4.4 Overall Standard Deviation
The value of overall standard deviation (So) in the AASHTO design procedure expresses the possible variation in traffic and material that is going to affect the pavement performance. The AASHTO guide states:
1. The estimated overall standard deviation for the case where the variance of the projected future traffic is considered (along with other variance associated with revised pavement performance models) are 0.39 for rigid and 0.49 for flexible pavements.
2. The estimated overall standard deviation for the case where the variance of the projected future traffic is not considered (along with other variance associated with revised pavement performance models) are 0.34 for rigid and 0.44 for flexible pavements.
The range of So value provided in AASHTO design guide are based on the values identified above.

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0.30 – 0.40 Rigid Pavement
0.40 – 0.50 Flexible Pavement
A value of 0.45 was used for the analysis.
4.5 Drainage Coefficient
The drainage coefficient is needed to study the impact of presence of water in the unbound base and sub base layers. This will result in an increase and decrease of the layer thickness depending upon the drainage quality. Drainage coefficient for the subject roads has been assumed as 1.0.
4.6 Granular Base / Sub base and Asphalt Layer
The specification for granular sub base states a CBR > 30 percent. For evaluating the existing structural capacity of the granular base, back calculation was used to estimate the resilient modulus. The layer coefficient is then estimated from the resilient modulus by the following correlation:

Where a3 is the layer coefficient for the sub base layer. In the above equation, Mr Value of 22,000 psi was used keeping in view availability of material that meets this specification value. Layer Coefficient is computed as 0.14.
The specification for the aggregate base requires a CBR > 100 percent. This corresponds to a modulus value of 45,000 psi approximately. The layer coefficient for the aggregate base is obtained by the following relationship.

For a modulus value of 45,000psi, the estimated layer coefficient is 0.17.
Finally the modulus of asphalt layer is estimated at 435,000 psi for new construction or for overlay condition. Since asphalt is a thermoplastic material that is its properties are dependent on temperature. The modulus of 435,000 psi is at 68oF, and this corresponds to a structural layer coefficient of 0.44.
4.7 Sub Grade Strength
For structural part, it is important that laboratory evaluation of sub grade soil be carried out. For the project road, destructive testing was carried out for determination of the CBR in the laboratory. The CBR Value @ 95% compaction of sub grade is 8.4 %. Summary of design parameters us as under:

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Location
(Km) Sieve Analysis
%
Passing Sieve: Plasticity
Tests AASHTO
Soil Classi -
fication Field
Density
Test Modified AASHTO
Compaction Test (T-180) Relative
Compact-ion
(%) Soaked CBR
at Compaction (%): Swell (%)
#10 #40 #200 LL
(%) PL
(%) Pl
(%) M-145 Field Moisture
Content (%) Field Dry
Density
(gm/cc) Optimum Moisture
Content (%) Max.
Dry
Density (gm/cc) 100 95 90 Relative
Compaction




1+000 98.6 86.9 48.0 Non-Plastic A-4(0) 8.9 1.8 11.8 1.9 91.6 10.9 9.0 5.9 7.1 1.23 to 1.58
2+700 22.5 18.5 4.0 A-4(1) 11.5 1.7 12.5 1.9 90.8 9.4 7.8 5.3 5.8 0.81 to 1.21
10.1 8.4
(S-4) 5.6 6.5
(S-3) Average

Summary of Design Parameters
Design Factor Review Recommendation
Design Life 10 years
Traffic (ESAL’s) 17.46 Million
Design Reliability (R) 90 %
Standard Deviation (So) 0.45
Initial Serviceability (Po) 4.2
Terminal Serviceability (Pt) 2.0
Drainage Coefficient 1.0
Subgrade CBR / Mr 8.4% /9,975 psi
Aggregate Base Course 45,000 psi (0.17)*
Subbase 22,000 psi (0.14)*
Asphalt Concrete Layer 435,000 psi (0.44)*
* Numbers in the parenthesis are layer coefficients





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4.8 Thickness Design:
For new construction / widening structural number required is:
Structural No. SN3 = 4.57
Layer coefficient a1 = 0.44
Layer coefficient a2 = 0.17
Layer coefficient a3 = 0.14
Design thicknesses based on above structural Number have been computed as Following:
Asphalt Wearing Course = 60 mm
Aggregate Base Course = 300 mm
Sub Base = 280 mm
SN provided = 4.59

4.9 CONSTRUCTION METHODOLOGY FOR REHABILITATION / WIDENING :
• For widening portions surplus / unsuitable material to be excavated and surface obtained as per design / drawing for sub grade level will be compacted as sub Grade @ 95% as per specifications.
• Sub Base (280mm thickness for 2 Lane carriageway & 200 mm for Service road / Utility corridor) and Aggregate Base Courses (300mm thickness for 2 Lane carriageway & 200 mm for Service road / Utility corridor) will be laid / compacted as per drawings and specifications.
• Existing road to be scarified and existing pavement structure will be removed / reuse as sub Base material in widening portion if found suitable as per design / drawing.
• After compacting scarified surface of existing road on sub grade level, Sub Base & Aggregate Base course will be compacted as per design / drawing & specification.
• Final blacktop layer (Asphaltic Wearing course 60 mm thick) will be provided over primed surface of approved Aggregate Base Course as per finished road levels as per design / drawing & specification.