Why Airport Construction Is Its Own Discipline

Airport construction is a fundamentally different engineering discipline from road or building work, because the loads, tolerances and safety expectations are incomparably higher. The main landing gear of a wide-body aircraft transfers tens of tonnes onto a small contact patch, and those loads repeat thousands of times every day. For this reason a runway, taxiway or apron is not merely a surface; it is a complete pavement system made of subgrade, sub-base, base and surface layers working together.

The second distinguishing feature is geometric precision. A runway longitudinal grade rarely exceeds 1.5 percent, while the transverse grade is held within roughly 1 to 1.5 percent; surface irregularities are dangerous for an aircraft rolling at high speed. Drainage, friction, sight lines and obstacle-free surfaces must all be satisfied simultaneously. As a result, airport construction sits at the intersection of geotechnical, transport, hydraulic and aviation standards.

Third comes operational continuity. When an existing airport is widened or rehabilitated, the runway may be closed at night and reopened by day, demanding concrete placed in tight windows and mixes that gain strength quickly. This operational pressure makes planning and site management far more critical than on an ordinary infrastructure project.

Finally there is the matter of scale. A single code-F runway can exceed 3,500 metres in length and 45 to 60 metres in width, with parallel taxiways, holding aprons, service roads and large drainage basins added alongside. That means hundreds of thousands of cubic metres of fill, tens of thousands of cubic metres of concrete and uninterrupted material logistics. Because even the smallest design error becomes a large cost at this scale, engineering decisions must be made carefully from the very start.

Site Investigation and Subgrade Preparation

Beneath every sound pavement lies a well-characterised soil. Runway construction design begins with boreholes, pressuremeter and plate-load tests, CBR (California Bearing Ratio) values and sieve analyses. A designer cannot calculate a single layer thickness without understanding the bearing capacity and settlement behaviour of the subgrade. As a rule, the higher the CBR, the thinner the layers required above it; a weak subgrade demands either thick layers or improvement.

The goal of subgrade preparation is a homogeneous, well-compacted and drained platform. The upper subgrade is usually placed at a high degree of compaction against the Standard or Modified Proctor test (often 95 percent and above); lift thicknesses are kept controlled and each layer is compacted separately. In clayey, settlement-prone soils, swelling, frost heave and moisture-content variation are also assessed.

Frost depth, capillary rise and drainage conditions are also fixed at this stage. In freezing climates the subgrade is protected with suitable granular material extending below the frost line; otherwise the seasonal freeze-thaw cycle heaves and cracks the surface. Likewise, where the water table is high, a drainage layer and collector pipes are placed under the pavement to keep water away from the structural layers.

The most common mistakes here are missing soil variability because of too few boreholes, planning drainage too late, and neglecting compaction control. A one-centimetre defect in the subgrade can translate into cracking and settlement in the expensive surface above; subgrade engineering is therefore the silent but decisive foundation of the entire project.

Ground Improvement Techniques

Airports are often built on flat, broad and unfortunately geotechnically awkward land such as reclaimed coast, old lake bed, alluvium or soft clay. In these cases the subgrade cannot be used as found, and ground improvement comes into play. The aim is to raise bearing capacity, bring settlements within acceptable limits and reduce liquefaction risk.

Common methods include surface and deep compaction, stone columns (vibro replacement), jet grouting, driven or bored piles, preloading and vertical drains. A frequent approach in soft clay is to place a temporary surcharge fill and accelerate water expulsion with vertical drains so that consolidation is completed before construction. In granular soils, dynamic compaction or stone columns provide stiffness.

In zones with liquefaction potential, improvement serves not only settlement but also seismic safety. Saturated loose sands can lose strength and behave like a fluid during an earthquake; stone columns, vibro-flotation or deep compaction densify these sands and open drainage paths, reducing the risk. Improvement design must therefore be considered together with the site seismic-hazard assessment.

Method selection depends on soil type, the depth of improvement needed, the time allowed and cost. The right solution is often a combination of several techniques. What matters is verifying the improvement afterwards with instrumentation (settlement plates, piezometers, additional CBR and seismic tests), because once a runway is cast it is practically impossible to fix the ground beneath it.

Rigid vs Flexible Pavement: Which, Where and Why

Airport pavements come in two main types. Rigid pavement is built from plain or reinforced concrete slabs and, because it spreads load over a wide area, resists high single-wheel loads and heat very well. Flexible pavement consists of an asphalt surface over granular layers; it distributes load from layer to layer down to the subgrade. Roughly speaking, in rigid pavement the slab carries the load, while in flexible pavement the layered system does.

In practice the choice follows the location. For apron construction and taxiway intersections, where aircraft stand, brake, turn and where fuel and oil spills are concentrated, rigid concrete is usually preferred because it resists static loads and hydrocarbons better than asphalt. Along the runway, flexible (asphalt) pavement is often used for fast placement, lower initial cost and easy repair, although concrete may appear at runway ends and turning pads.

The decision is made on life-cycle cost, not durability alone. Rigid pavement has a higher initial cost and construction time but low maintenance and a long life; flexible pavement starts fast and cheap yet needs periodic resurfacing. Climate, material supply, traffic composition and the future aircraft fleet all enter the equation. A mature design frequently uses both together.

The PCN-ACN Logic: Matching Aircraft Load to Pavement

Which aircraft a pavement can carry is not arbitrary; it is expressed through a standardised system. The ACN (Aircraft Classification Number) describes the relative effect of an aircraft on a given pavement-soil combination, while the PCN (Pavement Classification Number) numerically defines the pavement bearing capacity. The simple rule is that an aircraft is granted unrestricted operation only when its ACN is equal to or less than the pavement PCN. International aviation has been migrating this approach to the ACR-PCR system; the logic stays the same, the calculation method is updated.

A PCN report is not a single figure; it is a coded statement that also includes pavement type (rigid or flexible), subgrade strength category (high to low), allowable tyre pressure and evaluation method. This coding lets an operator anywhere in the world read, in a common language, whether an aircraft can safely use a runway.

For the designer, PCN-ACN is the criterion that drives pavement thickness. Layer thicknesses are sized considering the heaviest representatives of the target fleet, landing-gear geometry, repetition count and tyre pressure. A frequent mistake is designing only for today traffic and ignoring future, heavier aircraft, which leaves the runway prematurely under-capacity. Good design leaves a reasonable margin for growth.

The Build Sequence for Runway and Apron

Once site preparation and ground improvement are complete, the pavement rises layer by layer. First the compacted sub-base and base (granular or stabilised with cement or bitumen) are placed, each layer passing level and compaction checks. In flexible pavement, bituminous binder and wearing courses follow; in rigid pavement, concrete slabs do. Concrete is laid with slipform pavers, the joint layout (expansion, contraction and construction joints) is planned, and the surface is textured for friction.

Geometry and tolerances are decisive at this stage. Runway smoothness, transverse and longitudinal grades, surface texture and friction coefficient are measured continuously, because they directly concern flight safety. The drainage system is built together with edge channels, gullies and collectors to remove water from the surface quickly. In apron construction, fuel and service lines, ground power units, marking and lighting infrastructure are additionally laid in coordination with the slabs.

The final layers are invisible but vital: runway and apron markings, edge and centreline lights, PAPI approach lights, cable ducts and electrical infrastructure. All of these systems must run in parallel with the concrete and asphalt works; otherwise a freshly placed surface has to be broken open later, wasting both money and time. A good programme therefore interleaves pavement and electromechanical works.

Quality Control, Acceptance and Life Cycle

In airport pavements, quality is proven by measurement, not by eye. Core samples, compressive and flexural strength tests for concrete and asphalt, layer-thickness and degree-of-compaction measurements are standard practice. On the surface, smoothness (for example continuous profile measurement), transverse grade, friction and texture depth are tested. Runway friction is a critical parameter that must be re-measured at intervals because it governs skidding risk in wet conditions.

At acceptance, every placed layer is judged against design tolerances; non-conforming sections are milled and replaced. Joint sealing, surface seals and marking visibility are inspected separately. Certified quality-management systems such as ISO 9001 ensure these records are traceable and auditable, which is indispensable for public clients and aviation authorities.

The life of a pavement begins the day casting ends. Crack monitoring, joint maintenance, resurfacing, rubber-deposit removal and drainage upkeep are carried out within an asset-management plan. A well-designed and maintained runway can serve for decades, whereas a neglected surface needs rehabilitation far sooner than expected. What lowers life-cycle cost is the correct decisions made at the very beginning.

Field Experience: International Aviation Infrastructure

Airport projects demand contractors who can turn a paper design into reality on site with heavy-tonnage equipment, tight time windows and multi-disciplinary teams. Coordinating the geotechnical, pavement, drainage and electromechanical components of a runway and apron under a single programme requires experience and institutional memory. International aviation infrastructure is therefore the domain of firms with proven reference projects.

One concrete example of this experience is the work of KMB Metro Altyapı on the Indore Airport project in India. Born from the partnership of Troy from Türkiye (since 1996) and Kyivmetrobud from Ukraine (since 1949), with more than 75 years of accumulated know-how, the firm carries experience in heavy infrastructure across nine countries, including metro, railway, tunnel, bridge and airport works. Its ISO 9001 certification supports the traceable records and audit discipline that pavement works require.

The point here is not promotion but a reminder of the field reality: runway and apron engineering begins with theory yet matures in practice through equipment choice, logistics, quality control and time management. For investors and public bodies, choosing the right contractor is the second half of a project, as important as the design itself.