4.4PAVEMENT DESIGNThe pavement design process is the technique of developing a combination of top layers of different materials to cater for the total axle load over the design life of a road. In other words this is an art through which the stresses as induced in to layers of a road due to movement of heavy wheel load is disseminated and minimized to a safe level through selection of different type and appropriate thickness of pavement layers.In order to carryout pavement design, following parameters in terms of axle loading and soil strength are required:

4.4.1Equivalent Axle LoadsThe damage caused by vehicles to a road depends on the axle loads and wheel configuration of the vehicles. It is therefore important to determine the axle loads of heavy commercial vehicles in the projected traffic mix (Refer Traffic data) that is likely to use proposed roads.

In order to determine the cumulative axle load damage that a pavement will sustain during its design life, it is necessary to express the total number of heavy vehicles that will use the road during the design period in terms of the cumulative number of Equivalent Single Axles Load (ESALs).

I. Design LifeDesign life is the number of years reckoned from the completion of pavement construction and application of traffic load until the time when major maintenance is required so that it can continue to carry traffic satisfactorily for further period.A design period of 20 years has been adopted. However for asphalt layers, stage construction is suggested; the asphalt requirement for ten years design life is ascertained and shall be placed so that the pavement can perform satisfactorily for 10years. After 10 years, the fresh traffic count will be taken and pavement condition survey will be conducted to ascertain distress in the pavement. Accordingly asphaltic overlay will be placed without adding in the granular layers.II. Cumulative Equivalent Single Axle LoadsTraffic load is converted into Equivalent Single Axle Load (ESALs). ESALs is related to a standard axle of 8.16 tones (18000 lbs), using equivalence factors, which have been derived from empirical studies. The ESALs as worked out for roads with different ROWs within Design Life (Refer Annexure-A) are as under;Table 4.5 Equivalent Single Axle LoadsSr. No..R.O.W

ESALs

10 Years20 Years

161m 10.536

236m 3.3210

325m 2.37

4.4.2 Design CBRThe design of pavement is based on the Subgrade Soaked CBR 14 at 95% Modified AASHTO material for the project. The detail is provided in the Geotechnical Investigations Report attached as Annexure C.4.4.3 Design MethodologyThe pavement design has been carried out as per AASHTO guide (1993) based on the following main parameters.

4.4.3.1 AASHTO Procedure for Pavement Design

The AASHTO Guide for Pavement Design 1993 outlines this procedure for determination of flexible pavement thickness by solving AASHTO equations manually, by using different nomographs or by using the computer software. For accuracy the computer program is preferred. In all options basically the Structure Number (SN) required to be assigned to the proposed pavement structure for a given set of conditions is determined by solving the following numerical equation:

The estimated future traffic in terms of ESALs for the design period, W18The reliability level, R Standard normal deviate Value, ZRThe overall standard deviation, SoThe roadbed soil resilient modulus, MRThe design serviceability loss, PSI = Po Pt

The ESALs have been provided at Table 4.5. The other general design variables have been discussed in the following paragraphs.

4.4.3.2 Reliability (R)

Design reliability refers to the degree of certainty that a given design alternative will last for the entire design period. A design reliability level of 90% has been adopted for pavement design of the Project Road.

4.4.3.3 Standard Deviation (SO)

The reliability factor is a function of the overall standard deviation that accounts for standard variation in materials and construction, the probable variation in the traffic prediction and the normal variation in pavement performance for a given design traffic application. The recommended value of standard deviation for total variation in material properties and in traffic estimation for flexible pavement is 0.45 and has been adopted for pavement design of project road.

4.4.3.4 Standard Normal Deviation (ZR)

The value corresponding to reliability (R) of 90% is -1.282 which has been adopted in the design based on the recommended values of standard normal deviation (ZR) by AASHTO Guide 1993.

4.4.3.5 Performance Criteria

The serviceability of a pavement is defined as its ability to serve the type of traffic that uses the facility. Initial and terminal serviceability indices have been established to compute the total change in serviceability that will be used in the design equations.

I. Initial Serviceability Index (Po)

The initial serviceability index is a function of pavement design and construction quality. For flexible pavement design typical value as recommended by AASHTO Road Test is 4.2 which has been adopted.

II. Terminal Serviceability Index (Pt)

The terminal serviceability index is the lowest index that will be tolerated before rehabilitation, resurfacing or reconstruction becomes necessary and it generally varies with the importance or functional classification of the pavement. Recommended value of terminal serviceability index is 1.7 for the project road.

4.4.3.6 Resilient Modulus MR

The basis for material characterization in the AASHTO Guide 1993 is Elastic or Resilient Modulus (MR). In the absence of necessary equipment required to determine resilient modulus of subgrade, following correlation between CBR and MR has been used.

MR = 2555 (CBR) 0.64Where MR is resilient modulus in psi.

4.4.3.7 Computation of Required Pavement Thickness

The structural number (SN) requirement as determined through adoption of design parameters as discussed above is balanced by providing adequate pavement structure. Under AASHTO design procedure the following equation provides the means for converting the structural number into actual thicknesses of surfacing, base and subbase materials:SN = a1 D1 + a2 D2 m2 + a3 D3 m3where:

a1, a2, a3=layer coefficients representative of surface, base and subbase courses respectively

D1, D2, D3=actual thicknesses (in inches) of surface, base and subbase courses respectively

m2, m3=drainage coefficients for base and subbase layers respectively

4.4.3.8 Recommended Values of Layer Coefficients

Asphaltic Wearing Course, a1= 0.40 / inch(0.157 / cm)

Asphaltic Base Course, a1= 0.40 / inch (0.157 / cm)

Aggregate Base Course, a2= 0.13 / inch (0.051 / cm)

Granular Subbase, a3= 0.125 / inch (0.049 / cm)

4.4.3.9 Pavement Thickness

The pavement thicknesses thus worked out exploiting AASHTO approach for pavement design are as under, subject to enforcement of Load restrictions:

Table 4.6 Pavement Thickness for Road (ROW : 61 m)

LAYERLAYER THICKNESS (cm)

Asphaltic Concrete Wearing Course5

Asphaltic Concrete Base Course 12

Aggregate Base Course25

Subbase Course15

Subgrade CBR 14% at 95% MDD

Table 4.7 Pavement Thickness for Road (ROW : 36 m)

LAYERLAYER THICKNESS (cm)

Asphaltic Concrete Wearing Course5

Asphaltic Concrete Base Course 9

Aggregate Base Course25

Subbase Course15

Subgrade CBR 14% at 95% MDD

Table 4.8 Pavement Thickness for Road (ROW : 25 m and 18m)

LAYERLAYER THICKNESS (cm)

Asphaltic Concrete Wearing Course5

Asphaltic Concrete Base Course 8

Aggregate Base Course25

Subbase Course20

Subgrade CBR 14% at 95% MDD

GENERAL RECOMMENDATIONSThe geotechnical investigations revealed that soil consist of Lean Clay, Sandy Lean Clay & Sandy Silty Clay. For the construction of the sub grade soil with CBR value of 14 and 95% Modified AASHTO should be used. Whereas it would be desirable to use materials with minimum CBR values of 50% and 80% for sub-base and water bound macadam, respectively. For a roadway to perform well, it is imperative that the subgrade for the roadway should be competent to support the anticipated traffic loads. It is, therefore recommended that the subgrade should be properly prepared to meet the design CBR. In order to meet this requirement, all the areas that will support roadway, should be properly cleared and grubbed by removing any top soil. Any wet, soft or loose soils pockets should also be replaced with improved soil, as the result of proof-rolling.For layer thickness and compaction following levels are recommended for various pavement elements.Material TypeMaximum Compacted Layer Thickness (cm)Recommended Modified AASHTO Compaction (%)

Water Bound Macadam10100

Sub-base1098

Sab-base & general fillUpper 30 cm (subgrade)30 cm-70 cm (fill)Below 70 cm (fill)151515959390

4 - 2

i) Approach RoadsThe project site is adjacent to Motorway M-2.Two approach roads have been proposed to provide access to the apparel park. The one is proposed from existing bridge after 200m from Upper Chenab (UC) canal and second from Sheikhupura Interchange. Each approach road is about 2.5 km long. Approach roads are shown in Figure. 4.2

Figure 4.2 Approach Road

Design of interchange and approach road are being carried out by some other departments (C& W Punjab, NHA)

ii)Typical Cross SectionsRoad Cross-sections are shown in Figure 4.3 to Figure 4.5. Widths allocated to different components of the roads along with the cross-section are provided in the Table 4.5.

Table 4.5 Cross Sectional Details of Roads

ROADTYPER.O.WWIDTH OF CARRIAGE WAYLANE WIDTHWIDTH OF SHOULDERSWIDTH OF MEDIANWIDTH OFFOOTPATHWIDTH OF SERVICE ROADWIDTH OF UTILITY CORRIDOR

(m)(m)(m)(m)(m)(m)(m)(m)

Approach Roads/Main Roads6110.83.627.4275

Secondary367.23.624.62-4.5

Tertiary253.63.62-2-4.9

150mm

Figure 4.3 Typical X-Section of 25m R.O.W150mm

150mm

Figure 4.4 Typical X-Section of 36m R.O.W

Figure 4.5 Typical X-Section of 61m R.O.W