STUDY ON AXLE LOADS; TURNING CIRCLES; AND LANE WIDTHS FOR FREIGHT VEHICLES IN SUB-SAHARAN AFRICA: DRAFT FINAL REPORT

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1 Contract Report: CSIR/BE/ISO/ER/2010/0023/B (GW PTA Projects 73355) March 2010 STUDY ON AXLE LOADS; TURNING CIRCLES; AND LANE WIDTHS FOR FREIGHT VEHICLES IN SUB-SAHARAN AFRICA: DRAFT FINAL REPORT Authors: MP Roux PA Nordengen PREPARED FOR: TradeMark Southern Africa Programme Management Unit Box 317 PERSEQUOR PARK 0200 Tel: Fax: PREPARED BY: CSIR Built Environment PO Box 395 PRETORIA 0001 Tel: Fax:

2 TABLE OF CONTENTS 1 Introduction Background Project Objectives Methodology Introduction Task 1: Payload efficiency of different freight vehicles and freight vehicle combinations Task 2: Permissible maximum axle loads for axles fitted with wide base tyres Task 3: Preferred lane widths for all major routes Task 4: Preferred turning circles and turning corridors mepads and the SAMDM Software for calculation of Load Equivalency Factors Background to the SA Mechanistic Design Method (SAMDM) Approach followed in this study to calculate LEFs Approach followed to estimate road wear from LEFs Approach followed to estimate road wear per payload ton-km Example of calculating LEFs using mepads and estimating road wear Task 1: Payload Efficiency of Different Freight Vehicles and Vehicle Combinations Freight vehicles included in the analysis Axle and vehicle masses used in the analysis Tyre sizes and tyre inflation pressures applied in the analysis Pavement types evaluated in this study Calculated LEFS per vehicle class and pavement type Road wear per vehicle class and pavement type Road wear relative to payload Conclusions regarding the payload efficiency of interlinks Task 2: Permissible Maximum Axle Loads for Axles Fitted With Wide Base Tyres Introduction Tyre sizes and tyre inflation pressures applied in the analysis Axle and axle units included in the analysis Calculated LEFs per axle type and pavement Africa Draft Final Report i

3 5.5 Determination of proposed axle and axle unit load limits for axles fitted with wide base tyres Inclusion of tyre contact stress values in the calculation of LEFs Conclusions and recommendations regarding the use of wide base tyres Task 3: Preferred Lane Widths for Major Roads Introduction Lane and shoulder widths recommended in design guides and codes of practice Selected research on lane widths Recommended preferred lane widths for major roads Task 4: Preferred turning circles and turning corridors Introduction SANRAL Geometric Design Guide SATCC Code of Practice for the Geometric Design of Trunk Roads Turning circles and turning corridors for other vehicle combinations Conclusions and Recommendations References Appendix A Vehicle classes analysed in terms of payload efficiency... A-1 Appendix B Pavement structures used in the analysis of payload efficiencies... B-1 Appendix C Properties of the pavement structures used in the analysis of payload efficiencies. C-1 Appendix D Vehicle classes for which turning corridors and radii were developed... D-1 Appendix E Lay-outs of developed turning corridors... E-1 Africa Draft Final Report ii

4 LIST OF TABLES Table 1-1: Harmonised permissible maximum axle and axle unit masses in the ESA Region... 5 Table 3-1: Vehicle information used for mepads analysis (Vehicle 4 6-axle articulated vehicle). 13 Table 3-2: Standard axle information used in mepads analysis Table 3-3: LEFs calculated for Vehicle 4 and Pavement A (dry state) Table 4-1: Eleven typical freight vehicles used in the analysis of road wear impact Table 4-2: Axle and vehicle masses used in the analysis Table 4-3: Axle loads and tyre inflation pressures for 315/80R22.5 size tyres Table 4-4: Calculated LEFs per vehicle class and pavement type Table 4-5: Calculated road wear per vehicle class and pavement type Table 4-6: Payload per vehicle class Table 4-7: Calculated road wear per payload ton-km per vehicles class and pavement type Table 5-1: Tyre Load versus Tyre Inflation Pressure for five tyre sizes Table 5-2: Axles and axle units fitted with wide base tyres used in analysis Table 5-3: Legal axles and axle units used in the analysis Table 5-4: Calculated LEFs for axles and axles units fitted with conventional and wide base tyres. 29 Table 5-5: LEFs for legal axles and axles fitted with wide base tyres at different axle loads Table 5-6: LEFs for single axles and tandem axle units fitted with wide base tyres at various axle loads Table 5-7: Tyre Load versus Tyre Inflation Pressure for two tyre sizes Table 5-8: Axles and axle units analysed with the inclusion of tyre contact stress values Table 5-9: Measured tyre loads and equivalent uniform contact stresses obtained from TyreStress Table 5-10: Calculated LEFs for legal axles and axles fitted with 425/65R 22.5 wide base tyres using outputs from the TyreStress software Table 5-11: LEFs for single axles and tandem axle units fitted with 425/65R 22.5 wide base tyres at various axle loads Table 6-1: Table 6-2: Table 6-3: Shoulder widths for undivided rural roads as recommended in SANRAL s Geometric Design Guide Minimum width of travelled way and shoulders for rural collector roads and streets (from AASHTO Policy) Minimum width of travelled way and useable shoulder for rural arterials (from AASHTO Policy) Table 7-1: Dimensions of design vehicles included in the SANRAL Geometric Design Guide Table 7-2: Table 7-3: Table 7-4: Minimum turning radii for design vehicles included in the SANRAL Geometric Design Guide Dimensions of design vehicles included in the SATCC Code of Practice for the Geometric Design of Trunk Roads Minimum turning radii for design vehicles included in the SATCC Code of Practice for Geometric Design of Trunk Roads Africa Draft Final Report iii

5 LIST OF FIGURES Figure 3-1: 6 Axle Articulated Vehicle Figure 3-2: Load positions 6 Axle Articulated Vehicle Figure 3-3: Detail of Road Pavement Structure A Figure 4-1: Tyre Load versus Tyre inflation Pressure for 315/80R22.5 size tyres (from Michelin s Technical Truck Tyre Data for South Africa 01/2009) Figure 4-2: Calculated average LEFS per vehicle class and pavement types Figure 4-3: Calculated average LEFS per vehicle class Figure 4-4: Calculated average road wear per vehicle class and pavement type Figure 4-5: Calculated average road wear per payload ton-km per vehicle class Figure 5-1: Tyre Load versus Tyre Inflation Pressure for five tyre sizes Figure 5-2: LEFs for legal axles and axles fitted with wide base tyres at different axle loads Figure 5-3: LEFs for single axles fitted with wide base tyres at different axle loads Figure 5-4: LEFs for tandem axle units fitted with wide base tyres at different axle unit loads Figure 5-5: Tyre Load versus Tyre Inflation Pressure for two tyre sizes Figure 5-6: LEFs for single axles fitted with 425/65R 22.5 wide base tyres at different axle loads Figure 5-7: LEFs for tandem axle units fitted with 425/65R 22.5 wide base tyres at different axle unit loads Figure 7-1: Wheel tracks for rigid chassis vehicles (from the SATCC Code of Practice for the Geometric Design of Trunk Roads) Figure 7-2: Wheel tracks for articulated vehicles (from the SATCC Code of Practice for the Geometric Design of Trunk Roads) Figure 7-3: Turning corridor for a 22 m interlink (with a short truck-tractor) doing a 90 turn Figure 7-4: Turning corridor for a 22 m interlink (with a short truck-tractor) doing a 180 turn Africa Draft Final Report iv

6 1 INTRODUCTION 1.1 Background Member states of the Regional Economic Communities (RECs) of the Eastern and Southern Africa (ESA) region, namely the Common Market for Eastern and Southern Africa (COMESA), the Southern African Development Community (SADC) and the East African Community (EAC) have agreed to harmonise permissible maximum axle and axle unit masses as summarised in Table Table 1-1: Harmonised permissible maximum axle and axle unit masses 2 in the ESA Region Permissible Maximum Axle or Axle Unit Mass No of (kg) Tyres/axle Steering Non-steering Single Axle Single Axle Tandem Axle Unit Tridem Axle Unit Two Four n.a In addition, the RECs agreed to a maximum permissible combination mass of kg. Although the RECs have agreed to harmonise permissible maximum masses there remain a number of outstanding issues to resolve in the implementation of these common rules and procedures, including: Not all countries abide by the regionally agreed permissible axle and axle unit masses; Not all countries abide by the regionally agreed permissible maximum combination mass of 56 t and there have been instances where national governments have arbitrarily reduced the permissible maximum combination mass to lower than 56 t; and Not all countries accept interlinks on their roads and there is the misconception that 56-ton interlinks do more road damage than 56-ton truck and trailer combinations. A further issue is the permissible maximum axle and axle unit masses for axles fitted with single tyres, which effectively limit the use of wide base tyres (the so-called super single tyres ). These tyres are wider than conventional tyres and according to the tyre manufacturer s specification can carry loads of more than 4 t per tyre. The permissible maximum axle and axle unit masses for axles fitted with single tyres (see Table 1-1) limits the load to 4 t per tyre. The smallest tyre that can carry a load of 4 t has a width of 315 mm. Wide base tyres with widths of 385 mm, 425 mm, 445 mm, 455 mm and 495 mm are available. Because of the wider base, the contact area between these tyres and the road surface should be larger than that of a 315 mm wide tyre for example. Transporters are always looking for ways to reduce the tare (or unladen) mass of their vehicles as this will allow them to legally increase their payloads and, in turn, bring down the cost of transport. The use of super 1 At a meeting of the SADC ministers responsible for transport and meteorology, held on 15 May 2009 in Swakopmund, Namibia; as well as at the SADC Roads Infrastructure, Transport and Traffic Committee (RSCom) meeting, held in Swakopmund on 11 and 12 May These permissible maximum axle and axle unit masses are actually the road wear limits, as the tyre manufacturer s ratings and the vehicle manufacturer s ratings for the axle and axle units should be taken into consideration to arrive at the real permissible axle or axle unit mass. The term road wear limit is therefore used in some cases in the report to refer to these limits. 5

7 single tyres instead of dual tyres will result in a reduction in the vehicle tare mass but, at present, axle load limits are set according to the number of axles on a vehicle and the number of tyres on an axle. Therefore, there would need to be a change in legislation to allow the use of super single types on an inter-link (or double semi-trailers), which is the main vehicle combination for heavy vehicle transport in the Southern African region. It is generally agreed that super singles should be given a higher load limit than for conventional tyres but the issue to be agreed is what that load should be so as to ensure the vehicle does not cause more road wear than a vehicle fitted with conventional single tyres, but at the same time allow the vehicle to carry a higher payload. Lane widths should be based on traffic volume and vehicle type and speed. The regionally agreed maximum width of heavy goods vehicles and buses is 2.6 m so, according to the SATCC Code of Practice for the Geometric Design of Trunk Roads of 2001, for the sake of safety, a lane width should not be less than 3.1 m and not more than 3.7 m (with no operational or safety benefit accruing from lane widths wider than 3.7 m) and so give adequate clear space on either side of trucks and buses. Intermediate conditions of volume and speed, as are the norm on the NSC, can be adequately catered for by a lane width of 3.4 m. New border facilities are being constructed along the North South Corridor. Design codes, including the SATCC Code of Practice for the Geometric Design of Trunk Roads 2001, only provide for Semi-Trailers WB 50 and SU Truck and Trailer. Turning circles and corridors for these vehicles are inadequate for interlinks. 1.2 Project Objectives The main objective of this study is to provide scientific and researched-based information to address the issues discussed in Section 1.1. The project objectives therefore include: To illustrate the payload efficiency of interlinks with a permissible maximum mass of 56 t; To recommend permissible maximum axle masses that should be adopted for single and tandem axle units fitted with wide base tyres (super singles); To recommend a preferred lane width to be adopted on the regional trunk road network; and To recommend preferred turning circles and turning corridors to be adopted in the design of all major roads and all roads within a border facility, weighbridge or any other facility where large numbers of freight vehicles are managed. 6

8 2 METHODOLOGY 2.1 Introduction In order to address the project objectives as set out in Section 1.2, the following tasks were undertaken: Task 1: Task 2: Task 3: Task 4: Payload efficiency of different freight vehicles and freight vehicle combinations; Permissible maximum axle loads for axles fitted with wide base tyres; Preferred lane widths for all major routes; and Preferred turning circles and turning corridors. Tasks 1 and 2 both involved the use of the CSIR pavement design software mepads, which is the electronic version of the current South African Mechanistic Design and Analysis Methodology (SAMDM). An overview of mepads and the SAMDM as well as an example of the application of the software is presented in Section 3. The approach and methodology followed for each of the tasks are described in the following sections. 2.2 Task 1: Payload efficiency of different freight vehicles and freight vehicle combinations In order to quantify the relationship between total vehicle mass and number of axles and to dispel the misconception that 56-ton interlinks cause more road wear (damage) than 56-ton rigid vehicle/drawbar trailer combinations or smaller vehicle combinations, the concept of payload efficiency was used. The road wear caused by various vehicles and vehicle combinations was calculated as a monetary value per payload ton-km. In this way a direct comparison of the road wear caused by different vehicles and vehicle combinations could be made. The methodology followed to calculate the payload efficiency for various vehicles and combination of vehicles is as follows: 1. The vehicles and vehicle combinations to be analysed were identified and are presented in Appendix A; 2. Typical pavement structures to be analysed were identified and are presented in Appendix B; 3. Using mepads, the pavement life under each axle of the vehicle using the axle loads when the total vehicle mass is equal to the maximum permissible vehicle mass applicable to the particular vehicle and using the recommended tyre inflation pressure was calculated; 4. Using mepads, the bearing capacity of the pavement in terms of the standard 80 kn/520 kpa axle with four tyres (two dual sets) at a tyre inflation pressure of 520 kpa, was calculated; 5. The LEF of the vehicle was then calculated as the sum of the ratios (for all axles of a particular vehicle) between the bearing capacity of the pavement determined from the Standard 80 kn/520 kpa axle with four tyres (two dual sets) at an inflation pressure of 520 kpa, divided by the pavement life under each individual axle load and its associated tyre pressures when the vehicle mass is equal to the maximum permissible vehicle mass; 6. These calculations were repeated for each vehicle for each of the different pavement structures for both the dry and wet climatic conditions and an average LEF per vehicle was calculated; 7

9 7. The average LEF for each vehicle was then multiplied by an average cost estimate of one Standard Axle-lane-km to arrive at an average road wear cost per km for each vehicle or vehicle combination; 8. This average road wear cost per km was divided by the permissible maximum payload of the vehicle to arrive at a road wear cost per payload ton-km; and 9. The road wear cost per payload ton-km for the various vehicles and vehicle combinations was compared and the vehicles were ranked in terms of payload efficiency. The output from Task 1 is a list of vehicles and vehicle combinations containing information on their payload efficiency as well a graphical representation of the payload efficiency of the various vehicles and vehicle combinations. It also includes a table showing axle loads and total vehicle or combination mass for the various vehicle and vehicle combinations, firstly for the situation where the only restriction is the permissible maximum axle or axle unit masses and secondly for the situation with the additional restriction of a permissible maximum vehicle or combination mass. 2.3 Task 2: Permissible maximum axle loads for axles fitted with wide base tyres. In order to recommend permissible maximum axle masses that should be adopted for single, tandem and tridem axle units fitted with wide base tyres (super singles) the concept of Load Equivalency Factors (LEF) was used again. The objective was to quantify the potential road wear on a relative basis. The potential road wear caused by various axles and axle units fitted with wide base tyres was compared to the potential road wear caused by axles and axle units loaded to the current maximum permissible loads of 8 t (single axle with single wheels); 9 t (single axle with dual wheels); 10 t (single axle with dual wheels); 18 t (tandem axle unit with dual wheels); and 24 t (tridem axle unit with single and dual wheels). The potential road wear caused by various axles and axle units fitted with wide base tyres was calculated at various axle masses to determine at what mass the same road wear is caused as the equivalent legal axle or axle unit. This mass would then be the recommended permissible maximum mass for the axle or axle unit fitted with wide base tyres. The methodology followed to arrive at recommended permissible maximum axle masses that should be adopted for single, tandem and tridem axle units fitted with wide base tyres (super singles) is as follows: 1. Calculate the pavement life under each of the following axles/axle units by calculating the total life of each layer in the pavement under static loading conditions and using the critical layer life (i.e. the particular layer with the shortest life) as the pavement life: a kg single axle fitted with single wheels (315/80R-22.5 at 850 kpa); b kg tandem axle unit fitted with single wheels (315/80R at 850 kpa); and c kg tridem axle unit fitted with single tyres (315/80R at 850 kpa). 2. Calculate the pavement life under each of the following axles/axle units by calculating the total life of each layer in the pavement under static loading conditions and using the critical layer life (i.e. the particular layer with the shortest life) as the pavement life: a. A single axle with two wide based tyres (385 mm) at an axle mass of kg and kg and at the recommended tyre pressure; b. A tandem axle unit with two wide base tyres (385 mm) per axle at an axle unit mass of kg, kg and kg and at the recommended tyre pressure; and c. A tridem axle unit with two wide base tyres (385 mm) at an axle unit mass of kg and at the recommended tyre pressure. 3. Repeat for wide base tyres with widths of 425 mm and 445 mm; 8

10 4. Plot the pavement life per axle/axle unit against the axle/axle unit mass for the various tyre sizes and using the pavement life calculated for the equivalent axles/axle units fitted with 315/80R-22.5 tyres, read from the graph the axle/axle unit mass of the axle/axle unit fitted with wide base tyres giving the same pavement life. The output from Task 2 is recommended permissible axle/axle unit masses for single and tandem axle/axle units fitted with wide base tyres of different widths. 2.4 Task 3: Preferred lane widths for all major routes. The preferred lane widths for all major roads is determined from available literature by relating recommended lane widths to the situation on the North South Corridor in terms of traffic volumes; speed; vehicle type; and vehicle widths. The methodology to be followed to arrive at the recommended preferred lane widths for all major routes is as follows: 1. Confirm the maximum permitted vehicle width throughout the region; 2. Obtain data on speed limits throughout the region; 3. Summarise the lane width recommendations contained in available geometric design guides from countries in the region, including the SANRAL Geometric Design and the SATCC Code of Practice for Geometric Design; 4. Review research on the relationship between vehicle widths, speed limits and lane widths; and 5. Make a recommendation on preferred lane widths. The output from Task 3 is the recommendation on preferred lane widths. 2.5 Task 4: Preferred turning circles and turning corridors The preferred turning circles and turning corridors to be adopted in the design of all major roads and all roads within a border facility, weighbridge or any other facility where large numbers of freight vehicles must be accommodated are determined using software that simulates the movement of vehicles. This is done for typical vehicles operating on the North South Corridor in terms of vehicle types and dimensions. The methodology to be followed to arrive at the preferred turning circles and turning corridors to be adopted in the design of all major roads and all roads within a border facility, weighbridge or any other facility where large numbers of freight vehicles are managed is as follows: 1. Obtain information on the different vehicles and vehicle combinations to be provided for, including dimensions of such vehicles and combinations (the vehicle combinations analysed are presented in Appendix A); 2. Use software that simulates the movement of vehicles to plot the movements of all the cardinal points of each vehicle combination doing a 90 and a 180 turn; 3. Use these plots to determine the dimensions of the turning corridor for each vehicle combination and prepare drawings for each vehicle combination showing turning corridors and turning radii for 90 and 180 turning movements. The output from Task 4 are drawings showing turning corridors and turning radii that can be used in the design of all major roads and all roads within a border facility, weighbridge or any other facility where large numbers of freight vehicles must be accommodated. 9

11 3 mepads AND THE SAMDM 3.1 Software for calculation of Load Equivalency Factors The software package used for the calculation of Load Equivalency Factors (LEFs) is the Pavement Analysis & Design Software package (mepads) that is based on the SA Mechanistic Pavement Design Method. The software combines a stress-strain computational engine with pavement material models developed at the CSIR. The Windows Graphical User Interface enables any pavement system and vehicle load configuration to be defined and analysed for bearing capacity and design reliability. Amongst others, the design outputs include pavement layer lives and contour plots of stresses and strains. In this study, the critical pavement layers were used for calculating the LEFs for each vehicle/pavement combination for both relatively dry and relatively wet pavement conditions. The mepads software of the SAMDM is discussed by Theyse and Muthen (2000). The basic mechanistic-empirical methodology is freely available within South Africa from the CSIR Built Environment unit (mepads, 2008) - see website: Background to the SA Mechanistic Design Method (SAMDM) The SAMDM was developed over the past three decades and includes both flexible and semi-rigid pavement types. An overview of the method is given by Theyse et al., (1996). This method takes into account factors relating to design strategy, including road category, traffic volumes and structural design period, and considers material types, environment, drainage, compaction and cost analysis. A simpler approach is based on a catalogue of designs, which is typically used as a preliminary assessment of the pavement type required. For the detailed background on the SAMDM the reader is referred to De Beer (1992), SARB (1995), Theyse et al., (1996) and Theyse and Muthen (2000). Through the use of mepads, the SAMDM pavement design methodology was used to estimate the LEFs, based on critical pavement layer life, under static loading conditions. The LEFs were calculated from estimated ratios of critical pavement layer life for each individual vehicle relative to the Standard Axle (80 kn, 520 kpa) bearing capacities for each of the typical road pavement structures presented in Appendix B. LEFs were calculated for both the relatively dry and relatively wet pavement conditions and averages of the dry and wet values were used in the further analysis. The SAMDM pavement design methodology is based on the philosophy of Equivalent Pavement Response - Equivalent Pavement Damage (EPR-EPD). With the EPR-EPD approach, no fixed equivalencies are used, per se, and each vehicle is considered with its full axle/tyre configuration (i.e. tyre/axle loading and its associated tyre inflation pressure) as input into the SAMDM. From this input, the total life of each layer in the pavement is calculated under static loading conditions, and the pavement life is equal to the critical layer life (i.e. lowest life found for a particular layer in the pavement). With the EPR-EPD approach the stresses and strains (i.e. mechanistic pavement response parameters) under each wheel of the vehicle are calculated and then directly related through the associated transfer functions for pavement damage to layer life. The transfer functions are the typical linear-log damage functions obtained (and calibrated) from experience including results of Heavy Vehicle Simulator (HVS) testing on the various pavement types carried out in South Africa since 1975 (see Theyse et al., 1996). In this methodology, the vehicle or combination of vehicles are therefore not reduced to an equivalent axle load of 80 kn (i.e. E80), based on the rather crude but well known so-called 4th power law of relative pavement damage. 10

12 3.3 Approach followed in this study to calculate LEFs 1. Using the full vehicle with the actual tyre loads, tyre inflation pressure and tyre spacing as input, a full mechanistic-empirical analysis was done with mepads per vehicle to calculate the layer life for each pavement layer under each axle of the vehicle; 2. The pavement life under each axle was then taken as being equal to the critical layer life (i.e. the particular layer in the pavement with the lowest life); 3. Using mepads, the layer life for each pavement layer under the Standard Axle (80 kn, 520 kpa) in the wet condition was calculated; 4. The bearing capacity of the specific pavement was then taken as being equal to the critical layer life (i.e. the particular layer in the pavement with the lowest life) under the Standard Axle; 5. The LEF for the particular vehicle was then calculated using the following equation: where: n = number of axles on vehicle Ncritical from Standard 80 kn/520 kpa Axle = Minimum layer life of pavement under the loading of the Standard axle of 80 kn and 520 kpa inflation pressure on 4 tyres (i.e. 20 kn per 520 kpa contact stress (= inflation pressure)) Ncritical from Axle i = Minimum layer life of pavement under the loading of Axle i of vehicle in question This calculation was done for each vehicle for each of the five pavements in both the dry and wet condition. 3.4 Approach followed to estimate road wear from LEFs The calculated LEF for a given vehicle and a given pavement structure, when multiplied by the number of vehicles of the same type, results in the number of Standard Axle (80 kn, 520 kpa) load applications that will have an equivalent effect on the performance of that pavement. A pavement is usually designed to have a specific bearing capacity which is expressed in terms of the number of Standard Axle load repetitions that will result in a certain condition of deterioration. This condition is normally considered to be the terminal condition, indicating that the pavement has structurally "failed", and can no longer support the functional service set by the service objective. For example, a pavement could have a bearing capacity of 1 million Standard Axle repetitions, indicating that the pavement will be able to carry a traffic spectrum up to the equivalent of 1 million Standard Axle loads (1 x 10 6 ESA). Each time a vehicle travels over the pavement, it uses up the bearing capacity of the pavement by an amount equal to the LEF of the vehicle. The road wear in monetary value caused by a vehicle can therefore be calculated by multiplying the LEF for a given vehicle by the average cost of one lane-km of road built to carry one Standard Axle (i.e. bearing capacity = 1.0). The cost should include both the construction and maintenance costs over the structural design period of the road. In this study an average cost of US$0.06/Standard Axle-km is used to calculate the road wear. This 11

13 value is based on the fee that is used in the calculation of the mass fees payable by abnormal vehicles by the nine provinces in South Africa (currently ZAR 0.45). A cursory analysis of the construction costs of a number of road construction projects in the Limpopo province in South Africa indicated that this value might be too low, but it is not considered to be important in the context of this study, as the value is only used to rank the various vehicles in terms of payload efficiency. Using a different value would have no effect on the ranking order. The equation to calculate road wear per vehicle-km is therefore a follows: where: LEF v = Load Equivalency Factor for the given vehicle and pavement structure MF = Mass Fee = Cost to construct and maintain one lane-km of road built to carry one Standard Axle = US$ Approach followed to estimate road wear per payload ton-km The purpose of freight vehicles travelling on the road is to transport freight and the road wear per ton of freight transported is therefore important. Ultimately this is the road wear cost that should be minimised. Dividing the road wear cost per km by the payload gives the road wear cost to transport 1 t of payload over a distance of 1 km. The equation to calculate road wear per payload t-km is therefore a follows: where: RW v = Road Wear per Vehicle-km in Rand PL = Payload of vehicle in ton 3.6 Example of calculating LEFs using mepads and estimating road wear To illustrate the use of mepads to calculate LEFs, the following worked example is presented. In this example, the LEF for Vehicle 4 (see Appendix A), the 6-axle articulated vehicle with dual tyres on the drive axle unit and the tridem axle unit is calculated. The calculations are done for Pavement A (see Appendix B) in the relatively dry state. The footprint of the vehicle (load positions) is presented in Figure 3-2. Figure 3-1: 6 Axle Articulated Vehicle 12

14 Y (mm) Load Positions: Vehicle 4-6 Axle Articulated Vehicle Figure 3-2: X (mm) Load positions 6 Axle Articulated Vehicle The vehicle information that is entered into the mepads software package is summarised in Table 3-1. Table 3-1: Vehicle information used for mepads analysis (Vehicle 4 6-axle articulated vehicle) Axle Tyre Tyre Load Tyre Pressure Tyre Position No No kg kn kpa X (mm) Y (mm) 1 R R R R R R R R R R R L L L L L L L

15 Axle Tyre Tyre Load Tyre Pressure Tyre Position No No kg kn kpa X (mm) Y (mm) 5 L L L L The pavement layer information that is entered in mepads is summarised in Figure 3-3. Pavement A Pavement type Pavement Class Design Bearing Capacity Base Subbase Granular Cemented ES3 1 to 3 million 80 kn axles/lane Pavement A Poisson's Elastic Moduli (Mpa) Ratio Phase 1 Phase 2 50 AG G C G Subgrade Figure 3-3: Detail of Road Pavement Structure A Using the vehicle information presented in Table 3-1 and the pavement layer information presented in Figure 3-3, mepads calculates the layer life 3 for each pavement layer under each individual axle. The layer with the shortest layer life is the critical layer and this critical layer life is taken as the pavement life under that particular axle. The pavement life under a standard axle is calculated in the same way using the input summarised in Table 3-2, but for the pavement in the relatively wet state. The LEF for each individual axle is then calculated by dividing the standard axle pavement life by the pavement life for that particular axle (see the LEF v -equation in Section 3.3). The LEF for the vehicle is the sum of the LEFs for each individual axle. The mepads output is summarised in Table 3-3. The LEF for this particular vehicle travelling on Pavement A in the relatively dry state is (see Table 3-3). This vehicle would therefore cause the same road wear as standard axles. Pavement A is a class ES3 pavement in terms of the TRH 4, which means that it has a design bearing capacity of 1 to 3 million standard axles. With an LEF of 2.818, it means that approximately vehicles of type Vehicles 4 can travel over this pavement in the relatively dry state before it would reach its terminal condition. 3 Layer life refers to the number of applications of the particular axle before the pavement layer reaches a terminal condition 14

16 Table 3-2: Standard axle information used in mepads analysis Axle Tyre Tyre Load Tyre Pressure Tyre Position No kg kn kpa X (mm) Y (mm) Standard axle R Standard axle R Standard axle L Standard axle L Table 3-3: LEFs calculated for Vehicle 4 and Pavement A (dry state) Axle/ Pavement Life Pavement Life Axle No. Axle Unit (Individual Axle) (Standard Axle) LEFs Steering Axle Drive Axle Unit Tridem Axle Unit Whole vehicle

17 4 TASK 1: PAYLOAD EFFICIENCY OF DIFFERENT FREIGHT VEHICLES AND VEHICLE COMBINATIONS 4.1 Freight vehicles included in the analysis The typical freight vehicles included in the analysis were chosen based on popularity, especially for long haul freight movement as well as to be able to compare the impact in terms of road wear of interlinks relative to articulated vehicles and rigid vehicle/trailer combinations. Eleven vehicle classes, as presented in Table 4-1, were analysed. Those vehicle classes containing tridem axle units were analysed first with dual wheels and then with single wheels on the tridem axle unit. Typical dimensions for the various vehicle classes were obtained from manufacturer s specifications. Table 4-1: Eleven typical freight vehicles used in the analysis of road wear impact Vehicle Vehicle Class Description Diagram axle rigid vehicle axle articulated vehicle 2a axle articulated vehicle (single wheels on tridem axle unit) axle articulated vehicle axle articulated vehicle 4a axle articulated vehicle (single wheels on tridem axle unit) axle rigid vehicle/drawbar trailer combination axle rigid vehicle/drawbar trailer combination 16

18 Vehicle Vehicle Class Description Diagram axle interlink axle interlink 8a axle interlink (single wheels on tridem axle unit) 4.2 Axle and vehicle masses used in the analysis The axle and vehicle masses used in the analysis are based on the harmonised permissible maximum axle and axle unit masses in the ESA Region as presented in Table 1-1. These permissible maximum masses were applied to the various vehicle classes included in the analysis and then summed to arrive at a total mass per vehicle class. In some cases the total mass per vehicle class arrived at was more than 56 t, which is the maximum permissible combination mass that was agreed to. For these vehicle classes, the individual axle and axle unit masses were adjusted to ensure that the total mass of the vehicle class did not exceed 56 t. The masses that were adjusted are highlighted in the table. The permissible and achievable axle and axle unit masses per vehicle class are summarised in Table 4-2. The achievable axle and axle unit masses were used for the further analysis. In practice, these achievable axle and axle unit masses should be adjusted to ensure correct load distribution, but it was considered not necessary for this study. 17

19 Y (mm) Y (mm) Y (mm) Y (mm) Y (mm) Y (mm) Y (mm) Y (mm) Y (mm) Y (mm) Y (mm) Load Positions: Vehicle 1-3 Axle Rigid Vehicle X (mm) Load Positions: Vehicle 2-5 Axle Articulated Vehicle X (mm) Load Positions: Vehicle 2a - 5 Axle Articulated Vehicle X (mm) Load Positions: Vehicle 3-5 Axle Articulated Vehicle X (mm) Load Positions: Vehicle 4-6 Axle Articulated Vehicle X (mm) Load Positions: Vehicle 4a - 6 Axle Articulated Vehicle X (mm) Load Positions: Vehicle 5-6 Axle Rigid/Trailer Combination X (mm) Load Positions: Vehicle 6-7 Axle Rigid/Trailer Combination X (mm) Load Positions: Vehicle 7-7 Axle Interlink X (mm) Load Positions: Vehicle 8-8 Axle Interlink X (mm) Load Positions: Vehicle 8a- 8 Axle Interlink X (mm) Table 4-2: Axle and vehicle masses used in the analysis Vehicle Description Configuration Vehicle diagramme Footprint No of axles No of wheels Permissible Maximum Axle/Axle Unit Mass (kg) Steering Axle Drive Axle/ AxleUnit Axle/ Axle Unit Axle/ Axle Unit Sum of Permissible Maximum Axle/Axle Unit Mass Achievable Maximum Axle/Axle Unit Mass due to 56 t restriction on Permissible Maximum Combination Mass Steering Axle Drive Axle/ AxleUnit Axle/ Axle Unit Axle/ Axle Unit Sum of Achievable Maximum Axle/Axle Unit Mass 1 Three axle rigid vehicle Five axle articulated vehicle a Five axle articulated vehicle Five axle articulated vehicle Six axle articulated vehicle a Six axle articulated vehicle Six axle rigid vehicle/drawbar trailer combination Seven axle rigid vehicle/drawbar trailer combination Seven axle interlink Eight axle interlink a Eight axle interlink

20 Tyre Inflation Pressure (kpa) 4.3 Tyre sizes and tyre inflation pressures applied in the analysis The methodology used in this study to calculate LEFs takes the tyre inflation pressure into account. It was therefore necessary to establish the tyre sizes for the various vehicle classes and to obtain the recommended tyre inflation pressures at the different tyre loads. The most popular tyre sizes for the different vehicle classes were obtained from the Vehicle Cost Schedule, Edition 37 published by the Road Freight Association in April Based on this information it was decided to only use one tyre size, namely 315/80R22.5. The recommended tyre inflation pressures for this size tyre at the various axle loads were obtained from Michelin s Technical Truck Tyre Data (South Africa, 01/2009). The tyre inflation pressures versus the tyre loads for this tyre size, based on the Michelin information, are presented in Figure 4-1. The recommended tyre inflation pressures at the various axle loads used in the analysis are summarised in Table /80R22.5_Single Tyre Load (kg) 315/80R22.5_Double Figure 4-1: Tyre Load versus Tyre inflation Pressure for 315/80R22.5 size tyres (from Michelin s Technical Truck Tyre Data for South Africa 01/2009) Table 4-3: Axle loads and tyre inflation pressures for 315/80R22.5 size tyres Vehicle Vehicle Class Steering Axle Loads (kg) Tyre Inflation Pressure (kpa) Drive Semi-trailer 1 Trailer Semi-trailer 2 Trailer

21 Vehicle Vehicle Class 2a a a 1232 Steering Axle Loads (kg) Tyre Inflation Pressure (kpa) Drive Semi-trailer 1 Trailer Semi-trailer 2 Trailer Pavement types evaluated in this study Five (5) typical flexible pavements found in Southern Africa were used in the analysis. Details of the various pavements, showing the pavement layers and material properties of each layer, are presented in Appendix B, while the parameter values for the mepads analysis are summarised per layer in Appendix C. 4.5 Calculated LEFS per vehicle class and pavement type The LEFs per vehicle class and pavement type, calculated using the approach described in Section 3.3, are summarised in Table 4-4. The values for the pavements in the relatively dry state and relatively wet state are shown, as well as the averages of the dry and wet states. The average LEFs per vehicle class and pavement type are presented in Figure 4-2, showing the impact in terms of LEFs of the various vehicle classes relative to one another. In Figure 4-3, the average LEF per vehicle class is presented. The average LEF is the average for the five pavement types per vehicle class. The vehicle classes are ranked according to LEF value, showing that the three vehicle classes fitted with single wheel tridem axle units have the highest average LEFs, while the 3-axle rigid vehicle and seven-axle interlink are the vehicles with the lowest average LEFs. 20

22 DRY&WET WET DRY Vehicle 1 Vehicle 2 Vehicle 2a Vehicle 3 Vehicle 4 Vehicle 4a Vehicle 5 Vehicle 6 Vehicle 7 Vehicle 8 Vehicle 8a Table 4-4: Calculated LEFs per vehicle class and pavement type Climatic Condition Pavement Pavement A ES Pavement B ES Pavement C ES Pavement D ES Pavement E ES All Pavements (Dry) Pavement A ES Pavement B ES Pavement C ES Pavement D ES Pavement E ES All Pavements (Wet) Pavement A ES Pavement B ES Pavement C ES Pavement D ES Pavement E ES All Pavements (Wet&Dry) Vehicle 8a Vehicle 8 Vehicle 7 Vehicle 6 Vehicle 5 Vehicle 4a Vehicle 4 Vehicle 3 Vehicle 2a Vehicle 2 Vehicle LEFs Pavement E ES1 Pavement D ES1 Pavement C ES3 Pavement B ES3 Pavement A ES3 Figure 4-2: Calculated average LEFS per vehicle class and pavement types 21

23 DRY&WET WET DRY LEF Vehicle 1 Vehicle 8 Vehicle 6 Vehicle 7 Vehicle 2 Vehicle 3 Vehicle 4 Vehicle 5 Vehicle 8a Vehicle 2a Vehicle 4a LEFs Figure 4-3: Calculated average LEFS per vehicle class 4.6 Road wear per vehicle class and pavement type The road wear in US c/km per vehicle class and pavement type, calculated using the approach described in Section 3.4, are summarised in Table 4-5. The values for the pavements in the relatively dry state and relatively wet state are shown, as well as the averages of the dry and wet states. The average road wear per vehicle class and pavement type are presented in Figure 4-4, showing the impact in terms of road wear of the various vehicle classes relative to one another. Table 4-5: Calculated road wear per vehicle class and pavement type 6 Road Wear (US cent/km) Pavement A ES Pavement B ES Pavement C ES Pavement D ES Pavement E ES All Pavements (Dry) Pavement A ES Pavement B ES Pavement C ES Pavement D ES Pavement E ES All Pavements (Wet) Pavement A ES Pavement B ES Pavement C ES Pavement D ES Pavement E ES Average (All Pavements)

24 Vehicle 8a Vehicle 8 Vehicle 7 Vehicle 6 Vehicle 5 Vehicle 4a Vehicle 4 Vehicle 3 Vehicle 2a Vehicle 2 Vehicle Road Wear (US cent/km) Pavement E ES1 Pavement D ES1 Pavement C ES3 Pavement B ES3 Pavement A ES3 Figure 4-4: Calculated average road wear per vehicle class and pavement type 4.7 Road wear relative to payload As stated previously, the purpose of freight vehicles travelling on the road is to transport freight and the road wear per ton of freight transported is therefore important. Ultimately this is the road wear cost that should be minimised. In this section the road wear per payload ton-km, calculated using the approach described in Section 3.5, is presented. The payload per vehicle class used in the calculations, are summarised in Table 4-6. The payloads were obtained from the Vehicle Cost Schedule, Edition 37 published by the Road Freight Association in April Table 4-6: Payload per vehicle class Vehicle Vehicle Class Diagram Payload (t)

25 Vehicle Vehicle Class Diagram Payload (t) 2a a a The road wear per payload ton-km in US c/km per vehicle class and pavement type are summarised in Table 4-7. The values for the pavements in the relatively dry state and relatively wet state are shown, as well as the averages of the dry and wet states. In Figure 4-5, the average road wear per payload ton-km per vehicle class is presented. The average road wear is the average for the five pavement types per vehicle class. The vehicle classes are ranked according to road wear per payload ton-km value, showing that the three vehicle classes with the highest road wear per ton of payload transported are the five-axle articulated vehicle (113) fitted with single tyres on the rear tridem axle unit; the six-axle articulated vehicle (123) fitted with single tyres on the rear tridem axle unit; and the six-axle rigid vehicle/trailer combination. The three vehicle classes with the lowest road damage per ton of payload transported are the eight-axle interlink; the seven-axle rigid vehicle/trailer combination; and the seven-axle interlink. 24

26 DRY&WET WET DRY Table 4-7: Calculated road wear per payload ton-km per vehicles class and pavement type Road Wear per Payload t-km (US cent/t-km) Pavement A ES Pavement B ES Pavement C ES Pavement D ES Pavement E ES All Pavements (Dry) Pavement A ES Pavement B ES Pavement C ES Pavement D ES Pavement E ES All Pavements (Wet) Pavement A ES Pavement B ES Pavement C ES Pavement D ES Pavement E ES Average (All Pavements) Vehicle 8 Vehicle 6 Vehicle 7 Vehicle 4 Vehicle 3 Vehicle 2 Vehicle 8a Vehicle 1 Vehicle 5 Vehicle 4a Vehicle 2a Road Wear (US cent/payload t-km) Figure 4-5: Calculated average road wear per payload ton-km per vehicle class 4.8 Conclusions regarding the payload efficiency of interlinks Based on the road wear analysis carried out, it can be concluded that interlinks, namely the eightaxle interlink and the seven-axle interlink are amongst the vehicle classes with the highest payload efficiency and it is recommended that the use of interlinks, especially for long distance freight movement should not be discouraged. It has also been shown that tridem axle units fitted with single tyres should be discouraged on freight vehicles that are used for mass transport. 25

27 5 TASK 2: PERMISSIBLE MAXIMUM AXLE LOADS FOR AXLES FITTED WITH WIDE BASE TYRES 5.1 Introduction In this section, the potential road wear caused by single, tandem and tridem axle units fitted with wide base tyres are compared with the potential road wear caused by axles and axle units loaded to the current maximum permissible loads of 8 t (single axle with single wheels); 10 t (single axle with dual wheels); 16 t (tandem axle unit with single wheels); 18 t (tandem axle unit with dual wheels); and 24 t (tridem axle unit with single and dual wheels). The axles and axle units fitted with wide base tyres are analysed at the same loads applicable to the legal axle and axle units to make a direct comparison of the road wear caused by the axle or axle unit fitted with wide base tyres relative to the road wear caused by legal axles and axle units subjected to the same load. The potential road wear is expressed in terms of Load Equivalency Factors (LEFs) calculated using the mepads software. 5.2 Tyre sizes and tyre inflation pressures applied in the analysis Three wide base tyres were included in this analysis, as follows: /65R /65R /65R22.5 For the current legal axles and axle units, the following two tyre sizes were used: 1. 11R /80R22.5. The recommended tyre inflation pressures for these tyres at different axle loads, as specified by the tyre manufacturer, were obtained from Michelin s Technical Truck Tyre Data (South Africa, 01/2009). The tyre inflation pressures versus the tyre loads for these five tyre sizes, based on the Michelin information, are summarised in Table 5-1 and shown graphically in Figure 5-1. Table 5-1: Tyre Load versus Tyre Inflation Pressure for five tyre sizes Tyre Size Axle Load (kg) Tyre load (kg) Tyre Inflation Pressure (kpa) 11R22.5 (dual) R22.5 (dual) R22.5 (dual) R22.5 (dual) R22.5 (dual) /80R22.5 (single) /80R22.5 (single) /80R22.5 (single) /80R22.5 (single)

28 Tyre Inflation Pressure (kpa) Tyre Size Axle Load (kg) Tyre load (kg) Tyre Inflation Pressure (kpa) 315/80R22.5 (single) /80R22.5 (dual) /80R22.5 (dual) /80R22.5 (dual) /80R22.5 (dual) /65R22.5 (single) /65R22.5 (single) /65R22.5 (single) /65R22.5 (single) /65R22.5 (single) /65R22.5 (single) /65R22.5 (single) /65R22.5 (single) /65R22.5 (single) /65R22.5 (single) /65R22.5 (single) /65R22.5 (single) /65R22.5 (single) (Source: Michelin s Technical Truck Tyre Data for South Africa 01/2009) R22.5 (dual) 315/80R22.5 (dual) 315/80R22.5 (single) 385/65R22.5 (single) 425/65R22.5 (single) 445/65R22.5 (single) Tyre Load (kg) Figure 5-1: Tyre Load versus Tyre Inflation Pressure for five tyre sizes (Source: Michelin s Technical Truck Tyre Data for South Africa 01/2009) 27

29 5.3 Axle and axle units included in the analysis The axles and axle units fitted with wide base tyres that were analysed are summarised in Table 5-2, including information on the axle loads and tyre inflation pressures (TiP) used in the analysis. The axle loads at which the various axles and axle units were analysed were kept within the ranges of axle loads specified by the tyre manufacturer (see Table 5-1) for the applicable tyre size. For example, a single axle fitted with 385/65R 22.5 tyres was not analysed for an axle load of 10 t, as the maximum axle load for this size tyre, as specified by the tyre manufacturer, is 9 t. Table 5-2: Axle/ Axle Unit Axles and axle units fitted with wide base tyres used in analysis No of Axles No of Wheels Tyre Size Axle/ Axle Unit Load (kg) Individual Axle Loads (kg) Wheel load (kg) Wheel Load (kn) Single Axle /65R Single Axle /65R Single Axle /65R Single Axle /65R Single Axle /65R Tandem /65R Tandem /65R Tandem /65R Tandem /65R Tandem /65R Tandem /65R Tridem /65R Tridem /65R Tridem /65R Information on the legal axles and axle units, including axle loads, tyre sizes and tyre inflation pressures are summarised in Table 5-3. TiP (kpa) Table 5-3: Axle/ Axle Unit Legal axles and axle units used in the analysis No of Axles No of Wheels Tyre Size Axle/ Axle Unit Load (kg) Individual Axle Loads (kg) Wheel load (kg) Wheel Load (kn) TiP (kpa) Single Axle /80R Single Axle R Tandem /80R Tandem R Tridem /80R Tridem R Calculated LEFs per axle type and pavement The LEFs calculated for the various legal axles and axle units and the axles and axle units fitted with wide base tyres are summarised in Table

30 AVERAGE WET DRY Single Axle - Legal (8t) SingleAxle_385mm_8t SingleAxle_425mm_8t SingleAxle_445mm_8t Single Axle - Legal (10t) SingleAxle_425mm_10t SingleAxle_445mm_10t Tandem - Legal (16t) Tandem_385mm_16t Tandem_425mm_16t Tandem_445mm_16t Tandem - Legal (18t) Tandem_385mm_18t Tandem_425mm_18t Tandem_445mm_18t Tridem - Legal (24t 6 tyres) Tridem_385mm_24t Tridem_425mm_24t Tridem_445mm_24t Tridem - Legal (24t 12 tyres) Table 5-4: Calculated LEFs for axles and axles units fitted with conventional and wide base tyres Climatic Condition Pavement Pavement A Pavement B Pavement C Pavement D Pavement E All Pavements Pavement A Pavement B Pavement C Pavement D Pavement E All Pavements Pavement A Pavement B Pavement C Pavement D Pavement E All Pavements In Table 5-5, the calculated LEFs for the various legal axles and axle units and the axles and axle units fitted with wide base tyre are summarised according to the current legal axle loads for the different axle types. The same information is presented in Figure 5-2. Table 5-5 and Figure 5-2 allow one to compare the road wear caused by axles and axle units fitted with wide base tyres with the road wear caused by a legal axle or axle unit carrying the same load. The following observations can be made from the information presented in Table 5-5 and Figure 5-2: Axles and axle units fitted with wide base tyres cause less road wear than the equivalent legal axle or axle unit fitted with conventional single tyres at the same axle load; Axles and axle units fitted with wide base tyres cause more road wear than the equivalent legal axle or axle unit fitted with dual tyres at the same axle load; The current legal axle and axle units fitted with conventional single tyres cause far more road wear than legal axles and axle units fitted with dual tyres at a higher axle load for example, the LEF for a single axle fitted with single tyres at an axle load of 8 t is 2.7, while for a single axle with dual tyres at an axle load of 10 t, the LEF is 1.44 (53 % lower). This same trend is observed for tandem and tridem axle units; The LEF for a single axle fitted with wide base tyres at an axle load of 10 t is higher than the LEF of a single axle fitted with conventional tyres at an axle load of 8 t; and The LEF for a tandem axle unit fitted with wide base tyres at an axle unit load of 18 t is higher than the LEF of a tandem axle unit fitted with conventional single tyres at an axle unit load of 16 t. Based on these observations, the following conclusions and recommendations can be made: The use of axles and axle units fitted with conventional single tyres should be discouraged on vehicles transporting mass freight. This could be achieved by lowering the road wear limits for these axles and axle units to such an extent that their use would only be viable on vehicles transporting volume freight; 29

31 If the road wear of axle and axle units fitted with wide base tyres are to be limited to the same road wear caused by current legal axles and axle units fitted with conventional single tyres, the road wear limits for axles and axle unit could be somewhat higher than the current legal road wear limits of 8 t for single axles and 16 t for tandem axle units (the determination of these higher limits is presented in Section 5.5); and The road wear limit for tridem axle units should remain at 24 t, even when fitted with wide base tyres. Table 5-5: LEFs for legal axles and axles fitted with wide base tyres at different axle loads Axle Type Single Axle Load (kg) Legal Axle Axles fitted with wide base tyres Diagramme Tyre Size LEFs Diagramme Tyre Size LEFs 385/65R /80R /65R /65R /65R 22.5 n.a 315/80R /65R /65R /65R /80R /65R Tandem 445/65R /65R R /65R /65R /65R /80R /65R /65R Tridem 385/65R R /65R /65R

32 Axle/Axle unit Loads 24t Tridem axle with dual tyres (11R 22.5) 24t Tridem axle with single tyres (315/80R 22.5) 18t Tandem axle with dual tyres (11R22.5) 16t Tandem axle with single tyres (315/80R 22.5) 10t Single axle with dual tyres (315/80R 22.5) 8t Single axle with single tyres (315/80R 22.5) LEFs 445/65R /65R /65R 22.5 Legal Axle Figure 5-2: LEFs for legal axles and axles fitted with wide base tyres at different axle loads 5.5 Determination of proposed axle and axle unit load limits for axles fitted with wide base tyres In order to determine the proposed axle and axle unit load limits for single axles and tandem axle units fitted with wide base tyres, the LEFs were calculated for single axles at axle loads of 8 t, 9 t and 10 t for the three wide base tyre sizes and for tandem axle units at of 16 t, 17 t and 18 t. These LEFs are summarised in Table 5-6 and are plotted against the axle loads for single axles in Figure 5-3 and tandem axle units in Figure 5-4. The horizontal line on Figure 5-3 represents the LEF of 2.7 calculated for a single axle with single tyres (315/80R 22.5) at an axle load of 8 t, which is currently a legal axle (see Table 5-5). The axle load values at the points where the lines of the three wide base tyres intercept this line represent the axle loads at which a single axle fitted with the applicable wide base tyres would cause the same road wear as the currently legal axle. These values are approximately as follows: 385/65R 22.5: kg 425/65R 22.5: kg 445/65R 22.5: kg The horizontal line on Figure 5-4 represents the LEF of 5.4 calculated for a tandem axle unit with single tyres (315/80R 22.5) at an axle load of 16 t, which is currently a legal axle (see Table 5-5). The axle load values at the points where the lines of the three wide base tyres intercept this line represent the axle loads at which a tandem axle unit fitted with the applicable wide base tyres would cause the same road wear as the currently legal axle. These values are approximately as follows: 385/65R 22.5: kg 425/65R 22.5: kg 31

33 LEFs 445/65R 22.5: kg Based on the axle load values presented above, it could be recommended that the following permissible maximum axle mass loads be allowed for single axles and tandem axle units fitted with wide base tyres: Tyre size: 385/65R 22.5: Single Axle: kg Tandem Axle Unit: kg Tyre size: 425/65R 22.5: Single Axle: kg Tandem Axle Unit: kg Tyre size: 445/65R 22.5: Single Axle: kg Tandem Axle Unit: kg Table 5-6: LEFs for single axles and tandem axle units fitted with wide base tyres at various axle loads Axle Axle/Axle Tyre Size Type Unit Load 385/65R /65R /65R 22.5 Single Single Single n.a Tandem Tandem Tandem Axle Load (kg) Legal LEF 385/65R /65R /65R 22.5 Figure 5-3: LEFs for single axles fitted with wide base tyres at different axle loads 32

34 LEFs Axle Unit Load (kg) Legal LEF 385/65R /65R /65R 22.5 Figure 5-4: LEFs for tandem axle units fitted with wide base tyres at different axle unit loads In these calculations of the LEFs, it is assumed that the tyre inflation pressure (TiP) is equal to the tyre contact stress. Numerous publications have however shown that the tyre-road pavement contact stresses are neither uniform nor circular in shape and that they depend to a large extent on the tyre loading and tyre inflation pressure level of a particular tyre. It was also found that the average vertical contact stress is much lower than the maximum vertical contact stress, which can be as much as twice the tyre inflation pressure. To analyse the road wear caused by wide base tyres versus conventional tyres in more detail, the tyre contact stress should be determined more accurately. This is presented in Section Inclusion of tyre contact stress values in the calculation of LEFs During the past 18 years (since 1994) the CSIR has focused research on the measurement of tyrepavement contact stresses for use on flexible (and rigid) road pavements in South Africa. This research is based on the well known Stress-In-Motion (SIM) technology platform at the CSIR. Currently a basic database of twenty two (22) tyres with corresponding information on loading/inflation pressure exists at CSIR Built Environment. This database constitutes the main source of data for a Tyre Contact Stress Information System (T-CSIS) under development at the CSIR. Of the 22 tyres in the database, 10 were tested in South Africa, while 12 were tested in SIM studies done on tyres internationally, such as at the University of California, Berkeley (UCB, USA), Texas Transportation Institute (TTI, Texas, USA) and TuDelft (in The Netherlands). A tyre contact stress viewer software product dubbed TyreStress has been developed at CSIR Built Environment. The TyreStress software provides 3 dimensional tyre contact stress data for both SIM measured cases as well as interpolated cases (i.e. tyre contact stress data not measured with SIM but generated by interpolation). 33

35 This software package has been used to calculate tyre contact stress and tyre loading for more detail analysis of the road wear caused by axles fitted with wide base tyres. The database contains three size 315/80R22.5 tyres and six size 425/65R22.5 tyres. The tyres in the database are from different manufacturers and the recommended tyre inflation pressures for these size tyres at different axle loads were obtained from Goodyear s Truck Tyre Inflation Table, which is based on ETRTO Standards. This table is published on Goodyear s website ( and was accessed on 18 March The tyre inflation pressures versus the tyre loads for these two tyre sizes, based on the ETRTO Standards contained in the Goodyear table are summarised in Table 5-7 and shown graphically in Figure 5-5. Table 5-7: Tyre Load versus Tyre Inflation Pressure for two tyre sizes Tyre Size Axle Load (kg) Tyre load (kg) Tyre Inflation Pressure (kpa) 315/80R22.5 (single) /80R22.5 (single) /80R22.5 (single) /80R22.5 (single) /80R22.5 (single) /80R22.5 (single) /80R22.5 (single) /80R22.5 (single) /65R22.5 (single) /65R22.5 (single) /65R22.5 (single) /65R22.5 (single) /65R22.5 (single) /65R22.5 (single) /65R22.5 (single) /65R22.5 (single) (Source: Goodyear s Truck Tyre Inflation Table) 34

36 Tyre Inflation Pressure (kpa) /80R /65R Tyre Load (kg) Figure 5-5: Tyre Load versus Tyre Inflation Pressure for two tyre sizes (Source: Goodyear s Truck Tyre Inflation Table) The axles and axle units that were analysed with the inclusion of tyre contact stress values are summarised in Table 5-8 including information on the axle loads used in the analysis and the corresponding tyre inflation pressures. Table 5-8: Axle/ Axle Unit Axles and axle units analysed with the inclusion of tyre contact stress values No of Axles No of Wheels Tyre Size Axle/ Axle Unit Load (kg) Individual Axle Loads (kg) Wheel load (kg) Wheel Load (kn) TiP (kpa) Single Axle /80R Single Axle /65R Single Axle /65R Single Axle /65R Tandem /80R Tandem /65R Tandem /65R Tandem /65R Using the wheel loads and tyre inflation pressures presented in Table 5-8 as input, the TyreStress software was then utilised to extract the SIM measured wheel loads and equivalent uniform contact stresses for the three 315/80R22.5 and six 425/65R22.5 tyres in the database. Average values of the measured wheel loads and equivalent uniform contact stresses were then calculated for the three 315/80R22.5 tyres and for the six 425/65R22.5 tyres and these average values are presented in Table 5-9. These measured wheel loads and equivalent uniform contact stresses were then used as 35

37 AVERAGE WET DRY Single Axle - Legal (8t) SingleAxle_425mm_8t SingleAxle_425mm_9t SingleAxle_425mm_10t Tandem - Legal (16t) Tandem_425mm_16t Tandem_425mm_17t Tandem_425mm_18t input for the mepads software to re-calculate the LEFs as before. Table 5-9: A x l e / Axle Unit No of Axles Measured tyre loads and equivalent uniform contact stresses obtained from TyreStress N o o f Wheels Tyre Size A x l e / Axle Unit Load (kg) Wheel load (kg) Wheel Load (kn) TiP (kpa) S I M Measured Tyre Load (kn) Equivalent Uniform Contact Stress (kpa) Single Axle /80R Single Axle /65R Single Axle /65R Single Axle /65R Tandem /80R Tandem /65R Tandem /65R Tandem /65R The re-calculated LEFs for legal axles and axles fitted with 425/65R 22.5 wide base tyres, using the SIM measured wheel loads and equivalent uniform contact stresses obtained from the TyreStress software, are presented in Table Table 5-10: Calculated LEFs for legal axles and axles fitted with 425/65R 22.5 wide base tyres using outputs from the TyreStress software Climatic Condition Pavement Pavement A Pavement B Pavement C Pavement D Pavement E All Pavements Pavement A Pavement B Pavement C Pavement D Pavement E All Pavements Pavement A Pavement B Pavement C Pavement D Pavement E All Pavements

38 LEFs The average LEF per axle fitted with 425/65R 22.5 wide base tyres are summarised in Table The average value per axle is the average of the values for the five pavements in both the dry and wet conditions. Table 5-11: LEFs for single axles and tandem axle units fitted with 425/65R 22.5 wide base tyres at various axle loads Axle Axle LEFs Type Load Single Single Single Tandem Tandem Tandem These average LEFs are plotted against the axle loads for single axles in Figure 5-6 and tandem axle units in Figure Axle Load (kg) Legal LEF 425/65R 22.5 Figure 5-6: LEFs for single axles fitted with 425/65R 22.5 wide base tyres at different axle loads The horizontal line on Figure 5-6 represents an LEF of 1.75 calculated for a single axle with single tyres (315/80R 22.5) at an axle load of 8 t, which is currently a legal axle (see Table 5-5). The axle load value at the point where the line of the 425/65R 22.5 wide base tyre intercepts this line represents the axle load at which a single axle fitted with 425/65R 22.5 wide base tyres would cause the same road wear as the currently legal axle. This values is approximately kg. 37

39 LEFs Axle Unit Load (kg) Figure 5-7: Legal LEF 425/65R 22.5 LEFs for tandem axle units fitted with 425/65R 22.5 wide base tyres at different axle unit loads The horizontal line on Figure 5-7 represents an LEF of 3.45 calculated for a tandem axle unit with single tyres (315/80R 22.5) at an axle unit load of 16 t, which is currently a legal axle unit (see Table 5-5). The axle load value at the point where the line of the 425/65R 22.5 wide base tyre intercepts this line represents the axle unit load at which a tandem axle unit fitted with the 425/65R 22.5 wide base tyres would cause the same road wear as the currently legal axle unit. This values is approximately kg. Based on the axle load values presented above, it could be recommended that the following permissible maximum axle mass loads be allowed for single axles and tandem axle units fitted with 425/65R 22.5 wide base tyres: Single Axle: Tandem Axle Unit: kg kg The value for a single axle of kg is similar to the value of kg that was previously arrived at with the assumption that the tyre inflation pressure (TiP) is equal to the tyre contact stress. For the tandem axle unit the value of kg is the same as the previous value arrived at with the assumption that the tyre inflation pressure (TiP) is equal to the tyre contact stress. 5.7 Conclusions and recommendations regarding the use of wide base tyres If the principle is that axles and axle units fitted with wide base tyres should not cause more road wear than similar axles fitted with conventional single tyres, it can be recommended that the road wear limit for single axles fitted with wide base tyres with a width of 425 mm and wider could be 38

40 increased by 500 kg to kg, while the road wear limit for tandem axle units fitted with wide base tyres with a width of 425 mm and wider could be increased by kg to kg. It has however been shown that the current legal axle and axle units fitted with conventional single tyres cause far more road wear than legal axles and axle units fitted with dual tyres at a higher axle load and it is therefore recommended that the use of axles and axle units fitted with conventional single tyres should be discouraged on vehicles transporting mass freight. This could be achieved by lowering the road wear limits for these axles and axle units to such an extent that their use would only be viable on vehicles transporting volume freight. If this recommendation is accepted, the principle should then be that axles and axle units fitted with wide base tyres should not cause more road wear than similar axles and axle units fitted with dual tyres. This would however not be achievable, as it has been shown that axles and axle units fitted with wide base tyres cause considerably more road wear than similar axles fitted with dual tyres (see Table

41 6 TASK 3: PREFERRED LANE WIDTHS FOR MAJOR ROADS 6.1 Introduction Task 3 involved carrying out a desk study of all available literature including Codes of Practice, latest research etc with a view to making a firm recommendation with regard to preferred lane widths for all major roads The selection of lane width is based on traffic volume and vehicle type and speed. Higher volumes and speeds require wider lanes. It is further important to establish consistency between lane width design standards and design speed. Lane widths should induce operating speeds compatible with the selected design speed. Wider lane widths on roadways with lower design speeds may therefore be undesirable. Lane widths must further be evaluated in conjunction with the width of the roadway or carriageway, which include the shoulders (paved or unpaved). Lane widths can vary from 2.7 m to 3.7 m on rural roads, although the most common widths are 3.1 m; 3.4 m and 3.7 m. Research has indicated that the crash rate starts to show a marginal increase above a lane width of 3.6 m and lane widths significantly greater than 3.7 m are therefore not desirable. Lane widths of more than 3.7 m are found in the urban environment, but these are usually on roads fulfilling both mobility and access functions. A wider lane allows vehicles wanting to access properties adjacent to the road to do this more safely while allowing through traffic to pass without encroaching onto the oncoming lane. The number of lanes to be provided is largely determined by traffic flow and the desired Level of Service that the road is to provide. Where traffic volumes are such that a multi-lane or divided crosssection is required, 3.7 m lanes are generally provided. It is therefore mostly on two-lane rural roads where the lane widths would vary and the focus of this task was for this reason mostly on two-lane rural roads. 6.2 Lane and shoulder widths recommended in design guides and codes of practice SANRAL Geometric Design Guidelines Minimum lane width: 3.1 m Maximum lane width: 3.7 m (for multi-lane/divided cross section roads and high speeds) For intermediate volumes and speed, a 3.4 m lane width is recommended Minimum shoulder widths: 1 m Maximum shoulder width: 3 m Intermediate widths of 1.5 m and 2.0 m Paved widths between 1.5 m and 2.5 m should be avoided Dual carriageway roads should have a 3 m outer shoulder and a 1 m inner shoulder Included in the SANRAL guidelines is Table 6-1 with recommended shoulder widths for different design speeds and design hour volumes. Design speed is defined as the speed selected as a safe basis to establish appropriate geometric design elements for a particular section of road and which should be a logical one with respect to topography, anticipated operating speed, the adjacent land use and the functional classification of the road. The design hour is defined as the hour in which the condition being designed for, typically the anticipated flow, is expected to occur. This is often the thirtieth highest hour of flow in the design year. 40

42 Table 6-1: Shoulder widths for undivided rural roads as recommended in SANRAL s Geometric Design Guide SATCC Code of Practice for the Geometric Design of Trunk Roads Minimum lane width: 3.1 m Maximum lane width: 3.7 m For intermediate volumes and speed, a 3.4 m lane width is recommended For multi-lane or divided cross section roads a lane width of 3.7 m is recommended Minimum shoulder widths: 1 m Maximum shoulder width: 3 m Intermediate widths of 1.5 m and 2.0 m Dual carriageway roads should have a 3 m outer shoulder and a 1 m inner shoulder AASHTO Policy on Geometric Design of Highways and Streets (2001) The AASHTO policy makes provision for lane widths from 9 to 12 feet, which has been converted to 2.7 m to 3.6 m, and mentions that a 3.6 m lane is predominant on most high-type highways. It states that the extra cost of providing a 3.6 m lane width, over the cost of providing a 3.0 m lane width is offset to some extent by a reduction in cost of shoulder maintenance and a reduction in surface maintenance due to lessened wheel concentrations at the pavement edges. The wider 3.6 m lane provides desirable clearances between large commercial vehicles travelling in opposite directions on two-way rural highways when high traffic volumes and particularly high percentages of commercial vehicles are expected. In general, 3.6 m lane widths are desirable in both rural and urban facilities, but there are circumstances where lanes less than 3.6 m wide should be used. These are: 3.3 m lanes: Urban areas where pedestrian crossings, right-of-way, or existing development become stringent controls; 3.0 m lanes: Acceptable on low speed facilities; and 2.7 m lanes: Appropriate on low-volume roads in rural and residential areas. 41