Flexible pavement design Flexible pavements are so named

Flexible pavement design Flexible pavements are so named because the total pavement structure deflects, or flexes, under loading. Design procedures For flexible pavements, structural design is mainly concerned with determining appropriate layer thickness and composition. The main design factors are stresses due to traffic load and temperature variations. Two methods of flexible pavement structural design are : Empirical design and mechanistic empirical design.

Empirical design An empirical approach is one which is based on the results of experimentation or experience. CBR method is widely known and will be discussed. Mechanistic-Empirical Design Empirical-Mechanistic method of design is based on the mechanics of materials that relates input, such as wheel load, to an output or pavement response. In pavement design, the responses are the stresses, strains, and deflections within a pavement structure.

Traffic and Loading Equivalent single wheel load To carry maximum load with in the specified limit and to carry greater load, dual wheel, or dual tandem assembly is often used. Equivalent single wheel load (ESWL) is the single wheel load having the same contact pressure, which produces same value of maximum stress, deflection, tensile stress or contact pressure at the desired depth.

Boyd and Foster method where P is the wheel load, S is the center to center distance between the two wheels, d is the clear distance between two wheels, and z is the desired depth.

Example 1 Find ESWL at depths of 5 cm, 20 cm and 40 cm for a dual wheel carrying 2044 kg each. The center to center tyre spacing is 20 cm and distance between the walls of the two tyres is 10 cm.

Repetition of Loads It is required to carry out traffic surveys for accounting the factor of repetition for wheel loads in the design of pavement. Such collected data are converted to some constant equivalent wheel loads. Equivalent wheel load accounting for repetition of load are those which require same thickness and strength of pavements. If the pavement structure fails with N 1 number of repetition of load W 1 and for the same failure criteria if it requires N 2 number of repetition of load W 2, then W 1 N 1 and W 2 N 2 are considered equivalent. An equivalent axle load factor (EALF) defines the damage per pass to a pavement by the ith type of axle relative to the damage per pass of a standard axle load.

Mc. Leod has given a procedure for evolving equivalent load factors for designing flexible pavements. Mc. Leod assumes that the pavement thickness which are deigned for a given wheel load would support 1000000 repetition of such load during the life of pavement. For one load application, the pavement thickness so required is only one fourth the pavement thickness designed for 106 load repetitions.

Wheel Load kg Repetition to failure, Number Equivalent to 2268 kg Equivalent Load factors 2268 105000 1. 0 1 2722 50000 2. 1 2 3175 22500 4. 7 4 3629 13000 8. 2 8 4082 6500 16. 3 16 4536 3300 32 4993 1700 61. 76 62 5443 1000 105 Equivalent load factors are employed to convert daily traffic count for each category of wheel load for design purposes.

Example: Calculate design repetitions for 20 year period for various wheel loads equivalent to 2268 kg wheel loads using the following traffic survey data on a four lane road. Wheel Average Daily Traffic Percent of Equivalent Load kg (both directions) total traffic Load volume factors 2268 13. 17 1 15. 30 2 11. 76 4 3629 14. 11 8 4082 6. 21 16 4536 5. 84 32 2722 3175 Total volume (considering traffic growth) 215

Wheel Load kg 1 A. D. T. Percent (both for each direction load s) 2 Days Number Equivale /year of years nt Load factors 3 4 5 6 2268 215 13. 17/100 365 20 1 2722 215 15. 30/100 365 20 2 3175 215 11. 76/100 365 20 4 3629 215 14. 11/100 365 20 8 4082 215 6. 21/100 365 20 16 4536 215 5. 84/100 365 20 32 Design repetitions equivalent of 2268 kg load 7=(2× 3× 4× 5× 6) Total estimated repetitions (two directions) = Design repetitions equivalent of 2268 kg wheel load per lane = /4

Design of Flexible Pavement by California Bearing Ratio Method: Design curves correlating CBR value with total pavement thickness cover were developed by California State Highway Department for wheel loads of 3175 kg and 5443 kg representing light and heavy traffic. Later the design curve for 4082 k wheel load was obtained by interpolation for medium traffic.

CBR Testing Machine Definition: It is the ratio of force per unit area required to penetrate a soil mass with standard circular piston at the rate of 1. 25 mm/min. to that required for the corresponding penetration of a standard material.

Equipments For CBR Test Cylindrical mould : Inside dia 150 mm , height 175 mm, detachable extension collar 50 mm height detachable perforated base plate 10 mm thick. Spacer disc 148 mm in dia and 47. 7 mm in height along with handle. Metal rammers. Weight 2. 6 kg with a drop of 310 mm (or) weight 4. 89 kg a drop 450 mm. Weights. One annular metal weight and several slotted weights weighing 2. 5 kg each, 147 mm in dia, with a central hole 53 mm in diameter. Loading machine. capacity of atleast 5000 kg , movable head or base that travels at an uniform rate of 1. 25 mm/min. Metal penetration piston 50 mm dia and minimum of 100 mm in length. Two dial gauges reading to 0. 01 mm. Sieves. 4. 75 mm and 20 mm Sieves.

Load vs Penetration The standard loads adopted for different penetrations for the standard material with a C. B. R. value of 100% Penetration of plunger (mm) Standard load 2. 5 1370 5. 0 2055 7. 5 2630 10. 0 3180 12. 5 3600 (kg)

Permissible Variation in CBR Value CBR (%) Maximum Variation in CBR Value 5 +_ 1 5 -10 +_ 2 11 -30 +_ 3 31 and above +_ 4

Following figure shows different curves based on traffic volume. This design chart is similar to the one followed in U. K.

It is possible to extend the CBR design curves for various loading conditions, using the following expression developed by U. S. Corps of Engineers: However these expressions are applicable only when the CBR value of the sub-grade soil is less than 12 percent. Here, t = pavement thickness, cm P = wheel load, kg CBR = California Bearing Ratio, percent p = tyre pressure, kg/cm 2 A = area of contact, cm 2

Determination of pavement thickness. Example: The CBR value of sub-grade soil is 5% calculate total thickness of pavement using: (i) Design curve developed by California State Highway Department. (ii) Design chart similar to the one followed in U. K. (iii) Design formula developed by U. S. Corps of Engineers. Assume 4100 kg wheel load or medium traffic or 200 commercial vehicles per day. Tyre pressure 6 kg/cm 2.

Solution: (i) 38 cm (ii) 37. 5 cm (iii) t = 35. 5 cm

Example: Design the pavement section using CBR charts similar to the one used in U. K. CBR values and traffic details are as under: (i) Soil sub grade with 4% CBR, (ii) Compacted soil with 7% CBR, (iii) Poorly graded gravel with 20% CBR, (iv) Well graded gravel with 95% CBR, (v) Minimum thickness of bituminous concrete surfacing may be taken as 5 cm. (vi) Number of vehicles for design 3260 veh/day.

Solution: 8 cm bituminous surfacing 13 cm well graded gravel CBR = 95% 19 cm poorly graded gravel CBR = 20% 15 cm compacted Soil CBR = 7% Soil Sub-grade CBR = 4% 21 cm 55 cm 40 cm

Example: Find out the total thickness of a flexible pavement to carry an average traffic of about 250 vehicles exceeding 3 tonne loaded weight. The CBR value of the sub-grade is 4%. Assume the wheel load as 4100 kg and tyre pressure as 6 kg/cm 2. Compare results as obtained by California State Highway charts and by U. S. Corps of Engineer formula.