 to 20)

Note:- Minimum possible value of G.I is zero and the maximum possible value of G.I is 20. Higher the value of G.I poorer is the soil as subgrade material.


A) Shear test – Carried out on relatively small soil sample in the laboratory like Direct shear or Tri-Axial Compression test (Part of Soil Mechanics)
B) Bearing Test – Carried out in situ with Plate load test.

Plate Load Test-

  • Used to evaluate the supporting power of subgrade for use in pavement design by using relatively large diameter plate.
  • First time used by Weter-Guard for determining the modulus of subgrade reaction (k) which is defined as “ The pressure sustained per unit deformation of subgrade at specified deformation using specified plate size (75cm diameter)
  • Pressure p corresponds to a settlement of 0.125cm is read and the k value is calculated by the reaction


  • Correction for the size of plate When the load is not adequate to cause 0.125cm deflection of 75 cm plate, then a smaller plate may be used( generally taken 30 cm plate). The corrected value of k1 is computed as where a1 is the radius of 75 cm plate.

Plate Load Test is a field test to determine the ultimate bearing capacity of the soil and the probable settlement under a given loading. The test essentially consists of loading a rigid plate at the foundation level and determining the settlement corresponding to each load increment. The ultimate bearing capacity then takes as the load at which the plate starts sinking at a rapid rate. The bearing plate is squared, of minimum size, 30 cm2 and the maximum size is 75 cm2. The thickness of the steel plate should not be less than 25 mm.

C) Penetration test – Carried out in situ or in laboratory

CBR Test: –

This method is used for evaluating the stability of soil subgrade and other flexible pavement materials. The California bearing ratio test is a penetration test meant for the evaluation of the subgrade strength of roads and pavements. The results obtained by these tests are used with the empirical curves to determine the thickness of pavement and its component layers. This is the most widely used method for the design of flexible pavement.

  • The test consist a cylindrical plunger of 50mm diameter to penetrate material at 1.25mm/minute
  • The standard load values obtained from tests on crushed stones are 1370 (70kg/cm2 ) and 2055 (105kg/cm2) at 2.5 and 5 mm penetration respectively at 50 mm dia plunger

 cpr=\frac{Lode/Pressure\: sustained\: at\: 2.5\:or\:5mm}{Lode/Pressure\: sustained\:by\:std.\:aggregate\: at\:2.5 \:or5mm}

Higher CBR is taken as final value

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