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**Engr. Ejaz Ahmad Khan Deputy Director Pakhtunkhwa Highways Authority**

PRESENTATION ON ROAD PAVEMENT DESIGN BY Engr. Ejaz Ahmad Khan Deputy Director Pakhtunkhwa Highways Authority

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CEE 320 Steve Muench

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**OUTLINE Pavement Structure Design of Pavement Structure**

Section - 1 Pavement Structure Section - 2 Design of Pavement Structure Section - 3 Flexible Pavement Design Section - 4 Section - 5 How to Design Practical Example

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Section - 1 Pavement Structure

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PAVEMENT : Combination of various layers between road top surface / Finished Road Level (FRL) and the subgrade is known as pavement structure. Pavement Structure:

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**CHAPPAR - DARBAND ROAD (30 KM) PHASE-I**

PAVEMENT PURPOSE Load support Skid Resistance Good ride Less VOC Time Saving Drainage CHAPPAR - DARBAND ROAD (30 KM) PHASE-I

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**PHILOSOPHY OF PAVEMENTS**

Pavements are subjected to moving traffic loads that are repetitive in nature. Each traffic load repetition causes a certain amount of damage to the pavement structure that gradually accumulates over time and eventually leads to the pavement failure. Thus, pavements are designed to perform for a certain life span before reaching an unacceptable degree of deterioration. In other words, pavements are designed to fail. Hence, they have a certain design life.

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**PAVEMENT TYPES Flexible Pavement Hot mix asphalt (HMA) pavements**

Called "flexible" since the total pavement structure bends (or flexes) to accommodate traffic loads The load transmit to the subgrade through particle to particle contact. Rigid Pavement Portland cement concrete (PCC) pavements Called “rigid” since PCC’s high modulus of elasticity does not allow them to flex appreciably The load transmit to subgrade through beam action.

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**FLEXIBLE PAVEMENT Structure Surface course Base course Subbase course**

Subgrade

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RIGID PAVEMENTS In rigid pavements the stress is transmitted to the sub-grade through beam/slab effect. Rigid pavements contains sufficient beam strength to be able to bridge over localized sub-grade failures and areas of inadequate support. Thus in contrast with flexible pavements the depressions which occur beneath the rigid pavement are not reflected in their running surfaces.

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**RIGID PAVEMENT Structure Surface course Base course Subbase course**

Subgrade

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**Design of Pavement Structure**

Section - 2 Design of Pavement Structure

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**PAVEMENT THICKNESS DESIGN**

Pavement Thickness Design is the determination of required thickness of various pavement layers to protect a given soil condition for a given wheel load. Given Wheel Load 150 Psi 3 Psi Given In Situ Soil Conditions Asphalt Concrete Thickness? Base Course Thickness? Subbase Course Thickness?

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DESIGN PARAMETERS Subgrade Loads Environment

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**Picture from University of Tokyo Geotechnical Engineering Lab**

SUBGRADE Characterized by strength and/or stiffness California Bearing Ratio (CBR) Measures shearing resistance Units: percent Typical values: 0 to 20 Resilient Modulus (MR) Measures stress-strain relationship Units: psi or MPa Typical values: 3,000 to 40,000 psi Picture from University of Tokyo Geotechnical Engineering Lab

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**SUBGRADE Some Typical Values Classification CBR MR (psi)**

Typical Description Good ≥ 10 20,000 Gravels, crushed stone and sandy soils. Fair 5 – 9 10,000 Clayey gravel and clayey sand, fine silt soils. Poor 3 – 5 5,000 Fine silty sands, clays, silts, organic soils.

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**TRAFFIC LOADS CHARACTERIZATION**

Pavement Thickness Design Are Developed To Account For The Entire Spectrum Of Traffic Loads Cars Pickups Buses Trucks Trailers

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**Axle loads bigger than 8.2 tons cause damage greater than one per pass **

EQUIVALENCY FACTOR Equivalent Standard ESAL Axle Load Ibs (8.2 tons) Damage per Pass = 1 Axle loads bigger than 8.2 tons cause damage greater than one per pass Axle loads smaller than 8.2 tons cause damage less than one per pass Load Equivalency Factor (L.E.F) = (? Tons/8.2 tons)4

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**Consider two single axles A and B where:**

EXAMPLE Consider two single axles A and B where: A-Axle = 16.4 tons Damage caused per pass by A -Axle = (16.4/8.2)4 = 16 This means that A-Axle causes same amount of damage per pass as caused by 16 passes of standard 8.2 tons axle i.e., 16.4 Tons Axle = 8.2 Tons Axle

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**AXLE LOAD & RELATIVE DAMAGE**

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**SERVICEABILITY CONCEPT OF PAVEMENT**

Serviceability is the ability of a pavement to serve the commuters for the desired results for which it has been constructed within the designed life and without falling the Terminal level (TSI). Present Serviceability Index (PSI) Present Serviceability is defined as the adequacy of a section of pavement in its existing condition to serve its intended use. Terminal Serviceability Index (TSI) It is defined as that stage of the pavement condition after which it is not acceptable for the road users.

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**SERVICEABILITY CONCEPT OF PAVEMENT**

Defined by users (drivers) Develop methods to relate physical attributes to driver ratings Result is usually a numerical scale From the AASHO Road Test (1956 – 1961)

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**Present Serviceability Index (PSI)**

Values from 0 through 5 Calculated value to match PSR SV = mean of the slope variance in the two wheel paths (measured with the profile meter) C, P = measures of cracking and patching in the pavement surface C = total linear feet of Class 3 and Class 4 cracks per 1000 ft2 of pavement area A Class 3 crack is defined as opened or spilled (at the surface) to a width of in. or more over a distance equal to at least one-half the crack length A Class 4 is defined as any crack which has been sealed. P = expressed in terms of ft2 per 1000 ft2 of pavement surfacing.

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PSI vs. Time p0 Serviceability (PSI) p0 - pt pt Time

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**PAVEMENT THICKNESS DESIGN Comprehensive Definition**

Pavement Thickness Design is the determination of thickness of various pavement layers (various paving materials) for a given soil condition and the predicted design traffic in terms of equivalent standard axle load that will provide the desired structural and functional performance over the selected pavement design life i.e. the serviceability may not fall below the TSI.

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Section - 3 Flexible Pavement Design

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Flexible Pavements A flexible pavement absorbs the stresses by distributing the traffic wheel loads over much larger area, through the individual layers, until the stress at the subgrade is at an acceptably low level. The traffic loads are transmitted to the subgrade by aggregate to aggregate particle contact. A cone of distributed loads reduces and spreads the stresses to the subgrade.

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**TYPICAL LOAD & STRESS DISTRIBUTION IN FLEXIBLE PAVEMENTS.**

Wheel Load Bituminous Layer Vertical stress Foundation stress Sub-grade

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**EFFECT OF PAVEMENT THICKNESS ON STRESS DISTRIBUTION**

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**BASIC EQUATION OF AASHTO PROCEDURE FOR FLEXIBLE PAVEMENT DESIGN.**

The various terms/parameters which are used in the basic equation of AASHTO Procedure for the Design of flexible pavements are: i). W18 (ESAL): It is the accumulated traffic load converted to 18-kips or 8.2 tons. This is also known as 18-kips Equivalent Standard Axle Load (ESAL). That the pavement will experience over its design life.

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**ii). Standard Deviation (S0):**

Standard deviation accounts for standard variation in materials and construction, the probable variation in traffic prediction and variation in pavement performances for a given design traffic application. The recommended value of S0 for flexible pavement is 0.4 to 0.5. iii) Reliability (R): Design Reliability refers to the degree of certainty that a given pavement section will last for the entire design period with the traffic & environmental condition. The recommended level of reliability for freeways in rural areas varies from 80% to 95%. A high reliability value will increase the thickness of pavement layer and will result in expensive construction.

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**TABLE FOR REALABLILITY Recommended Level of Reliability ( R )**

Functional Classification Reliability (%) Urban Rural Interstate and Other Freeways Principal Arterial 80-99 75-95 Feeders 80-95 Local 50-80

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**Standard Normal Deviate (ZR)**

iv). Standard Normal Deviate (ZR): It is defined as the probability that serviceability will be maintained at adequate levels from a user’s point of view throughout the design life of the facility. This factor estimates the probability that the pavement will perform at or above the TSI level during the design period and it accounts for the inherent uncertainty in design. The relationship of reliabilities with ZR is given in the table: Value of (ZR) Reliability R (%) Standard Normal Deviate (ZR) 50 0.000 60 -0.253 70 -0.524 75 -0.674 80 -0.841 85 -1.037 90 -1.282 91 -1.340 92 -1.405 93 -1.476 94 -1.555 95 -1.645 96 -1.751 97 -1.881 98 -2.054 99 -2.327 99.9 -3.090 99.99 -3.750

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**Definition of Structural Number**

v). Structural No (SN): Structural No is the total structural strength value required to cater for the cumulative equivalent standard axles load (CESAL) during design life so that the serviceability may not fall below the Terminal serviceability Index (TSI) Definition of Structural Number Subgrade Structural Coefficient (a): a = fnc (MR) SN = SN1 + SN2 + SN3

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**vi) Loss of Serviceability Index ∆ PSI.**

∆ PSI = Initial Serviceability Index – Terminal Serviceability Index The recommended value for initial serviceability index is 4.2 while for terminal serviceability index it is to 2 to 2.5. ∆ PSI = 4.2 – 2.5 = 1.7 p0 Serviceability (PSI) p0 - pt pt Time

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**vii). Resilient Modulus (MR):**

It is defined as repetitive or cyclic stress divided by recoverable strain. Resilient Modulus is a measure of stiffness of the soil. MR = Repetitive stress / recoverable strain MR can be determined from the resilient modules test in the laboratory or from the following equations: MR = 1500 * CBR for CBR < 10 % MR = 2555 (CBR)0.63 for any value of CBR

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**viii). Computation of Required Pavement Thicknesses**

The structure 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 for the means for converting the structural number into actual thickness of surfacing, base and sub base materials. SN = a1D1 + a2D2m2 + a3D3m3 _______________ (2) a1, a2, a3 = Layer coefficients representative of surface, base and subbase courses respectively. It depends upon the modulus of resilient. D1. D2, D3 = Actual thicknesses (in inches) of surface, base and subbase courses respectively. m2, m3 = Drainage coefficients for base and subbase layers respectively.

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This equation does not have a single unique solution. There are many combinations of layer thicknesses that can be adopted to achieve a given structural number. There are, however, several design, construction and cost constraints that may be applied to reduce the number of possible layer thicknesses combinations and to avoid the possibility of constructing an impractical design. According to this approach, minimum thickness of each layer is specified to protect the under lying layers from shear deformation. ix). Recommended Value of Layer Coefficients Asphaltic Wearing Course, a1 = 0.44/inch (0.1732/cm) Asphaltic Base Course, a1 = 0.40/inch (0.1575/cm) Water Bound Macadam, a2 = 0.14/inch (0.0551/cm) Granular Subbase, a3 = 0.11/inch (0.0433/cm) OR Nomograph can be used to work out SN.

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NOMOGRAPH

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Section - 4 How to Design

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How to Design Step 1. Fix the design life of the pavement. Step 2. Work out MR value of the subgrade MR = CBR for CBR <10% OR MR = (CBR)0.63 for CBR > 10 Work out MR in the laboratory. Step 3. Conduct 7-days traffic count. Step 4. Classify the traffic and consider the commercial vehicles i.e. Bus, Tractor , Trolley, 2-Axle, 3-Axle, 4-Axle, 5-Axle and 6-Axle Trucks. Step 5. Take Growth rate from the table on the next slide.

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**Growth Rate S. No Vehicle Class Growth Rate 1 Bus 8.4% 2**

Tractor Trolley 7.9% 3 Mini Truck 4 2-Axle 7.0% 5 3-Axle (Single) 6 3-Axle (Tandem) 7 4-Axle 8 5-Axle 9 6-Axle

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**CONVERT THE TRAFFIC TO EQUIVALENT STANDARD AXLE LOAD. **

ESAL = TRAFFIC X EQUILLANCY FACTOR , EQUIVALENCY FACTOR FOR VARIOUS CLASSES OF VEHICLES ARE GIVEN IN THE FOLLOWING TABLE. S. No Vehicle Class Equivalency Factor (Empty) Equivalency Factor (Loaded) 1 Bus 0.939 2 Tractor Trolley 0.1 1.19 3 Mini Truck 0.0172 2.596 4 2-Axle 0.043 6.49 5 3-Axle (Single) 0.072 16.62 6 3-Axle (Tandem) 18.48 7 4-Axle 0.206 19.00 8 5-Axle 0.084 19.59 9 6-Axle 0.165 27.96

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**Total Traffic for 10 Years**

Calculation of CESAL Vehicle Type ADT Annual Traffic Growth Rate % Growth Factor Total Traffic for 10 Years ESA Factor CESAL for 10 Years 80% 20% Loaded Empty Buses 20 7300 6 13.18 96,214 0.939 72,276 Tractor Trolly 139 50735 668,687 1.19 0.1 649,964 Trucks2XL 500 182500 2,405,350 6.49 0.043 12,509,263 Trucks 3XL 250 91250 18.48 0.072 17,797,666

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Cumulate the future traffic throughout the design life with the help of the selected growth rate. Following is the simple relation to project the traffic to any selected year. Pn = (1 + r)n – 1 Where Pn = Projected traffic for nth year r = Growth rate n = year of consideration Add all the yearly traffic from base year to the last year of the design life.

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Step 6. Fix the parameter like R, ZR, So, ∆ PSI etc. The generally taken value of the above parameters is listed below: ∆ PSI = 1.7 R = 90% So = 0.45 ZR = Step 7. Put these values in equation 1 and use trial & error method or Nomograph to work out the SN SN = a1D1+a2D2m2+a3D3m3 Step 8. Take the value of m2 and m3 from the table on the next slide.

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**TABLE FOR QULALITY OF DRAINAGE**

QUALITY OF DRAINAGE Quality of Drainage Water Removed within Excellent 2 hours Good 1 day Fair 1 week Poor 1 month Very Poor water will not drain

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Quality of Drainage Percent of Time Pavement Structure is exposed to Moisture Levels Approaching Saturation Less Than Greater Than 1% 1 - 5% 2 - 25% 25% Excellent 1.20 Good 1.00 Fair 0.85 Poor 0.60 Very Poor 0.40

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**Select the most appropriate and economical combination of thicknesses.**

Put the above values in equation at step No. 07, to find out the various combination of thicknesses, keeping in view the minimum thicknesses requirements as mentioned below: Minimum Asphalt wearing course thickness = 5 Cm Minimum asphaltic base course thickness = 7.5 Cm Minimum unbound base course thickness = 15 Cm Minimum unbound sub base thickness = 15 Cm Select the most appropriate and economical combination of thicknesses.

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Section - 5 Practical Example

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Practical Example Let us work out the thicknesses of various layers for the Pavement of Topi Bypass Road. The ADT is given as follow. Vehicle Type ADT COASTER/ FLYING COACH 250 BUSSES 25 Tractor Trolley 36 Trucks2XL 110 Trucks 3XL 2 Trucks 4XL 5

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**THE CESAL IS WORKED OUT AS FOLLOW:-**

Vehicle Type ADT Annual Traffic Growth Rate % Growth Factor Total Traffic for 10 Years ESA Factor ESAL for 10 Years 80% 20% Loaded Empty COASTER/FLYING COACH 250 91250 8.4 14.78 1,348,675 0.939 1,013,125 BUSSES 25 9125 134,868 101,312 Tractor Trolly 36 13140 7.9 14.423 189,518 1.19 0.1 184,212 Trucks2XL 110 40150 7 13.82 554,873 6.49 0.043 2,885,673 Trucks 3XL 2 730 18.48 0.072 149,295 Trucks 4XL 5 1825 19 0.385 385,309 Total 4,718,925 ESAL by taking 100 % of directional factor = 4.719 million ESAL by taking 80 % lane factor 3.775 CESAL =3.775 million

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**California Bearing Ratio (CBR)= 30 % at 95% MDD**

MR= (CBR)0.63 for CBR > 10 Putting the value CBR , MR= Psi Keeping the value of various parameters as follow. R= 90% So=0.45 ∆Psi=1.7 Using Nomograph to work out the SN

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Required SN = 3.35

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**Using the following the equation**

SN = a1D1+a2D2m2+a3D3m3 Given Data: a1=0.44, a2=0.14 , a3=0.11 and m2,m3=1 from Nomograph SN= 3.35 Putting these values and assuming d1=2 inch, d2=10 inch and d3=10 inch 3.35=0.44*2+0.14* *10 3.35≈3.38 Hence the Design thickness are ACWC= 5cm WBM=25cm Granular sub base=25cm

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Thank You

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**SERVICEABILITY AND PERFORMANCE CONCEPT**

• AASHO Road Test performance based on user assessment: – Difficult to quantify (subjective) – Highly variable – Present Serviceability Rating (PSR) Performance Measurements 0-1 – V. Poor 1-2 – Poor A panel of experts drove around in standard 2-3 – Fair vehicles and gave a rating for the pavement 3-4 – Good 4-5 – V. Good • Measurable characteristics (performance indicators): – Visible distress (cracking & rutting) – Surface friction – Roughness (slope variance)

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