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KPSC JE 2016: Specific Paper

Option 1 : CBR test

ST 1: Building Material and Concrete Technology

18104

20 Questions
20 Marks
12 Mins

__Concept:__

The following methods are used for the design of flexible pavements:-

**1. Group-Index Method:**

This method was devised by the U.S. Highway Engineers (Highway Research Board) in 1945. The group index of a soil is an arbitrary index assigned to different soil types based on their percentage of fines, liquid limit and plasticity index and is defined by:

\({\bf{GI}} = 0.2{\bf{a}} + 0.005{\bf{ac}} + 0.01{\bf{bd}}\)

Where,

a = percentage of fines passing through 75μ sieve ≯ 40%

b = percentage of fines passing through 75μ sieve ≯ 40%

c = represents liquid limit percentage = w_{L} – 40 ≯ 20%

d = represents plasticity index percentage = I_{P} -10 ≯ 20%

Also, a, b, c, d ≮ 0

- G
_{max}= 20 and G_{min}= 0

**The higher the group index value, the weaker is the subgrade soil**.

**2. CBR (California Bearing Ratio) Method:** (IRC 37-1984)

The CBR method was developed originally by the California State Highway Department.

The advantage of the CBR method is that it can be used to find the total thickness of the pavement and that of individual courses in addition to thickness of the subgrade soil (provided the CBR values of the materials of the courses are also known).

**Note:**

**Group Index method of design of flexible pavement has been obsolete nowadays.****In the recent revision of IRC 37-2018, IITPAVE software is used to determine the thickness of compositions of pavement based on the CBR values.**- CBR can be defined as the ratio of load at penetration of 2.5 mm or 5.0 mm to the standard load (i.e. 1370 kg for 2.5 mm & 2055 kg for 5.0 mm) for same penetration.
- These graphs are prepared using the formula-

\({\bf{T}}\left( {{\bf{cm}}} \right) = \;\sqrt {\left( {\frac{{\left( {1.75{\bf{P}}} \right)}}{{{\bf{CBR}}\% }} - \frac{{\bf{A}}}{{\bf{\pi }}}} \right)}\)

Where, P – Wheel load (kg), A – Contact area (m2)

- CBR value is inversely proportional to the thickness of the pavement.

∴ CBR is more appropriate answer.