Introductiony A highway pavement is a structure consisting of

superimposed layers of processed materials above the natural soil sub-gradey Primary function is to distribute the applied vehicle

loads to the sub-grade.

Highway Materialsy Pavements are a conglomeration of materials. y These materials, their associated properties, and their

interactions determine the properties of the resultant pavement.

Highway Materialsy Subgrade Soil y Road aggregates y Bituminous materials

Soily Soil is an accumulation or deposit of earth material,

derived naturally from the disintegration of rocks or decay of vegetation, that can be excavated readily with power equipment in the field or disintegrated by gentle mechanical means in the laboratory.

SUBGRADE SOILy The supporting soil beneath pavement and its special

under courses is called sub grade.y Function- To provide support to the pavement from

beneath.

y Natural sub grade -Undisturbed soil beneath the

pavement.y Compacted

sub

grade

Soil

compacted

by

controlled movement of heavy compactors.

Classified based on grain size as

y Gravel y Sand y Silt y Clay

Desirable PropertiesThe desirable properties of sub grade soil as a highway material are:y Stability(resistance to permanent deformation under loading) y Incompressibility (min. variation in volume) y Permanency of strength y Minimum changes in volume and stability under

adverse conditions of weather and ground watery Good drainage, and y Ease of compaction

Subgrade Soil StrengthStrength characteristics of soil depends ony Soil type y Moisture content y Dry density y Internal structure of the soil y Type and mode of stress application

Evaluation of Soil StrengthThe tests used to evaluate the strength properties of soils may be broadly divided into three groups:y Shear tests y Bearing tests y Penetration tests

Shear testsy Shear tests are usually carried out on relatively small soil

samples in the laboratory.y In order to find out the strength properties of soil, a

number of representative samples from different locations are tested.y Eg: Direct shear test, Triaxial compression test, and

Unconfined compression test. Vane shear test- soil sample or In situ

Bearing testsy Bearing tests are loading tests carried out on sub grade

soils in-situ with a load bearing area.y The results of the bearing tests are influenced by

variations in the soil properties within the stressed soil mass underneath and hence the overall stability of the part of the soil mass stressed could be studied.y Eg: Plate Bearing Test

Penetration testsy Penetration tests may be considered as small scale

bearing tests in which the size of the loaded area is relatively much smaller and ratio of the penetration to the size of the loaded area is much greater than the ratios in bearing tests.y The penetration tests are carried out in the field or in

the laboratory.y Eg: California Bearing Ratio Test (CBR), Cone

penetration tests

CBR Testy California Bearing Ratio (CBR) test was developed by

the California Division of Highway as a method of classifying and evaluating soil-sub grade and base course materials for flexible pavements.

CBR Test contd

y CBR is a penetration test wherein a standard piston,

having an area of

50 mm diameter , is used to

penetrate the soil at a standard rate of 1.25 mm/minute. The pressure up to a penetration of 12.5 mm and it's ratio to the bearing value of a standard crushed rock is termed as the CBR.

CBR Test contd

y California bearing ratio is defined as the ratio

(expressed as percentage) between the load sustained by the soil sample at a specified penetration of a standard plunger and the load sustained by the standard crushed stones at the same penetration.

CBR Test contd

CBR Test contd

CBR Test contd

Test Procedurey The laboratory CBR apparatus consists of a mould 150

mm diameter with a base plate and a collar, a loading frame and dial gauges for measuring the penetration values and the expansion on soaking.y Heavy compaction- 5 layers 56 blows, 4.89 kg hammer,

45cm fally Light compaction- 3 layers 56 blows, 2.6 kg hammer,

31cm fall

CBR Test contdy The specimen in the mould is soaked in water for four

days and the swelling and water absorption values are noted. The surcharge weight is placed on the top of the specimen in the mould and the assembly is placed under the plunger of the loading frame.y Load is applied on the sample by a standard plunger

with dia of 50 mm at the rate of 1.25 mm/min.

CBR Test contd

y Load

corresponding

to

penetration

values

of

0.0,0.5,1.0,1.5,2.0,2.5,3.0,4.0,5.0,7.5,10,1nd 12.5 mm are noted.y Calibration chart is used to convert division to load. y A load penetration curve is drawn. The load values on

standard crushed stones are 1370 kg and 2055 kg at 2.5 mm and 5.0 mm penetrations respectively.

CBR Test contd

y CBR value is expressed as a percentage of the actual

load causing the penetrations of 2.5 mm or 5.0 mm to the standard loads mentioned above. Therefore,

CBR Test contd

Design CBRy Two values of CBR will be obtained. y If the value of 2.5 mm is greater than that of 5.0 mm

penetration, the former is adopted.y If the CBR value obtained from test at 5.0 mm

penetration is higher than that at 2.5 mm, then the test is to be repeated for checking.

CBR Test contd

y If the check test again gives similar results, then higher

value obtained at 5.0 mm penetration is reported as the CBR value.y The average CBR value of three test specimens is

reported as the CBR value of the sample.

CBR Test contd

Initial ConcavityThe load penetration curve may show initial concavity due to the following reasons:y The top layer of the sample might have become too

soft due to soaking in watery The surface of the plunger or the surface of the sample

might not be horizontal

CBR Test contd

Correctiony Draw a tangent to the load-penetration curve where it

changes concavity to convexityy The point of intersection of this tangent line with the

x-axis is taken as the new originy Shift the origin to this point (new origin) and correct

all the penetration values

CBR Test contd

Design CBRy The average CBR values corresponding to 2.5 mm and 5

mm penetration values should be worked out.y If the average CBR at 2.5 mm penetration is more than that

at 5 mm penetration, then the design CBR is the average CBR at 2.5 mm penetration.y If the CBR at 5mm penetration is more than that at 2.5 mm

penetration, then the test should be repeated. Even after the repetition, if CBR at 5mm is more than CBR at 2.5 mm, CBR at 5 mm could be adopted as the design CBR.

CBR Test contd

Limitationsy Cannot be used to evaluate soil properties like

cohesion or angle of internal friction or shearing resistance.y Material passing 20mm sieve only is used in the test.

Modulus of Subgrade Reactiony Modulus of subgrade reaction is the reaction pressure

sustained by the soil sample under a rigid plate of standard diameter per unit settlement measured at a specified pressure or settlement.y IRC specifies that the K value be measured at 1.25 mm

settlement.

Modulus of Subgrade Reaction

Plate Bearing Testy Plate Bearing Test is conducted to calculate the

Modulus of Subgrade Reaction.y In this a compressive stress is applied to the soil

pavement layer through rigid plates of relatively large size and the deflections are measured for various stress values.y The exact load deflection behavior of the soil or the

pavement layer in-situ for static loads is obtained by the plate bearing test.

Plate Bearing Test contd

Apparatusy Bearing Plates: Consist of mild steel plate 75 cm in

diameter and 0.5 to 2.5 cm thickness and few other plates of smaller diameters (usually 60, 45, 30 and 22.5 cm) used as stiffeners.y Loading Equipment: Consists of a reaction frame

and a hydraulic jack. The reaction frame may suitably be loaded to give the needed reaction load on the plate.

Plate Bearing Test contd

y Settlement Measurement: Three or four dial gauges

fixed on the periphery of the bearing plate. The datum frame should be supplied far from the loading area.

Plate Bearing Test contd

Plate Bearing Test contd

Plate Bearing Test contd

Procedurey Prepare the test site and remove the loose material so

that 75 cm diameter plate rests horizontally in full contact with the soil sub-grade.y Place the plate accurately and then apply a seating load

equivalent to a pressure of 0.07 kg/cm2 (320 kg for 75 cm diameter plate) and release it after few seconds.y The settlement dial readings are adjusted to zero for

zero load.

Plate Bearing Test contdy A load is applied by means of the jack, sufficient to

cause an average settlement of about 0.25 mm.y When there is no perceptible increase in settlement or

when the rate of settlement is less than 0.025 mm per minute, the load dial reading and the settlement dial readings are noted.y The average of the three or four settlement dial

readings is taken as the average settlement of the plate corresponding to the applied load.

Plate Bearing Test contd

y The load is increased till the average settlement

increased to a further amount of about 0.25 mm and the load and the average settlement readings are noted as before.y The procedure is repeated until the settlement is about

1.75 mm.

Plate Bearing Test contd

Calculationy A graph is plotted with the mean settlement (cm) on x

axis and load (kg/cm2) on y-axis.y The pressure p corresponding to a settlement of .125

cm is obtained from the graph. The modulus of subgrade reaction K is calculated from the relation ,y K = P/0.125 kg/cm2 /m or kg/m3 y K = P/0.00125 kN/m2 /m or kN/m3

Pl t B ri g

st c

t

Corrections(1) Allowance for worst subgrade moisturey The modulus of subgrade reaction found by this

method depends on the moisture content of the soil at the time of testing.y The minimum value of K is obtained at the time of

soaking condition.y For K value in soaking condition, a correction factor

must be applied to the K value found at the prevailing field moisture content.

Allowance for worst subgrade moisture:(Contd .)y Two samples are collected from the site where the plate

load test was conducted.y One sample is subjected to laboratory compression

test under unsoaked condition and the other in soaked condition.y Pressure sustained by the soil samples at different

settlements is plotted.

y The correction factor for soaking is taken as Ps/P

which will be less than 1.0 .y Therefore, Modulus of Subgrade for soaking condition

is,

(2) Correction for plate sizey A heavy reaction load is required for soils with high K

value when a plate of diameter 75 cm is used for testing.y If the reaction load is to be reduced a plate of smaller

diameter has to be used.

y If K1 is the modulus of subgrade of smaller size plate

and a1 is the diameter of the smaller plate, then the K value corresponding to the standard plate (diameter a) is,

THANK YOU