Utilizing AR in Electrical Cable Common Duct Construction: Enhancing Construction Efficiency and Accuracy through Position Guidance

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

In utility-duct construction that advances the undergrounding of utility lines (eliminating roadside utility poles), coordination with many stakeholders and existing infrastructure is required, and construction involves rigorous precision management. Although efforts toward undergrounding have been pursued since the 1980s, roughly 36 million utility poles still remain nationwide, and their number is not decreasing but actually increasing. Constructing new common utility ducts involves installing long conduits to house multiple power and communications cables and large precast chambers (CC boxes), requiring precise work within the limited road space where existing underground utilities such as water, sewer, and gas pipes are densely interwoven. If excavation or installation is carried out in the wrong location, there is a risk of damaging adjacent buried utilities or causing problems such as manhole components failing to connect as designed. Therefore, it is essential to accurately know what is buried where underground and to minimize construction errors. For example, in the past many trial excavations (test pits) were necessary to locate buried objects that were not shown on drawings. However, in recent years GNSS (satellite positioning)-compatible detection devices have made it possible to map the positions of buried utilities and reduce the number of trial excavations. In one actual project, where ten trial excavations were originally planned, this high-precision surveying reduced the number to four, achieving about a three-day schedule reduction and roughly 2 million yen in cost savings. Thus, in common utility duct construction, precision management that consistently pursues positional accuracy from preliminary surveys through construction is indispensable to proceed safely and efficiently.

Conventional surveying and positioning methods and their limitations

Conventional civil surveying and staking out work relies on time-consuming manual labor, and common utility duct construction for power lines is no exception. Typically, a surveyor and an assistant working as a two-person team operate equipment such as total stations, setting out stakes and batter boards (boards that serve as height and position references) on site. Even positioning a single point requires the cumbersome procedure of measuring distances 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 take place at night, crews can struggle because adequate batter boards cannot be pre-installed under traffic control. Human error inherent in manual work is unavoidable; misreading drawings or recording mistakes that cause stakes to be driven in the wrong place can later lead to rework or construction defects. There are also major accuracy challenges: inexpensive handheld GPS units can have errors of several meters and cannot be used for precise positioning, so in practice projects have had to rely on expensive surveying instruments costing several million yen and the experience of veteran technicians. Furthermore, in common utility duct construction it is necessary to measure the dimensions of the finished structure (as-built) and take photographs at each stage, but traditionally this relied on the inefficient method of pausing work for multiple people to take measurements and record them. In this way, conventional surveying and staking out methods were reliant on individuals, time- and labor-intensive, and a cause of delays and reduced accuracy in work.

A new position guidance system combining AR and high-precision positioning (RTK-GNSS)

In recent years, the combination of high-precision GNSS and AR (Augmented Reality) has been attracting attention as a technology that can completely change 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”; in particular, by using the RTK method it is possible to achieve positioning with errors of only a few centimeters. AR technology overlays three-dimensional design data and annotations onto camera images from smartphones or tablets, enabling virtual guides and models to be rendered in real time on the real-world scene. By combining these two, an entirely new positioning guidance mechanism is realized at construction sites.

Specifically, coordinates for structures and cable-routing paths from the construction design drawings are first imported into a dedicated smartphone app. A small RTK-GNSS receiver attached to the smartphone continuously measures the current position with centimeter-level accuracy, and the app displays in real time the direction and distance the worker should move, such as "5 cm east and 10 cm north to the target point." By following the on-screen instructions and taking just a few steps, the worker can arrive exactly at the intended position. Tasks that previously required several people for staking out can now be completed by one person with a smartphone in hand. Moreover, by switching the camera view to AR mode, virtual objects such as "drive a stake here" or "excavate to this position" are overlaid onto the real-world view, allowing workers to intuitively grasp the work points. Even without experienced intuition or advanced surveying knowledge, workers can perform high-precision pile-driving and layout marking simply by following the on-screen guide, greatly reducing measurement-point errors and rework. The accuracy of this GNSS×AR automatic guidance is on the order of a few centimeters horizontally and vertically, rivaling total stations, and it also offers high work efficiency because it can perform continuous measurements without re-setting the equipment even in locations with poor line-of-sight. This new workflow—truly a kind of smartphone surveying—is bringing innovation to the field, and it aligns with the i-Construction initiative promoted by the Ministry of Land, Infrastructure, Transport and Tourism. In fact, from fiscal 2023 trials of ICT construction (utilizing 3D design data and as-built measurement) for joint conduit construction for power lines have begun, and industry-wide on-site DX using 3D data is accelerating.

What AR display with a smartphone + GNSS device can do (virtual display of cable routing positions, interference checks with temporary structures)

Using AR displayed on a smartphone and a GNSS device, the design positions and structures can be virtually "visualized" on site. For example, if the ducts (cable routing paths) of a multi-utility trench are shown as lines on the ground with AR, you can immediately understand where to dig and where to run the conduit before excavation. In multi-utility trench work—where more than ten ducts for power and communications are buried simultaneously—there are many complex situations that require routing ducts with horizontal and vertical curves to avoid existing buried utilities, and even veteran workers may find it difficult to mentally picture the post-installation piping layout. By displaying new ducts and existing buried pipes 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 piping work progress has improved as a result. In this way, AR can share a completed-image on site that is hard to convey with drawings alone, making a significant contribution to 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 in AR based on design data, you can confirm in advance the clearance (gap) between new and existing structures. If a planned route risks intersecting or contacting temporary shoring or other buried objects, AR makes that immediately visible so countermeasures can be taken before construction. There are actual examples where BIM/CIM models and AR were used to check interference with overhead lines during crane operations in confined sites, helping to improve safety.

Furthermore, in terms of safety management, performing machine excavation while visualizing the locations of underground utilities on the surface with AR can prevent accidental damage to water and gas pipes. For example, if you hold up a tablet or AR glasses before work, existing underground pipes appear transparent, allowing operators to identify points requiring caution at a glance. In this way, visualization using a smartphone + GNSS + AR can be applied widely—from pre-construction review to guidance during actual operations and safety checks—becoming a powerful tool to make shared electrical duct construction sites smarter.

Effects of labor reduction and quality improvement by combining position guidance and AR verification

Combining position guidance from high-precision GNSS with visual verification using AR enables both labor reduction and quality improvement in construction. First, as an effect of labor reduction, positioning tasks that previously required multiple people can be done by a single person, allowing work to proceed efficiently even on sites facing labor shortages. In fact, at a power-line common duct construction project in Niigata Prefecture, a one-man surveying system was introduced and they achieved no-stake construction (construction without installing stakes), increasing daily construction extension from the conventional 4 m to 6 m. In the later stages, once they became accustomed, the subcontractor foreman reportedly mastered the device himself and was able to continue construction without the need for a surveyor (a specialist in surveying). Also, dramatic efficiency gains are seen in as-built measurements. Traditionally, as-built measurement tasks that required 2–4 people using surveying instruments and scales and took 10–15 minutes including photography can be completed in the new method using the iPhone’s LiDAR scanner in about 3 minutes by one person. These time savings significantly improve overall construction productivity and reduce the workload and required personnel per site.

On the other hand, the effect of quality improvement should not be overlooked. Following AR guidance greatly reduces human error, which in turn helps prevent variability in as-built conditions and construction defects. Because work can be carried out at the exact design positions without relying on a veteran’s intuition, the incidence of rework and touch-ups also decreases. Furthermore, on-site verification via AR contributes to quality assurance by enabling immediate detection and correction of discrepancies between the plan and the actual situation during construction. For example, if a conduit’s slope or depth differs from the design, workers can notice it as an inconsistency in the AR display during work, without having to wait for the later as-built management stage discussed below. Traditionally, mistakes were sometimes found only after completion when surveys were compared with drawings, but combining AR makes it possible to prevent rework. In this way, combining high-precision positional guidance with AR verification allows both efficiency gains from reduced manpower and strengthened quality control.

Integration with As-Built Surveys and Records: Visualizing the Gap Between Design and the Site On-Site

In shared underground duct construction for utility cables, verifying and recording the as-built condition (final shape) is extremely important, and the introduction of AR and RTK-GNSS is transforming that process into a real-time, highly advanced workflow. Traditionally, work would be stopped at the end of each stage, staff would measure dimensions with tape measures and record the values, and the completed shape would be checked against the design at a later time based on those recorded numbers. In contrast, systems now exist that let you scan a site with a smartphone or tablet to capture point cloud data (three-dimensional measurement data) and immediately compare it with a 3D design model to evaluate the as-built condition.

For example, if you circle-scan a trench after excavation using the LiDAR function of an iPhone/iPad, you can obtain a high-accuracy point cloud model of the entire site aligned to a public coordinate system in just a few minutes. From that data, excavation width and depth, conduit burial locations, and other features are automatically extracted and can be checked against pre-prepared BIM/CIM design models almost in real time. As a result, productivity has been shown to increase dramatically compared with the as-built measurement and documentation work that previously required several people using tape measures and leveling instruments.

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

Furthermore, photos taken with an RTK-GNSS–enabled smartphone are automatically tagged with latitude, longitude, elevation, and camera orientation, so as-built photo records reliably preserve exactly where and in which direction each shot was taken. These geotagged photos can be uploaded immediately to a cloud database (digital ledger), making it easy to view site conditions on a map from an office PC. What used to be difficult—identifying locations later from paper photo albums—can now be managed clearly with photos linked to map plots. With all on-site information, including survey data and point clouds, aggregated and shared in the cloud, the preparation of as-built documentation and reporting to clients becomes smoother. Even subtle discrepancies between design values and as-built conditions can be exposed and recorded on the spot—this level of advanced as-built management is becoming achievable through the combination of smartphones, RTK-GNSS, and AR.

Ripple Effects on the Maintenance and Management Phase through AR and RTK

High-precision positioning information and on-site visualization using AR also have significant ripple effects on post-construction maintenance operations. The precise 3D data and coordinate information acquired during construction can be capitalized as a digital ledger for maintenance, directly supporting future inspections and repairs. For example, when inspecting a road after completion, it is not easy to accurately identify the location of buried utility ducts on site. However, by using AR functions on tablets or smartphones, underground conduits can be projected onto the surface, making it immediately clear where the ducts run. This reduces the risk of accidentally damaging the ducts or other buried assets when the road is excavated for other work. Moreover, routine inspections can be made smart by utilizing GNSS and AR. Traditionally, infrastructure inspections typically involved personnel recording observed anomalies on site with notes and photos and later annotating the drawings, but this approach left photos without precise position or orientation information, making it difficult for others to determine where a photo was taken. In contrast, photos with location tags indicate the shooting point as latitude/longitude or as a point on a map, so anyone can identify the photographed location. Furthermore, with AR the locations of previously taken photos can be displayed as icons in physical space, making it easy during inspection to stand at the same point as before through the smartphone screen and take a new photo from the same angle. For example, in inspecting cracks inside a duct manhole, taking a new photo matched to the previous shooting position allows immediate comparison of crack progression. Thanks to the consistency provided by high-precision positioning, records can be taken with exactly the same composition and distance each time, enabling quantitative tracking of long-term changes. Also, if needed, scanning the structure's surface with a smartphone's LiDAR and saving it as point cloud data enables detailed 3D analysis later of shape displacement and damaged areas. All such inspection data can be uploaded to the cloud from the field and shared and viewed instantly from the office, facilitating report preparation and communication among stakeholders. In this way, applying AR＋RTK technology to the maintenance phase can smarten the entire process, from locating underground structures to routine inspection records, greatly contributing to preventing oversights and improving recording accuracy.

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

The ultra-compact RTK-GNSS receiver for smartphones, the “LRTK Phone.” By simply attaching the pocket-sized unit—about 150 g in weight and 1.3 cm thick (the blue device shown in the photo)—to your smartphone, the phone instantly becomes a centimeter-level positioning device. Thanks to these smartphone-linked RTK solutions, the era is approaching in which the construction DX that leverages smartphone + RTK + AR, as we have seen so far, can be easily put into practice by anyone. With systems like LRTK, a single smartphone can utilize a wide range of functions, from surveying (single-point coordinate measurement) and point cloud acquisition to layout marking (positioning) and AR-based construction simulation. You can also use the acquired data to calculate distance, area, and volume, and the positioning information obtained on-site can be sent to the cloud and shared with the office instantly, enabling seamless collaboration. Because specialized equipment is no longer necessary and operation is simple, the concept of "one-person-one-device surveying", in which each field staff member can autonomously work using their own high-precision positioning tool, has become realistic. In practice, the ease of being able to perform positioning, measurement, and AR display anytime, anywhere with just a smartphone and an ultra-compact GNSS unit has led not only surveyors but also construction management engineers and workers themselves to increasingly carry out surveying and inspections. In infrastructure fields that require precise construction and record-keeping—such as shared conduit construction for power lines—the benefits of these smartphone RTK + AR technologies are particularly significant. With solutions that can cover everything described above—from more efficient position guidance and improved as-built management to applications in maintenance—on a single device, the barriers to on-site DX have been dramatically lowered.

A workflow centered on smartphones that leverages high-precision positioning and AR is becoming the new norm on construction sites. Not limited to utility duct construction for power lines, smartphone RTK + AR technology will increasingly spread as the key to balancing improved efficiency and quality assurance across all civil engineering works. By actively embracing the latest digital technologies, a future in which “everything from surveying to construction management can be completed with a single smartphone” may truly become a reality. The use of AR in utility duct construction is expected to lead the digital transformation (DX) of construction sites and significantly change how infrastructure is developed and maintained going forward.