Five indicators for detecting signs of ground cracking at solar power plants using drone surveys

At solar power plants, it is important not only to inspect the power generation equipment itself but also to continuously monitor changes to the site's ground and developed surfaces. In particular, signs of ground cracking can lead to slope failure, concentration of rainwater, subsidence, poor drainage, and deformation around racking foundations. Walking the site for visual checks is necessary, but to compare the entire plant under the same conditions, aerial records captured by drone surveys are useful.

Table of Contents

• The significance of confirming signs of ground fissures through drone surveying

• Don't miss linear changes on the Earth's surface

• Confirm the extent of settlement and level differences.

• Overlay the flow of rainwater and erosion traces

• Check for abnormalities in the mounting racks and around cables.

• Organize indicators based on differences from past data

• Precautions when using drone surveys to inspect ground fissures

• Summary

The significance of using drone surveying to confirm signs of ground fissures

Solar power plants are installed on a variety of land types, such as forested areas, developed land, fallow land, sloping terrain, landfill sites, and locations near valley topography. Because the sites are large and equipment is arranged at regular intervals, small changes in the ground surface can be difficult to detect through on-site inspections alone. In particular, areas under the panels, between rows of mounting racks, alongside fences, around drainage channels, and at the top and bottom edges of slopes are hard to check with a normal line of sight.

Ground fissures do not necessarily appear suddenly or on a large scale. At first they may show up as thin linear cracks, openings in the soil, disturbances in vegetation, the beginning of a step, or traces where rainwater has collected. If such changes are noticed at that stage, on-site inspections, reviewing drainage, considering the scope of repairs, and explaining the situation to clients or managers become easier. On the other hand, if detection is delayed, cracks may widen after rainfall, concerns may arise about the stability of slopes or developed surfaces, and it may become necessary to consider impacts on equipment.

The advantage of drone surveying is that it allows you to record the entire site from an overhead perspective. By using images taken from above and the orthophotos, point clouds, and three-dimensional models created from them, it becomes easier to confirm linear changes and topographic irregularities that are easily overlooked on site. Rather than viewing only specific locations as points, you can clarify where deformations are occurring across the entire site and whether they are related to drainage routes or slopes.

However, the safety of the ground cannot be determined by drone surveys alone. Features that appear to be cracks in images may instead be shadows, flattened vegetation, ruts from construction vehicles, surface cracking caused by drying, or traces from construction work. Conversely, vegetation or the shadows of panels can obscure the ground surface and prevent actual cracks from being captured. For these reasons, drone surveys should not be used as a substitute for on-site investigations; they should be used to prioritize on-site inspections and to objectify records.

When looking for signs of ground cracking at a solar power plant, simply taking photographs is not sufficient. If shooting coverage, flight altitude, overlap, weather, sunlight conditions, the handling of reference points, and the method for comparing with previous data are not standardized, it becomes difficult to tell whether differences are actual changes or merely variations in shooting conditions. Especially in plant maintenance, where the same locations are often inspected repeatedly, it is important to create data from the first survey with future comparisons in mind.

Don't miss linear changes on the Earth's surface

The first thing to check when looking for signs of ground cracking is any linear changes that appear on the ground surface. Cracks may extend as long, straight lines or may appear as a series of short fractures. At the edges of the developed surface, at the boundary between fill and cut, at slope shoulders, at slope toes, around drainage channels, near vehicle traffic routes, and along rows of supports, linear changes can appear where the surface is weak or where water tends to collect.

In drone surveying, orthophotos allow you to view the ground surface from above in plan view. With standard oblique photos, the appearance of panels and mounting racks can make it difficult to pinpoint locations, but orthophotos can be overlaid with site maps and equipment layout plans to make positional confirmation easier. If a line that appears to be a ground crack is found, it also becomes easier to explain between which rows of racks it lies, how far it is from the drainage channel, and its positional relationship to fences and slopes.

When checking linear changes, it is important not to judge based only on how the image looks. Lines where the soil color has changed, lines where grass is flattened, vehicle tire tracks, smoothing marks from construction, rills where rainwater has flowed, and shadow boundaries can all appear similar to cracks in images. Especially at solar power plants, because panel shadows fall in a regular pattern, the appearance of the ground can change significantly depending on the time of day. In the low sun of morning and evening, shadows become longer, and thin linear shadows can look like ground fissures.

Therefore, when taking images, choose conditions that make the ground surface as visible as possible. Times of day when shadows are too long, or conditions immediately after rain when there are many puddles and reflections, can make it difficult to discern linear changes on the ground surface. Conversely, when the surface layer is overly dry, fine cracks increase and it can become difficult to distinguish them from cracks that require management attention. If the purpose is to track signs of ground cracking, keep the shooting conditions consistent each time and record the shooting date, weather, recent rainfall, and the local soil condition, as this will make later judgment easier.

Also, linear changes should be noted not only for their length but also for their orientation. Cracks that run laterally across a slope, cracks that follow drainage paths, lines that continue parallel to rows of mounting racks, and lines that run along fences each suggest different causes. Aerial imagery makes it easier to confirm overall orientation, so it is important to place such features in the context of the site’s overall topography and grading rather than viewing them as isolated cracks.

When recording linear changes on the ground surface, it is desirable not only to mark the discovered locations on images but also to link them with the results of field inspections. For example, confirm on site the locations extracted from images as crack candidates and record whether they are actually open, merely surface drying, or exhibiting changes in depth or width. By correlating drone survey results with on-site photographs, you can present the aerial positional relationships together with the ground conditions when explaining to clients or managers.

Check the extent of settlement and level differences

Ground cracks can appear not only as lines on the ground surface but also in combination with subsidence or offsets. Especially at solar power plants, localized settlement can occur due to factors such as the post-construction compaction state, infiltration of rainwater, uneven drainage, changes in topography, and the settling of fill areas. Even when there are cracks on the surface without an offset, and when one side has dropped along a crack, the priority for on-site inspection differs.

Using drone surveys to create point clouds and elevation data makes it easier to identify variations in ground elevation. Although panels and mounting structures can hide portions of the ground surface, within areas with line of sight — between rows of mounts, along walkways, on slopes, around drainage channels, and alongside fences — you can discern terrain trends. In particular, if you survey the same location regularly, it becomes easier to see which areas have become lower compared with the previous survey and whether any steps or offsets have widened.

When assessing settlement or level differences, it is important not to make judgments based on a single elevation value. Elevation data from drone surveys are influenced by imaging conditions, the placement of control points, analysis methods, how the ground surface appears, and the presence or absence of vegetation. Even if numerical differences appear, you need to confirm whether they represent actual terrain changes or are due to differences in surveying or processing conditions. For comparison, it is important to capture data under as similar conditions as possible and to produce results using the same references.

Signs of subsidence appear not only as ground cracks but also as puddles, accumulations of mud, patterns in vegetation growth, gaps around mounting foundations, depressions in walkways, and similar features. In aerial imagery they can show up as differences in surface color or as areas where water remains. If, after rainfall, water remains only in specific spots, there may be a subtle depression at that location. Overlaying the positions of such puddles with candidate ground cracks makes it easier to judge whether the surface changes are merely superficial cracks or involve changes in topography.

When checking for level differences, on-site safety checks are also essential. After identifying locations likely to have level differences from images and point clouds, on-site inspections proceed while confirming footing safety, distances to racking and electrical equipment, access routes, and whether entry onto slopes is permitted. At solar power plants, equipment is densely arranged, so forcing entry under panels can be dangerous. By narrowing down the locations of anomalies in advance using drone surveying, it becomes easier to plan on-site inspection routes.

When explaining the extent of settlement and step-like offsets, it is useful to indicate not only the location of ground cracks but also their relationship to the surrounding terrain. For example, whether the changes are continuous from the top of the slope down to the walkway, concentrated near drainage channels, or aligned along a row of support-structure foundations will affect which causes and countermeasures should be considered. Using results from drone surveys makes it easier to organize the extent and spatial relationships that are difficult to convey with site photographs alone.

Overlaying Rainwater Flow and Erosion Traces

When checking for signs of ground cracking, the flow of rainwater is an important consideration. Areas of the surface with cracks can allow rainwater to penetrate more easily. Also, where rainwater concentrates, topsoil can be washed away, forming narrow rills or scours, and cracks may develop around those areas. At solar power plants, rainwater falling from panel surfaces, the gradient of walkways, the capacity of drainage channels, slope treatment, and the grade of the developed surface are all interrelated in complex ways, so examining ground cracks alone can sometimes make it difficult to determine the cause.

Viewing a site from above with drone surveying makes it easier to identify the directions in which rainwater is likely to flow. In an orthophoto you can observe traces of soil runoff, mud streaks, gravel displacement, disturbed vegetation, and marks left by puddles. Using three-dimensional data, you can determine the terrain’s slopes and low-lying areas and consider where rainwater is likely to accumulate. By overlaying this information with the locations of potential ground cracks, it becomes easier to see whether the deformations are related to water flow.

Pay particular attention to the area near the lower edge of the panel rows. If rainwater that hits the panels continues to fall repeatedly at the same spot, it can erode the ground or wash soil away. In locations where ground protection is insufficient, narrow channels can form along the line where the water falls, and cracks or step-like offsets may develop around them. In drone aerial images, these may appear as color differences or gully-like changes running along the panel rows.

The areas around drainage channels are also important. Conditions such as blocked drainage channels, insufficient slope causing water to stagnate, overflow at the outlet, or water flowing directly onto slopes can lead to surface erosion or ground cracking. Drone surveys can broadly check the continuity of drainage channels, sediment deposition, discoloration of surrounding ground surfaces, and water paths onto slopes. However, because the interiors of drainage channels and dark areas may not be visible from aerial images alone, locations where anomalies are suspected in images need to be inspected on site.

When observing the flow of rainwater, it is effective to use surveys conducted in clear weather and inspections carried out after rainfall. Clear weather makes it easier to see ground surface shapes and linear changes, while after rainfall it becomes easier to identify areas where water remains and traces of flow. However, flying during rain or strong winds poses safety issues, so operations should be carried out after the weather has stabilized. For post-rain inspections, recording when the rain occurred, how heavy it was, and how much time had passed at the time of imaging will help with later assessments.

When assessing the relationship between signs of ground cracking and rainwater, rather than simply recording "there is a crack," record the surrounding conditions together—for example, "a flow path upslope of the crack," "sediment accumulating near a drainage channel," "mud streaks at the bottom of the slope," and "topsoil washing along the lower edge of the panel." Doing so makes it easier when considering repairs or improvements to determine whether it is sufficient to fill the crack itself or whether the water flow needs to be redirected.

Check for abnormalities around mounting racks and cables

When checking for signs of ground cracking at a solar power plant, it is necessary to inspect not only the ground surface but also the effects around the racking and cables. The management significance differs depending on whether a fissure is located away from equipment or near racking foundations, support posts, cable routes, junction boxes, or inspection walkways. Ground deformation around equipment can lead to foundation tilting, voids around support posts, increased tension on cables, exposure of protective conduits, and heightened passage risks during inspections.

Drone surveying allows you to verify the positional relationship between the overall equipment layout of a power plant and potential ground fissures. By using orthophotos, you can identify which row of mounting racks the cracks lie between, how close they are to walkways, and whether they intersect cable routes. During on-site inspections, attention tends to focus on visible anomalies right in front of you, but viewing from above makes it easier to confirm their position within the equipment layout.

Around the mounting structure, what you should watch for are changes to the ground surface at the bases of the support posts and around the foundations. Conditions such as gaps forming around posts, soil subsiding on only one side, water pooling, or the ground cracking along rows of posts should be given high priority for on-site inspection. Drone imagery can make it difficult to detect fine gaps, but differences in ground color or trends in step changes along the rows of posts can sometimes be identified. A practical workflow is to extract candidates from aerial images and then perform close-up ground verification where necessary.

Around cables, check whether buried sections or protective parts have been exposed by ground cracks or erosion, and whether there is any soil runoff around the wiring route. Cables are placed differently depending on the site—underground, near the ground surface, under racks, inside conduits, etc. While drone surveys cannot directly confirm the condition of every cable, overlaying the locations managed as cable routes with candidate ground-crack locations allows you to narrow down the areas that require on-site inspection.

Also, ground cracks near inspection paths and vehicle traffic routes should not be overlooked. Even small cracks can widen due to the passage of inspection vehicles or workers on foot. At the edges of paths, in areas prone to becoming muddy, where crushed stone is shifting, or where ruts are deepening, not only surface cracking but also subgrade settlement and poor drainage should be suspected. A drone survey that provides an overview of the entire path can reveal not only localized deterioration but also overall trends in drainage and gradient.

When recording anomalies around equipment, you must consider the operation and safety of the power generation equipment. Do not make judgments based solely on what is visible from above; if it is necessary to approach electrical equipment, carry out the inspection in accordance with the facility’s management rules. The closer a ground crack is to the equipment, the more important it is not to have the surveying personnel handle it alone; information should be shared with relevant parties such as maintenance staff, civil engineering personnel, and electrical personnel.

Organize indicators by differences from historical data

When managing signs of ground fissures, what’s important is not a one-time inspection but observing changes over time. Even if an image at a single point in time shows a line that looks like a crack, without historical data it’s difficult to tell whether it was present before, formed recently, or is spreading. If drone surveys are carried out regularly, you can compare the same location and determine whether changes have occurred.

When comparing with past data, it is first important to match the acquisition conditions as closely as possible. If flight altitude, image overlap, flight extent, shooting direction, control points, the coordinate system of the outputs, or data processing methods differ significantly, it becomes difficult to determine whether observed differences are actual changes or merely differences in the data-creation conditions. This is especially true when tracking narrow changes such as ground cracks: large positional shifts between the previous and current data reduce the reliability of the comparison.

Products used for comparison include orthophotos, point clouds, elevation data, and three-dimensional models. Orthophotos make it easier to compare linear changes on the ground and color differences. Elevation data makes it easier to confirm the extent of subsidence and steps or level differences. Point clouds and 3D models make it easier to grasp shape changes on slopes and around drainage channels. Depending on the objective, it is important to combine visual comparisons and height comparisons.

When examining differences, it is necessary to avoid drawing overly definitive conclusions about changes. For example, a crack may appear to have newly formed if the appearance of the ground changes before and after mowing. Conversely, if grass grows and covers the ground, a crack that was previously visible may appear to have disappeared. Moisture content, sunlight, shadows, soil color, and the camera angle can also alter how features appear in images. Therefore, rather than treating the difference results as facts, judgments should be made in conjunction with on-site verification.

When documenting signs of ground cracking, it is useful to keep a record of changes over time. Recording the discovery date, photo date, location, type of deformation, approximate length and width, surrounding facilities, relationship with rainwater, results of on-site inspections, and response status makes it easier to track progress later. Even if you believe you are inspecting the same location on each patrol, judgments can vary when the person in charge changes. Linking drone survey results with the record sheets makes it easier to reduce subjective, person-dependent judgments.

Differences from past data are also useful when explaining things to clients and managers. Simply reporting that there are ground cracks may not convey the urgency or the scope of the response. If you can explain it in terms such as “linear changes not seen in the previous survey were observed this time,” “the water flow path has become longer than before,” or “the extent of settlement has expanded,” it becomes easier to share the need for action. Conversely, being able to show that there has been no significant change since the previous survey can serve as a basis for deciding to continue monitoring.

Precautions When Using Drone Surveys to Inspect Ground Fissures

Drone surveying is a convenient method, but there are several precautions when checking for signs of ground cracking. First, do not determine ground stability or hazard level solely from images or three-dimensional data obtained by a drone. Ground cracks may be only a surface phenomenon, or they may be related to changes within the ground or the effects of water. What can be confirmed from images is mainly the surface condition, and, as necessary, specialized on-site investigations, surveys, and geotechnical inspections should be combined.

Next, thorough safety management is required at solar power plants. When operating drones, verify the locations of power generation equipment, transmission equipment, mounting racks, fences, surrounding roads, neighboring facilities, and workers, and conduct operations in accordance with the facility’s management rules. Even if the flight area is large, do not try to capture everything in a single flight; plan within a range that can be safely managed. In strong winds, bad weather, or poor visibility, not only survey accuracy but also safety will be affected.

In the imaging plan, we check in advance whether the ground surface will be visible. Because solar power plants have panels widely installed, the ground beneath the panels may be difficult to see from above. The area that can be confirmed varies depending on the spacing between mounting rows, panel height, grass growth, and how shadows fall. If locations with a high likelihood of ground cracking are under panels or in the shade of equipment, we use not only aerial imagery but also ground photographs and close-up inspections.

The influence of grass and debris is also an important consideration. When grass is overgrown, cracks and unevenness on the ground surface can be hidden. Conversely, immediately after mowing the ground surface becomes easier to see, but the swaths of cut grass and the tracks left by the work can sometimes look like cracks. If the goal is to continuously monitor ground cracks, recording the timing and condition of mowing can help reduce misinterpretation when comparing data.

Also, the results of drone surveys need to be managed with attention to positional accuracy and reproducibility. Even if a potential ground fissure is discovered, continued management becomes difficult if the same location cannot be checked next time. It is important to use control points and markers appropriately, and to organize how result data are stored, file names, capture dates, coverage, responsible personnel, and survey conditions. As data increase, the time spent searching past results and the burden of comparison work grow, so establishing management rules from the early stages makes them easier to handle in practice.

Furthermore, in reports and inspection records, clearly categorize any observed anomalies. Mixing items such as suspected ground cracks, signs of erosion, suspected settlement, suspected drainage problems, and inspection points around equipment makes it difficult for readers to assess. When marking images, avoid treating everything the same; distinguish whether an item has been confirmed on site, is only a candidate identified in the image, or is under observation, as this makes follow-up actions easier.

Summary

To check for signs of ground fissures at a solar power plant, it's important not to look only at cracks on the ground surface, but to comprehensively assess the site's overall topography, rainwater flow, subsidence and unevenness, impacts around racking and cables, and differences compared with past data. Using drone surveying allows you to record a large plant from above and more easily confirm positional relationships and the extent of changes that are difficult to grasp from on-site inspections alone.

The five items for observing signs of ground cracking are: first, do not overlook linear changes; next, check for subsidence and steps; furthermore, it is important to cross-check rainwater flow and traces of erosion. In addition, verify impacts around mounting racks and cables, and organize the history of changes by comparing with past data. By building these up one by one, you produce survey results that are useful for maintenance management and explanations to clients, rather than mere photographic records.

On the other hand, it is necessary to avoid determining the safety of the ground based solely on drone surveys. Lines visible in images are not necessarily ground cracks; shadows, grass, vehicle tracks, surface soil drying, and construction marks can produce misleading appearances. Conversely, cracks hidden beneath panels or vegetation can be difficult to see from above. Therefore, a practical approach is to use drone surveys to extract candidate locations and combine them with on-site inspections to make a judgment.

At solar power plants, detecting early signs of ground cracking makes it easier to review drainage, consider the scope of repairs, verify safety around equipment, and plan ongoing monitoring. By conducting drone surveys regularly under the same conditions and accumulating orthophotos and three-dimensional data, it becomes easier to identify where deformations have occurred and to describe their progression. If you want to efficiently detect signs of cracking, settlement, or poor drainage in site maintenance, it is important to establish a system that can integrate flight planning, on-site inspections, record organization, and reporting to stakeholders.