Using AR in Shared Duct Construction for Electrical Cables: Enhancing Construction Efficiency and Accuracy with Position Guidance

Construction Challenges and the Importance of Precision Management in Shared Cable Duct Construction

In the construction of joint utility ducts to promote the undergrounding of utility lines along roads (burial of overhead cables), many stakeholders and existing infrastructures must be coordinated, and the work requires highly precise accuracy management. Although the undergrounding initiative has been pursued in a planned manner since the 1980s, about 36 million utility poles still remain in the country, and their number is not decreasing but increasing. New joint utility duct installations involve the placement of long conduits and large precast vaults (CC boxes) that accommodate multiple power and communication cables, requiring precise construction within limited road spaces where existing buried facilities such as water and sewer pipes and gas lines are densely interwoven. If excavation or installation is carried out in the wrong position, there is a risk of damaging adjacent buried facilities or causing manhole components to fail to connect as designed. Therefore, it is extremely important to accurately know what is buried underground and where, and to minimize construction errors. For example, traditionally it was necessary to perform numerous trial excavations (test pits) to locate buried objects not shown on drawings. In recent years, however, GNSS (satellite positioning)-compatible survey equipment has been used to map buried asset locations, reducing the number of trial excavations. In one actual project, where 10 trial excavations had originally been planned, this high-precision survey reduced the number to four, achieving a schedule reduction of about 3 days and cost savings of 2 million yen. Thus, in joint utility duct construction, accuracy management that consistently pursues positional accuracy from preliminary surveys through construction is indispensable for proceeding safely and efficiently.

Conventional Surveying and Layout Methods and Their Limitations

Traditional civil engineering surveying and layout work relies on time and manpower, and common utility duct construction for power lines is no exception. Typically, a surveyor and an assistant operate equipment such as total stations as a two-person team, installing stakes and batter boards (boards that serve as height and position references) on site. Even for laying out a single point, complicated procedures are required—measuring the distance from a reference point with a tape measure and driving a stake while signaling—and if there are many survey points it is not uncommon for the work to take an entire day. In roadworks that often occur at night, crews may struggle to pre-install adequate batter boards under traffic restrictions. Human errors inherent to manual work are unavoidable, and if a stake is driven in the wrong position due to misreading drawings or recording mistakes, it can lead to later rework or construction errors. Accuracy is also a major issue: inexpensive handheld GPS units can have errors of several meters (several ft), making them unsuitable for precise layout, so in practice there is no choice but to rely on expensive surveying equipment costing several million yen and the experience of veteran technicians. Furthermore, in common utility duct construction for power lines it is necessary to measure the dimensions of the as-built structure and take photographs at each stage, but traditionally this relied on inefficient methods such as temporarily halting work and having measurements and records taken by multiple people. In this way, traditional surveying and layout methods were person-dependent, time- and labor-consuming, and a cause of delays and reduced accuracy in work.

A new positioning guidance system that combines AR and high-precision positioning (RTK-GNSS)

In recent years, the combination of high-precision GNSS and AR (Augmented Reality, augmented reality) has been attracting attention as a technology that can transform this situation. GNSS is a technology that measures one’s position by receiving radio signals from multiple satellites such as GPS and Japan’s quasi-zenith satellite Michibiki, and in particular, using RTK enables positioning with errors of a few centimeters. AR technology overlays three-dimensional design data and annotations on the camera images of smartphones or tablets, allowing virtual guides and models to be rendered in real space in real time. By combining these two, an entirely new mechanism for positioning guidance at construction sites can be realized.

Specifically, first import into a dedicated smartphone app the coordinate data of structures and cable-routing routes based on the construction design drawings. A small RTK-GNSS receiver attached to the smartphone continuously measures the current position at centimeter-level (half-inch accuracy), and the app displays in real time the direction and distance the worker should move, such as “5 cm (2.0 in) east, 10 cm (3.9 in) north.” The worker only needs to take a few steps following the on-screen instructions to arrive precisely at the target. Positioning work that previously required several people can now be completed by one person with a smartphone in hand.

Furthermore, if you switch the camera view to AR mode, virtual objects such as “drive a stake here” or “excavate to this point” are overlaid on the real-world image, allowing intuitive identification of work points. Even without experienced intuition or advanced surveying knowledge, simply following the on-screen guides enables high-precision stake-driving and marking, greatly reducing surveying point errors and rework.

The accuracy of this GNSS×AR automatic guidance is comparable to total stations, etc., with horizontal accuracy of several centimeters (a few inches) and vertical accuracy of several centimeters (a few inches), and it also offers high work efficiency because it can perform continuous measurements without re-setting the equipment even in areas with poor line of sight. This innovative workflow—properly called smartphone surveying—is bringing innovation to construction sites and aligns with the i-Construction initiative promoted by the Ministry of Land, Infrastructure, Transport and Tourism. In fact, from fiscal 2023 a trial of ICT construction (use of 3D design data and as-built measurement) for shared conduit construction for power lines has also begun, and industry-wide on-site digital transformation (DX) using 3D data is accelerating.

What can be done with AR display using a smartphone + GNSS receiver (virtual display of cable routing positions, interference checks with temporary structures)

By using AR display with a smartphone and a GNSS terminal, the design positions and structures can be virtually “visualized” on site.

For example, if the conduit (cable routing path) of a shared utility duct for power and communications is displayed as a line on the ground in AR, you can understand at a glance where to dig and where to route the ducts before excavation.

In shared utility duct construction that simultaneously buries 10 or more ducts for power and communications combined, there are many complex situations where ducts must be routed with curves in both vertical and horizontal directions to avoid existing buried utilities, and even veteran workers may find it difficult to visualize the post-construction routing in their heads.

By displaying new conduits and existing buried ducts in 3D with AR and checking them in advance, even less experienced workers can more intuitively grasp the routing, and there have been reports that the progress of duct installation improved as a result.

In this way, AR enables on-site sharing of a finished image that is difficult to convey with drawings alone, and has a large effect on aligning the team's understanding and optimizing construction planning.

AR is also useful for interference checks with temporary structures and surrounding equipment. By displaying models of temporary retaining structures and existing structures on AR based on design data, you can confirm the clearance (gap) between new and existing structures in advance. If the planned route is likely to intersect or come into contact with temporary shoring or other buried utilities, it can be seen immediately on AR, allowing countermeasures to be taken before construction. There are actual cases where using BIM/CIM models and AR helped check interference with overhead lines during crane operations in confined sites, contributing to improved safety.

Furthermore, from the perspective of safety management, performing excavations with heavy machinery while visualizing the positions of underground buried utilities on the ground surface with AR can prevent accidental damage to water and gas pipes. For example, if a tablet or AR glasses are held up before work, existing underground pipes appear to be visible through the ground, so the operator can instantly see where extra caution is needed. In this way, visualization using a smartphone + GNSS + AR can be widely applied—from pre-construction review to guiding actual work and safety checks—and becomes a powerful tool that makes joint utility duct construction sites for power lines smarter.

Labor savings and quality improvement effects from combining positioning guidance and AR-based verification

By combining position guidance using high-precision GNSS with visual verification via AR, construction labor savings and quality improvement are achieved simultaneously. First, as an effect of labor savings, site layout tasks that previously required multiple people can be performed by a single person, allowing work to proceed efficiently even on sites with labor shortages. In fact, on a joint utility conduit project in Niigata Prefecture, a one-man surveying system was introduced and batter-board-less construction (construction without installing batter boards) was achieved, with daily construction extension improving from the previous 4 m (13.1 ft) to 6 m (19.7 ft). In the later stages, once they became accustomed to it, the subcontractor’s foreman himself mastered the equipment and reportedly continued construction without the need for technicians (surveying specialists). Moreover, dramatic efficiency gains are seen in as-built measurements as well. Whereas conventional as-built measurements of each process—conducted by 2–4 people using surveying instruments and scales and taking 10–15 minutes including photographing—the new method using an iPhone’s LiDAR scanner completes them in about 3 minutes by one person. These time reductions have greatly improved overall construction productivity and reduced the workload and required personnel per site.

On the other hand, the effect on quality improvement should not be overlooked. Following AR guidance greatly reduces human error, which in turn helps prevent variability in as-built results and construction defects. Because work can always be performed at the design-specified location without relying on a veteran’s intuition, the incidence of rework and touch-ups also decreases. Furthermore, on-site verification with AR contributes to quality assurance by allowing discrepancies between the plan and the actual situation to be detected and corrected immediately during construction. For example, if the slope or depth of piping differs from the design, you can notice it as an anomaly in the AR display during work, without having to wait for the as-built management stage described below. Traditionally, mistakes were sometimes discovered only after completion when surveys were compared to drawings, but by combining AR you can achieve prevention of rework. In this way, by combining high-precision positional guidance with AR-based verification, it is possible to both improve efficiency through reduced staffing and strengthen quality control.

Integration with as-built surveys and records: Visualize the gap between design and the site on-site

In shared underground duct construction for power lines, verification and recording of as-built (finished) shapes are also extremely important, and with the introduction of AR and RTK-GNSS that process is becoming real-time and more sophisticated. Traditionally, work was stopped at the end of each phase, staff measured dimensions with tape measures, and the recorded numbers were later used to verify whether the finished shape matched the design. In contrast, systems now exist that let you scan the site with a smartphone or tablet to obtain point cloud data (3D measurement data) and immediately compare it with the design 3D model to evaluate the as-built. For example, if you use an iPhone/iPad LiDAR function to walk all the way around and scan the excavated trench, you can obtain a high-precision point cloud model of the entire site aligned to the public coordinate system in just a few minutes. From that data, excavation width, depth, and conduit burial positions are automatically extracted and can be compared with the pre-prepared BIM/CIM design model almost in real time. This has been shown to dramatically improve productivity compared to the previous as-built measurement and documentation tasks, which required several people using tape measures and levels.

Right after backfilling the buried conduit section of a shared utility duct for electrical wiring, a construction management engineer is scanning the excavation surroundings with a smartphone LiDAR (left). A high-density point cloud dataset of the entire site (right) is then generated in the public coordinate system, enabling immediate as-built comparisons with the design model. From the vast information contained in the point cloud data, required dimensions such as excavation width and depth and the conduit slope are automatically extracted, and analyses such as detecting deviations from the design values are also automated. Using an AR-based as-built simulation feature, these point cloud measurement results and the design 3D model can be overlaid on-site, allowing the human eye to intuitively grasp the gap between the design and the actual site. For example, if the installed pipe’s position or elevation deviates from the design, the virtual pipe will appear to float above or sink into the actual ground in AR, so you can immediately see where corrections are needed. Conventionally, the workflow was to perform a separate survey after construction was completed, check consistency on the drawings, and if defects were found, carry out corrective work. However, by combining AR and digital measurement, instant verification and immediate correction during construction can be performed, enabling shorter schedules while preventing rework and quality degradation.

Furthermore, photos taken with an RTK-GNSS–linked smartphone are automatically tagged with latitude, longitude, elevation and camera orientation, so as-built photo records reliably show exactly which point and which direction were photographed. Such coordinate photos can be uploaded immediately to a cloud database (digital ledger), and it is easy to check site conditions on a map from an office PC. What used to be difficult—identifying locations later from paper photo albums—can now be clearly managed with photos tied to map plots. Because all information collected on site, including survey data and point clouds, is aggregated and shared in the cloud, creating as-built documentation and reporting to the client is also smooth. Even slight differences between design values and as-built conditions can be detected and recorded on the spot—this level of advanced as-built management is becoming achievable through the combination of a smartphone, RTK-GNSS and AR.

Ripple Effects of AR and RTK on the Operation and Maintenance Phase

High-precision positioning information and on-site visualization using AR also bring significant spillover effects to post-construction maintenance operations. The precise 3D data and coordinate information acquired during construction can be directly capitalized as a digital ledger for maintenance management and used for future inspections and repairs. For example, when inspecting a road after construction is complete, it is not easy to accurately ascertain the on-site location of buried shared ducts for electrical cables. However, by using AR functions on tablets or smartphones, underground pipelines can be projected onto the ground surface, making it immediately clear where the shared duct runs. This reduces the risk of accidentally damaging the shared duct or other buried objects even when excavating the road for other works.

Also, for periodic inspections, smart inspections using GNSS and AR are possible. Traditionally, infrastructure inspections typically involved personnel recording observed abnormalities in the field with notes and photographs, and later marking the locations on drawings; however, this method leaves photos without accurate location or orientation information, making it difficult for others to determine later where the photos were taken. On the other hand, with geotagged photos, the shooting location is indicated by latitude/longitude or a point on a map, so anyone can pinpoint the photographed location. Furthermore, using AR, the locations of previously taken photos can be displayed as icons in the real-world space, allowing inspectors to stand at the same point as last time through the smartphone screen and easily take new photos from the same angle. For example, when inspecting cracks inside a shared duct manhole, taking a new photo aligned with the previous shooting position allows immediate comparison of crack progression. Thanks to the consistency afforded by high-precision positioning, records can be made with exactly the same composition and distance each time, enabling quantitative tracking of long-term changes.

Also, if needed, scanning the structure surface with a smartphone’s LiDAR and saving it as point cloud data allows detailed 3D analysis later of deformations and damaged areas. All such inspection data are uploaded from the field to the cloud and can be shared and viewed instantly from the office, facilitating report creation and communication among stakeholders. Thus, applying AR＋RTK technology to the maintenance phase can smarten the entire process—from locating underground structures to recording regular inspections—and greatly contribute to preventing oversights and improving record accuracy.

Fusion of Smartphone RTK + AR Guidance + Point Cloud Acquisition: LRTK Paving the Way for the Future of On-site DX

The ultra-compact RTK-GNSS receiver "LRTK Phone" that attaches to a smartphone. Simply mounting the pocket-sized unit (the blue device in the photo), which weighs approximately 150 g and is about 1.3 cm (0.5 in) thick, instantly transforms a smartphone into a positioning device with centimeter-level accuracy (cm level accuracy (half-inch accuracy)). Such smartphone-linked RTK solutions are ushering in an era in which the construction DX using smartphone + RTK + AR we have seen so far can be easily implemented by anyone. With systems like LRTK, a single smartphone can access a wide range of functions—from surveying (single-point coordinate measurement) and point cloud acquisition to layout marking (position setting) and AR-based construction simulation. You can also use the acquired data to calculate distances, areas, and volumes, and positioning information obtained on site can be sent to the cloud immediately and shared with the office seamlessly. Because dedicated equipment is no longer necessary and operation is simple, the concept of "one-device-per-person surveying"—in which each field worker autonomously performs tasks using their own high-precision positioning tool—has become more realistic. In fact, because of the convenience of being able to position, measure, and display AR anywhere and anytime with just a smartphone and an ultra-compact GNSS terminal, not only site surveyors but also construction management engineers and workers themselves are increasingly performing surveying and inspection. It can be said that these smartphone RTK + AR technologies offer great benefits especially in infrastructure sectors where precise construction and record-keeping are required, such as joint conduit construction for power lines. With solutions emerging that can cover everything from more efficient position guidance and advanced as-built management to applications in maintenance management—all with a single device—the barriers to on-site DX have been significantly lowered.

Now, a workflow centered on smartphones for high-precision positioning and AR utilization is becoming the new norm at job sites. Not limited to utility conduit construction for power lines, as a key to balancing efficiency gains and quality assurance in all civil engineering works, smartphone RTK + AR technology will continue to become more widespread. By actively adopting the latest digital technologies, a future in which "a single smartphone completes everything from surveying to construction management" may truly become a reality. The use of AR in utility conduit construction for power lines is expected to significantly change how infrastructure development and maintenance are conducted going forward, serving as an example that drives the DX of construction sites.