Technical details of runway layer work, compared to roads

Choice of materials for runways

The choice of materials used for constructing runways is made by the engineers after assessing the local ground condition. Often, for a busy airport where ground conditions are normal, concrete is the first choice of pavement as it can be easily maintained as compared to other construction materials.

Occasionally, runways have expansion joints which allow a relative moment of the concrete slabs placed inside the concrete pavement to accommodate peak demand periods.  Runways with unstable ground conditions are usually made of asphaltic concrete surface as it is easier to scrap periodically [2]. For airports with light traffic and light aircraft, the use of a sod surface is recommended. Some runways have been reported to use salt flat runways as well to cut down the cost.

Pavements Design

Designing pavements is a complex process as it takes into account the subgrade condition. Engineers come up with the specifications after analysing the bearing capacity of the subgrade. A pavement thickness between 10 inches and 4 ft (including subgrade) is recommended for commercial aircraft, regardless of what the top surface is.

There are two approved methods for designing Airport pavements:

Westergaard method

It is assumed that the pavement is an elastic plate with a uniform reaction coefficient (K value). However, research has shown that the formula that K values does not apply to aircraft using the latest technology.

California bearing ratio

California bearing ratio was developed in the late 1940s from a calculation of the value of a function outside the range of known values. This method applies to modern aircraft.

Some designs have been developed by combining the above two methods to achieve better results. Based on the vehicle response as an important design parameter, an analytical system has been designed recently. This new method has proved to be a great success as it considers air traffic conditions, materials used in the construction and the response of vehicles.

Manufacturers usually tend to minimise the stress on the pavement as it is very expensive to construct. The latest aircraft come with numerous tyres to support the weight of the place[3]. Effects on the pavement can also be minimised by paying close attention to the characteristics of the landing gear.

Pavement surface

The pavement surface is designed carefully after considering the fact it has to provide maximum friction for wheel braking. Furthermore, to minimise the effects of aqua planning that prevents the vehicle from responding to control inputs, the surface is grooved so the rainwater can flow into them and the peaks of the grooves remain in contact with the aircraft tyres.

To maintain required FAA friction levels, the maintenance staff is responsible for removing airfield rubber or hydro-cleaning. In areas where the soil is at or below the freezing point, the construction of runways is extremely difficult as compared to temperate regions. Since differences in natural surface can give rise to the variable therregimesgime and slope stability, engineers have to give a solution to avoid serious and progressive consequences.

The state of subgrade soil plays a key role in determining the final design. The design of pavement sections also depends heavily on the type of subgrade soil Design problems rarely occur in frost conditions as they do no tend to change their stability or load/pressure handling abilities with seasons.

Typical equipment required

Model Maker and Road Maker software are two of the most widely used software for designing runways. Machinery, including the Topcon ATG 6 automatic levels and Topcon GTS series are typically used to assess the bearing capacity of the runways. GPS Machine Control System helps in completing the layer works. FC 200 Controller along with Topsurv, pocket 3D and 3DMC software also among the tools required to successfully design a runway.

Design Process

Consultants are usually responsible for delivering the final design. Before the design is released, freeman extracts information from the drawings and uploads it onto the Model Maker and Road Maker software.

To generate the design model, details from the original ground level must be passed on to the engineers and consultants. Freeman is typically responsible for checking the original ground level (OGL) for the consultants to identify the potential problems beforehand.

Contractors are not supposed to bring in their controls as they are required to have over the control to the consultants.

If the job involves renewing an old runway, the analysis should be done to determine if the old runway has deteriorated to a point where it has to be rebuilt from the scratch[4]. In some cases where underlying foundation layers have, the old runway is stripped off and digs in down to the stable layer.

Dynamic compaction should be applied to the main runway and taxiways in dolomite areas to avoid sinkholes forming. A geotechnical company should be hired to do the dynamic compaction work.

The process of dynamic compaction is very interesting as it involves dropping a significant weight with the use of a crane. The impact of the weight creates tremendous stress on the surface to compress the weaker patches which have the potential to collapse.

Chapter 2; Roads

Introduction

The construction of roads can be defined as preparing a hard surface for the travellers by overcoming any obstacles in the right of way, in consideration of the state of law. The process of road construction starts with the design of road patterns best suited for residents concerning their travel needs. The first step taken during the construction phase is the removal of earth rocks by digging, before laying on the pavement and road building equipment materials.

Once the design and environmental and legal issues are approved/considered, the road alignments are finalized by the civil works team management; the design gradients and radii of the road are set as per best-fit methods concerning the site ground levels. This activity ensures minimize cut and fill for civil works and enhances diver’s safety.

Roads are primarily constructed for the use of motor vehicles and pedestrians. Therefore, rain storm drain systems along with other environmental impacts are considered a major parameter in the road designs. Sediment, erosion and other control systems are built to neutralize detrimental effects [5]. Drain lines are constructed along the road to assure stormwater is collected safely at the nearest sewerage system. The Darin lines are built in road easements with joined seals to ensure a smooth flow of water. This system needs approval by the local development authorities, to guarantee that the design flow is good enough for the actual drainage system. 

Road Construction Process

The following processes are taken during the construction of roads;

• Excavation
• Filling
• Compacting
• Construction

Roads are constructed of various layers of asphalt and aggregates. Excavation is carried out to remove low compact and unrequited material from the land/site, and the moisture content is balanced as per the designed value. This is then compacted with standardized fill with the relative compact percentage of 85-95. This process is completed by the compacted layer method, in which the fill layer is spread along the designated area and it is then compacted through heavyweight machinery[6]. The fill–compaction process is undertaken until the desired specifications of the road are achieved. Following is an example of road material specification details, provided by the road construction agency in the UK. The thickness of layers may vary depending on the specifications of the heavy traffic and native soil

[1] International Civil Aviation Organisation, available online at http://www.icao.int/Pages/default.aspx, retrieved on 12/12/12

[2] Hassan, Y, El Halim, A, Razaqpur, A, Bekheet & W, Farha, M, ‘Effects of Runway Deicers on Pavement Materials and Mixes: Comparison with Road Salt’, 2002, Journal of Transportation Engineering. 391

[3] Trimbath, K,  ‘Link established between pavement deicers and runway cracking’, 2006, Civil Engineering (08857024), 76(5), 29-31.

[4] Corsi, S, Geis, S, Loyo-Rosales, J, Rice, C, Sheesley, R, Family, G. & Cancilla, D, ‘Characterization of Aircraft Deicer and Anti-Icer Components and  Toxicity in Airport Snowbanks and Snowmelt Runoff’, 2006, Environmental Science & Technology, 40, 3195-3202

[5] AASHTO, ‘Chapter 3 Elements of Design’, A Policy on Geometric Design of Highways  and Streets, 6 th edition, 2011, American Association of State Highway & Transportation  Officials (AASHTO), Washington, D.C.

[6] MassDOT Project Development & Design Guide, 2006, available online at http://www.mhd.state.ma.us/default.asp?pgid=content/designGuide&sid=about, retrieved on 12/12/12

Figure 1: Details of Material specifications with respect to layers 

The fill material used for the road construction is without any organic material and has the desired/designed stress bearing ratio and minimum plasticity index. The aggregate base mixtures is generally comprised of a mixture of sand and very fine gravel, which helps to maintain the bottom surface free from any organic matter growth such as plants and serves as lower stratum drainage.

The next layer fill is composed of broken rocks, gravel and /or sand clay of certain sizes as shown in figure 1 above. The roadbed is rolled for each layer of fill, which compacts the fill and norms the particulate distribution throughout the road. 

Various pavement layers are used to improve the quality of the road. These pavement layers consist of geosynthetics which are of extreme importance in improving the quality of the roads. These geosynthetics help to increase the road/layer strengths, and enhance reinforcement attributes, drainage and filtration. Therefore, pavement performance is optimized concerning overall construction, decrease in costs and maintenance

The road's final finishing is done by paving. The road surface is finalized after having considered factors such as expected usage and other economic factors.

Road pavement design criteria  

This section provides technical details of standard pavement design criteria for road construction.  The design criteria can be set for twenty years of pavement life provided that the maintenance work is carried out to maintain the road surface after completion of the road. Equivalent Daily Load application and Design Traffic numbers are considered in the design calculation for the roads.  The following classifications can be considered as minimum limits for pavement sections during the design of the roads. Final designs should be based on real-site test results for subgrade support[1][2]

Table 1; Pavement design classifications

Excavation requirements

Moisture conditioned fill is put in to replace the expansive soil sub-grades. Expansive Soil sub excavation requirements are as follows;

Table 2; Excavation requirements