PAVEMENT DESIGN

- PAVEMENT STRUCTURE- TYPES (Flexible & Rigid pavements) - TERMINOLOGIES- TRAFFIC VOLUME ESTIMATION- DESIGN METHODS - Empirical Designs (e.g. California Bearing Ratio,

CBR, method & AASHTO Road Test; Dynamic Cone Penetrometer, DCP, Design Method)

- Mechanistic Design (“balanced design” in which no single layer in the payment is subjected to excessive stress or strain)

- Catalogue Method of Design (e.g. TRH 4)

- Surfacing (the wearing course)

Pavement structure

Some management issues

PAVEMENT DESIGN TERMINOLOGIES

• Structural Design & Analysis Periods • Structural Design Period:

- The chosen minimum period during which the pavement is designed to carry the traffic in the prevailing environment with a reasonable degree of confidence that structural maintenance will not be requiredAnalysis Period:

- The useful life over which the total present cost of the pavement (comprising construction costs, maintenance costs (including user costs), & salvage value) is determined so that the whole-life-cycle cost comparisons can be made between alternative design options)

Illustration of DSP & AP of flexible pavements and design strategies (TRH 4)

Terminologies (cont’d)

• Structural distress: visible manifestation of deterioration of the pavement with respect to either the serviceability or the structural capacity

• Design Reliability: the extent to which the pavement can carry the design traffic without deterioration over the design life.

Road category and reliability

Terminologies (cont’d)

• Standard Axle Load (SAL) : 80 kN single axle dual wheel configuration. Tyre contact stress is 520 kPa (The maximum pemissible single axle load (4 or more tyres) is 88 kN.

• Equivalent Standard Axle (Load) / ESA/ESAL (aka: E80s): - All axle loads are converted to an equivalent number of standard axle loads, ESAs, and pavement design is usually based upon the total cumulative ESAs that the pavement will have to carry over its design life.

• Equivalence Factor, EF: The pavement damaging effect of an axle in relation to the damage caused by a standard axle which has a load of 80kN.

• Vehicles per day (vpd): The total daily number of motorised vehicles in both directions (i.e. two-way flow)

• Vehicles per lane per day (vpld): This is usually the total daily number of motorised vehicles in one direction or lane

• CV = Commercial vehicles• PCU = Passenger car units

Traffic volume

• ADT / AADT: (Annual) Average Daily Traffic - The total annual traffic in both directions divided by 365. Hence it is an average 240hour daily traffic volume. - It is important to note that this is a TWO-WAY

traffic flow. For pavement design purposes, ONE-WAY traffic flow is normally used.

- Pavement design is normally based on the lane with the highest traffic loading.

Table 1a: Manual classified traffic count data – combined data (for a two-way flow)

Number of Commercial Vehicles for each hour of the dayTime Mon Tue Wed Thurs Fri Sat Sun00.00 – 01.00 - 3 - - - 1 -01.00 – 02.00 - 2 - - - 2 -02.00 – 03.00 - 0 - - - 0 -03.00 – 04.00 - 1 - - - 3 -04.00 – 05.00 - 4 - - - 1 -05.00 – 06.00 - 12 - - - 4 -06.00 – 07.00 - 23 - 20 24 7 -07.00 – 08.00 - 45 - 47 52 12 -08.00 – 09.00 - 56 - 48 60 8 309.00 – 10.00 - 64 - 63 71 13 510.00 – 11.00 - 61 - 65 66 15 811.00 – 12.00 - 55 - 52 59 18 412.00 – 13.00 - 49 47 52 - 16 213.00 – 14.00 - 51 44 47 - 12 614.00 – 15.00 - 52 45 51 - 15 715.00 – 16.00 - 61 55 57 - 21 916.00 – 17.00 - 65 61 58 - 24 -17.00 – 18.00 - 51 54 56 - 14 -18.00 – 19.00 - 29 - 42 - 8 -19.00 – 20.00 - 22 - 21 - 5 -20.00 – 21.00 - 13 - 8 - 6 -21.00 – 22.00 - 7 - 6 - 3 -22.00 – 23.00 - 4 - - - 1 -23.00 – 24.00 - 2 - - - 0 - No. survey hours: 24 6 16 6 24 824 hour count: 732 A B C 209 D

• • A – estimated traffic count for Wednesday:•

• = (actual traffic count for 12.00 – 18.00) x total 24-hour count

(traffic count for 12.00-18.00 of 24-hour count)

Weight of selected vehiclesVEHICLE TYPE CURB WEIGHT kN EF = (CB/80KN)^4

(Tare mass, kg) 2010 VW Passat 2.3T Sport Combi Wagon 1532 15.028 0.001

2013 Cadillac ATS 1542 15.128 0.001

Mini Cooper S Countryman ALL4 1476 14.482 0.001

Chevrolet Cruze LTZ RS 1455 14.273 0.001

Chevrolet Corvette Z06 Carbon Ltd. 1455 14.269 0.001

Hyundai Tucson GLS 1443 14.158 0.001

Hyundai Elantra GLS 1246 12.222 0.001

VW Jetta, SEL 1425 13.975 0.001

VW Jetta, TDI 1442 14.144 0.001

Nissan Juke, SV 1330 13.045 0.001

Ford Fiesta, SES 1157 11.355 0.000

2011 Subaru Outback 2.5i 1536 15.065 0.001

Volvo S40 1360 13.342 0.001Volvo V40 1380 13.538 0.001

Weights of selected vehiclesVEHICLE TYPE CURB WEIGHT kN EF = (CB/80KN)^4 Tare mass, kg 2011 Volvo XC 60 3.2 1795 17.609 0.0022012 BMW 528i (AT) 1730 16.971 0.0022012 Volvo XC90 3.2 2155 21.141 0.0052012 BMW 760 Li F02 series 2250 22.073 0.006

2006 /8 Range Rover Sport Model 2586 25.369 0.010Light Australian vehicles (<) 4500 44.145 0.093

Heavy Australian Vehicles (HAV) (>) 4500 44.145 0.093 5000 49.050 0.141

SANRAL Two axle unit veh (restriction) 7850 77.009 0.859Max limit (HAV) 8000 78.480 0.926SANRAL )Maximum load 9000 88.290 1.483

Single axle with 4 wheels (restriction) 10200 100.062 2.447Vehicle load GVM 12000 117.720 4.689

Truck load 24000 235.440 75.017Restriction on (Truck) 30400 298.224 193.113

Principles of pavement design

• Introduction - need for rational design of pavements- design parameters- pavement performance - functional and structural pavement failures

● Design approaches to pavement

Introduction

• Pavement- components (foundation in cut /fill; subgrade (prepared

foundation; subbase, base)

● Pavement types- Surface roads (bituminous roads, concrete pavements)- Unsurfaced roads (earth roads, gravel roads)- Block pavements

Broad classification:- Flexible pavements (earth roads, gravel roads, bituminous

pavements)- Rigid pavements (concrete roads)

Need for good road designs

• Pavement failures occur gradually and cumulatively

• Bad roads trigger- economic loss- loss of life

Design issues

• Complexity in traffic loading conditions

• Complexity in material behaviours

• Availability of adequate information for modelling and accurate prediction of pavement behaviour over time

Pavement design parameters (1)• Type of pavement

- number of layers- layer thickness- material selection

● Subgrade - soil type- strength- drainage characteristics

● Traffic loading- volume of traffic- types of vehicle- loads carried- speed

Pavement design parameters (2)

• Climatic conditions- Average rainfall (seasonal distribution)- Temperature (maximum, minimum, variations – daily & seasonal) (NB: effects on materials)- Snow

EMPIRICAL PAVEMENT DESIGN

• California Bearing Ratio, CBR, design method• Dynamic Cone Penetrometer, DCP, design method• Empirical design is based on observed service life of a

pavement (i.e. experience / statistical data of pavement performance – no theory behind it, except observation only)

• Empirical design approach cannot be extrapolated with confidence beyond the range of experience

• May not work for new materials under new traffic conditions, and new climatic conditions

• To replicate empirical design methods, conditions must be the same

CBR design method

• CBR expresses the bearing strength / capacity of a compacted road layer as a percentage

• CBR (@ optimum soil density) = (load carried by selected specimen) x 100%

(load carried by a standard crushed stone specimen)

CBR Values of road layers & material selection

G1 to G4

G5

G7 material

Material selection & surfacing (TRH4 Tables 13 & 14)

CBR design (cont’d)• CBR strengths is tested at 3 different densities on soaked material• Variation of CBR with density

CBR

Density

• CBR of pavement subgrade material (at field density – usually 93% mod.AASHTO density) is determined from the CBR vrs. Density curve

Pavement thickness design

31 kN light wheel load

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40 kN medium wheel load

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53 kN heavy wheel load

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Thickness design (cont’d) • Decide whether to base design on a 32; 20; or

53 KN wheel load ( depending on the whether the proposed road is a minor road or heavily trafficked road)

• From the CBR – depth of construction curve, read off the amount of material cover required to protect the roadbed (in-situ subgrade) from the damaging effects of traffic

Example

• Given:- In-situ subgrade CBR = 5- Expected heavy traffic = 53KN wheel load- Selected subgrade CBR = 15- Minimum CBR of Base-course = 80- Minimum CBR of sub-base = 45

Design the thickness of the above proposed pavement.

• Solution:- From CBR – thickness design curve, read off the total cover above

the natural / in-situ subgrade = 420 mm (equals the combined thickness of selected subgrade, subbase, base and surfacing)

- Selected Subgrade of CBR 15: Read off the required thickness = 225 mm (i.e. combined thickness of subbase, base and surfacing)

- Subtract 225mm from 420mm, the thickness of the selected subgrade layer = 195 mm

- Cover needed for the subbase of CBR 45 (from curve) = 100mm (combined thickness of base & surfacing)

- Subtract 100 from 225 to obtain the thickness of the subbase to be 125mm

- NB: the curve does not give cover required by the base, so that neither the base nor surfacing can be found by the CBR method

EMPIRICAL PAVEMENT DEISGN METHOD USING DYNAMIC CONE PENETROMETER

(DCP)

The DCP instrument consists of

- An 8 kg hammer which is made to drop through a height of 575 mm

- A cone angle of 60◦ (can also be 30

◦)

- A clamped one metre rule

Uses of the DCP

• Establish un-compacted soil strength for new road design across a green field

• Most useful in measuring in-situ soil strength of an existing road for upgrading /rehabilitation

• Usually driven to its full length of 800 mm

and ‘pavement structure number, DSN800” - (number of blows to 800 mm depth) read off

Uses of the DCP (cont’d)

• From calibrated design charts of CBR / DCP numbers, the following can be estimated:– Approximate traffic capacity– Pavement layers can be identified – Layer thickness can be measured

NB: The DCP is sensitive to soil moisture content & density

- It is therefore a good practice to confirm DCP results with a few laboratory CBR

tests

Features of the DCP instrument (Ref: Paige-Green & Du Plessis, 2009, CSIR)

DCP test procedure

• After assembly of the portable instrument, first record the zero reading

• Ideally, three operators are required for a DCP survey• Recommended readings (depth of penetration):

- after every 5 or 10 blows (for good quality road layers)

- after every 1 or 2 blows (for weak subbase and subgrade layers)

(Ref: TRL/DIFID – ORN 18)

Analysis of DCP results

• Plot a graph of Number of Blows versus Depth of penetration (mm)

• Identify each layer• Find the slope of each layer – The DCP number

(mm per blow)• Read off the corresponding soil strength -

California Bearing Ratio (CBR) - from the CBR/DCP chart

Analysis of DCP results (Cont’d)

• Convert DCP number (in mm/blow) for each layer using the empirical formula

Log10 (CBR) = Log10 (401.8) – 1.284Log10 (DCP №)

(Van Steenderen, W.P.C. 1994, School of Civil Engineering,

Wits University)

Typical DCP results (Ref: TRL/DFID – ORN 18)

DCP – CBR Relationships (Ref: TRL/DFID – ORN 18)

Pavement Design methods (Cont’d)

• Mechanistic design approach

• Uses mechanical properties of pavement materials and their ability to resist stress and strain

• CBR design (i.e. empirical design which relies on observations / experience) ignores fatigue ( cumulative effects of repeated axle loadings)

• Therefore, mechanistic design is preferable• However, empirical method is always a good starting

point (in the absence of nothing)

Pavement load transfer

• The angle, α, is dependent on the relative stiffness of each layer

• For example, if subbase is stabilised (or stiff), it will resist load; therefore pressure on the layer beneath will be very much less. Hence the angle flattens out to probably 30o

• Water-bound Macadam (roads – WBM)( See page 36 of TRH) – research by Phillips (1994) at Wits showed that WBM basecourses can withstand significantly higher shear stresses than crushed stone base courses.

Catalogue design method (based on TRH 4)

• Combination of empirical and mechanistic design methods (i.e. empirical-mechanistic design)

• TRH 4 is based on research and local experience (by carried out CSIR & Road Authorities)

(Pavement) Layer-works and materials Layerworks Material / thicknessBASECOURSE Very strong material; suitably graded

crushed rock or high quality natural gravels; spreads traffic loads so that subbase can carry; usually, 100-150 mm thick

SUB-BASE Good quality natural or processed gravels (may be stabilised with small quantities of cement/lime/ other agents); One or two layers; 100-150 mm thick

SELECTED SUBGRADE Generally natural soils & sands with low plasticity; one or two layers; 100-150 mm thick

Classification of pavements & traffic for structural design purposes

• TRH 4 divides pavements into 10 different classes, from ES 0.003 to ES 100

• ES 0.003 indicates extremely light trafficked road

• ES 1000 indicates extremely heavy trafficked road

Pavement classification (TRH4, Table 4)

Example:• Given: vpd/lane = 200• 10% heavy vehicles from the vpd/lane count• Each heavy vehicle = 1.2 E80• Annual growth in E80 s = 4%• Design period = 20 years• Question: determine the pavement class

Solution• Equivalent design traffic = 200 x 10% x E80s x

traffic growth factor fy (from Table 12)= 200 x 0.1 x 1.2 x 11303 E80s= 271272E80s = 0.271272 X 106 E80s

Therefore, the bearing capacity of the pavement is between 0.10 - 0.30 million 80 kN axles per lane (from Table 4) = Pavement class ES0.3

Important selected Tables (from TRH4)

Table 13 (cont’d)

TRH4 Subgrade design

Climatic conditions & pavement design

• Climatic conditions (moisture and temperature) affect choice and stability of materials

• RSA divides climatic regions into 3:- a relatively large dry region- a moderate region- a few small wet regions

Pavement layer selection

Design for Moderate / dry region

Design for wet regions