Seven checkpoints when deciding pile length for utility-scale solar PV construction

In solar PV construction, it's easy to focus on selecting the racking, modules, and electrical equipment, and to assume that pile length is simply "the length shown on the foundation drawings." In reality, however, pile length varies greatly with site soil conditions, topography, earthwork history, groundwater, load conditions, and construction method. If this is treated superficially, it can lead not only to poor installation quality or unexpected lack of bearing capacity during construction, but also to differential settlement, tilting, pullout, and increased maintenance burden after completion. Although PV equipment looks light, ground conditions can cause differential settlement, and when pile foundations are used it is organized that resistance to not only compression but also uplift and lateral forces must be provided. NEDO +1

Moreover, piles used for PV foundations differ in character from the long piles used in general buildings. Recent technical documents indicate that piles used for PV are relatively small in cross section and are mostly friction piles that do not reach a bearing stratum; penetration depths are often shallow, typically about 1-3 m (3.3-9.8 ft). For that reason, small differences in soil or earthwork conditions tend to have a direct effect on decisions about pile length. It is not as simple as "deeper is safer"; it is important to determine at what depth, in which soils, and how resistance will be provided. NEDO +1

Furthermore, current design and construction guidelines require that when pile foundations are adopted, it must be confirmed from ground investigations that adequate bearing capacity can be expected, and as a rule test piles should be installed on site and load tests carried out to verify safety. If bearing capacity cannot be expected in soft ground, or if the ground is too stiff or contains many boulders, shallow foundations should be selected instead. In other words, examining pile length is as much an assessment of whether pile foundations are appropriate in the first place as it is a task of choosing the pile length. NEDO +1

This article organizes and explains seven checkpoints practitioners should understand when deciding pile length during utility-scale solar PV construction. It summarizes, from a construction viewpoint, not only desk calculations but what to look for on site, what to suspect, and how to connect observations to decisions. To avoid later difficulties from setting pile length uniformly, these are items you should confirm before mobilization. NEDO +1

Table of contents

• Why confirming pile length is important in solar PV construction

• Point 1 First confirm land formation and site history

• Point 2 Don’t assume uniform ground across the site; decide investigation points

• Point 3 Decide pile length for uplift and lateral resistance as well as compression

• Point 4 Understand pile type and support mechanism to decide depth

• Point 5 Avoid uniform pile length on cut/fill, valley lines, and soft ground

• Point 6 Incorporate seasonal variations such as groundwater, drainage, and frost heave

• Point 7 Make the final decision with test piles and load tests

• Common characteristics of sites prone to failure in pile-length decisions

• Shared precision during construction raises the quality of pile-length decisions

Why confirming pile length is important in solar PV construction

Confirming pile length is important because piles are not merely members supporting equipment from below; they are the element most sensitive to differences in site conditions. Ground-mounted solar PV systems are relatively small-scale and are easily affected by near-surface ground conditions. On sites such as soft ground, reclaimed land, filled ground, engineered fill, slopes, or valley-bottom lowlands, there is concern that sufficient foundation bearing capacity may not be obtained, so more detailed investigation is required. Pile length is an important adjustment variable to accommodate such ground differences. NEDO

Also, the external forces transmitted to the foundation are not only compressive loads. Current guidelines categorize external forces from the racking to the foundation as compression, uplift, lateral force, and moment, and treat pile foundations as structures that respond with end-bearing, skin friction, and lateral resistance, respectively. In other words, pile length should be decided not by "how deep it reaches" but from the perspective of "which resistance, in which soil layer, and how much" will be secured. NEDO

Furthermore, pile length directly affects constructability and cost. Longer piles increase installation load and the risk of hitting hard layers or boulders. If piles are too short, bearing capacity or uplift resistance may be insufficient and later remediation may be necessary. Therefore, pile-length confirmation should be considered at the intersection of safety, constructability, and cost, rather than merely following the dimensions on the drawings. Because it is necessary to select construction methods that can be managed reliably and appropriately with maintenance in mind, pile-length decisions are decisions that directly affect long-term operation. NEDO

Point 1 First confirm land formation and site history

The first point is to check the land formation and site history before looking at ground investigation numbers. Current guidelines organize that document surveys such as geological maps and old maps should be used to check land formation, surface geology, the presence of special soil layers, and whether adjacent reinforcement works exist; and that field surveys should check signs of differential settlement in nearby houses and roads, abnormalities in drains and retaining walls, site history, thickness of fill, and clarity of cut/fill boundaries. Pile length is a judgment made on top of these histories. You cannot determine appropriate lengths by looking only at the current surface without knowing the site's past. NEDO +1

For example, land that looks flat today may originally have been rice paddies, wetlands, ponds, valleys, or riverbanks, leaving weak layers that are not apparent in the top few tens of centimeters. Conversely, residential development fills or old fills may have heterogeneous compaction and be prone to local settlement beyond what is visible. Such information becomes a basis for the judgment that "this site should not be treated uniformly" even before deciding how many meters to drive piles. If the initial assessment is naive, you will be more likely to be thrown off by variability in installation results later. NEDO +1

Also, topography and history help predict construction-stage troubles. In old valley topography or reclaimed land, differential settlement is likely; near cliffs or slopes, scour or collapse; in engineered fill areas, deformation at cut/fill boundaries—how problems manifest becomes easier to imagine in advance. When deciding pile length, instead of "this length is fine for average ground," first check the site's history to find "which blocks are most likely to deviate from average." NEDO

Point 2 Don’t assume uniform ground across the site; decide investigation points

The second point is to consider pile-length decisions from how investigation points are placed, rather than assuming the site is uniform ground. Current guidelines indicate that in-situ tests should be conducted based on preliminary survey results to grasp geotechnical properties required for design, and that the number of investigation points should be increased when site scale, topography, or expected ground composition changes warrant it. Utility-scale PV sites are large and conditions can change even within the same row, so deciding a uniform pile length across the entire site based on only a few investigation points is dangerous. NEDO +1

Pay particular attention to cut/fill boundaries, valley lines, microtopography boundaries, and transitions in earthwork. These locations are prone to changes in ground composition and thus to changes in pile resistance conditions. Current guidelines also state that investigations should be increased when ground composition cannot be estimated or when changes are expected. If pile length is determined by averaging from a small number of points, problems can occur both ways: installation difficulties in stiff areas and insufficient bearing in soft areas. NEDO +1

Also be aware of the strengths and weaknesses of each investigation method. The SWS test is inexpensive and simple and provides continuous depth data, but is weak in gravel and rubble, cannot collect soil samples, and its results require attention to rod skin friction effects. By contrast, the standard penetration test (SPT) is advantageous for identifying layers, groundwater level, and soil classification, and helps detect hard bearing layers that SWS tests may miss. In other words, investigations to decide pile length should not rely on a single inexpensive method but should be combined according to what you need to know. NEDO

Point 3 Decide pile length for uplift and lateral resistance as well as compression

The third point is to decide pile length considering uplift and lateral forces as well as compression. Current guidelines categorize external forces on foundations as compression, uplift, lateral force, and moment, and treat pile foundations as structures that resist compression with end-bearing and skin friction, uplift with skin friction, and lateral force with lateral resistance. Thus, pile length must be seen not just as a length to resist downward loading but as a length to resist uplift and lateral motion as well. NEDO

In PV systems, uplift due to negative pressure from wind can be dominant in some situations. Current guidelines specifically state that pile foundations for PV racking should be designed with particular attention to uplift from negative pressure during strong winds. Therefore, choosing a shorter pile length based only on compression may appear acceptable under normal conditions but could result in a design that is prone to pullout during typhoons or gusts. When deciding pile length, always confirm not only how much is needed to resist downward loads but also how much is needed to prevent pullout. NEDO

Viewing lateral forces is also important. On slopes or in windy locations, lateral forces and moments tend to increase. If pile length is insufficient, piles may resist compression and uplift yet undergo large lateral displacement, which can affect alignment of racking and stability of rows. When deciding pile length, you need to look not only at whether bearing capacity is sufficient but also at how much horizontal movement is possible. Pile length should be the result of deciding which resistances to secure against the external forces, not merely a safety factor number. NEDO +1

Point 4 Understand pile type and support mechanism to decide depth

The fourth point is to decide pile length after understanding the chosen pile type and support mechanism. Recent validation documents show that piles used for PV are typically small-section friction piles that penetrate shallow layers of about 1-3 m (3.3-9.8 ft), unlike foundation piles for buildings. This means the building-pile mindset of "always reach the bearing layer" does not necessarily apply. You must understand whether the PV pile is an end-bearing pile that reaches a bearing stratum or a friction pile that relies on skin friction, and then decide the required length. NEDO +1

What matters is that different pile types produce different resistance behavior even at the same length. Technical documents show different support calculation approaches for closed-end piles, open-end piles, H-section piles, and screw piles; in particular, for screw piles the outer surface of the blades is used in calculating skin friction. In other words, you cannot judge depth appropriateness by comparing numbers alone—you must understand which pile resists in which soil layer and how. NEDO

Current guidelines also require that preliminary ground investigation determine soil stiffness and that an appropriate pile method and installation method be selected accordingly. When deciding pile length, always consider constructability as well as length. In very stiff ground or ground with many boulders, piles assumed to be long may not be installable on site; in soft ground, increasing length may not produce the expected skin friction. Therefore, pile length must be determined by combining pile type, installation method, and soil layering. NEDO +1

Point 5 Avoid uniform pile length on cut/fill, valley lines, and soft ground

The fifth point is to avoid using a uniform pile length on cut/fill boundaries, valley lines, and soft ground. Current guidelines note that soft ground, which is common in lowlands composed of clays or peat, tends to exhibit large differential settlement and earthquake-induced shaking; reclaimed land risks subsidence, cracking, and settlement that could damage racking and foundations. Filled ground is prone to consolidation settlement, and when the soft layer is thick the likelihood of differential settlement increases. In such locations, applying the same pile length as other areas is dangerous. NEDO

In particular, in mountain and hill development areas the boundary between cut and fill is the most likely place for differential settlement, and it is necessary to solidify the fill side by ground improvement or to take measures with retaining walls or piles. In valley-bottom lowlands, the valley-centered soft layers are prone to greater settlement toward the centerline and require measures such as supporting with piles. In other words, while standardizing pile length looks easier to manage, adjustments tailored to the most problematic locations are actually necessary. NEDO

In practice, it is tempting to decide "we'll use 2 m (6.6 ft) piles for this site," but that only works if the site's ground conditions are truly uniform. Utility-scale PV sites are large and it is not uncommon for multiple topographic and geologic conditions to coexist under a single site name. Therefore, when examining pile length, even if you set a standard length you should plan from the start where change zones will be located. Missing cut/fill boundaries or valley lines can lead to later partial remedial or reinforcement works, making the schedule and cost more burdensome. NEDO

Point 6 Incorporate seasonal variations such as groundwater, drainage, and frost heave

The sixth point is to incorporate seasonal variations such as groundwater, drainage, and frost heave into pile length. Current guidelines state that groundwater investigations mainly examine groundwater levels, and that based on boring results it is important to identify groundwater position and distribution, the influence of meltwater in spring, and the presence of springs. In other words, pile-length decisions must consider not only static soil properties but also seasonal water conditions. Areas that appear dry most of the year may experience a rise in groundwater level during snowmelt or prolonged rain, which could affect bearing conditions and constructability. NEDO +1

Drainage relationships are also important. On slopes, slope protection and securing appropriate drainage paths are essential, and where rainwater from racking and modules concentrates, measures are needed to prevent soil erosion or scour. Even if pile length alone ensures foundation safety, poor surrounding drainage can wash soil away around piles and change conditions. Therefore, when deciding pile length, look not only at soil stiffness but also at whether that soil will remain in the same condition after construction. NEDO

Frost heave cannot be ignored in cold regions. A 2025 technical document confirmed in full-scale model tests that shallowly buried piles experience larger frost heave, while deeply buried piles experience smaller frost heave. The tests also indicated that regardless of pile type, when burial depth is roughly twice or more the maximum frost penetration depth, frost heave is greatly reduced. When deciding pile length in cold regions, it is important to consider not only bearing capacity but also the relationship with frost penetration depth. NEDO

Point 7 Make the final decision with test piles and load tests

The seventh point is to make the final decision with test piles and load tests. No matter how much document and ground investigation you do, there remain elements on PV sites that cannot be known until construction. Current guidelines state that when adopting pile foundations, installing on-site test piles and performing load tests to verify safety is fundamental. This means it is important not to determine pile length solely from drawing figures but to verify on site whether that length is truly appropriate. NEDO

Recent bearing capacity verification studies also organize that while bearing capacity checks for PV piles are based on design formulas using SPT N-values, it is not appropriate to use SWS-converted N-values directly as design values; soil constants should ideally be obtained from laboratory tests or SPT. In other words, do not decide pile length solely from simple survey numbers; proceed to more reliable verification as needed. Test piles are an important step to make that on-site judgment. NEDO

Test piles are also useful for verifying constructability, not just bearing capacity. They show whether the assumed pile length can actually be installed, whether piles are likely to stop on hard layers or boulders, whether the planned method will reliably install piles, and whether installation variability appears. On site, pile lengths that looked reasonable on paper may be too long to install or too short to provide a sense of hold. Therefore, test piles and load tests should be considered not just as a confirmation step but as the final decision of pile length. NEDO +1

Common characteristics of sites prone to failure in pile-length decisions

Sites that are prone to failure in pile-length decisions share common characteristics. The most frequent is assuming the whole site has uniform ground and deciding a single length from a representative value. Current guidelines state that detailed investigations should be carried out in places like soft ground, reclamation areas, filled ground, engineered fill, slopes, and valley-bottom lowlands. In other words, the more a site contains ground types that require attention, the less you should standardize pile length. NEDO

The second common issue is judging based only on compression and downplaying uplift and lateral forces. Wind loads have a large effect on PV systems, and uplift resistance provided by skin friction is particularly important for pile foundations. A pile length chosen only for downward resistance may be fine under normal conditions but reveal weaknesses under strong winds or eccentric loads. While shorter piles seem easier to install, misjudging external force conditions leaves problems in places that are hardest to fix. NEDO

The third major problem is fragmentation of information among investigation, design, and construction. If site history and ground cautions are not sufficiently reflected in the design drawings, if construction crews do not know where cut/fill boundaries are, or if test-pile results are not propagated to all blocks, pile-length judgment tends to vary on site. Pile-length decisions are not completed by numerical calculations alone; they only make sense when investigation results are shared on site. NEDO +1

Shared precision during construction raises the quality of pile-length decisions

On large PV sites, it is important that stakeholders can quickly share which blocks are cut/fill boundaries, which rows follow valley lines, where soft ground is suspected, and where test piles were installed. Even if pile-length decisions are correct in the design documents, if that information is not tied to positions and shared on site, construction and remediation decisions are likely to be delayed. The more a site has pile-length change zones, the more the positional sharing precision affects quality. NEDO +1

In such cases, using LRTK (iPhone-mounted GNSS high-precision positioning device) to share investigation points, test-pile locations, cut/fill boundaries, and caution blocks via iPhone makes it easier to translate pile-length decisions to the field. While pile length itself is determined by ground investigation and design, on large sites the quicker you can share "where conditions change," the more stable construction responses will be. Deciding pile length correctly and enabling the field to use that decision correctly are equally important.