How Do Solar Farm Foundations Differ from Building Foundations?
The difference is the load that governs design. Building foundations mainly carry compression, while solar farm piles mainly resist wind uplift and lateral push. In a building, the weight of the structure, floors, walls and live loads presses down at all times, so the piles are pushed into the ground and uplift plays almost no role.
Solar farm foundations are the reverse. The mounting structure is a very lightweight steel frame, but the solar panels are wide, flat surfaces tilted toward the sun. When wind blows under the panels, it creates uplift and lateral forces that transfer directly to the piles. The small self-weight of the structure is not enough to counteract the uplift.
As a result, solar farm piles are usually governed by uplift and lateral loads, not compression. A pile that carries compression comfortably can still be pulled out under the design wind gust. Solar farm pile testing therefore uses a different set of tests from ordinary buildings.
Which Pile Tests Does a Solar Farm Need?
There are 3 core items: soil investigation, uplift testing and lateral load testing. Each answers a different question and is done at a different stage of the project, as shown in Table 1.
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| Item | What it answers | When | Reference standards |
|---|---|---|---|
| Soil investigation (Soil Boring + SPT) | What are the soil layers across the site, where are the soft clay or loose sand zones, and what is the groundwater level? | Before foundation design | ASTM D1586 (SPT) |
| Uplift test (Uplift / Tension Test) | Does the pile resist the design uplift, and at what load does it start to move upward? | After installing test piles, before proceeding with production piles across the site | ASTM D3689/D3689M-25 |
| Lateral load test (Lateral Load Test) | How far does the pile move laterally at the design load, and is that within the limit the mounting structure can accept? | Together with or after the uplift test | ASTM D3966/D3966M-22 |
Some projects also add a compression test to ASTM D1143/D1143M-20e1, especially when concrete piles are used in soft soil areas or there are heavy structures such as inverter buildings and transformers. The principle of this test is explained in What Is a Static Load Test. For projects using driven steel piles or ground screws, the tests that decide the design are usually uplift and lateral load. Read the details of each test method in Pile Uplift Test (Tension Test) and Lateral Load Test for Piles.
The diagram below summarizes everything in one figure. The left panel shows the three forces at the head of a solar panel pile and the soil resistance against them. The right panel matches each force with its test method, standard and what is measured. The bottom strip lists the project's test sequence in time order, from soil investigation to random testing of working piles.
Why Does Uplift Govern Solar Farm Pile Design?
Uplift governs design because fewer mechanisms resist uplift than resist compression. When a pile is pushed down, load transfers to the soil in two ways: skin friction along the pile shaft and end bearing at the pile toe. When a pile is pulled up, end bearing disappears entirely, leaving only skin friction plus the pile's own weight.
In addition, skin friction in tension is often lower than in compression, because pulling reduces the stress in the soil around the pile. The same pile therefore usually carries clearly less uplift than compression. Usable values must come from on-site testing, not from taking the compression capacity and multiplying it by a ready-made factor.
Most solar farm piles are short with little contact surface, so the skin friction resisting uplift is limited. Adding just a little pile length can often change test results significantly, which is why testing should be done before ordering piles for the whole project.
How Is Testing Different for Steel Piles, Ground Screws and Concrete Piles?
Solar farms in Thailand use 3 main pile systems: driven steel piles, ground screws and concrete piles. Each system has different points to watch during testing.
1. Driven Steel Pile
These are H, C or W structural steel sections driven straight into the ground. They install quickly and can be removed. The point to watch is corrosion in saline or highly acidic soil, which affects the project's service life of 25 years or more. For uplift testing, the pile-head gripping detail must be designed so the steel does not twist or tear before the soil fails.
2. Ground Screw / Helical Pile
This is a steel shaft with helical plates screwed into the ground. The helices increase uplift resistance compared with a straight pile of the same size, making them suitable where high uplift capacity is needed without going very deep. Testing must also record the installation torque, because it is an indicator of installation quality that can be compared across the whole site.
3. Concrete Pile
Used when soft soil requires deeper piles or when steel corrosion is a concern. Its self-weight helps resist uplift, but the weak points are splices and the reinforcement embedment at the pile head, because uplift pulls directly on the reinforcement. If the development length is insufficient, failure occurs at the pile head before the soil fails, and the soil capacity cannot be read.
Which Soils Pose a Risk to Solar Farm Piles?
There are 3 soil groups to watch: saline soil that corrodes steel, loose sand, and flood-prone areas or expansive soil. Solar farms are usually built on land outside towns, which often has soil conditions unsuitable for ordinary construction, so all three groups must be checked clearly from the soil investigation stage.
Saline and steel-corrosive soils Areas near the coast, former salt fields, or areas with saline groundwater greatly accelerate corrosion of steel piles. The soil investigation should collect samples to test soil resistivity, pH, and chloride and sulfate content, so the designer can choose between thicker galvanizing, a corrosion allowance in thickness, or switching to concrete piles. Read how it is measured in Soil Resistivity Test for Solar Farms.
Loose sand provides low skin friction and is sensitive to vibration. Uplift capacity measured in loose sand is often lower than expected from SPT values alone, so actual test results matter more than formula estimates. Read how to interpret SPT values in How to Interpret SPT N-Values.
Flood-prone areas and expansive soil Where the groundwater level rises and falls with the seasons, soil strength changes back and forth, so values measured in the dry season may not apply in the rainy season. Clays that swell and shrink with moisture can push or pull short piles. The report should always state the groundwater level encountered and the testing period. The installation level of panels and equipment in flood-risk areas is set from the flood study results (Flood Study).
If the project is still at the land selection stage, checking soil conditions before deciding to buy greatly reduces risk. Read the items to check in Before Buying Factory Land: An Engineer's Soil Checklist, which applies the same principles to solar farm land.
How Should Pile Test Locations Be Planned Across a Solar Farm Site?
The principle is to divide the site into zones based on soil investigation results and then spread test locations to cover every zone, rather than using results from one location to represent the whole site. Solar farms cover very large areas, some projects several hundred rai, and the soil layers can differ completely from one corner to another. Using results from a single borehole or a single test pile for the whole site is therefore a risk not worth taking.
The common approach is to divide the area into zones by topography and soil investigation results, then spread test locations to cover every zone, especially zones where boring results differ from the rest. The appropriate number of boreholes and test piles depends on site size, soil variability, and the requirements of the designer or project owner. Read the principles for setting the number of boreholes in How Many Boreholes and How Deep.
Test locations should also be separated by pile system, because different pile types and lengths give very different results. If a project uses more than one pile system or several lengths, the test results of one type cannot represent another. For projects using ground screws, the recorded installation torque helps compare installation quality across the whole site.
The sequence that makes test results most useful is: soil investigation before design → install test piles and complete uplift and lateral load tests before ordering piles for the whole project → proceed with production pile installation → randomly test working piles at the proportion set by the specification. Projects that test after all materials have been ordered have far fewer options if the results are not as expected. The difference between these two stages is explained in Test Piles vs. Working Piles: What Is the Difference.
The 3D view below models one block of a solar farm divided into 3 soil zones. Note that every zone has its own boreholes and its own uplift and lateral test locations, while compression test locations are only at some points. The box on the right summarizes 5 principles for placing test locations; the actual number of locations must be set for each project.
How Much Does Solar Farm Pile Testing Cost, and How Much Time Should You Allow?
Solar farm pile testing is priced per project based on the actual scope, because it depends on site size, number of soil zones, number of test locations and maximum test load. For reference, SPN's soil investigation for general projects starts at about THB 20,000-40,000 (2-3 boreholes, including basic laboratory testing and a report certified by a civil engineer licensed by the Council of Engineers Thailand (COE)). For solar farm projects requiring many boreholes, plus uplift and lateral load testing, please contact us for a free quotation within 24 hours.
Note: This price excludes travel and equipment mobilization costs, which are charged according to the distance to the site. Most solar farm projects are in the provinces, so state the site location when requesting a quote to get a realistic figure.
For scheduling, plan soil investigation and test pile testing ahead of the pile ordering date, because you need to wait for the results and for the designer to review them. The actual duration depends on the number of zones and test locations, so it is stated in each project's quotation. SPN provides pile testing for uplift, lateral and compression loads nationwide across all 77 provinces.
This content explains the principles for selecting tests for solar farm projects for general understanding only. Design uplift and lateral loads, pile length and size, number of test locations, and acceptance criteria must be set by the project's design engineer based on actual test results.