Why soil confirmation is important in PV plant construction and 6 ways to assess it

In PV plant construction, attention tends to focus on racking, foundations, earthworks, drainage, access paths, and electrical equipment layout, while soil confirmation is often separated out as “something for the geotechnical investigation team to handle.” In reality, misreading soil conditions can lead to a chain of quality issues across the site: insufficient foundation bearing capacity, pile pullout or lack of lateral resistance, differential settlement, poor drainage, scour, deformation of filled ground, and worse walking conditions during maintenance. Even though PV equipment appears lightweight, uneven settlement can occur depending on ground conditions, and when using pile foundations it must be ensured the piles resist not only compressive forces but also pullout and horizontal forces. NEDO +1

Soil confirmation does not end with pre-construction checks. In the maintenance and inspection concept for ground-mounted systems, it is recommended to continuously monitor for soil erosion near foundations, ground settlement, expansive soils, frost depth influence, snow-related settlement, unequal settlement, foundation scour due to inadequate drainage design, collapse of fill ground, and vegetation interference. In other words, soil confirmation is not only a task for design and construction; it also determines how easy the site will be to operate after completion. JEMA The Japan Electrical Manufacturers' Association

The current guidelines for ground-mounted systems treat preliminaries as consisting of document review, visual site inspection, geotechnical investigation, and land-use and surrounding environment surveys. They recommend tracing a site’s history using geological maps and old maps and checking surface geology, special soil layers, nearby past records, groundwater level, nearby reinforcement works, place names and vegetation, signs of differential settlement, cut-and-fill forms, reclamation timing, drainage planning, and potential for inflow/outflow of sediment. Soil confirmation is crucial in PV construction because these checks collectively determine not only the safety of foundations but also the preconditions for earthworks, drainage, and maintenance. NEDO

This article summarizes why soil confirmation is important in PV plant construction and then explains six practical “ways to assess” that can be used on site. The intended readers are field personnel searching for information with the query “PV plant construction.” Rather than deeply investigating complex geotechnical theory, this article clearly summarizes what to look for on site, where to suspect danger, and how to translate the findings into construction decisions. NEDO +1

Table of contents

• Why soil confirmation is important in PV plant construction

• Way 1: Assess from landform and the site’s origin

• Way 2: Suspect soft ground, reclaimed land, and filled ground

• Way 3: Assess from features of valley bottoms, slopes, and logged forests

• Way 4: Assess from groundwater, drainage, and liquefaction potential

• Way 5: Assess from bearing capacity, settlement, pullout, and lateral resistance

• Way 6: Combine investigation methods by purpose

• Commonalities of sites that tend to fail at soil confirmation

• How to link soil confirmation to construction quality

• Faster site response by improving shared precision of soil confirmation

Why soil confirmation is important in PV plant construction

Soil confirmation is important in PV plant construction because even seemingly light equipment can reveal ground weaknesses as various on-site problems. Soft ground, filled ground, earthworks, slopes, and valley bottoms can raise concerns about foundation bearing capacity and differential settlement, so special attention is required at the design stage. Even if the site looks tidy during construction, lax soil assessment can later appear as misaligned rows, settlement around foundations, muddy access paths, and poor drainage. NEDO

Moreover, soil confirmation is not just for selecting foundation types or pile lengths. During document and site surveys it is recommended to identify landform and ground features, surface geology, special soil layers, groundwater level, past ground improvement works, and signs of nearby differential settlement. Those results should be used to conduct geotechnical investigations and soil testing efficiently and effectively. In short, soil confirmation determines where effort is needed, where risks are, and where to spend money. NEDO +1

Looking at maintenance and inspection concepts, soil influences persist after completion. Since it is recommended to check for soil erosion near foundations, ground settlement, foundation scour due to inadequate drainage design, and collapse of filled ground, it is clear that soil judgments made during construction significantly affect ease of long-term operation. Therefore, soil confirmation is not something to be checked just once before construction; it should be managed integrally across construction planning, as-built verification, and the initial operation period. JEMA The Japan Electrical Manufacturers' Association

Way 1: Assess from landform and the site’s origin

The first way to assess soil is to look at landform and the site’s origin before examining boring logs. Current guidelines recommend tracing how the land was formed from geological maps and old maps, and investigating landform and ground features, surface geology, past local records, place names and vegetation, and signs of differential settlement. This is because site soils are not determined solely by conditions directly under investigation points but are strongly linked to land history and surrounding topography. NEDO

For example, in landform classification, valley-bottom plains, backswamps, and wetlands are considered extremely soft, while alluvial fans and natural levees are relatively better but require attention to subsurface flow. This classification is not intended to judge a place as simply good or bad by name alone; rather, it is a starting point to decide where to focus detailed investigations. A seemingly flat site can be a filled valley or an old river terrace, and problems that arise during construction differ greatly between those cases. NEDO

Place names, vegetation, and deformation of neighboring houses and roads are also valuable clues. The checklist for preliminary surveys suggests interviewing neighbors about site history and investigating nearby house and road differential settlements and deformations to assess the risk of ground settlement, and checking cut-and-fill forms for differential settlement risk. Sites that grasp the land’s quirks—unseen on drawings—early tend to have more stable subsequent construction decisions. NEDO

Way 2: Suspect soft ground, reclaimed land, and filled ground

The second way is to start by suspecting soft ground, reclaimed land, and filled ground. Current guidelines classify land consisting of mud or peat, and land reclaimed from swamps or the sea, as soft ground, and indicate these grounds tend to produce differential settlement or large shaking during earthquakes, so countermeasures are necessary. Reclaimed land may experience subsidence, ground cracking, or sinking in earthquakes, potentially tilting racks or damaging foundations. It is dangerous to think PV equipment is safe simply because it is lighter than buildings. NEDO

Filled ground requires similar caution. Fill has significant self-weight, and placing fill on soft ground causes consolidation settlement of the underlying soft layer. Thick soft layers are prone to differential settlement, and in mountain or hilly earthworks, the boundary between cut and fill is particularly likely to trigger differential settlement. In such cases, either improve the fill side by ground improvement or use retaining walls or piles as countermeasures. Thus, sites that cannot identify cut-and-fill boundaries on earthwork drawings already carry a major weakness. NEDO

The document review section further explains that for artificial fill sites it is important to confirm fill height and extent, pre-fill ground slope, overlay new and old topographic maps, and verify by field survey; older fills tend to be more prone to sliding or collapse. In other words, the risk of fill is understood only when you know not just that it exists but when and how it was placed. In practice, do not be reassured by superficial flatness—always confirm the history of cuts and fills. NEDO

Way 3: Assess from features of valley bottoms, slopes, and logged forests

The third way is to read the soil from the characteristics of valley bottoms, slopes, and logged forests. In valley bottoms, organic soft layers similar to peat often accumulate along valley courses below hills and plateaus. If fill is placed on such layers, settlement tends to concentrate toward the valley centerline and the likelihood of differential settlement increases, so measures such as pile support may be needed. Because PV sites are sometimes developed by large, unified earthworks, the presence of valley-bottom terrain within part of the site can be easily overlooked. NEDO

Slopes are not only about gradient. Current guidelines emphasize slope-face protection and securing appropriate drainage paths on slopes, and caution that locations where rainwater from racks or modules concentrates above foundations require measures to prevent sediment runoff and scour. Thus, soil checks on slopes must consider not only bearing capacity but also how water flows and where erosion will occur. If you rely solely on bearing capacity on a slope, you may face scour around foundations and slope failures after completion. NEDO

Logged forests also require caution. Forests suppress surface erosion from rain and tree roots help prevent shallow-slope failures. Therefore, when land is created after forest clearing, erosion and collapse risks increase, especially on slopes. In practice, do not simply place equipment on cleared ground as-is; plan assuming soil movement and surface stability will change. It is important to recognize that conditions differ even for the same soil before and after logging. NEDO

Way 4: Assess from groundwater, drainage, and liquefaction potential

The fourth way is to read soil from groundwater, drainage, and liquefaction potential. Soil confirmation often focuses on soil stiffness and bearing capacity, but groundwater conditions greatly affect foundation planning, constructability, and maintenance. Investigation methods recommend using groundwater surveys to determine water table and groundwater flow, and to set drainage structure placement plans and design conditions. Groundwater checks are not ancillary information but the foundation of the drainage plan itself. NEDO

Document review basics include investigating rainfall amounts needed for design and construction of drainage facilities. The checklist for preliminary surveys also includes forming a drainage plan and checking the potential for sediment inflow and outflow. This indicates soil confirmation is a task of looking at soil together with water. In practice, if you do not check for springs, post-rain muddiness, positions of existing watercourses, and low areas where water tends to remain, post-construction issues such as puddles, scour, and weakening of fills are more likely to occur. NEDO +1

Liquefaction potential must not be overlooked. Current guidelines warn that loose saturated sand layers can liquefy under strong shaking such as earthquakes, causing structures on the ground to settle, so attention is required. It is often assumed PV plants are less affected by liquefaction because they are lighter than buildings, but if racks or foundations settle, row alignment and the stability of electrical equipment are affected. When assessing the ground, it is important to look not only at “whether you can stand now” but also at “whether the state changes during earthquakes or heavy rain.” NEDO

Way 5: Assess from bearing capacity, settlement, pullout, and lateral resistance

The fifth way is to include not only bearing capacity and settlement but also pullout and lateral resistance in the assessment. Even for lightweight racking, differential settlement can occur depending on ground conditions, and when adopting pile foundations sufficient resistance to compressive, pullout, and lateral forces is required. Thus, soil confirmation cannot restrict itself to assessing only the “supporting force.” If you do not evaluate how the foundation will withstand wind loads, eccentric loads, and seismic horizontal forces, you may choose an inappropriate foundation type. NEDO +1

Current guidelines indicate that frictional resistance at the foundation base is obtained by multiplying the vertical force by the ground’s friction coefficient, and when no soil tests are carried out, reference tables may be used to set values. Those tables treat rock, rock fragments, gravel, and sand as relatively high, sandy soils as intermediate, and silt and clay as lower, showing that sliding resistance and horizontal resistance assumptions change with soil type. This means you should understand differences between soil classes before applying a single foundation type uniformly across all panels. NEDO

Regarding allowable bearing capacity, laws and regulations basically require it to be determined based on geotechnical investigation results, and the approach of deciding from surveys and loading tests is recommended. Furthermore, if weak layers that self-penetrate in SWS tests exist near the foundation base, attention is needed to avoid harmful damage from self-weight settlement or ground deformation. In practice, do not stop at “we drove piles” or “we placed footings”—verify whether the soil will hold long-term, settle, or be pulled out by wind. NEDO

Way 6: Combine investigation methods by purpose

The sixth way is to combine investigation methods by purpose. Current guidelines organize ground investigation methods such as boring, sounding, geophysical surveys, in-situ tests, sampling, groundwater surveys, and laboratory soil tests, and indicate the information obtainable and intended uses for each. The key is not to try to judge everything with just one method. Each method reveals different information, so you must combine them according to site conditions and the items you need to decide. NEDO

For example, boring can confirm stratigraphy, N-values and other ground parameters, groundwater environment, and sample characteristics, and can lead to setting support layers or to in-situ tests using the borehole. Sounding provides penetration resistance values to see vertical distributions of ground parameters and can be used for liquefaction assessment or bearing capacity estimation. Geophysical surveys are effective for grasping planar ground structure, groundwater distribution, and horizontal distribution of ground parameters. In short, use methods that are good at looking deeply at points and methods that are good at seeing variations across areas. NEDO

Moreover, in-situ tests help capture deformation and strength characteristics and support capacity estimation, while laboratory tests provide physical, mechanical, and deformation properties and support capacity estimation, and can also be used to estimate pile pullout and lateral resistance. Therefore, choose investigation methods based on “what you want to decide,” not merely on cost. If you are afraid of differential settlement, need to assess pile pullout, or are concerned about drainage conditions, the required investigations differ. NEDO

The guidelines also state that because it is difficult to allocate a large budget for geotechnical investigations in PV projects compared with mid-to-large buildings, it is all the more necessary to conduct thorough preliminary surveys and then plan geotechnical investigations and soil tests efficiently and effectively based on those results. This does not mean randomly increasing the number of investigation points; it means identifying risky locations and allocating more investigation there. The planning of investigations matters more than the sheer amount. NEDO +1

Commonalities of sites that tend to fail at soil confirmation

Sites that often fail at soil confirmation share several common traits. First, treating the entire site as a single uniform ground. Current guidelines and maintenance concepts repeatedly show that within the same site conditions can vary—fill, cut, valley bottoms, slopes, reclaimed land, etc. If you base an entire site on only a few investigation results, you are likely to miss local weaknesses. Such weaknesses commonly appear after completion as differential settlement, scour, or mud. NEDO +1

Second, focusing only on bearing capacity while downplaying drainage, groundwater, and the soil’s history. Document review items include drainage plans, sediment inflow/outflow, rainfall, groundwater level, fill timing, and signs of neighboring differential settlement. Ignoring these and looking only at bearing capacity makes it difficult to prevent post-rain scour, fill weakening, or deformation around foundations. Sites with weak soil confirmation often view soil only in mechanical terms and drop the water and topography perspectives. NEDO +1

Third, failing to monitor changes after construction starts. Maintenance and inspection guidance recommends continuously checking for soil erosion, ground settlement, foundation scour due to drainage failures, collapse of filled ground, and vegetation interference. This indicates that ground quality cannot be fully fixed by pre-construction surveys alone; it requires observing and correcting changes during construction and in the early operation period. Sites that fail at soil confirmation tend to treat investigations as a one-time task. JEMA The Japan Electrical Manufacturers' Association

How to link soil confirmation to construction quality

To link soil confirmation to construction quality, do not confine investigation results to reports; concretely reflect them in foundation types, pile lengths, ground improvement, drainage plans, access plans, equipment layout, and maintenance block settings. The current guideline design flow treats document review, site survey, geotechnical investigation, earthworks/drainage planning, layout planning, foundation design, and maintenance planning as a continuous sequence. In other words, soil confirmation is not just a civil engineering issue but a common foundational condition that relates to structural design, layout, and maintenance. NEDO

In practice, it is important to convert soil confirmation results into language usable on site. For example: “This is a valley-bottom area—watch for differential settlement,” “This is a cut-and-fill boundary—re-check need for piles or ground improvement,” “Groundwater is high here—take care with drainage and vehicle access,” “Soft layers are thick here—do not apply a uniform specification.” Organizing site-specific precautions speeds decisions. Sites that share not just numbers but construction implications of investigation results have fewer reworks. NEDO +1

Also, linking soil confirmation to construction quality requires keeping the post-handover period in view. Early detection of signs such as ground settlement, scour, collapse of filled ground, and vegetation interference during initial operation inspections makes countermeasures easier. Therefore, if you organize “areas requiring attention” during construction, prioritizing maintenance inspections becomes simpler. The value of soil confirmation grows when considered not only for construction safety but also for long-term preventive maintenance. JEMA The Japan Electrical Manufacturers' Association +1

Faster site response by improving shared precision of soil confirmation

On large PV sites, it is important that stakeholders can quickly share which blocks are at cut-and-fill boundaries, which areas are suspected of soft ground, and where groundwater or drainage caution is needed. Even if soil confirmation quality is high, if findings are not shared on-site with their positions, construction and corrective decisions tend to be delayed. On especially large sites, sharing problematic locations only on drawings makes back-and-forth with the field time-consuming. NEDO +1

In such cases, using systems like LRTK (iPhone-mounted GNSS high-precision positioning devices) to make it easier to share investigation locations and caution areas via iPhone helps link soil confirmation results to construction decisions. For example, if you can organize cut-and-fill boundaries, valley traces, suspected soft-ground spots, drainage caution points, and maintenance priority blocks together with location information, field explanation and verification become faster. The value of soil confirmation depends not only on report precision but also on how easily it can be shared on site.