Cable Trench vs Direct Burial
Two underground cable installation methods used across AGL series-circuit infrastructure. Cable trench routes the cable through a deliberately engineered, more accessible corridor — often with structured bedding, protection and access provisions. Direct burial places the cable straight into a prepared route in the ground, backfilled without a dedicated accessible structure. Both can be used on an airfield; the right choice depends on maintainability, pavement conditions, cable protection, drainage and the applicable standard — not on installation cost alone.
At a Glance
| Parameter | Cable Trench | Direct Burial |
|---|---|---|
| Installation method | Cable installed in a prepared trench/duct arrangement | Cable placed directly in the ground |
| Cable protection | Generally greater and more controllable | Depends strongly on soil and protective layers |
| Accessibility | Better | Limited after backfilling |
| Installation complexity | Higher | Generally simpler |
| Future cable replacement | Easier in accessible systems | More difficult |
| Mechanical protection | Can include ducts, covers, slabs or other protection | Relies more on burial depth and protective layers |
| Inspection | More accessible | Requires excavation in many cases |
| Initial cost | Generally higher | Often lower |
| Route modification | Easier in planned accessible systems | More difficult |
| Typical use | Complex or maintainable infrastructure routes | Suitable permanent underground routes |
What Is Cable Trench & Direct Burial?
Cable Trench
A cable trench is a deliberately excavated, engineered route in which the cable is installed as part of a planned system — often with defined bedding, protection, separation and access arrangements. The route can be backfilled, ducted or covered, but its defining feature is that it was engineered as a corridor rather than a simple excavation.
Typically includes
- Sand or approved bedding
- Ducts or conduits
- Concrete protection or covers
- Warning tape and markers
- Draw pits / junction structures and spare ducts, where provided
Direct Burial
Direct burial places the cable straight underground in a prepared route, without a dedicated accessible trench structure once installation is complete. The cable itself must be suitable for direct burial, and it is protected by burial depth, bedding and surrounding backfill rather than by a maintained corridor.
Typically includes
- Selected bedding / soil
- Selected backfill
- Warning layer / tape
- Adequate burial depth
Terminology note: Direct-buried cable is itself laid in an excavated trench — in FAA practice (AC 150/5370-10, Item L-108) that trench also carries the counterpoise, marking tape and specified fill, and is then backfilled. Cable in conduit or duct banks is specified separately (Item L-110), with electrical manholes and junction structures under Item L-115. On this page, "cable trench" means a route engineered to stay accessible or replaceable after installation (ducts, duct banks, covers, pits); "direct burial" means cable backfilled in the ground with no such provision. Terminology varies between specifications, so confirm how your contract defines each term.
Engineered Corridor vs Backfilled Route
The simplest way to remember the difference between cable trench and direct burial:
Cable Trench
Structured + More Accessible Cable Route
Direct Burial
Simpler Underground Installation + Less Accessibility
Engineering point: A trench can still be backfilled and covered, and a direct-buried cable can still be well protected. The real distinction is whether the route was engineered to remain maintainable, not whether the cable is visible.
Basic Difference in Cross-Section
Cable Trench
An engineered, deliberately excavated corridor prepared as part of the installation system.
Direct Burial
The cable is installed in an excavated trench and covered with suitable bedding and backfill.
Easy memory: Trench = engineered cable corridor. Direct burial = cable placed underground and backfilled.
Schematic only, not to scale. Layer order, depths and clearances follow the applicable specification and project drawings.
What Protects the Cable in Each Method
Cable trench may incorporate
- Sand or approved bedding
- Ducts or conduits
- Concrete protection
- Protective covers
- Warning tape
- Cable markers
Direct burial must protect against
- Stones
- Mechanical damage
- Accidental excavation
- Ground movement
- Moisture conditions
- Rodent / termite attack (site-dependent)
In both methods the cable itself must be rated for the intended installation — AGL series cable (for example FAA L-824 type) is designed for underground use, but confirm the cable datasheet and project specification for direct-burial versus duct use — and the surrounding protective arrangement must suit the actual soil, loading and route conditions.
Lightning protection applies to both: Where the specification requires a counterpoise, it is a bare copper conductor laid along the route above the cable or duct. FAA Item L-108 calls for bare No. 6 AWG copper, at least 8 in below grade and at least 4 in above the cable or duct bank in a shared trench. In the FAA isolation method the counterpoise is not bonded to the light base; a separate grounding electrode is provided at each base. Whichever method is chosen, the counterpoise and base grounding must be part of the design. Verify the current edition for your project.
Reaching the Cable After Installation
Cable trench
Fault identified and located
Open cover, pit or duct access
Inspect / repair / re-pull cable
Direct burial
Fault identified and located
Confirm exact route and depth
Open excavation at fault
Splice / replace and reinstate
Main advantage: Where the trench or associated duct system provides genuine access, cable trench systems generally offer better maintainability than direct burial.
This matters more on a series circuit: all fixtures share one loop, so a single cable fault can affect the whole circuit. Fault location (insulation-resistance testing and cable-fault locating) is done before any excavation in both methods, and every repair splice must use an approved method and be re-tested before the circuit is re-energised.
Step-by-Step Installation
Cable trench sequence
Survey route
Excavate trench
Prepare bedding
Install cable / duct system
Protect (covers, tape, counterpoise)
Test before closing
Backfill / restore, record as-built
Direct burial sequence
Survey route
Excavate route
Prepare bedding
Place cable
Warning tape / counterpoise
Test before backfill
Backfill, mark route, record as-built
Direct burial can reduce infrastructure complexity, but it requires careful installation because access will be limited once the route is closed. In both methods, insulation-resistance testing before closing the route, and again before energising, is standard commissioning practice, and an accurate as-built record of route, depth and splice positions is essential for future fault location.
Behaviour Under External Load
Cable trench
The engineered installation — ducts, covers or slabs — can provide additional, controllable protection against external loads above the route.
Direct burial
Protection depends heavily on burial depth, soil condition, cable construction, protective layers and route marking rather than an engineered structure.
Where This Fits in the AGL Cable System
For airfield ground lighting, either method can carry the underground route between the constant current regulator and the fixture:
Constant current regulator feeds the series loop
Underground route — cable trench or direct burial
Isolation transformer at each fixture position
AGL light fixture
The installation method chosen must account for series-circuit cable requirements, isolation transformers, light bases, pavement crossings, drainage and future maintenance access.
Crossing Beneath Critical Pavement
Cable trench
Crossings beneath runways, taxiways and aprons are normally built as ducts, conduits or concrete-encased duct banks (FAA Item L-110), often with spare ducts and junction structures, so cable can be replaced without cutting the pavement.
Direct burial
Beneath critical pavement, direct burial is generally avoided because a cable failure would force excavation through the pavement structure. It is more typical in grassed and unpaved areas; where it is considered under pavement, depth, structural loading and future excavation difficulty must be assessed against the specification.
Initial Cost vs Long-Term Cost
Cable trench cost drivers
- Excavation
- Civil work
- Protection
- Access system
Direct burial cost drivers
- Excavation
- Cable
- Backfill
- Marking
Engineering note: A lower initial cost can result in higher future repair costs if excavation becomes necessary. Initial cost should not be the only selection factor.
Full Engineering Parameters
| Parameter | Cable Trench | Direct Burial |
|---|---|---|
| Installation infrastructure | Higher | Lower |
| Cable accessibility | Better | Limited |
| Initial civil work | Higher | Generally lower |
| Future maintenance | Easier | More difficult |
| Mechanical protection | Can be high | Installation-dependent |
| Fault repair | Potentially faster | Often requires excavation |
| Route modification | Easier | More difficult |
| Initial cost | Generally higher | Generally lower |
| Surface restoration after repair | Less disruption possible | Often significant |
| Best advantage | Maintainability | Simplicity |
Cable Trench vs Direct Burial: When to Choose Each
Selection question: Is future access and maintenance important? If yes, a trench or other accessible system may be preferred. If no, direct burial may be suitable.
Also weigh cable type, soil conditions, pavement loads, drainage, future expansion, maintenance access, and the applicable standard together — not in isolation.
Consider cable trench when
- Future access and maintenance are important
- The route crosses runways, taxiways or aprons, or is complex or heavily used
- Mechanical protection needs are high
- Route modification is likely later
Consider direct burial when
- A simpler, permanent underground route is suitable
- Soil and depth conditions support adequate protection
- Lower initial installation cost is a priority
- The applicable standard permits it for the route
Myths vs Facts
Not necessarily. A trench can be backfilled, covered or ducted. The important distinction is the engineered route and installation arrangement, not visibility.
Incorrect. A direct-buried installation may still include selected bedding, protective layers, warning tape, markers and suitable burial depth.
Not always. A completely backfilled trench without ducts or accessible structures may still require excavation. The real maintenance advantage depends on whether the design provides genuine future access.
Lower initial cost can be offset by higher repair costs later if excavation is needed. Long-term cost depends on fault frequency and route location.
Which Method Applies Where?
Cable trench is a planned, engineered underground route that can provide greater cable protection, organisation and future maintainability. Direct burial is a cable installed directly into the ground with suitable bedding, depth and protection, offering a simpler installation but more difficult physical access after backfilling.
For AGL systems, the choice should be based on maintainability, pavement conditions, cable protection, drainage, future expansion and the applicable airfield standard — not simply on initial installation cost.
Engineering & Expert Notes
Choose Based on Maintenance Expectations
SelectionTrenches ease access and future change; direct burial saves cost but makes repairs harder. Decide with the lifecycle in view.
Plan Drainage in Either Method
WaterWater in a trench or in the soil attacks joints. Design drainage and check it during inspection.
Survey and Record Routes Accurately
RecordsAccurate as-built routes shorten fault finding. Record depths and markers.
Protect Against Future Civil Works
ProtectionUse markers and records so later excavations avoid the cable.
Frequently Asked Questions
No. A trench can be backfilled, covered or ducted. The defining feature is the engineered, planned route — not whether it remains open.
No. Direct-buried cable is still protected through selected bedding, backfill, burial depth, warning tape and route marking.
Direct burial generally has a lower initial cost. However, a lower initial cost can lead to higher repair costs later if excavation becomes necessary — so cost alone should not drive the decision.
Yes. Both methods can be used depending on the airfield design, with the choice made route by route based on pavement crossings, drainage, maintenance access and the applicable standard.
Only if the trench or duct system provides genuine access. A fully backfilled trench without ducts or accessible structures may still need excavation for repairs.
Maintainability, pavement conditions, cable protection, drainage, future expansion and the applicable airfield standard — not simply the initial installation cost.
Under pavement, cable is normally installed in ducts, conduits or duct banks so it can be replaced without cutting the pavement. Direct burial is more typical in unpaved areas. Always follow the project specification and the applicable standard.
Governing Documents
For an actual airport project, verify the current applicable editions of the relevant standards before using this comparison for project design.
ICAO Annex 14, Volume I — Chapter 8
Aerodrome Design and Operations: electrical systems for aerodrome visual aids (power supply, system design and monitoring). Annex 14 sets these system requirements; it does not prescribe cable installation methods.
ICAO Aerodrome Design Manual, Doc 9157 Part 5
Electrical Systems: guidance on electrical practices and installations for aerodromes, including circuit wiring. Guidance material, not a Standard.
FAA AC 150/5340-30
Design and Installation Details for Airport Visual Aids — cable, counterpoise, light base and installation details for airfield lighting.
FAA AC 150/5370-10 — Items L-108, L-110, L-115
Standard Specifications for Construction of Airports. L-108: underground power cable (direct-buried trench, counterpoise, marking, testing). L-110: underground electrical duct banks and conduits. L-115: electrical manholes and junction structures.
FAA AC 150/5345-7
Specification for L-824 Underground Electrical Cable for Airport Lighting Circuits — the cable type used in AGL series circuits.
IEC 61820 series & IEC 61821
Electrical installations for AGL at aerodromes (including requirements for series circuits), and maintenance of AGL constant current series circuits.
DGCA / AAI Requirements (India)
For Indian airport projects, verify the applicable DGCA aerodrome requirements and AAI technical requirements and specifications.
Project Specification & Manufacturer Data
Contract specifications, approved drawings, and cable and duct manufacturer installation instructions govern burial depth, bedding, bend radius and clearances on the actual project.
Cable Trench → Structured + Engineered + Accessible route. Direct Burial → Simple + Permanent + Depth-and-bedding-protected route.
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