5 checks for inspecting the surroundings of retaining walls at solar power plants using drone surveys

In solar power plants, attention tends to focus on inspections of panels, mounting racks, and electrical equipment, but it is also important to check the condition of the site's developed ground, drainage facilities, slopes, and areas around retaining walls that support the plant. In particular, retaining walls are structures that manage the site's elevation differences and contribute to the stability of the developed surfaces, access routes, and equipment placement, so the ability to detect surrounding changes early affects the quality of operation and maintenance. By using drone surveying, it becomes easier to comprehensively inspect the entire retaining wall and surrounding terrain from above and to identify water flow, signs of settlement, soil movement, and vegetation changes that are easily overlooked during on-foot inspections.

Table of Contents

• The Importance of Inspecting Areas Around Retaining Walls with Drone Surveying

• Check 1: Examine the positional relationship of retaining walls and the overall shape

• Check 2: Inspect drainage routes and locations where puddles form

• Check 3: Inspect the backfill ground for signs of settlement or cracking

• Check 4: Examine the relationship with slopes and soil/sediment movement

• Check 5: Assess the impact on inspection access routes and equipment

• Precautions When Inspecting Areas Around Retaining Walls Using Drone Surveys

• Summary

The Importance of Inspecting the Surroundings of Retaining Walls with Drone Surveying

The areas around retaining walls at solar power plants are locations where conditions should be continuously checked not only immediately after construction but also after operations begin. Solar plants are sometimes installed in mountainous areas, on slopes, on reclaimed land, or near valley terrain, and the site may combine retaining walls, slopes, drainage channels, side ditches, and maintenance access paths to handle elevation differences. These elements do not exist merely as individual installations; they influence the overall stability of the plant while interacting with rainwater flow, the degree of ground compaction, vegetation growth, vehicle traffic, and the movement routes of maintenance operations.

When trying to check the area around a retaining wall using only on-site patrols, the field of view is limited by the inspector's position. Looking from below the wall makes it difficult to understand the upper ground and how drainage concentrates, while looking from above can reduce overall visibility of the wall face. In addition, during periods of dense vegetation growth or after rain, mud and slopes can make some locations hard to approach. As a result, even if point-based observations are collected, the retaining wall and surrounding topography are often not captured as continuous surfaces.

Drone surveying is an effective means of compensating for this shortfall. By using images captured from above, generated orthophotos, and terrain elevation data, you can get an overview of the retaining wall’s alignment, the extent of the ground behind it, the location of drainage channels, and the relationship with slopes and pathways. Even in places that are difficult for people to access, if flight conditions permit, it becomes easy to assess the situation without approaching. In particular, if the same area is photographed regularly, comparing with previous images makes it easier to detect sediment accumulation, scouring, changes in vegetation, and locations where puddles form.

However, it is advisable to avoid making a definitive judgment on the integrity of a retaining wall based solely on drone surveying. Images and survey data mainly reveal changes that have appeared on the surface and trends in the terrain. Structural safety, internal condition, verification against design conditions, and the need for repairs should be assessed in combination with specialized investigations and on-site inspections. Drone surveying serves as an entry point for remotely identifying locations suspected to be hazardous, and it is easier to incorporate into practice when used as material for determining patrol plans and prioritizing detailed inspections.

When inspecting the area around a retaining wall, it is important not to look only at the wall face but to observe the top, bottom, ends, back, drainage facilities, adjacent slopes, maintenance access paths, and even the mounting racks and cable routes. Problems at a solar power plant are not isolated to a single location; they can affect other places via the direction of rainwater flow or slight changes in the ground. If you use drone surveying, it is important to record the surroundings of the retaining wall from a wide perspective and manage the data so it can be compared later.

Check 1: Examine the relative positions of retaining walls and the overall shape

First, what you want to confirm is where the retaining wall is located within the overall power plant and how it relates to the surrounding equipment and topography. A retaining wall is not a standalone structure; it is connected to the developed surface above, access paths and drainage channels below, adjacent slopes, rows of mounting racks, fences, and maintenance roads. With drone surveying, aerial images make it easier to verify the retaining wall’s alignment and end positions, bends, steps, and spatial relationships to nearby equipment.

When inspecting the overall shape of a retaining wall, first check whether parts that should have been built straight show any unnatural bending or misalignment. If a clear change is visible in the image, determining whether it is the as-built shape or a change that occurred during operation requires comparison with past data. If images from immediately after construction or from the previous inspection are available, overlaying them with data covering the same area makes it easier to determine whether any change has occurred.

In drone surveys, the area around the top (crest) of retaining walls is also a target for inspection. If signs such as steps, settlement, disturbances in pavement or ground surface, or gaps are observed near the crest, they may be related to rainwater inflow or movement of the backfill/ground behind the wall. However, because depth and causes cannot be determined from images alone, it is realistic to first identify locations where abnormalities are suspected and then perform close-up on-site inspections. The role of drones is to pick up areas of concern across a wide area and improve inspection efficiency.

The ends of retaining walls are also important. The ends are often connected to the natural ground, adjacent slopes, drainage facilities, and walkways, and they tend to be locations where water flow and sediment movement concentrate. From above, you can determine whether sediment has accumulated around the wall ends, whether vegetation is locally overgrown, or whether the shape encourages water to collect. Because the condition of the ends is more easily overlooked than the wall face itself, it is advisable to include them in the coverage during flight planning.

Also, verify the positional relationship between the retaining wall and the panel rows. If mounting frames, foundations, cable racks, junction boxes, or maintenance walkways are located near the retaining wall, changes in the ground can affect the maintainability of the equipment. When equipment is close to the retaining wall, it can be difficult to secure footing for inspection work or there may be insufficient workspace for repairs. Keeping bird’s-eye images from drone surveys can also be useful for future repair planning and for reviewing inspection routes.

What is important in this inspection is not to view the retaining wall’s shape in isolation, but to compare it with the power plant’s overall layout plan and past records. Even if an image shows something that looks odd, it is not necessarily a problem if that is the original design shape. On the other hand, if you observe soil protrusions that were not present before or disturbances near the top of the wall, it is worth conducting an on-site inspection promptly. By accumulating records from drone surveys, changes around the retaining wall become easier to treat as comparative material rather than as impressions.

Check 2: Inspect drainage routes and locations where puddles form

What should be given particular attention during inspections around retaining walls is the flow of rainwater. Because solar power plants cover a large area, depending on the direction in which rainwater gathers and its flow velocity, slope erosion, clogging of drainage channels, concentration of water behind the retaining wall, and muddy paths can occur. Even if the retaining wall itself shows no major deformation, poor drainage around it can, over time, affect the ground and the facilities.

In drone surveying, you can broadly observe drainage paths from above. If you capture images shortly after rain, it becomes easier to see where puddles remain, where wet ground continues in strips, traces of sediment flow, and the extent of deposition around drainage ditches and catch basins. It isn’t always possible to fly after rain, but even in dry images you can sometimes infer areas prone to water accumulation from low-lying terrain, variations in vegetation density, and differences in soil color.

When water accumulates on the top of a retaining wall, it can easily affect the condition of the ground behind it. For example, puddles remaining near the top of the wall, water that should flow to a drainage channel becoming stalled partway, or drainage outlets being obscured by sediment or fallen leaves—these are points that should be carefully noted and recorded. There are limits to what can be confirmed from images, but by identifying where water is concentrating over a wide area, you can narrow down the locations to prioritize during on-site inspections.

Don't overlook the lower portion of the retaining wall. If water continues to collect beneath the wall face, it can lead to muddy walkways, scour or washout, and the loss of soil around the foundation. If there is a drainage channel at the base, visually check that sediment and vegetation are not accumulating around the channel and that a clear path exists for water to drain to the outlet. Pay particular attention around the power plant's perimeter and on the valley side, where inflow from off-site or outflow off-site may occur.

The timing of imaging is also important when checking water flow. Images taken only during dry seasons can make it difficult to see rainwater-related problems. On the other hand, immediately after heavy rain there may be constraints on site safety and flight conditions. Therefore, it is practical to keep baseline images from normal conditions and, as needed, perform additional imaging after rain or after a typhoon has passed. Comparing images from normal conditions and after rain makes it easier to identify water channels and deposition areas that are not usually visible.

When checking drainage routes, merely looking for puddles is insufficient. It is important to clarify where the water is coming from, where it is intended to exit, and how retaining walls, slopes, pathways, and facilities relate along that route. If you overlay the points to be checked onto the aerial images created by drone surveys and manage them, inspection personnel can verify from the same perspective even if the person in charge changes. Rainwater problems can often be resolved with minor cleaning or sediment removal if addressed early, but if left unaddressed they can affect a wide area. Around retaining walls, visualizing drainage becomes the starting point for maintenance and management.

Check 3: Inspect the backfill soil for signs of settlement or cracking

The ground behind a retaining wall is difficult to assess without an overhead view. During on-site inspections, checks are often carried out only partially from walkways or around equipment, so it is not easy to uniformly view long stretches along the retaining wall. Using drone surveying allows continuous observation of the ground at the top and rear of the retaining wall, making it easier to detect settlement, changes in level, cracks, soil loosening, and signs of concentrated rainwater.

Settlement of backfill ground does not necessarily appear as a large collapse in the early stages. It may manifest as slight undulations on the surface, streaky disturbances on pavement or crushed-stone surfaces, vegetation growing differently from the surrounding area, or drainage traces concentrated in certain spots. It can be difficult to judge from a single drone image, but regularly photographing the same area makes it easier to notice differences from earlier images.

If linear changes resembling cracks are visible on the rear face of a retaining wall, it is worth following up with an on-site inspection. In images, the actual width, depth, and continuity may not be accurately determined, but they can help pinpoint the location. Pay particular attention to and carefully record linear changes running parallel to the wall, steps or offsets near the top (crest), and groove-like traces where rainwater flows in. These do not necessarily indicate danger, but it is advisable to manage them as items to monitor for progression.

When checking the ground at the rear, comparing terrain data is also effective. If data obtained from drone surveys can capture trends in surface elevation, comparing it with past data makes it easier to identify local tendencies toward subsidence or heaving. However, because the appearance can change due to grass height, shooting conditions, processing methods, and the accuracy of control points, small differences should not be immediately taken as ground displacement. It is safer to treat comparison results as clues for carrying out on-site verification.

Changes in the ground behind a retaining wall also affect nearby facilities. If a maintenance access path is located close to the retaining wall, settlement or surface irregularities can interfere with vehicle passage and workers' ability to walk. If mounting foundations or cable routes are nearby, ground disturbance affects the ease of inspection and repair. Conducting drone surveys to obtain an overhead view makes it easier to grasp not only the condition of the retaining wall but also the impact on maintenance and operational routes within the power plant.

Grass, fallen leaves, and deposited soil can accumulate on the backfill ground, making changes to the surface difficult to see. If you take photos only during the growing season, it can be hard to verify the condition of the ground surface. When necessary, taking baseline images after weeding or during periods of low grass height makes later comparisons easier. At solar power plants, weed-control plans and inspection plans tend to be considered separately, but for checks around retaining walls, post-weeding visibility is directly linked to inspection accuracy.

The important point in this check is not to jump to conclusions based on visual anomalies. Changes seen in drone surveys can include shadows, vegetation, surface wetness or dryness, and differences in appearance due to the shooting angle. Therefore, organize the coordinates, photo numbers, and capture dates for any suspicious locations, and establish a process to verify them on site. By continuing to record, it becomes easier to determine whether the changes are merely temporary surface variations or changes that are progressing over time.

Check 4: Examine connections with slopes and sediment movement

The condition around a retaining wall is closely related to the adjacent slopes and the movement of soil and sediment. At solar power plants, slopes may be graded during site development to secure panel installation surfaces and maintenance access routes. Near locations with retaining walls there are often cut or fill slopes, drainage channels, changes in level, and perimeter fences, and rainwater and sediment move across and over them. Even if you look closely at the retaining wall alone, it can be difficult to notice that soil is flowing from upstream slopes or that scour is advancing downstream.

The strength of drone surveying is that it allows retaining walls and slopes to be viewed as a single, continuous terrain. From above, it becomes easier to identify sediment flow lines, denuded areas on slopes, spots where vegetation has thinned, valley-like places where rainwater collects, and sediment deposited around drainage channels. Especially after heavy rain or typhoons, small changes on the slope can affect the areas around retaining walls. Early detection makes it easier to guide decisions on cleaning, repairs, and enhanced inspections.

If soil is flowing from the slope toward a retaining wall, drainage channels or side ditches can become clogged, causing rainwater to flow in unintended directions. As a result, water may tend to accumulate at the back or ends of the retaining wall. Conversely, if it appears that soil is being washed out from the base of the retaining wall toward the outside, it is necessary to check for scour at the base or poor drainage. When you find soil movement in images, it is important to organize the source, flow direction, and deposition locations as a continuous sequence.

Changes in vegetation can also provide important clues. Conditions such as grass on a slope being locally flattened, colors differing from the surrounding area, expanding bare ground, or trees and weeds growing densely near a retaining wall can be related to water flow or the accumulation of sediment. Of course, you cannot determine ground problems solely from differences in vegetation, but they provide material for identifying locations that should be checked during inspections. Comparing drone images season by season makes it easier to separate the effects of vegetation from changes in topography.

Near retaining walls, small slope failures and the buildup of earth and sand may not immediately affect power generation equipment. However, they can lead to indirect problems such as access paths being blocked, drainage channels becoming clogged, sediment accumulating along fences, and reduced safety during vegetation-cutting operations. In the maintenance of solar power plants, it is important to identify early not only malfunctions that directly cause power outages but also factors that could make future maintenance work more difficult.

When inspecting slopes and soil movement, it’s important not to limit the photography area to just around the retaining wall. Including the upstream and downstream sides of the retaining wall, the left and right ends, and the outlet of the drainage flow makes it easier to trace cause-and-effect relationships. Even if you zoom in and photograph only the spot where a problem occurred, you may not be able to determine why sediment accumulated there or where the water came from. In drone surveying, combining wide-area overhead views with detailed shots of key locations increases the value of the inspection records.

Additionally, the results of sediment movement checks are useful for prioritizing field work. Deposits that immediately block passages, sediment clogging drainage channels, and flow traces that are directing water toward retaining walls should be considered for prompt intervention. Conversely, minor surface changes can be managed as items to be monitored at the next inspection. By using drone survey results to sort issues by urgency in this way, it becomes easier to allocate limited personnel and time efficiently.

Check 5: Assess the impact on inspection access routes and equipment

Changes around retaining walls affect not only the power generation equipment itself but also the ease of inspection paths and maintenance work. At solar power plants, various tasks such as patrols, weed control, electrical equipment inspections, checks of mounting structures, and cleaning of drainage channels are carried out regularly. If the ground around retaining walls becomes disturbed, paths narrow, or puddles remain, they can impede the movement of workers and vehicles. Drone surveying can provide an overhead view to identify such issues along circulation routes.

First, what you want to check is whether the maintenance path near the retaining wall is in a condition that is safe to pass. From the image you can confirm whether the path’s width has been narrowed by soil or grass, whether puddles or mud remain on the surface, and whether collapsed soil has flowed onto the path. Rather than looking at only a part of the path, viewing it as a continuous section along the retaining wall makes it easier to grasp problems across the entire inspection route.

Next, check the relationship with equipment such as mounting racks, foundations, cable routes, and junction boxes. If topographical changes around retaining walls are approaching the equipment, it does not necessarily mean equipment failure will occur immediately, but it is worth recording as locations to pay attention to during inspections. For example, situations where sediment has flowed up close to rows of mounting racks, drainage tends to collect near cable routes, or vegetation is proliferating between the retaining wall and the equipment should be checked on site.

Fences and perimeter areas are also subject to inspection. When a retaining wall is close to the perimeter, movement of soil or concentrated drainage can cause deposits to accumulate along the fence, or water from outside can flow into the site. By including the perimeter in drone surveys, you can see terrain connections that are not apparent from inside the power plant alone. The perimeter requires long walking distances during patrols and is prone to oversights, so verification using overhead images is helpful.

In terms of maintenance work, it is also important how close work vehicles and materials can get when repairs are needed. Even if signs of distress are found around a retaining wall, conditions such as narrow access routes, steep slopes, muddy ground, or nearby equipment can delay the response. Using images from drone surveys makes it easier to assess workspaces and access routes before going to the site. This is useful not only in emergencies but also when planning scheduled repairs and preventive maintenance.

When checking the impact on inspection paths and equipment, it is important not just to look for abnormalities but also to consider future accessibility for maintenance. Even if an area is currently passable, places where paths become hard to see during the growing season, spots that tend to get muddy after rain, or areas where sediment gradually accumulates will increase the maintenance burden. By using drone surveys to record the same area regularly, you can detect declines in accessibility early and more easily reflect measures such as weeding, cleaning, and drainage improvements in your plans.

The point to keep in mind in this check is to view retaining walls not only as civil engineering structures but as part of power plant operations. To preserve a power plant’s value, in addition to ensuring that equipment continues to generate power, it is necessary to maintain an environment that allows safe inspections. Small changes around retaining walls can make routine inspections more difficult, increase work time, and impose constraints during repairs. Using drone surveys to confirm conditions from a broader perspective makes it easier to consider equipment management and civil engineering management together rather than separately.

Precautions When Inspecting Areas Around Retaining Walls with Drone Surveying

When inspecting the area around a retaining wall using drone surveying, it is important to organize the imaging plan in advance. Retaining walls are often long structures, and if you include the top, bottom, ends, backside, and surrounding drainage, the inspection area becomes wide. If you capture images haphazardly, you may find that necessary parts were not photographed or that you end up with images taken from angles that are difficult to compare later. Organizing the areas you want to check in advance using drawings and past patrol records, and deciding on the flight route and imaging coverage, makes the process more efficient.

To continuously compare the same location, it is important to keep shooting conditions as consistent as possible. If flight altitude, image overlap, shooting time, weather, or the condition of the grass differ significantly, the appearance in the images will change, making it difficult to judge changes. Although it is not possible to make conditions exactly the same, deciding on a standard shooting method makes comparison with past data easier. In particular, times with strong shadows or periods when grass is tall can make the condition of the ground surface and retaining walls hard to see.

Safety considerations are also necessary. Areas around retaining walls have changes in elevation, locations where wind flow is easily altered, and places where obstacles such as trees, power lines, fences, and mounting frames are nearby. Before flight, you must check surrounding obstacles, takeoff and landing sites, the positions of operators, and whether any third parties are present, and ensure conditions that allow safe flight. It is also essential to confirm flight-related conditions relevant to the site—such as the Aviation Law, the Act on Prohibition of Flight of Small Unmanned Aerial Vehicles, etc., and the rules of municipalities and facility managers—and to complete any necessary procedures and safety management. The purpose is not to fly a drone for its own sake, but to obtain useful data safely.

When using it as survey data, the concept of positional accuracy should also be clarified. When managing changes around retaining walls on a map or comparing them with past data, not only the captured images but also reference positional information and on-site checkpoints are important. For applications that require high accuracy, appropriate establishment of control points, selection of positioning methods, and verification after processing are indispensable. On the other hand, when used as an aid for routine patrols, it can be effective to roughly identify the locations of anomalies. It is important to determine the level of accuracy required according to the purpose.

Care is also required when interpreting images and terrain data. Shadows visible in drone images, flattened grass, differences in soil color, and traces of puddles can be clues that suggest the possibility of an anomaly, but these alone cannot determine the cause or the level of risk. If you discover an area of concern, it is safer to carry out a close on-site inspection and, when necessary, obtain an expert’s assessment. Drone surveying is a means of increasing the information available for decision-making and does not replace on-site verification or confirmation of design conditions.

The method of record-keeping is also important. If you organize the date the photos were taken, the weather, the scope of the photos, any abnormalities confirmed, the results of on-site inspections, and the history of responses, it will be easier to compare them at the next inspection. Even if you only save images, their value as maintenance management documentation falls if it becomes unclear where they were taken and for what purpose. Organize records by retaining wall, by section, and by drainage system, and make them shareable with patrol personnel and managers so they are easy to use for ongoing management.

At power plants, the type and scale of retaining walls, surrounding topography, drainage plans, and inspection frequency can vary greatly. Therefore, rather than mechanically applying the same checklist to every site, it is necessary to translate the items into inspection points tailored to your company's power plants. For example, plants located on steep slopes should emphasize soil movement and drainage, while plants on nearly flat terrain should emphasize puddles and muddy access paths; priorities change according to site conditions. Drone surveying allows the survey area to be set flexibly, making it an easily applicable method for addressing site-specific risks.

Summary

When inspecting the retaining walls around a solar power plant using drone surveying, it is important to view not only the retaining wall itself but also the surrounding terrain, drainage, the ground behind the wall, slopes, soil movement, inspection access routes, and their relationship with equipment as a whole. A retaining wall is not a component of the power generation equipment itself, but it is an important factor affecting site stability and the ease of maintenance work. By utilizing aerial overview images and survey data, you can more easily organize changes over a wide area that are difficult to grasp by on-foot inspections alone.

In Check 1, confirm the positional relationships and overall shape of the retaining wall. It is important to grasp the retaining wall’s direction of extension, its ends, the top edge, and the distances to surrounding equipment, and to record them in a way that allows comparison with past data. In Check 2, inspect the drainage routes and locations where puddles form. By taking an overview of where rainwater comes from and where it drains, you can more easily detect early signs of poor drainage or sediment accumulation.

Check 3 involves confirming signs of settlement and cracking in the ground behind the structure. Slight undulations or linear changes should not be judged from images alone, but should be cataloged as items for on-site verification. Check 4 examines the connection to slopes and soil movement. Problems around retaining walls can originate from distant slopes or drainage channels, so it is important to trace causes and effects with wide-area photography. Check 5 confirms impacts on inspection access routes and equipment. Understanding passability, workspace, and proximity to equipment makes it easier to plan maintenance and management.

Drone surveys are an effective means of visualizing conditions around retaining walls and organizing inspection priorities. However, it is important not to determine structural safety based solely on images and terrain data, but to use them in combination with on-site verification and professional judgment. Continuously imaging the same area and keeping track of imaging dates, verification results, and response histories makes it easier to track long-term changes in the power plant's condition.

Areas around retaining walls at solar power plants tend to be deprioritized during routine inspections, yet they are places where problems related to rainwater, ground conditions, sediment, and access paths readily accumulate. By utilizing drone surveying to detect changes early and linking that information to on-site inspections and repair planning, it becomes easier to stabilize the overall operation and maintenance of the plant. If you want to streamline inspections and surveys of solar power plants, including areas around retaining walls, it is important to consider implementation from both safety and practicality perspectives, while establishing surveying methods, flight plans, and record-management systems tailored to site conditions.