Six Basic Items to Consider for Slope Countermeasures in Solar Power Plant Construction

In the construction of solar power plants, attention tends to focus on panel and racking layout, pile and foundation accuracy, drainage planning, and access road preparation, while slope protection measures that determine the overall stability of the site are often postponed. However, in practice, if slope planning is inadequate, various problems can arise both during construction and after completion, such as erosion by rainwater, shallow surface slips, sediment runoff, reduced operability of construction equipment, and worsened safety during maintenance and inspections.

In particular, solar power plants often involve earthmoving and site leveling over large areas, and are frequently planned not only on flat land but also on hilly or sloping terrain. Therefore, both cut slopes and fill slopes may occur, and measures must be developed according to the varying soil types and groundwater conditions at each location. Furthermore, because solar power plants are facilities that operate for long periods after completion, slope planning must look beyond construction and extend to the maintenance stage; otherwise future repair burdens and operational risks will increase.

In on-site practice, it is important to consider slope protection measures not as a standalone civil-engineering issue but in conjunction with site development planning, drainage planning, equipment layout, construction logistics, safety management, and maintenance. Stabilizing slopes is not simply about preventing collapse. It forms the foundation that supports the overall quality of the site, including preventing work stoppages, avoiding rework, protecting equipment, minimizing impacts on the surrounding environment, and ensuring stable long-term operation.

This article organizes and explains the six key items that field personnel involved in solar power plant construction should keep in mind when considering slope countermeasures, from the perspectives of construction planning and on-site operations. By understanding what needs to be checked during the design phase, what should be finalized before construction, and what kinds of oversights are likely to occur on site, you can prevent problems caused by slopes.

Table of Contents

• Why slope countermeasures are important in solar power plant construction

• Basic Item 1 Accurately ascertain the topography and the extent of site development

• Basic Item 2 Assess soil and ground conditions and the influence of groundwater

• Basic Item 3 Consider stormwater management and drainage planning together with slope design

• Basic Item 4 Set slope gradients and select slope protection works according to site conditions

• Basic Item 5 Finalize construction safety management and temporary works planning in advance

• Basic Item 6 Plan with post-construction inspection and maintenance in mind

• Approach to linking slope countermeasures to construction quality

Why slope protection measures are important in solar power plant construction

In solar power plant sites, it is important not only to consider the panels' power generation efficiency and equipment layout but also to carry out construction that does not place undue stress on the ground and terrain. In particular, slopes are often artificial inclines newly formed as a result of land development work and tend to be less stable than the original natural terrain. On cut slopes, excavation alters the balance of the natural ground, and on fill slopes, inadequate compaction or drainage can cause settlement or sliding.

In solar power plants, because equipment is spread widely across the site, it is not uncommon for slope faces to be located close to the power generation equipment itself. When racking foundations, maintenance access paths, or collection equipment are placed near the crest or toe of a slope, slope destabilization can directly lead to equipment damage and reduced maintainability. Even a small collapse can cause muddy water to run off and make paths slick or allow soil and sediment to pile up around equipment, hindering patrols and inspections.

Another point that is easy to overlook is that slope protection measures are not finished once they are implemented. Solar power plants are exposed to wind and rain for long periods after completion, so surface erosion, changes in vegetation, and deterioration of drainage functions gradually progress. Even if the slopes look tidy at completion, early rainfall can cause fine scouring, and major repairs may be required several years later. Therefore, slope protection measures should be evaluated not by their appearance at the time of construction but by their durability over time.

Also, in recent years at construction sites, due to shortened schedules and labor shortages, slope treatment tends to be pushed to later stages. However, if site formation is followed by prioritizing equipment installation and slope measures are handled all at once at the end, surface runoff from rain and disruption of heavy equipment traffic routes can occur first, resulting in unnecessary repair work. Slopes should be stabilized concurrently with site formation and should not be left to the end of the construction schedule.

As such, slope protection measures are the foundation that affects safety, quality, construction processes, and maintenance. It is therefore necessary to take a comprehensive approach that considers site conditions and operational conditions, rather than focusing solely on the slope shown on the design drawings.

Basic Item 1 Accurately understand the terrain and the extent of site development

The starting point for slope protection measures is to accurately understand the site's topography. This may seem obvious, but in practice it is an area that tends to be surprisingly neglected. Because solar power plants occupy large sites, if you proceed with the overall plan by looking only at some representative cross-sections and major elevation differences, you may overlook local steep slopes, valley landforms, weathered sections along ridgelines, and interfaces with existing waterways.

In slope planning, it is necessary to decide early which areas will be cut, which will be filled, and which will be left as the existing ground. If this decision remains unclear and equipment layout is prioritized, impractical slopes may arise later, the amount of adjustment to mounting-frame heights may increase, and access-way gradients may become excessive. As a result, earthwork quantities increase and the overall construction becomes unstable.

What you should pay particular attention to is not to look at a slope in isolation but to consider it in relation to the entire development area. For example, even if the slope in a given lot appears stable at first glance, if there is terrain above that tends to collect water, during rainfall more surface water than expected can flow in. Also, if the tie-in with the finished-ground heights of adjacent lots is poor, water can easily accumulate at the slope shoulder and trigger a collapse. The stability of a slope is not determined by the shape of that slope alone. You cannot make a correct judgment without considering the surrounding topography and water flow.

A common occurrence on site is that, even though slopes appear gentle on the drawings, in reality the way small benches are formed and the treatment of the slope toe are inadequate, causing problems for heavy equipment operations and maintenance. When reviewing the cross‑section plans for grading, it is important to prioritize and verify constructability, drainage, and maintainability over how neat it looks. You must consider whether construction equipment can enter safely, whether there will be an unreasonable close approach near the slope shoulder, and whether people can walk safely during inspections.

When determining the extent of earthworks, it is also important to consider how much of the existing terrain to retain. If you try to make everything flat, cut-and-fill volumes increase and slopes proliferate, raising not only mitigation costs but also construction risks. At solar power plants, depending on equipment specifications and layout, a certain gradient may be acceptable, so rather than forcing large-scale grading, it can be more rational to plan in accordance with the terrain. The best way to reduce slope stabilization measures is to avoid creating excessive slopes in the first place.

The important thing here is to incorporate slopes as constraints from the early stages of site planning, rather than treating them as issues to be addressed later. The more accurately the terrain is understood, the fewer modifications will be needed in later stages, and slope countermeasures will be more realistic.

Basic Item 2: Assessing Soil and Ground Conditions and the Influence of Groundwater

One of the major factors controlling slope stability is soil properties and the condition of the natural ground. Even when constructed to the same gradient, a compact natural ground and a loose embankment will have completely different stability. Furthermore, even if the surface appears dry, the slope can be more unstable than it looks when there are layers that readily retain water internally or when weathered ground is present.

At solar power plant sites, soil conditions are not necessarily uniform across a wide area. Even if ridge areas are relatively stable, weak soils can remain in valleys or in backfilled portions of former topography. During the site investigation stage, it is important to verify not only the surface topography but also what kinds of layers are present, whether the material can be reused as fill material, and whether any soils prone to collapse or instability are present.

On cut slopes, fractures and weathered zones in the natural ground may appear after excavation. Even slopes that appeared stable before construction often reveal continuous weak layers the moment the excavation face is exposed. Therefore, rather than deciding slope gradients solely on desk-based assumptions, it is necessary to have a system in place to observe and make judgments during construction. Being prepared so that unexpected seepage, loose stones, or friable layers can prompt a rapid decision to change the gradient or revise reinforcement methods is an important point in site management.

In embankment slopes, the quality of materials and compaction control are particularly important. In power plant construction, on-site excavated soil is sometimes used to balance earthwork quantities, but forcing the use of soil with poor moisture conditions or a skewed particle-size distribution can lead to insufficient compaction and retention of seepage water, making the slope prone to loosening. Even if only the surface is shaped, if the interior is unstable, deformations will appear over time. Embankment slopes should be judged by the quality of their contents rather than by their as-built shape.

Another factor not to be overlooked is the influence of groundwater and springs. Many slope problems involve water. Even if surface water flow is not visible, in geological conditions where water tends to accumulate within the slope, soil strength decreases and can cause shallow surface failure and sliding. Even if only a small damp spot is observed during construction, conditions can rapidly worsen after rainfall. Therefore, it is important not to judge based solely on site inspections during dry conditions, but to be aware of and understand post-rainfall conditions and how water drains.

In practice, it is often thought that surface protection alone is sufficient to prevent slope failure, but when soil properties or groundwater conditions are poor, surface measures alone are not enough. If the reason a slope becomes unstable is not determined from the soil’s characteristics, surface treatments may not address the root cause. It is necessary to recognize that slope countermeasures are not cosmetic finishing work but stabilization designs tailored to ground conditions.

Basic Item 3 Integrate stormwater treatment and drainage planning with slope design

When considering slope protection measures, water management is one of the most important items. At solar power plant construction sites, surface water during rainfall can concentrate on slopes, causing erosion and scour even in a short period. Especially during construction, the ground surface tends to be bare, and because vegetation and protective works are unfinished and exposed to heavy rain, insufficient drainage planning can allow a single rainfall event to severely damage the slopes.

It is important here not to treat the drainage plan as something to be installed only after the slope is completed. Slopes are affected by water from immediately after earthworks. Therefore, it is necessary to plan not only the permanent drainage facilities but also temporary drainage measures during the construction phase. As the work progresses and the terrain changes, you must consider in stages when and where water will collect and where it should be discharged.

Water accumulating at the slope shoulder is particularly dangerous. When water flowing from above falls directly onto the slope face, it runs down while scouring the surface soil, making rill-like erosion likely. If this condition is left unaddressed, it will eventually develop into localized deep scouring and undermine the overall stability of the slope. Measures to prevent water from being received at the slope shoulder, or to promptly divert any collected water into safe channels, are essential.

On the other hand, treatment of the slope toe is also important. If water flowing down from above cannot be properly captured at the slope toe, it will wash out together with soil and contaminate nearby access paths and planned equipment sites. If muddy water flows in, the working conditions for subsequent work will deteriorate and the quality of paving and equipment installation will be affected. Drainage is not just a slope-face issue but a factor that influences the construction environment of the entire site.

At solar power plants, attention must also be paid to the fact that panel layout alters how water flows during rainfall. If panels are arranged so that runoff from the panel rows concentrates in particular locations, large loads can be placed on slopes or parts of access paths after completion. Even if problems are not apparent during site preparation, it is necessary to check drainage routes, taking into account that water flow can change after equipment is installed. Slope protection measures are not just a matter of civil engineering work but are also connected to equipment layout.

Also, even if you plan drainage facilities, they will not function if they become clogged with sediment or fallen leaves. If you look ahead to maintenance and management, operational aspects—such as whether the structure is easy to clean, whether it is situated in a location that is easy to inspect, and whether you can identify places where sediment tends to accumulate—are also important. Drainage is not finished once constructed; it only has meaning when its function can be maintained.

At many sites where slopes fail, it is a misreading of water flow—rather than the slope itself—that is the cause. For this reason, slope design and drainage planning should not be treated separately but should be considered together as an integrated whole.

Basic Item 4: Match slope settings and selection of slope protection measures to site conditions

When considering slope protection measures, how steep to set the gradient and what protective works to adopt are central issues. However, the important point here is not to apply standard cross-sections or past cases wholesale. Site conditions at photovoltaic power plants vary greatly depending on terrain, soil properties, rainfall conditions, the movement paths of construction equipment, and the ease of operation and maintenance. Therefore, the selection of slope gradients and protective works must be based on realistic combinations tailored to each site.

Making the slope gentler generally tends to increase stability, but it also increases the area occupied. At power plants, because the usable site is preferred to be devoted to equipment layout as much as possible, making slopes overly gentle can reduce layout efficiency. Conversely, if the slope is made too steep to prioritize site area, the burden on protective works and reinforcements increases and constructability worsens. In other words, slope setting is not merely a figure from stability calculations but is determined in balance with land-use planning.

The same is true for protective measures. Vegetative surface protection may be appropriate at some sites, while at others, due to rainfall intensity and soil conditions, a more erosion-resistant method should be chosen. The important thing is not to decide based only on appearance or initial ease of construction. If you do not consider how to get through the period until vegetation is fully established, whether the site is prone to drying out, whether flow velocities will be high, and the sunlight and maintenance conditions, the measure may not perform as expected.

In addition, slope protection works are not necessarily applied uniformly across the entire slope. Because the loads differ near the crest, the central part, and the toe of the slope, it can be effective to consider differentiated, targeted measures as needed. In particular, areas where water tends to collect and locations adjoining inspection walkways may require local reinforcement. In many cases, it is more rational to identify weak points and treat them than to apply uniform measures over the whole surface.

Furthermore, the timing of construction is also important when selecting slope protection measures. If construction takes place close to the rainy season, methods with low initial protective performance carry a risk of being damaged immediately after construction. Conversely, if construction can be carried out during a relatively stable period, it becomes easier to choose methods that prioritize long-term appearance and maintainability. Which construction method is best is influenced not only by site conditions but also by scheduling conditions.

When considering slopes and protective works, on-site discussions often proceed based solely on the feasibility shown on the drawings. However, what truly matters is whether that slope will hold up during construction and continue to function after completion. Even if a design is feasible on paper, if the safety margin is small when affected by construction errors or rainfall, it will be unstable in the field. In practice, choosing a plan that is less likely to fail on site, rather than optimizing on paper, ultimately leads to improved quality.

Basic Item 5: Finalize safety management during construction and temporary works planning in advance

When it comes to slope protection measures, attention tends to be directed at the stability of the finished slope, but on actual sites accidents and troubles are more likely to occur during construction. During excavation, during embankment construction, before protective works are installed, and immediately after rainfall, the period during which slopes remain unstable is long, and if safety management and temporary works planning during that time are inadequate, there can be major impacts on both the schedule and the quality.

In particular, during solar power plant construction, multiple tasks may proceed concurrently across a large site. When earthworks, drainage works, access road preparation, racking-related work, and the like overlap, situations increase in which heavy machinery and vehicles repeatedly pass near slopes. As a result, excessive loads may be applied near the slope crest, or the slope toe may be disturbed, creating instability factors that were not anticipated during the planning stage.

From a safety-management perspective, the first priority is to clearly identify areas where entry requires caution. Areas near slopes can appear passable at first glance, but the ground underfoot may be loose or the area may become prone to collapse after rain. If work proceeds without clearly defining workers' movement routes, the turning radius of heavy equipment, and temporary material storage locations, loads can unknowingly be imposed on the slope shoulder and the slope itself. Safety cannot be maintained by warnings alone; it needs to be supported by routing plans that make hazardous behavior unlikely.

Temporary drainage is directly linked to safety during construction. When water flows over slopes or work passages, it creates mud and slippery conditions, leading to falls and unstable operation of heavy equipment. Moreover, those conditions disturb the soil and can easily trigger a vicious cycle that further degrades the slope. Considering how to divert water at the temporary stage will not only enhance slope protection but also contribute to overall stability of site operations.

Decisions about construction that take weather conditions into account are also essential. Risks for slope work change significantly between clear weather and after rainfall. A slope that posed no problem until yesterday can have its surface loosened by rain overnight and become dangerous by the next morning. Therefore, safety management during construction requires not only routine inspections but also a system for rechecking in response to weather changes. Rather than prioritizing site progress and continuing work, temporarily stopping to reassess the situation will ultimately better protect the overall project.

Furthermore, it is important to maintain close information sharing between the teams responsible for slope countermeasures and the teams carrying out equipment installation and site development. Materials temporarily placed near a slope by one crew can affect another crew’s work, and changes to temporary drainage that are not shared can flow into other sections — this becomes more likely on larger sites. Even if a slope is only part of the site, problems can spread to other operations. For that reason, safety management around slopes must be positioned within the overall project schedule.

Basic Item 6 Plan with an Eye Toward Inspections and Repairs After Completion

When it comes to slope protection measures at solar power plants, you must not be satisfied merely that they are stable at the time of completion. Rather, what truly matters is whether they can withstand long-term operation after completion. Once a plant is finished, it is a facility that will be maintained over a long period, and the slopes will be exposed to wind and rain throughout that time. Even if there were no problems during construction, it is entirely possible that signs of distress or deformation will appear months or years later.

Therefore, when planning slopes it is important to incorporate ease of inspection from the outset. For example, consider from where the condition of the slope can be checked, whether there are access routes that allow early detection of abnormalities, and whether the slope toe and drainage facilities can be approached easily. Locations that are difficult to see or access tend to delay discovery of abnormalities, and small deformations can lead to major repairs.

Common occurrences after completion include clogging of drainage facilities, expansion of surface erosion, uneven vegetation, local settlement, and opening of the slope shoulder. These tend not to occur suddenly on a large scale, but often progress gradually from small signs. Therefore, rather than responding after abnormalities appear, it is important to establish conditions that allow early detection of signs of deformation so repairs can be made while they are still small.

Also, when considering operation and maintenance, ease of repair should be an important part of slope planning. If you emphasize only the initial appearance or constructability and end up with special details or complex drainage routes, repairs in later years will become difficult. Slope measures should ideally be configured so they can be handled by people over the long term. Even if partial repairs or reshaping are required in the future, planning them to be relatively easy to address will reduce downtime and inspection burdens.

Furthermore, maintenance staff at power plants are not necessarily civil engineering specialists. During routine inspections, it may be equipment maintenance or patrol personnel who detect abnormalities. Therefore, it is also important to make it easy to clearly share which conditions warrant attention. If you organize readily observable, easy-to-assess signs on site—such as cracks in slopes, drainage overflows, localized scouring, and sediment runoff onto pathways—it will help enable an early response.

If slope protection is treated as work that ends at completion, it tends to leave risks that are not visible during construction. Conversely, if you plan by working backwards from maintenance, important elements such as inspectability, repairability, and drainage are naturally organized. This perspective is essential for creating power plants that can operate stably over the long term.

How to Link Slope Protection Measures to Construction Quality

The six items we've looked at so far may appear to be independent, but in reality they are closely interconnected. If topographic understanding is insufficient, appropriate slope settings cannot be made. If soil characteristics or groundwater are misjudged, drainage plans and protective works will not perform effectively. If safety management during construction is lax, no matter how good the plan, it will fail on site. If the ease of inspection after completion is not considered, problems will become apparent during long-term operation.

In other words, slope countermeasures should be approached from overall optimization rather than local optimization. In site practice, it is important not to treat slope work as merely an ancillary task. From the land development planning stage, by anticipating equipment layout, drainage, construction traffic flow, and maintenance, and by identifying in advance where risks will be concentrated, you can ultimately reduce rework and the incidence of problems.

In solar power plant construction, because a large site must be organized in a short period, plans that work on drawings often leave little margin on site. Precisely in such cases, it is important not to treat slopes as something to be adjusted at the end, but to regard them as a prerequisite for stabilizing the entire construction. When slopes are stable, access roads are stable, drainage is in order, and equipment installation proceeds more smoothly. Conversely, if slopes are unstable, the overall construction schedule is prone to collapse.

Also, to reliably advance slope countermeasures, it is important to accurately understand the site’s terrain and construction extent and to detect early signs of deformation. In that respect, an environment that enables efficient on-site coordinate verification and as-built confirmation contributes to improved construction management accuracy. For example, in situations where you need to quickly confirm the positions of slope crests and toes, drainage facilities, and maintenance access routes, a system that can handle location information with high precision aids on-site decision-making. On large sites such as solar power plants, where it is necessary to carefully grasp the positional relationships among slopes, drainage, and equipment, using devices like LRTK (iPhone-mounted GNSS high-precision positioning device) can more easily lead to improved accuracy in construction and maintenance management. Rather than leaving slope countermeasures as desk studies, it is advisable to also consider employing such high-precision positioning as a means to ensure they are reliably implemented on site.