Reducing Construction Errors by Visualizing the Underground Structure of Utility Common Ducts with AR

What a utility common duct is: an underground shared conduit supporting pole elimination

A utility common duct is a shared conduit for housing multiple lines such as power lines and communication cables underground. By embedding concrete boxes or pipes beneath roads and running multiple operators’ cables through them, the number of utility poles and overhead wires is reduced, contributing to improved streetscape and disaster resilience. This is a representative method for promoting the elimination of utility poles (so-called “undergrounding”), and development is progressing nationwide, especially in urban areas. Utility common ducts reduce obstacles on sidewalks and lower the risk of poles falling during disasters, making them indispensable infrastructure for safe and attractive urban development.

However, developing utility common ducts requires high expertise and careful planning. Because new structures are installed underground, detailed preliminary surveys and designs are essential. The positions relative to existing buried infrastructure such as water, sewer, and gas pipes must be coordinated, and the limited underground space must be used effectively. Construction periods and costs can also be large, and when roads are excavated for construction, impacts on traffic must be considered. In some cases, developing hundreds of meters of duct can take several years, and for such long-term, large-scale projects, rework caused by mistakes must be avoided at all costs. While utility common duct development brings significant benefits, its realization involves various challenges.

Key points to keep in mind for design and construction of utility common ducts

At the planning and design stage of a utility common duct, detailed wiring plans must be developed in coordination with multiple operators. Rules for wiring—such as cross-sectional dimensions of the duct, internal compartment allocation, and which cables are placed in which compartments—must be established, and the design should allow margin for future expansion. The installation location is also determined by considering road width and relationships with other underground utilities. For example, when installed under a sidewalk, it is necessary to keep a certain distance from private property boundaries and ensure separation distances from other conduits (to prevent interference). Coordinates and elevations should be determined precisely in the design stage, and clearance in the vertical direction and protection measures should be planned at planned crossing points.

There are many points to watch during construction as well. Because utility common ducts include not only straight sections but also curves and branched/merged sections (special parts such as manholes and outlets), accurate onsite positioning and advanced construction techniques are required. When close to existing lifelines, vibration and excavation effects must be considered to carry out work safely. In particular, when using pipe jacking (jacking method) to pass conduits under a road, even a slight deviation in launch or arrival positions or direction can lead to significant rework, so precise surveying and control are indispensable. It is also important to share design intent among workers and strictly adhere to wiring rules when placing cables. Neglecting these design and construction points can lead to defects or problems after completion.

Construction errors caused by complex underground structures

Because underground structures in utility common duct projects can be extremely complex, there is a higher tendency for construction errors to occur. Typical examples of such mistakes include the following:

• Mistakes at crossing points: At locations where the utility common duct crosses other buried pipes, planned clearances (vertical separation) may not be maintained, causing conduits to interfere with one another. For example, where a certain elevation difference was planned for crossing, measurement errors or insufficient excavation accuracy on site can result in shallower-than-planned placement, leading to contact with other pipes. Crossing points are invisible underground and crews tend to rely on feel, but even slight deviations can become major problems later.

• Interference with other facilities: Underground spaces contain many utilities besides the common duct, such as water and sewer pipes, gas pipes, and communication conduits. Even if plans on paper appeared to clear these, unexpected obstacles or existing structures can appear on site. As a result, routes may need to be altered or ducts detoured, producing wiring routes that differ from the plan. If sufficient adjustments are not made to avoid existing structures, issues can arise after completion—such as inspection openings not aligning, or bends in conduits being too severe for cables to pass.

• Deviations from wiring rules: Inside the common duct, compartments are divided among power companies and communication companies to house their respective cables. If cables are not placed in the designated positions according to wiring rules, future maintenance or expansion work will be impeded. Mistakes can also occur if site decisions change wiring positions, or if a cable is mistakenly routed through another operator’s compartment. In cases where site confusion or communication errors lead to "Company A’s cable, which should be in the right-hand compartment, being placed on the left," correcting the issue later can require significant effort.

As shown, utility common duct construction carries various risks of construction errors due to the complexity of underground structures. Even if plans look neat on paper, small onsite deviations or human errors can accumulate and turn into major defects after completion.

Rework and accidents caused by the invisibility of underground structures

A major barrier in utility common duct construction is that the underground is invisible. During construction and after backfilling, the underground situation cannot be directly observed, making mistakes hard to detect; by the time problems surface, it may be too late. For example, discovering a wiring position error after burial and needing to excavate and redo work causes schedule delays and additional costs. Such avoidable mistakes that reduce overall project efficiency are a significant loss.

The invisibility of underground structures also poses safety risks. There are reported cases in which excavation accidentally cut an existing communication cable causing an outage, or an unexpected gas pipe was encountered and work was temporarily halted. In many cases, construction proceeds relying on drawings indicating buried object locations or markings from surveys, but when things can’t be seen directly, a degree of ‘‘gut feeling’’ remains. As a result, misjudging positions can lead to human errors that damage buried assets or cause construction mistakes.

Moreover, invisibility is a problem for maintenance and other works after completion. When a road is re-excavated, lacking accurate records of buried positions can lead to surprising appearances of cables or pipes at unexpected locations due to discrepancies between past drawings and reality. If this causes accidental damage to infrastructure, the impact on users—such as power outages or communication failures—can be significant and escalate into a societal issue.

Accurately preserving location information of underground structures is not easy. After burial, photos and surveys must be taken and compiled into drawings, and as a field workaround, crews sometimes paint the routes of buried pipes and cables on temporarily restored road surfaces. This illustrates how difficult and laborious the task of ‘‘visualizing’’ and communicating hidden underground structures is.

Thus, the fundamental problem that the underground is invisible is the root cause of rework and accidents in utility common duct construction.

Supporting construction by visualizing underground structures with AR

In recent years, initiatives using AR (augmented reality) technology to solve the problem of underground invisibility have attracted attention. AR overlays digital data onto the real world through a smartphone or tablet camera. In utility common duct construction, AR can project underground structure data and wiring plans created in the design phase onto the site, making it possible to "visualize" unseen underground structures on the spot.

Specifically, a dedicated AR app is loaded onto a smartphone or tablet with the 3D design model of the common duct and burial route information. When the device is pointed at the site, the model of the common duct and cables to be installed underground is displayed superimposed on the live view of the actual road. It appears as if seeing through the ground, with underground structures floating in the real environment. This enables intuitive onsite understanding of underground structures without relying solely on 2D drawings or mental images: you can grasp the underground layout while standing at the location.

AR visualization dramatically improves construction management and worker understanding. What used to be an exercise in imagining "the common duct should be around here" while holding drawings can now be checked as a life-size 3D model using AR. It is also easy to verify onsite whether the actual position and dimensions match those assumed in the design. For example, if a plan has the duct passing over a sewer at a certain point, AR lets you visually confirm the positional relationship and whether sufficient clearance is maintained per the design.

In practice, pilot cases of AR have begun appearing in utility common duct projects commissioned by the Ministry of Land, Infrastructure, Transport and Tourism. In one national highway project, combining a smartphone with high-precision GNSS to overlay the 3D design model in the field enabled even young engineers to immediately grasp design intent, earning high marks from onsite construction management. This technology has made it easier to share underground structure images that were hard to understand from drawings alone, and it is being applied to other sites. AR visualization is bringing a new dynamic to utility common duct construction. Such advanced technology adoption aligns with the Ministry’s i-Construction and construction DX (digital transformation) initiatives, and use cases are expected to increase.

Specific examples of AR-based construction support

Using AR technology on site enables the following specific supports for utility common duct construction:

• Onsite verification of design data: You can directly compare plans on drawings with site conditions. By viewing the AR-projected duct position, you can check whether it fits with the actual road width and surrounding structures, and whether it will be accommodated as designed. If the design model appears to overlap with site structures in AR, the need for design changes can be identified on the spot. Identifying discrepancies between site and drawings in advance can prevent major rework.

• Visualization and guidance of burial routes: Visualizing the route of the underground duct from aboveground allows workers to intuitively locate where to excavate and lay infrastructure. For example, AR can display which line on the road to follow for digging, enabling workers to trace digital guides accurately instead of relying on stake driving or spray markings. If depth information is included in the AR display, it can also serve as a reference when adjusting excavation depth to achieve the specified burial depth. As a result, mistakes such as misplacement or tilting of the duct can be reduced, improving construction accuracy.

• Pre-checks at crossing points: At locations that cross or are near other buried utilities, AR allows prechecking of positional relationships so safe construction procedures can be planned. For example, if an AR marker indicates "gas pipe crossing point at ○m ahead," workers can switch to careful probing or cautious excavation before reaching that spot. AR can also indicate where protective coverings or reinforcement materials need to be installed at crossings, helping to prevent onsite omissions. Visualizing relationships with unseen utilities helps prevent human errors and damage accidents at crossing points.

Main effects of introducing AR

Introducing AR into utility common duct construction is expected to produce the following benefits:

• Improved construction accuracy: Because AR enables construction to be carried out at the planned positions and dimensions, dimensional and positional errors decrease. Working while directly viewing the model onsite reduces reliance on ‘‘gut feel,’’ minimizing deviations from planned values. The result is higher as-built accuracy and higher-quality infrastructure.

• Prevention of rework: AR can greatly reduce rework caused by unnoticed mistakes during construction that require later correction. Continuously comparing design and site with AR reduces the risk of proceeding in the wrong direction. Even if small discrepancies occur, they can be detected and corrected immediately onsite before escalating into major corrective work. Fewer reworks directly lead to shorter schedules and cost savings.

• Smoother information sharing and consensus building: AR-visualized information is easier for the entire site team to share. Not only veterans but also young or first-time site visitors can intuitively understand 3D visuals, reducing the chance of misaligned recognition. AR can also be used to show completion images and construction ranges to clients and nearby residents on the spot, facilitating consensus building. Additionally, AR enables real-time data sharing between site and office, speeding up reporting, communication, and consultation.

High-precision positioning with LRTK and the convenience of smartphone AR

High-precision positioning is key to fully leveraging AR on site. Ordinary smartphone GPS has errors on the order of meters, making it unsuitable for accurately overlaying large structures. Recently, centimeter-level positioning with RTK-GNSS (real-time kinematic satellite positioning) has gained attention. The RTK method uses correction information from a reference station to greatly reduce positioning errors, enabling location determination within a few centimeters. The approach that applies this technology to site AR use is called LRTK.

By using LRTK, combining a compact high-precision GNSS receiver with a smartphone makes centimeter-level positioning easily achievable for anyone. With a dedicated app linked to the phone, one’s coordinates are calculated in real time with high accuracy, allowing AR models to be precisely aligned with the design. Without complicated machinery or specialized knowledge, users can simply point the device to display accurate AR, enabling site workers themselves to use AR without relying on surveying equipment operators. Traditionally, survey specialists would set out positions using transits or total stations and instruct workers. With LRTK and smartphone AR, a single smartphone can serve both as a surveying instrument and as the drawing, enabling immediate onsite position checks and construction guidance.

LRTK positioning is available over wide areas as long as satellite signals are available, so it can be used continuously even for long utility common duct projects. For example, on routes extending over 100 m, AR models can be displayed continuously on the smartphone according to the designated coordinate system, enabling high-accuracy guidance across the entire construction area. The equipment is compact and easy to carry or set up, making it practical for checking while moving between sites. Thus, LRTK brings the benefits of high-precision positioning to the field and is a key enabler for making AR-based construction support easily accessible to everyone.

Conclusion: Toward zero mistakes in utility common duct construction with AR

The ability to visualize underground structures in real time during construction is no longer a dream but is being realized on site.

Efforts to "visualize" the underground structure of utility common ducts with AR hold great potential to reduce construction errors and enable safe, efficient work by making what was previously unseen visible. In underground infrastructure development, AR technology that enables intuitive sharing of design information will play an increasingly important role. In particular, the emergence of easy-to-use, high-precision positioning solutions like LRTK has significantly lowered the barrier to onsite AR use. Supplementing veteran intuition and experience with digital technology to create a work environment where everyone can construct accurately will be key to future construction DX. These digital supports can also address aging skilled workers and labor shortages, helping even less experienced workers maintain quality on site.

Technicians and construction managers involved in utility common duct projects are encouraged to try visualizing underground structures with AR. Onsite discoveries and insights not available from drawings will bring you closer to error-free, reliable construction. Actively adopting advanced technologies will help turn "mistakes happen because the underground is invisible" into "zero mistakes because the underground is visible." The fusion of utility common duct construction and AR technology is expected to become the standard for safe, smart infrastructure development in the future.