Mainly this presentation covers about how to understand and analyse soil as highway sub-grade material.. discussed about the basic properties of soil, classification of soils, tests to conduct on soil and how soil can be selected as highway material..

1. HIGHWAY
MATERIALS1 . S O I L
2 . S TO N E A G G R E G AT E S
3 . B I T U M I N O U S M I X E S
4 . P O R T L A N D C E M E N T A N D C E M E N T C O N C R E T E
R AV I K U M A R G A R R E

2. SOIL
• In the structure of highway, embankment and
subgrade are to be constructed with soil.
• The soil is considered as one of the principal highway
materials
• Soils are mainly of mineral matter formed by the
disintegration of rocks, by the action of water, frost,
temperature and pressure.

3. BASIC CHARACTERISTICS OF SOIL
• Based on individual grain size, soils are, gravel, sand,
silt, and clay
• The characteristics of soil grains depend upon the size,
shape, texture, chemical composition and electrical
charges on the surface of fine soil particles.
• Moisture and dry density influence the engineering
behaviour of soil mass

4. DESIRABLE PROPERTIES
If a soil is to be used as highway material, it should possess the following properties:
1. Stability
2. Incompressibility
3. Permanency of strength
4. Minimum changes in volume and stability
5. Good drainage
6. Ease of compaction

5. SOIL CLASSIFICATION
Soil classification systems are:
1. Grain size soil classification
2. Textural soil classification
3. Unified soil classification
4. BIS soil classification
5. HRB soil classification

6. GRAIN SIZE SOIL CLASSIFICATION
Based on grain size of individual particles, soils has been classified in to following
categories:
Gravel, sand, silt and clay
Gravel – size particle > 2mm
Sand – size of particle in between 0.2mm to 2mm
Coarse sand – 0.6mm to 2mm
Medium sand – 0.2mm to 0.6mm
Fine sand – 0.06mm to 0.2mm

7. GRAIN SIZE SOIL CLASSIFICATION
(CONTD.)
Silt – size of particle in between 0.006mm to 0.06mm
Coarse silt – 0.02mm to 0.06mm
Medium silt – 0.006mm to 0.02mm
Fine silt – 0.002mm to 0.006mm
Clay – size of particle < 0.002mm
Coarse clay – 0.0006mm to 0.002mm
Medium clay – 0.0002mm to 0.0006mm
Fine clay – less than 0.0002mm

8. TEXTURAL SOIL CLASSIFICATION
Textural classification system is based on grain size of different soil components which
contains different proportions of sand, silt and clay
Depend upon the fraction of soil component in the soil aggregate mix, soil classified as
 Loam
 Sandy loam
 Silty loam
 Silty clay loam
 Sandy clay loam

9. TEXTURAL CLASSIFICATION
US BUREAU OF PUBLIC ROADS

10. UNIFIED SOIL CLASSIFICATION
• The unified soil classification system has been very widely
accepted in general for the classification of soils for civil
engineering purposes
• According to this classification system,
the soils are divided into two broad groups,
based on grain size: Coarse grained soils and
Fine grained soils
• This soil classification system makes the use of results of sieve
analysis, liquid limit and plastic limit.

11. COARSE GRAINED SOILS SYMBOLS
1. Coarse grained soils: Gravels and Sands
Gravel (G) – Well graded (W)
Poorly graded (P)
Clay binder (C)
Containing considerable portion of silt (M)
Sand (S) – Well graded (W)
Poorly graded (P)
Clay binder (C)
Containing considerable portion of silt (M)

12. FINE GRAINED SOILS
• Generally silt and clay come under this category
• The soils with more than 50% passing 0.075mm sieve come under this
category
• Fine grained soils sub divided into two groups:
1. Soils with liquid limit <50% (or) soils with low compressibility (L)
2. Soils with liquid limit >50% (or) soils with high compressibility (H)

13. FINE GRAINED SOILS SYMBOLS
Soils with low compressibility (L) – Very fine sand (M)
Inorganic clay (C)
Organic soils (O)
Soils with high compressibility (H) – Very fine sand (M)
Inorganic clay (C)
Organic soils (O)

14. HRB CLASSIFICATION
• Highway Research Board (HRB) soil classification method is also Revised Public Roads
Administration (PRA) soil classification system
• This system of soil classification system is based on mainly three laboratory tests:
Sieve analysis
Liquid limit
Plastic limit
• Soils are divided in to seven groups A-1 to A-7.
• The soil groups A-1, A-2 and A-3 are granular soils, the proportion of fines passing
0.075mm sieve being less than 35%
• The soil groups A-4, A-5, A-6 and A-7 are fine grained soils, passing 0.074mm sieve
being greater than 35%

15. GROUP INDEX OF SOIL
• Fine grade soils of each classification group exhibit a wide range of properties
as subgrade material.
• 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, called as Group Index (GI).
• GI depends upon:
 % soil passing 75micron sieve
 Liquid limit
 Plastic limit

16. GROUP INDEX
GI = 0.2a+0.0005a.c+0.01b.d
Where,
a = that portion of % of soil passing IS 75 micron sieve greater than 35 and
not exceeding 55% (0 to 40).
b = that portion of % of soil passing IS 75 micron sieve greater than 15 and
not exceeding 55% (0 to 40).
c = that portion of liquid limit greater than 40% and not exceeding 30% (0
20).
d = that portion of the plasticity index greater than 10 and not exceeding
(0 to 20).

17. GROUP INDEX
• If GI value worked out is negative, it is reported as 0.
• GI values varies from 0 to 20.
A soil with GI = 0, is best soil
A soil with GI = 20, is poor soil for pavement
• If GI = 0 to 4, the soil is rated as “ excellent to good”.
If GI = 4 to 20, the soil is rated as “ fair to poor”.

18. INDEX PROPERTIES OF SOIL
• The proportion on which identification and classification of soils are based, are known
as index properties.
• Index properties which are usually used are:
 Grain size distribution
 Atterberg limits

19. GRAIN SIZE DISTRIBUTION
• The components of soils analysed through grain size distribution
 Coarse grained soils – sieve analysis
for non-cohesive soils – dry sieve analysis
for cohesive soils – wet sieve analysis
 Fine grained soils – Sedimentation analysis
Pipette method
Hydrometer method
• Grain size analysis is carried out to determine
Gradation of soils
Proportion of sand, silt and clay present in a soil sample

20. ATTERBERG LIMITS
• Mostly used for fine soils, Atterberg divide soil, the entire range from liquid state to
solid state into four states:
a) Liquid state
b) Plastic state
c) Semisolid state
d) Solid state
• Atterberg limits are the water contents at which soil mass passes from one state to the
next.
a) Liquid limit d) Plasticity index
b) Plastic limit e) Free swell index
c) Shrinkage limit

21. COMPACTION OF SOIL
• Compaction of soil is a mechanical process, by which the soil particles
are constrained to be packed more closely together by reducing the air
voids.
• Degree of compaction is usually measured quantitatively by dry density.
Objects of Compaction:
• Soil compaction causes decrease in air voids, increase in dry density and
increase in shearing strength
• The possibility of future settlement or compressibility decreases and also
the tendency for subsequent changes in moisture content decreases.

22. METHODS OF COMPACTION
In the field soil compaction may be carried out by different methods, such as:
1) Applying pressure on soil layers by means of static rollers
2) Ramming
3) Applying pressure and vibratory effect, using vibratory rollers

23. SUBGRADE SOIL STRENGTH
The factors on which the strength characteristics of soil depend are:
• Soil type
• Moisture content
• Dry density
• Internal structure of soil
• Type and mode of compaction

24. CBR TEST
• The California Bearing Ratio (CBR) test was developed by the California state highway
department, to evaluate the strength of: subgrade soil, stone aggregates and
other pavement materials
The CBR test method also standardized by BIS.
• CBR test results cannot be related accurately with any fundamental property of soil or
pavement material.
Principle:
The basic principle – is by causing penetration with a cylindrical plunger
of 50mm diameter into the specimen of soil/ pavement material at a rate of
1.25mm per minute.

25. CBR TEST (CONTD.,)
• The loads required for 2.5mm and 5.0mm penetration of the plunger into the soil
tested are recorded.
• The CBR value of the material tested is expressed as a percentage of standard load
value in a standard material.
• The standard values given below may directly be used to compute the CBR value of
the test material.
Penetration (mm) Standard load (kg) Unit standard load
(kg/cm2)
2.5 1370 70
5.0 2055 105

26. CBR METHOD – LABORATORY
EQUIPMENT MODEL APPARATUS
• Mould of 150mm diameter, with
a base plate and collar
• A loading frame with a
cylindrical plunger of 50mm
diameter
• Dial gauge to read deformation
• Proving ring attachment

27. CBR TEST PROCESS
• The specimen in the mould is compacted to maximum dry density at the optimum
moisture content. After subjecting the specimen for soaking of four days, the swelling
and water absorption values are noted.
• The values are noted corresponding to penetration values of 0.0, 0.5, 1.0, 1.5, 2.0, 2.5,
3.0, 4.0, 5.0, 7.5, 10.0 and 12.5mm.
CBR (%) =
𝑙𝑜𝑎𝑑 𝑎𝑡 2.5 𝑜𝑟 5.00𝑚𝑚 𝑝𝑒𝑛𝑒𝑡𝑟𝑎𝑡𝑖𝑜𝑛
𝑠𝑡𝑎𝑛𝑑𝑎𝑟𝑑 𝑙𝑜𝑎𝑑 𝑎𝑡 𝑐𝑜𝑟𝑟𝑒𝑠𝑝𝑜𝑛𝑑𝑖𝑛𝑔 𝑝𝑒𝑛𝑒𝑡𝑟𝑎𝑡𝑖𝑜𝑛
𝑋100
• CBR test is an arbitrary strength test, hence cannot be used to evaluate the soil
properties like cohesion, angle of internal friction or shear resistance.
• Presence of coarse grained particles would result in poor reproducibility
of CBR results. Material passing 20mm sieve is only used in the test.

28. PLATE BEARING TEST
• Plate load test is originally devised
to find the modulus of subgrade
reaction in westergaard’s analysis
for wheel load stresses in rigid
pavements.
• The plate load test is used to
evaluate the support power of
subgrade in-situ for use in
pavement design by using large
diameter plates.

29. PLATE LOAD TEST (CONTD.,)
The test apparatus consists of
• a set of plates of diameter 750,
600, 450 and 300mm.
• A loading device consisting of
jack, proving ring and a reaction
frame
• A datum frame far from loaded
area
• Dial gauges to measure
settlement of loaded plates

30. PLATE LOAD TEST (CONTD.,) – TEST METHOD
• The test site is levelled and the plate is properly seated on the prepared
surface.
• The stiffening plates of decreasing diameters are placed and the jack
and proving ring assembly are fitted to provide reaction against the
frame.
• Three or more dial gauges are fixed on the periphery of the plate from
the independent datum frame to measure the settlement values.
• A seating load to cause the pressure of 0.07kg/sq.cm (i.e. 320kg for
75mmØ plate) is applied and released after a few seconds. Settlement
measuring dial gauges are set to zero.

31. PLATE LOAD TEST (CONTD.,) – TEST METHOD
• A load sufficient to cause 0.25mm
average settlement is applied, readings
of the dial gauges are noted. The
corresponding load value from
proving ring also noted.
• The average settlement is determined
from the dial gauge readings.
• Again the load is increased till
settlement value increases to a further
amount of about 0.25mm and the
average settlement and load values are
noted.
• The procedure is repeated till the
settlement reaches 1.75mm (0.175cm).

32. MODULUS OF SUBGRADE REACTION
• Modulus of subgrade reaction (K), may be defined as
the pressure sustained per unit deformation of
subgrade at specified deformation or pressure level,
using specific plate size.
• The diameter of standard plate for finding K-value of
the subgrade is 750mm.
• But for finding highway pavements, a smaller plate of
300mm diameter is also used.