Technical details of runway layer works, compared to roads

Chapter 1; Runways

Introduction

According to International Civil Aviation Organisation, a rectangular area used for the landing and takeoff of aircraft is known as the runway.

Airport runway is an amazing piece of construction. A typical runway is 2 miles long, as wide as 16 lane highways and 3 feet thick. Runways are an integral part of the airports all across the world. Considering a fully loaded 350,000 kilograms 777 hitting hard, it is to understand why the runways have to be strong and reliable to take that strain without cracking and collapsing.

The bigger the plane is, the bigger the runway size should be to handle the extra stress. Civil engineers have to be extra careful while designing this massive infrastructure. Several important parameters like number of tyres on the place, distance between the tyres and their size have to be considered.

It takes months and sometimes years to complete a runway that meets international standards. Typically, runways are a man made surface consisting of asphalt, concrete, and natural surface such as salt, gravel, ice, dirt and grass. According to the data revealed by the Denver International Airport in the United States, nearly 2.5 million cubic yards of concrete was used to construct five 12,000 foot runways and taxi-ways.

Prior to adding a 17 inches concrete paving, the engineers put down compacted soil, a foot deep layer of soil with 8 inch thick cement base on top. Main runways are designed in the direction of wind so they could assist the aircraft in landing and take-off. It is the duty of the air traffic controller to determine which runway will be used for landing and which for take take-off after taking into account the wind speed, weather conditions and air traffic conditions.

Engineers sometimes design runways crossing each other to ensure the air traffic controllers are paying more attention.  Taxi runways are used by the pilots to take the plane from main gate to the runway for take-off and from runway to main gate as it lands.

Choice of materials for runways

The choice of materials used for constructing runways in 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 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 on a periodic basis. For airports with light traffic and light aircrafts, 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 aircrafts, regardless of what the top surface is.

There are two approved methods for designing Airport pavements:

1. 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 is not applicable with aircrafts using the latest technology.

2. California bearing ratio

California bearing ratio was developed in the late 1940’s from a calculation of the value of a function outside the range of known values. This method is applicable on modern aircrafts.

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 response of vehicles.

Manufacturers usually tend to minimise the stress on the pavement as it is very expensive to construct. Latest aircrafts come with numerous tyres in an effort to support the weight of the place. Effects on the pavement can also be minimised by paying close attention to the characteristics of the landing gear.

Pavement surface

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 rain water can flow into them and the peaks of the grooves remain in contact with the aircraft tyres.

To maintain required FAA friction levels, 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 the temperate regions. Since differences in natural surface can give rise to variable thermal regime 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 a pavement sections also depends heavily on the type of subgrade soils. 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 softwares 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 softwares also among the tools required to successfully design a runway.

Design Process

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

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 own controls as they are required to have over the control to the consultants.

If the job involves renewing an old runway, analysis should be done to determine if the old runway has deteriorated to a point where it has to be rebuild from the scratch. 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 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.