Visualizing Underground Structures of Shared Conduits for Power and Communication Cables with AR to Reduce Construction Errors

What a shared conduit for power and communication cables is: an underground common duct supporting pole removal

A shared conduit for power and communication cables (densen kyōdōkō) is a common duct for housing multiple types of electrical and communication lines underground. Concrete boxes or pipes are buried beneath roads, and cables from multiple utilities are routed through them to reduce utility poles and overhead lines, contributing to improved cityscapes and disaster resilience. It is a representative method for promoting so-called "pole removal" and is being developed nationwide, especially in urban areas. Shared conduits reduce obstacles on sidewalks and lower the risk of poles falling during disasters, making them indispensable infrastructure for safe and attractive urban development.

However, constructing shared conduits requires high expertise and careful planning. Because new structures are being placed underground, detailed preliminary surveys and design are essential. The positions relative to existing buried infrastructure such as water and sewer lines or 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, traffic impacts must be considered. In some cases, developing hundreds of meters of shared conduit can take several years, and in such long-term, large-scale projects rework due to mistakes must be avoided at all costs. Although shared conduit installation offers great benefits, its realization comes with various challenges.

Key points to consider in designing and constructing shared conduits

When planning and designing shared conduits, it is necessary to develop detailed wiring plans while coordinating with multiple utilities. Design rules should define the conduit cross-sectional dimensions, internal compartmenting, and which cables go into which compartment, and the design should allow margin for future expansions. The installation location is also determined with consideration of road width and the positions of other buried utilities. For example, when installing under a sidewalk, it is required to keep a certain distance from private property boundaries and to ensure separation distances from other pipes (clearances to prevent interference). Coordinates and elevations should be determined precisely at the design stage, and at planned crossing points vertical clearances and protective measures should be planned.

There are many points to watch during construction as well. Shared conduits include not only straight sections but also curves and branch/junction sections (special parts such as manholes and access points), so accurate on-site positioning and advanced construction techniques are required. When working close to existing lifelines, vibrations and excavation impacts must be considered to carry out work safely. Especially when installing ducts under road crossings using jacking methods, even a slight deviation in the launch/arrival positions or directions can lead to major rework, so precise surveying and control are indispensable. It is also important to ensure that workers share the design intent and strictly follow wiring rules when placing cables. Failure to address these design and construction points can lead to defects or troubles after completion.

Construction errors caused by complex underground structures

Because underground structures in shared conduit projects can become very complex, construction errors are prone to occur. Typical examples of such errors include the following:

• Errors at crossing points: At locations where the shared conduit crosses other buried pipes, planned clearances (vertical separation) may not be maintained, causing interference between pipes. Where a certain elevation difference should be provided at a crossing, surveying errors or insufficient excavation accuracy at the site can result in shallower burial than planned and contact with other pipes. Because crossings are carried out underground and not visible, work can tend to rely on intuition, but small deviations can lead to major problems later.

• Interference with other facilities: Underground spaces are crowded not only with shared conduits but also with water and sewer pipes, gas pipes, and communication ducts. Even where plans on drawings appeared to be clear, unexpected obstacles or existing structures may be encountered in the actual field. As a result, routes may need to be changed or the conduit routed around obstacles, causing actual wiring paths to differ from the plan. If sufficient adjustments are not made to avoid interference with existing structures, problems may arise after completion such as mismatched access point locations or excessive bending of ducts that prevent cables from being pulled through.

• Deviation from wiring rules: Inside shared conduits, compartments are allocated for each electricity or communication company, and cables are housed accordingly. If cables are not placed in their designated positions according to wiring rules, maintenance and future expansions can be hindered. However, on-site decisions that change cable placement or mistakes that route cables into another operator’s compartment can occur. In cases of on-site confusion or communication errors—e.g., “Company A’s cables, which should have been in the right compartment, were placed on the left”—correcting the error later can require considerable effort.

Thus, in shared conduit work, the complexity of underground structures creates various risks of construction errors. Even if plans are neatly laid out on paper, small on-site deviations and human errors can accumulate into serious defects after completion.

Rework and accidents caused by invisible underground structures

A major barrier in underground shared conduit construction is that the underground is "invisible." During construction or after backfilling, the underground situation cannot be directly visually confirmed, so errors are hard to detect and problems may only surface when it is too late to remedy them. For example, discovering after burial that cable positions were incorrect and having to excavate and redo the work results in schedule delays and additional costs. Such avoidable errors can significantly reduce overall construction efficiency.

The invisibility of underground structures also poses safety risks. There are reports of accidents such as accidentally cutting existing communication cables during excavation, causing communication outages, or unexpected gas pipes appearing and temporarily suspending work. In many cases construction relies on drawings or markings from surveys that indicate buried object locations, but when workers cannot actually see underground, some degree of "guesswork" remains. As a result, misjudgments about locations can lead to human error, damage to buried facilities, and construction mistakes.

Furthermore, the invisibility of underground structures also causes problems during maintenance or other later construction. When re-excavating roads, lack of accurate records of buried positions can cause disparities between past drawings and reality, resulting in cables or pipes unexpectedly appearing at unanticipated locations. If these are accidentally damaged, the impact on users—such as power outages or communication failures—can be significant and potentially escalate into a social issue.

Accurately recording positions of underground structures after installation is not easy. After burial, tasks such as photographing and surveying and compiling the results into drawings are required, and there are cases where workers even paint the routes of buried ducts and cables on temporarily restored road surfaces as a site-specific effort to leave a record. This illustrates how difficult and laborious it is to "visualize" and convey hidden underground structures.

This fundamental problem—that the underground is unseen—is thus the root cause of rework and accidents in shared conduit construction.

Supporting construction by visualizing underground structures with AR

In recent years, new initiatives using AR (augmented reality) technology have attracted attention as a way to solve the problem of underground invisibility. AR is a technology that overlays digital data onto real-world scenes through a smartphone or tablet camera. In shared conduit construction, AR can project design data and wiring plans for underground structures onto the site view, making it possible to visualize the invisible underground structures on the spot.

Specifically, a dedicated AR app is loaded on a smartphone or tablet with the 3D design model of the shared conduit and the buried route information. When the device is held up on site, the model of the conduit and cables that are supposed to be installed underground is displayed composited with the real road scene on the screen. As if by x-ray vision, underground structures appear to float in the real environment. This allows workers to intuitively grasp underground structures on site without relying solely on flat drawings or mental images.

AR visualization dramatically improves construction management and workers’ understanding. What used to require looking at drawings and imagining “the conduit should be around here” can be confirmed as a life-size 3D model with AR. It is also easy to verify on site whether positions and dimensions assumed in the design fit in reality. For example, if a plan states that “at this point the conduit is designed to pass above a sewer pipe,” AR lets you visually confirm that relationship and immediately see whether sufficient clearance is maintained per the design.

In fact, pilot uses of AR have already begun in shared conduit works commissioned by the Ministry of Land, Infrastructure, Transport and Tourism. In one national highway project, combining a smartphone with a high-precision GNSS to overlay a 3D design model on site enabled even young engineers to instantly grasp design intent, earning high praise from site construction managers. This technology makes it easier to share underground structural images that were difficult to understand from drawings alone, and it is spreading to other sites. AR visualization is bringing a fresh perspective to shared conduit construction sites. Such advanced technology aligns with the Ministry’s i-Construction and construction DX (digital transformation) initiatives, and the number of use cases is expected to grow.

Concrete examples of AR-supported construction

Using AR on site can provide the following concrete support for shared conduit construction:

• On-site verification against design data: You can compare plans on drawings with on-site conditions instantly. While viewing the AR-projected conduit, you can check the relationship with actual road width and surrounding structures to confirm whether the design will fit. If the design model appears superimposed on existing structures in AR, you can recognize the need for design changes on the spot. Identifying discrepancies between the field and drawings in advance prevents large-scale rework.

• Visualization and guidance of buried routes: Because the route of the conduit to be buried can be visualized from above ground, workers can intuitively grasp where to excavate or lay components. For example, AR can show which line on the road surface to follow for excavation, allowing workers to trace an accurate digital guide instead of relying on stakes or spray markings. If depth information is included in the AR display, it can serve as a reference when adjusting excavation depth to achieve the specified burial depth. As a result, errors such as misplacement or tilting of the conduit body can be reduced and construction accuracy improved.

• Pre-checks for crossing points: At locations where the conduit crosses or is close to other buried facilities, AR can be used to confirm spatial relationships in advance and plan safe construction procedures. For example, if the AR marks “the point where it crosses a gas pipe ○ m (○ ft) ahead,” workers can switch to careful probing and cautious excavation before reaching that point. AR can also indicate where protective coverings or reinforcement materials are required at crossings, helping to prevent omissions on site. Visualizing the positions of unseen utilities helps prevent human error and damage accidents at crossing points.

Main effects of introducing AR

Introducing AR into shared conduit construction is expected to yield the following effects:

• Improved construction accuracy: AR enables construction to be carried out at the designed positions and dimensions, reducing dimensional and positional errors. By working directly while viewing the model on site, reliance on intuition is reduced and deviations from planned values minimized. As a result, as-built accuracy improves and high-quality infrastructure development is achieved.

• Prevention of rework: AR can greatly reduce rework caused by unnoticed mistakes during construction that require later corrective excavation. By continuously comparing design with field conditions through AR, the risk of proceeding in the wrong direction is reduced. Even if small discrepancies occur, they can be detected and corrected on the spot before escalating into major corrective work. Reduced rework directly shortens schedules and lowers costs.

• Smoother information sharing and consensus building: AR-visualized information is easier for everyone on site to share. Not only veterans but also young or first-time site visitors can understand intuitively through 3D visuals, reducing misalignment in recognition. For explanations to clients or nearby residents, being able to show the completion image and construction scope on site helps build consensus more smoothly. AR also enables real-time sharing of data between the site and the office, speeding up reporting and coordination.

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

High-precision alignment is key to full-scale AR use on site. Ordinary smartphone GPS has an error of several meters and is unsuitable for accurately overlaying large structures. Recently, RTK-GNSS (real-time kinematic satellite positioning) has attracted attention for providing centimeter-level positioning. RTK uses correction information from a reference station to greatly reduce positional errors and can determine current position with accuracy within a few centimeters (within a few inches).

The approach of applying this technology to on-site AR use is called LRTK. By combining a smartphone with a compact high-precision GNSS receiver, LRTK enables anyone to achieve centimeter-level positioning easily. Because the dedicated app linked with the smartphone calculates highly accurate coordinates in real time, AR models can be aligned exactly at the positions specified in the design. Without complex equipment operation or specialized knowledge, simply holding up the device yields accurate AR displays, allowing site workers themselves to use AR without relying on a surveying equipment operator. Traditionally, surveying specialists used instruments like transits or total stations to set positions and then instructed workers. But by combining LRTK with smartphone AR, a single smartphone can serve both as a surveying instrument and as the drawings, enabling instant position checks and construction guidance on site.

Moreover, LRTK positioning can be used across wide areas as long as satellite signals are received, so it can be applied continuously in long shared conduit projects. For example, even for conduit routes that extend over 100 m (328.1 ft) or more, models can be displayed continuously on a smartphone according to a prescribed coordinate system, enabling high-accuracy guidance across the entire construction area. The equipment is compact and easy to carry and install, so confirming locations while moving between sites is not problematic. LRTK thus brings the benefits of high-precision positioning to the field and is the key to making AR-based construction support easily accessible to everyone.

Conclusion: Toward zero errors in shared conduit construction with AR

Real-time x-ray–like visualization of underground structures during construction is no longer a pipe dream but is becoming a reality on site.

Efforts to visualize the underground structures of shared conduits with AR have great potential to reduce construction errors and achieve safer, more efficient work by making the previously invisible visible. AR technology that allows intuitive sharing of design information will play an increasingly important role in underground infrastructure development. The arrival of easy-to-use high-precision positioning solutions like LRTK has significantly lowered the barrier to on-site AR use. Complementing veteran intuition and experience with digital technology to create a work environment where anyone can construct accurately will be a key to future construction DX. Such digital support also helps address the aging skilled workforce and labor shortages, enabling even less experienced workers to maintain quality on site.

Engineers and construction managers involved in shared conduit projects are encouraged to try visualizing underground structures with AR. You are likely to discover insights on site that drawings alone cannot provide, bringing you closer to error-free, reliable construction. Actively adopting advanced technologies, let us transform “errors occur because the underground is invisible” into “errors are zero because the underground is visible.” The integration of shared conduits and AR technology is expected to become the standard for safe and smart infrastructure development.