2. Soil strength.

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Subgrade Strength


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Subgrade (SG) Soil Strength

The main factors affecting SG strength are:- Particle Size: SG strength increase with the increase in particle size,

Bulk Density: SG strength increase with the increase of its bulk

density which increase with compaction,

Moisture Content: SG strength increase with the decrease of moisture,

Permeability: SG strength increase with the increase of its permeability, but

Uniform grade are more permeable than well graded but well graded SG are stronger.

Well graded allows the voids between particle are filled with smaller particles whichgive strength.

Uniform graded give large voids which increase permeability but reduce its strength.


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Shear strength parameters (soil cohesion value and angle of friction), modulus ofsubgrade reaction and/or CBR values are used to describe SG strength. Four

groups are used include shear tests, bearing tests, penetration tests, and cyclic

tests.

Group I: Shear Tests

It used to determine shear characteristics of soil at a point such as:

- Triaxial compaction (lab test) to determine, (C, )

- Unconfined compaction test (lab test),

- Direct shear test (lab test), granular soil,

- Van shear test (field test).

Measuring of SG Soil Strength


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Triaxial compaction(Using Mohrs Circle)


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Direct shear test


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Group II: Bearing Tests- Plate loading test (field test) to determine the coefficient of subgrade reaction

(k, kg/cm3) ( ) which used for design of the rigid pavement.The result of this test are expressed as a relation between the acting

compression load and resulting deflection.


Deflection , cm

Stress, kg/cm2

P

0.125

K = P/0.12 5


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Group III: Penetration Tests

- These types of tests, such as CBR tests, can be conducted in both the

lab and the field .


7

ImmersionCompaction Load


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(OMC) :10 (100.%)

.

(2.)

2.5 .

15 .

(0.0250.050.0750.10.150.20.30.40.5).

.

.

(P) 0.1&0.2 (Ps.)


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0.50 0.4 0.3 0.2 0.1 (Inch)

2600 2300 1900 1500 1000 (PSI)

()


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Example0.50.40.30.20.1Pent. inch

900850800700550(Soil A), %850800700500210(Soil B), %

Find CBR for two soils:

Draw the charts for two soils

Soil A:CBR0.1 = (550/1000)*100 = 55%

CBR0.2 = (700/1500 )*100= 47%

Then CBR = 55%

Soil B:CBR0.1 = (330/1000)*100 = 33%

CBR0.2 = (550/1500 )*100= 37%

Then Repeat the test to take the higher value

If it gives the same results

CBR = 37%

AB


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Group IV: Cyclic Tests

- Is a lab tests conducted under cyclic loading to measure the SG

resilient modulus.

Mr = applied stress/elastic part of axial strain

Mr = 1500 CBR CBR10

Mr = 1155 + 55 (R-Value) R < 2

Measuring of SG Strength


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12

Compaction

GOOD COMPACTION LEADS

TO GOOD PERFORMANCE


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:-

.

(Soil Strength)

(Shear Strength.)


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:-

.1 (Sand Cone Test)

.2 (Proctor Test)

.3=( / (Proctor Test))100

.4 95.%


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()

Mould

ModifiedStandard

6

4

6

4

() 152.4101.60152.4101.60

() 116.43116.43116.43116.43

(3) 21249442124944

() 44.524.544.524.5

562556255533

() 457457305305


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4/3

(7). .556.

(4)...(500)

(110+5).(2% )

.

.


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d=w

1 + wc


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ModifiedStandard


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Mold No. units 1 2 3 4

Sample and mold mass g 12141.00 12498.00 12655.00 12553.00

Mold mass, g 7684.00 7684.00 7684.00 7684.00

Wet sample mass g 4457.00 4814.00 4971.00 4869.00

Wet density g/cm3 2.12 2.29 2.36 2.31

Moisture content % 3.93 5.79 8.12 8.63

Dry density g/cm3 2.04 2.16 2.19 2.13

Can No. units 1 2 3 4

Gross wet mass g 332.60 338.56 291.38 321.47

Gross dry mass g 321.09 321.60 271.46 298.19Moisture mass g 11.51 16.96 19.92 23.28

Tare mass g 28.20 28.59 26.14 28.48

Dry soil mass g 292.89 293.01 245.32 269.71

Moisture content % 3.93 5.79 8.12 8.63

Gs Specific Gravity of Soil Solids 2.60 2.65 2.70 2.70

Mold Volume = 2104


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2.038

2.1632.185

2.130

1.80

1.90

2.00

2.10

2.20

2.30

2.40

0.00 2.00 4.00 6.00 8.00 10.00 12.00

MOISTURE CONTENT %

DRYDENSITY(g/cm3


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2. Soil strength.

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