ANALYSIS OF THE STRUCTURAL BEARING CAPACITY OF AN AIRPORT USING RUDIMENTARY TEST RESULTS AS INPUT INTO THE SAMDM

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1 ANALYSIS OF THE STRUCTURAL BEARING CAPACITY OF AN AIRPORT USING RUDIMENTARY TEST RESULTS AS INPUT INTO THE SAMDM P.W. de Bruin 1, G.J. Jordaan 1 J. Andre 2, F. Francisco 2 and N.A.S. Domingos 3 1 Tshepega Engineering PO Box 35256, Menlopark, 0102, South Africa. debruinpw@tshepega.co.za and jordaangj@tshepega.co.za 2 Ministry of Public Works, Luanda, Angola. 3 Ministry of Airports, Luanda, Angola. ABSTRACT Logistical problems, in remote areas in Africa, make it extremely difficult if not impossible to use sophisticated testing methods to analyse the bearing capacity of roads and airport pavements. Hence, engineers have to be innovative in obtaining maximum information with easily transportable equipment and testing methods such as the Dynamic Cone Penetrometer (DCP). Information obtained from such testing can be used to get appropriate input values into existing design models in order to determine the bearing capacity and rehabilitation needs. This paper covers a preliminary analysis for an airport in Angola. The South African Mechanistic Design Method (SAMDM), together with the International Civil Aviation Organisation s (ICAO s) empirically design method/s (i.e. based on the US FAA CBR design method), were used to calculate the bearing capacity of the airport for the design aircraft. The results obtained from the SAMDM analyses are compared with the designs recommended by ICAO s and show great promise in analysing airport pavement structures. It is demonstrated that the results obtained using the SAMDM (with rudimentary test results as input) compare excellently with the design obtained using the ICAO method. The SAMDM procedure was also used to determine the Aircraft Classification Number (ACN) ratio s (or equivalent factors) for the different aircraft types for comparison reasons. Keywords: airport design, ACN, ICAO, DCP, structural bearing capacity 1. INTRODUCTION With the ageing of road networks and airport pavements all over the world, more and more emphasis is placed on the cost-effective rehabilitation of these pavements. A meaningful assessment methodology is therefore required, for the evaluation of the existing structural condition of pavements and the effect thereof on possible alternative rehabilitation options. The Mechanistic-Empirical (M-E) analysis of the bearing capacity of pavements is a methodology that can accurately take into account the effect of rehabilitation measures. This methodology is able to simulate any type of load and tyre pressure (which is a great benefit in analyzing different aircraft types) and the influence of such loading on the pavement layers in order to calculate the structural bearing capacity of the pavement. The South African Mechanistic Design Method (SAMDM) developed since 1982 (Jordaan, 1994), (Theyse et al, 1996) is well established and is widely used by many road designers for pavement rehabilitation design purposes. This method has been used extensively by researchers for new pavement designs and for the refinement of existing designs as well as by engineers for practical applications (Freeme et al, 1982). The current method of mechanistic analysis/design used in South Africa has been developed largely from the results of extensive Heavy Vehicle Simulator (HVS) testing and is based on the non-hereditary aspects of linear elastic stress/strain theory. Proceedings of the 8 th Conference on Asphalt Pavements for Southern Africa (CAPSA'04) September 2004 ISBN Number: Sun City, South Africa Proceedings produced by: Document Transformation Technologies cc

2 Currently, the structural bearing capacity of airport pavements is usually analysed using the International Civil Aviation Organisation s (ICAO s) empirically design method/s, which are based on the US Federal Aviation Administration (FAA) California Bearing Ratio (CBR) design method (ICAO, 1983). Although these empirically design methods are widely used and accepted, there is a tendency world-wide towards using the Mechanistic-Empirical (M-E) design analysis approach in design applications. In remote areas in Africa, logistical problems make it extremely difficult if not impossible to use sophisticated testing methods to analyse the bearing capacity of airport pavements. Hence, engineers have to be innovative in obtaining maximum information with easily transportable equipment and testing methods such as the Dynamic Cone Penetrometer (DCP). Information obtained from such testing is then used to get appropriate input values into existing design models in order to determine the bearing capacity and rehabilitation needs. The objectives of the paper are to: do a preliminary analysis and to compare the bearing capacity derived from the ICAO empirically design method with results obtained from the SAMDM, determine ACN ratio (or equivalent factors) for different aircrafts using the SAMDM and compare it with ACN ratio s calculated by using ICAO s traffic calculation method (UK CAN-PCN classification) (ICAO UK, 1989), and illustrate that the M-E design method can be used to accurately simulate aircraft loading and to determine the structural bearing capacity requirements of an airport pavement structure. For the purpose of this paper the preliminary design will mainly focus on the analysis of the runway. 2. BACKGROUND The airport runway identified for analysis purposes is situated in Angola and is approximately 3.4 kilometers long and at an altitude of about 1982 m above sea level. The runway was constructed in 1961 and was until recently mainly used as a military airport. The asphalt surfacing on the outer shoulders of the runway shows severe signs of distress. The centre 20 to 25 m of the main runway is in a better condition since the asphalt surfacing was replaced in 1994 using a 50 mm thick asphalt surfacing. This relatively new surfacing is not of a good quality and is also exhibiting distress in terms of inter alia cracking. The visual condition investigation / inspection of the airport showed clearly that the runway, taxiway and apron are in urgent need for rehabilitation. The existing asphalt surfacing (AC) layer varies in thickness between 50 and 70 mm. The base layer along the runway, taxiways and the apron consists of a large stone Waterbound Macadam (WBM) with sand used as a filler material. The layer varies in thickness between 80 and 100mm for the runway and 150 mm for the taxiways and aprons. This Waterbound Macadam (WM) layer was placed on a 150 mm subbase layer, which consists of an imported gravel sand layer (ferricrete). Due to logistical problems, it was not an option to use sophisticated testing methods (e.g. measurement of deflections etc) to characterize the pavement and to determine the material properties of the different pavement layers. Hence, the engineers were to obtain maximum information using easily transportable equipment and testing methods (non-destructive) such as the Dynamic Cone Penetrometer (DCP). In South Africa, the measurement of the in-situ shear strength of the pavement layers using a DCP is well established (Jordaan, 1994a), (Kleyn et al, 1987), (Kleyn, 1984), (Kleyn and Savage, 1982), (DOT, 1997). The DCP is used to measure the rate of penetration (DN) through the various components (layers) of the pavement structure. The penetration is a function of the in-situ shear strength of the material and the profile in depth thereof gives an indication of the effective in-situ properties of the materials in all

3 the pavement layers up to a depth of penetration (800 mm is recommended). The California Bearing Capacity (CBR) test, which are required as an input parameter into the ICAO design, also gives an indication of the shear strength of the material but has the typical limitation of all similar laboratory tests, such as unnatural conditions, which makes it difficult and time consuming to obtain the in-situ prevailing pavement condition. Although in principle the DCP and CBR both measure the shear strength of the material, the DCP has the advantage that it is non-destructive, easy to transport and use and allows for the detailed in-situ evaluation and analysis of pavement structures and their different layers (Jordaan, 1994a), (Kleyn et al, 1987), (Kleyn, 1984), (Kleyn and Savage, 1982). Good correlations were found and documented between the DCP measurements and the well known California Bearing Capacity (CBR) of granular materials and the Unconfined Compressive Strength (UCS) of cemented materials (Kleyn, 1984), (Kleyn and Savage, 1982), (DOT, 1997). The DCP test results, together with test pits, are also used to determine the thicknesses of the different layers, which have similar shear properties. The different layer thicknesses are of utmost importance in the design approach and required as input into both the SAMDM and ICAO design methods. Information obtained from the DCP testing is used to get appropriate input values (CBR and Stiffness (E-moduli)) into existing design models (e.g. the SAMDM and ICAO design method) in order to determine the bearing capacity and design needs. The material definitions and descriptions, e.g. Asphalt (ic) (Concrete) Surfacing (AC), Waterbound Macadam (WM), Graded Crushed Stone (G1&G2) and Natural Gravel (G6), used in this paper are well documented and described in TRH 14 (1987). 3. TRAFFIC LOADING 3.1 Number of Aircraft and Type The following initial information was received from the client as a basis for the preliminary analysis of the pavement structure: Type of aircraft (currently using the airport): IL- 76T aircraft of 171 ton (18 wheel configuration), and B (design aircraft) Frequency (Number of departures / day): 7 flights per day (given by client as a realistic scenario) Estimated ACN: 45 Design period: 20 years During discussions with the client it was made clear that the runway must be designed to allow its use by other types of aircraft. The following large aircrafts were identified by the authorities to be included in the design aircraft traffic mix (future): IL-76T aircraft, B aircraft, B aircraft, and B B aircraft.

4 4. DESIGN CRITERIA 4.1 Design Criteria Runway The ICAO manuals (ICAO, 1983), (ICAO, 1999), (ICAO, 1984) give guidelines for the planning of an airport (aerodrome) using certain aeroplane reference codes. The intent of the reference codes is to provide a simple method for interrelating the numerous specifications concerning the characteristics of aerodromes so as to provide a series of aerodrome facilities that are suitable for the aeroplanes that are intended to operate at the airport. The codes are not intended to be used for determining runway length or pavement requirements (ICAO, 1983). The code comprise of the following two elements: Code element 1 (Code number) - Aeroplane field length, Code element 2 (Code letter) - Wing span and outer gear wheel span. The particular specification of the airport is related to the more appropriate of the two elements or a combination of the two elements. The code letter or number is related to the critical aeroplane characteristics for which the facility is provided. The identified design aircraft (B ) have the following characteristics (ICAO, 1983): ACN (for flexible pavement) (CBR >15): 45 Wing Span: 32.9 Outer main wheel span 6.9 Taking the above into consideration as well as the characteristics of the other types of aircrafts identified in using the airport, the airport reference codes (ICAO, 1984) composed for the airport are as follows: Code element 1 - Aeroplane field length >1800m (3400m) Code number = 4, Code element 2 - Wing span (between 24m and 36m) and outer gear wheel span (between 6m and 9m) Code letter = C The characteristics of the various pavement layers of the runway were determined as discussed in Section 5 of this paper. It is clear that the design CBR as determined using the various percentile levels are all above a value of 15. Therefore, in terms of the ICAO definition there is no doubt that the in-situ material represents a subgrade with a high CBR. It follows that all calculations based on the ICAO manuals are done for the subgrade conditions with a High CBR. 4.2 Design Criteria Traffic Using the scenario for a subgrade with a high CBR, the following Aircraft Classification Numbers (ACN) are obtained for the various identified aircrafts: IL-76T ACN - 37 B ACN - 45 B ACN - 22 B B ACN - 42 It is clear that the ACN for the B aircraft is the highest, representing the aircraft that requires the runway pavement with the highest bearing capacity. Taking the above into consideration as well as the fact that it is envisaged that the B aircraft will most frequently used the airport facilities, it was agreed that the B will be the design aircraft for the airport.

5 5. GEOTECHNICAL INVESTIGATIONS 5.1 Geotechnical Information and Material Investigation Dynamic Cone Penetrometer (DCP) tests were done along the centre line of the runway during the visit to the airport. During this investigation data were gathered in order to: obtain a meaningful indication of the layer thicknesses, and determine the material quality and properties and uniformity of the pavement structure. Both these measurements are important input parameters required for the evaluation of the pavement structure, analysis of pavement bearing capacity and to determine an appropriate rehabilitation design for the pavement. The DCP tests were used to obtain the in-situ CBR of the materials in all the pavement layers. The in-situ CBR values of the different pavement layers were also used to derive the soaked CBR values of each pavement layer. According to sound pavement engineering, the results of geotechnical/material tests should be combined and statistically processed to obtain reliable design data based on a certain lower percentile level. Hence, the data was processed and combined to obtain a 90-percentile (lower) design profile (used in South Africa) (TRH12, 1997) of all the pavement layers of the runway in depth, as well as the lower mean minus one standard deviation design profile (recommended in the ICAO manual Federal Aviation Administration (FAA) design) (ICAO, 1983) of all the pavement layers of the runway in depth. These results are shown in detail in Figure 1 and summarised in Table 1. The in-situ material characteristics for the different pavement layers are used in various design methods as discussed in detail in the following sections dealing with the rehabilitation design of the runway. 5.2 Test Pit and Layer Information Due to the uniformity of the DCP measurements, only two tests pits were opened on the runway in order to identify the pavement structures and properties of the different layers. The following pavement structures were identified for the runway: Surfacing 50 mm severely cracked Asphalt (ic) (Concrete) (AC) surfacing (The centre m of the runway was repaired with a new asphalt in 1994 also cracked although less severe). Base 80 mm to 100 mm Waterbound Macadam (WBM) base Subbase 150 mm Imported Coarse Gravel Sand (G6) (ferricrete) Selected layer/s several imported/in-situ gravel/sand layers each about 100mm thick (G6) Subgrade In-situ sand 6. INVESTIGATION AND DESIGN 6.1 General Two methodologies or approaches are used to calculate the bearing strength or capacity (in terms of coverages of the design aircraft) of the rehabilitated pavement of the runway.

6 They are as follow: International Civil Aviation Organisation (ICAO) method (US FAA method) (ICAO, 1983) is based on empirically derived design curves based on the CBR method (flexible pavement). Mechanistic Design Method (SAMDM) (Jordaan, 1994), (Theyse et al, 1996) this method is based on sound engineering principles, using basic material properties and characteristics of the existing pavement structure together with the traffic loading to calculate the stresses and strains in each pavement layer. Both the abovementioned methods are internationally recognised as standard practice in pavement evaluation and design Depth (mm) % CBR (In-Situ & Soaked) 90th In-Situ CBR FAA Soaked CBR 90th Soaked CBR FAA In-Situ CBR Figure th Percentile CBR/layer and the FAA CBR/layer. 6.2 Evaluation Methodology and Analysis of Data ICAO (US FAA design) method The ICAO (US FAA) method is based on empirically derived design curves, which are based on the CBR method (flexible pavement) and the Unified Soil Classification System (ICAO, 1983). This method has been used extensively and is well known in the pavement industry. The material properties (CBR) calculated in the geotechnical investigation were used for analysis

7 purposes. The in-situ CBR values of the different pavement layers were also used to derive the soaked CBR values of each pavement layer. Table 1. Summary of material properties (CBR and E-moduli values) calculated statistically for the different pavement layers. Pavement layers CBR values calculated from the DCP testing results 90 th 90 th Percentile Percentile in-situ soaked CBR CBR CBR Layer thickness (mm) (FAA Std.) Mean 1xStdev in-situ (FAA Std.) Mean 1xStdev soaked CBR E-Moduli based on CBR values using E=10 x CBR (ICAO, 1983) Pavement layers Layer thickness (mm) 90 th Percentile in-situ (MPa) 90 th Percentile soaked (MPa) (FAA Std.) in-situ (MPa) (FAA Std.) soaked (MPa) E-Moduli based on penetration rate (DN) values using E eff =10 (DOT, 1997) Layer 90 th thickness Percentile (mm) soaked Pavement layers ( log (DN)) (FAA Std.) soaked (MPa) (MPa) Final input data for pavement layers used in the mechanistic design analysis programme Pavement layers Layer thickness (mm) E-moduli (MPa) Poisson ratio Material definitions according to TRH 14 (TRH14, 1987) New Asphalt (ic) (Concrete) Overlay (AC) Old cracked asphalt surfacing (AC) almost in an equivalent granular state (failure - deformation) properties similar to Graded Crushed Stone - analysed as a G2 graded crushed stone Waterbound Macadam (WM) properties similar to Graded Crushed stone (failure - deformation) analysed as a G1 graded crushed stone CBR>25 Natural gravel - (G6) CBR>25 Natural gravel - (G6)

8 Aircraft Type According to sound pavement engineering, the results of the geotechnical/material tests should be combined and statistically processed to obtain reliable design data based on a certain lower percentile level. Hence, the data was processed and combined to obtain a lower mean minus one standard deviation design profile of all the pavement layers of the runway in depth (recommended in ICAO manual (FAA) design) (ICAO, 1983). These results are shown in detail in Figure 1 and summarised in Table 1. The ICAO (CBR) design curves for the design aircraft (B dual wheel gear) require the following design input parameters: subgrade and subbase CBR (34 Table 1, minimum CBR of 20 (subgrade) was used for calculations), annual departures (1,785 for the design aircraft calculated in Table 2), gross aircraft mass (84, 005kg ICAO), and gear configurations (dual wheel gear - ICAO). Based on the above design input parameters and the evaluation of the bearing capacity of the pavement, the total required thickness (using Table 4-37 in the ICAO design manual), calculated for the design aircraft is ±380 mm (including the 100 mm bituminous surfacing layer). Thus, according to the ICAO design method a total thickness of ±150 mm asphalt surfacing (AC) is required for the design traffic loading and a design period of 20 years. Table 2. Traffic analysis for flexible pavements - based on ICAO design. (US FAA Practice) (20 year design traffic) (ICAO, 1983). Gear Type No of Daily Forecast Annual Conversion Factor to Gear Dual Gear Maximum Take-off Weight (MTOW) * (kg) Wheel Load (kg) Wheel Load - Design Aircraft (kg) Equivalent Annual - Design Aircraft IL-76T Dual-Tandem , ,000 10,150 19, B # Dual ,005 19,950 19, B Dual ,722 10,860 19, B B Dual-Tandem , ,778 17,670 19, # Design Aircraft, * MTOW = Maximum Take Off Weight, Depart = Departures 1, The South African Mechanistic Design Method (SAMDM) Methodology: The methodology and approach used for the evaluation and determination of the structural bearing capacity of the pavement (equivalent coverages) of the runway, using the SAMDM, are briefly described and involve the following steps: Collection of all available data. This included a geotechnical investigation during which available information on pavement structural data, test pits and material test results (DCP (CBR) testing) are obtained, Identification of uniform pavement sections. The identification of uniform pavement sections are based on a holistic approach taking into account all available data such as as-built data, material test results, geology and measurements giving an indication of the bearing capacity of the different pavement layers. According to sound pavement engineering, the results of geotechnical/material tests should be combined and statistically processed to obtain reliable design data based on a certain lower percentile level. Hence, the data was processed and combined to obtain a 90-percentile (lower) design profile (used in South Africa) (TRH12, 1997) of all the pavement layers of the runway in depth (Jordaan, 1994), (Theyse et al, 1996). The structural condition (visually detected as well as measured) do not vary along the length of the runway (similar geology along the runway length) and the runway was therefore not subdivided into several uniform sections for analysis purposes.

9 Characterisation of the pavement layers. The pavement layers are characterised in terms of the elastic modulus, Poisson ratio and thickness of each pavement layer. The material properties (i.e. effective elastic moduli (E-moduli) etc) derived from the material testing are summarised in Table 1 and involved the following steps: - First: Determination of the CBR values. The following empirical relationships between the DCP s penetration rate (DN value=penetration rate (mm/blow)) and CBR were used to determine the CBR values of the different pavement layers (DOT, 1997): CBR=66.66 DN DN DN<2, (Eq.1) CBR=410 DN 1.27 DN>2 (Eq.2) - Second: Determination of the Effective Elastic Moduli (E eff ). The following two empirical relationship between DCP and Effective Elastic Moduli were used to determine the stiffness (E eff ) of the different pavement layers: E=10 CBR (Recommended by ICAO), (ICAO, 1983) (Eq.3) E eff =10 ( x log (DN)), (DOT, 1997) (Eq.4) The final material properties that have been used as input into the M-E design model are summarised in Table 1. Due to the fact that it was extremely difficult to use sophisticated testing methods to accurately characterise and determine the material properties of the pavement layers (e.g. deflection measurements etc) the use of rudimentary tests (e.g. DCP) and input values derived from these tests need to be used with caution. Experience and engineering judgement were used to determine the final input values for the elastic modulus of the pavement layers derived from the DCP test results. Development of a M-E model (computer based) of the pavement structures using one of the available linear elastic software packages, allows for the simulation of the effect of any load (e.g. any aircraft) on the pavement structure. The following input data is required by the Mechanistic Analysis software programme for calculation purposes: - contact tyre pressure (1.02 MPa) and design load (198kN) (e.g. for the design aircraft B ), - number of wheels and position (dual wheel 86cm spacing), - material properties including the estimated effective elastic moduli (elastic stiffness). The effective elastic moduli were derived from the CBR results obtained from the geotechnical investigation as shown in Figure 1 and Table 1 using engineering judgement and procedures recommended by the SAMDM, - Poisson s ratio of each layer typical values used in South Africa, - layer type and description (Table 1), - number of pavement layers (Figure 1 and Table 1), - layer thicknesses (Figure 1 and Table 1), - calculation of the associated stresses and strains using a linear elastic software programme (e.g. Elsym 5 and CHEV15), Calculation of the structural bearing capacity (number of coverages) of the different pavement layers in the proposed pavement model. An important aspect in the calculation of the number of coverages of the design aircraft is to take lateral wandering into account. The lateral wander of loads over a trafficked section has a significant influence on the calculated pavement response. Unlike the random variation in other design parameters (which can cause either a decrease or increase in calculated pavement response parameters), the traffic wander always causes a decrease in the calculated stress or strain response, because any degree of wander constitutes a movement away from the evaluation positions. Lateral wander typically increases with lane width.

10 Because of the significant effect that traffic wandering has on the calculated pavement response, it is important to determine beforehand whether the transfer function that will be used for interpreting the calculated pavement response already takes into account the effect of lateral wander. If the transfer function is based on laboratory results, then it is possible that lateral wander is not taken into account in the transfer function. However, if the transfer function is based on long-term field evaluation (Heavy Vehicle Simulator (HVS) testing), coupled with mechanistic design calculations, then traffic wander has already been taken (indirectly) into account. Thus, lateral wandering has already been indirectly taken into account using the transfer functions in the SAMDM. A typical example of the output model obtained from the mechanistic analysis is shown in Figure 2. The mechanistic design method allow for the calculation of pavement life to a state of cracking or deformation originating in any of the pavement layers (inter alia, deflection based relationships are based on pavement life to a rut depth of 10 mm). Transfer functions are used to determine the fatigue life of the different pavement layers (failure curves) in order to calculate the bearing strength capacity of the pavement. Failure criteria, given in Table 3, are used to determine the total number of coverages / repetitions of the design loading that any of the pavement layers in the pavement model can carry before reaching a defined level of distress. The transfer functions and the failure criteria used in the mechanistic analysis are based on the SAMDM, which are internationally recognised and well documented (Jordaan, 1994), (Theyse et al, 1996). The transfer functions and failure criteria will not be discussed in detail in this paper. Table 3. Failure criteria for different material types (Jordaan, 1994), (Theyse et al, 1996). Material type Asphalt (ic) (Concrete) (AC) Overlay Bitumen Treated Layer (BTB or DBM) Large Aggregate Mixes for Bases (LAMBS) Mode of distress Cracking or Deformation Horizontal strain (ε r ) X Critical failure criteria Vertical strain (ε v ) Horizontal stress (σ) Emulsion Treated Material (ETB) Cracking X Cement Treated Materials (C1 C4) Cracking X Equivalent granular materials Deformation X (Post crack) (EG3-EG6) Granular Materials (G1 G6) Deformation X Subgrade or soils (G7-G10) Deformation X ICAO s FAA design procedure and approach (ICAO, 1983) was used to get a preliminary indication of the additional strength that will be required during the rehabilitation of the runway. Information obtained from the empirically derive curves indicated that the existing pavement structure will require an asphalt overlay of approximately 150 mm in order to be able to carry the future design traffic loading for a 20 year design period. Based on the FAA design output requirements, several mechanistic design analyses were done, inter alia, for a new 100 mm and 150 mm asphalt overlay. Details of the input parameters and the respective analysis are given in Table 1 and Figure 2 (a typical example). X

11 The design aircraft s (B ) bearing capacity (number of equivalent coverages) for the two proposed rehabilitated pavement structures, (using the SAMDM approach), are as follows: 150 mm overlay = 21,378 total equivalent coverages, 100 mm overlay = 15,738 total equivalent coverages. 6.3 Comparison Between Bearing Capacity and Design Life Obtained from the Different Methods In order to compare (validation) the two rehabilitation design method s output, the following two criteria were assessed for comparison: design life (number of years) of the pavement, and number of equivalent coverages. The basis used for comparison is founded in the outcome of the ICAO empirically design method, where the existing pavement requires a 150 mm asphalt surfacing over a design period of 20 years. Thus, the two design methods can be compared by calculating the design life (number of years) that can be expected for the same rehabilitation actions (e.g. 150 mm AC overlay) using different design methods (ICAO and mechanistic). This can be achieved by dividing the total number of equivalent coverages calculated by the mechanistic design for a 150 mm AC overlay (21,378) with the total number of equivalent annual coverages. The following two different approaches were used in order to assess and calculate the annual required equivalent coverages: Using ICAO s United Kingdom (UK) ACN-PCN Classification to calculate the total equivalent annual coverages of the typical design aircraft (B ) (ICAO UK, 1989), Using the Mechanistic Design Method (SAMDM), as described in (Jordaan, 1994), (Theyse et al, 1996), for the recommended aircraft mix to calculate the annual equivalent coverages Calculation of total equivalent annual coverages for the typical design aircraft (B ) using ICAO S-UK ACN-PCN classification method The calculation of the total equivalent annual coverages for the typical design aircraft (B ) using ICAO s - UK ACN-PCN classification method (ICAO UK, 1989) is given in Table 4. The total annual equivalent coverages were both calculated for a 10 and a 20 year design period. Based on the analysis, the following annual equivalent coverages for the typical design aircraft were calculated: 10 year design 994 equivalent coverages/year, 20 year design 1,082 equivalent coverages/year. Using the above 10 year and 20 year annual equivalent coverages, an average equivalent coverage of 1,038 was calculated for the typical design aircraft (B ). Hence, the expected design life (number of years) for the proposed rehabilitation actions using the mechanistic design approach together with the average equivalent annual coverage derived from the ICAO-UK ACN-PCN classification method is as follows: 150 mm overlay = 20.6 years, 100 mm overlay = 15.2 years. This compares well with the 20 year design life obtained from the ICAO design.

12 6.3.2 Calculation of total effective coverages using mechanistic design analysis procedures for different aircraft types The same approach that has been followed to calculate the total equivalent coverages for the design aircraft has been used to calculate the total equivalent coverages for the different aircraft types in the aircraft design mix using mechanistic design analysis procedures. The total equivalent coverages for each aircraft type has been modelled in the mechanistic analysis programme using their unique load and tyre pressures as input into the programme. The calculated bearing capacity of the pavement structure for each aircraft type from the mechanistic design analysis was then used to calculate the equivalent repetition factor (ACN ratio) for each aircraft in terms of the design aircraft. The equivalent repetition factors calculated for the different aircraft types are summarised in Table 5. The total annual equivalent coverages were both calculated for a 10 and a 20 year design period. Based on the analysis, the following annual equivalent coverages for the typical design aircraft were calculated: 10 year design 1,009 equivalent coverages/year, 20 year design 904 equivalent coverages/year. The calculated equivalent repetition factors (equivalency factors), correlate well with the ACN ratio factors calculated in Table 4 using the ICAO s UK ACN-PCN classification method (ICAO UK, 1989). These repetition factors derived from the mechanistic analysis incorporates the effect of each aircraft and are now used to calculate the effective annual coverages of the design traffic loading. Using the above 10 year and 20 year annual equivalent coverages, an average equivalent coverage of 957 was calculated for the typical design aircraft (B ). The average total life (number of years) for typical rehabilitation actions, inter alia considered, is as follow: 150 mm overlay = 22.3 years, 100 mm overlay = 16.4 years. Taking all the above into consideration, both the identified methods (ICAO and the SAMDM) give similar estimated predicted life of the proposed rehabilitation actions. For the 100 mm overlay design it is estimated that the pavement will be able to carry the design traffic loading (equivalent coverages) for a pavement life of approximately between 15.2 and 16.4 years provided that adequate routine maintenance are provided. Likewise, for a 150 mm overlay the pavement life can be expected to be between 20.6 and 22.3 years. The mechanistic-empirical design method can be used to predict the design life (number of years) for a rehabilitation action proposed, which is not always possible with the ICAO (FAA) design method Comparison of the equivalent repetition factor (ACN ratio) derived from the SAMDM with the ICAO ACN factors The equivalent repetition factors (ACN ratios) derived from the SAMDM and ICAO s ACN ratio s are summarised in Tables 4 and 5 and diagrammatically shown in Figure 2. As can be seen from these results, the equivalent repetition factors (ACN ratio) calculated by using the SAMDM for the different aircraft in the design mix compare well with the ACN ratio calculated by using ICAO s UK ACN-PCN classification method (ICAO UK, 1989). These results are found to be comparable irrespective of the different assumptions built into each method. The advantage of calculating the equivalent repetition factors with the mechanistic-empirically design method is that an equivalent repetition factor for a specified aircraft design mix and a typical pavement structure can now be determined/simulated accurately.

13 Aircraft Table 4. Equivalent annual coverages calculated for the design aircraft (B ) for a traffic loading for flexible pavement using ICAO s UK ACN-PCN classification method (ICAO UK, 1989). Main Wheel Config *ACN Pass-to- Coverage Ratio No of Daily Annual Depart Coverage During Design Life (20 years) *ACN Ratio Flexible Mix Traffic Factor (FMTF) (20 years) Modified Flexible Mixed Traffic Factor (6)x(7) Equivalent Coverages - Design Aircraft (B ) (20 years) (1) (2) (3) (4) (5) (6) (7) (8) (9) IL-76T Dual- Tandem , ,581 B727- # 200 Dual , ,563 B Dual , B707- Dual- 320B Tandem , ,605 # Design Aircraft 2555 Total Coverages 21,646 *ACN - Aircraft Classification Number Total Annual Coverages (20 years) 1,082 = Departures; Config. = Configuration Total Annual Coverages (10 years) 994 Table 5. Effective annual coverages of the design traffic using equivalency factors derived from the SAMDM (100mm). Aircraft Main Wheel Configuration No of Daily No of Annual Pass-to- Coverage Ratio Effective Coverage / Year Equivalent Repetition Factor (*ACN ratio) Effective Annual Coverage / (Design Aircraft) Effective Annual Coverage (20 Year Design) IL-76T Dual-Tandem ,328 B # Dual ,563 B Dual ,049 B B Dual-Tandem ,237 # Design Aircraft; *ACN - Aircraft Classification Number; = Departures 1,009 20,176 Figure 2. Equivalency factors / ACN ratio s for the different aircraft as determined by different methods (SAMDM & ICAO).

14 7. CONCLUSIONS The following conclusions are made: It was demonstrated that excellent comparative results could be obtained between ICAO s empirically design method (FAA design method) and the mechanistic-empirical (SAMDM) design method. Both the two methods predict the same pavement design life and the total number of coverages for a similar proposed rehabilitation design action. The fact that rudimentary tests results, obtained from easily transported equipment that is non-destructive (Dynamic Cone Penetrometer (DCP), were used to characterize the different pavement layers and served as input into the Mechanistic-Empirical (M-E) (SAMDM) design method shows great promise in analysing airport pavement structures in remote areas. However, experience and engineering judgement are required to incorporate such results meaningfully. Equivalent repetition factors (ACN ratio s) calculated by using the SAMDM for the different aircraft in the design mix compare well with the ACN ratio calculated by using ICAO s UK ACN-PCN classification method (ICAO UK, 1989). The advantage of calculating the equivalent repetition factors with the mechanistic-empirically design method is that an equivalent repetition factor for a specified aircraft design mix and a typical pavement structure can now be determined/simulated accurately. The M-E design method can be used to accurately simulate different aircraft loadings and determine the bearing capacity (the total number of equivalent aircraft coverages) for a pavement structure. 8. ACKNOWLEDGEMENTS The paper is presented with the permission of the Angola Ministry of Public Works and the Ministry of Airports. The views expressed do not necessarily reflect those of the Ministries. 9. REFERENCES Jordaan, G.J. (1994). The South African Mechanistic Pavement Design Method. Research Report 91/242, Department of Transport, Pretoria, South Africa. Theyse, H. L., de Beer, M. D., and Rust, F. C. (1996). Overview of the South African Mechanistic Pavement Design Method. Transportation Research Record No 1539, TRB, National Research Council, pp.6-17, Washington, D. C. Freeme, CR, Maree, JH, and Viljoen, AW, (1982). Mechanistic design of asphalt pavements and verification of designs using the Heavy Vehicle Simulator. Proceedings of the 5th International Conference on the Structural Design of Asphalt Pavements, Vol1, Delft, Holland, pp , (CSIR reprint RR362). International Civil Aviation Organisation (ICAO) (1983). Aerodrome design manual Part 3. Second Edition. International Civil Aviation Organisation (ICAO UK) (1989), Directorate of Civil Engineering Services (Airfields Branch, United Kingdom). A Guide to airfield pavement design and evaluation. Croydon, United Kingdom. Jordaan, G.J. (1994a). Pavement design based on pavement layer component tests (CBR and DCP). Research Report 91/241, Department of Transport, Pretoria, South Africa. Kleyn, E.G., de Wet, L.F., and Savage, P.F. (1987). The development of an equation for the strength balance of road pavement structures. Transvaal Provincial Administration, Roads Branch, Report L7/87, Pretoria, South Africa.

15 Kleyn, E.G. (1984). Aspects of pavement evaluation and design as determined with the Dynamic Cone Penetrometer (DCP). M Eng Thesis (in Afrikaans), Faculty of Engineering, University of Pretoria, Pretoria, South Africa. Kleyn, E.G., and Savage, P.F. (1982). The application of the pavement Dynamic Cone Penetrometer (DCP) to determine the bearing properties and performance of road pavements. International Symposium on Bearing Capacity of Roads and Airfields, June 1982, Trondheim, Norway. Department of Transport (DOT) (1997). Short Courses on design of Flexible Pavements in South Africa. Volume 1 & to 28 November Council for Science and Industrial Research (CSIR), Roads and Transport Technology, Pretoria, South Africa, pp. 5-1 to Committee for State Road Authorities (CSRA document) (1987). TRH14: Guidelines for road construction materials. Pretoria, South Africa. International Civil Aviation Organisation (ICAO) (1999). Annex 14. International Standards and Recommended Practices - Aerodromes. Third Edition, July. International Civil Aviation Organisation (ICAO) (1984). Aerodrome Design Manual Part 1. Second Edition. Department of Transport (Colto) (1997). Draft TRH12: Flexible rehabilitation investigation. Pretoria, South Africa.