From Soil Test to Foundation Selection: The Solar EPC Engineer’s Guide to Geotechnical Investigation

#TLDR

Before you specify a single pile, you need three things from your geotechnical report: soil classification (USCS or ASTM D2487), bearing and skin-friction values, and friction angle or cohesion. These three values, combined with site-specific C-post pull-out test results at embedment depths from 1.5 m to 3.5 m, determine whether you drive a C-post, screw a helical pile, predrill a micropile, or pour a concrete ballast footing. Skipping this step costs more in change orders than it saves in survey fees.

Why Soil Type Determines Your Foundation — Not the Other Way Around

Every solar EPC project carries the same structural challenge: a 25-year asset sitting on ground you cannot fully control. Wind uplift, frost heave, seismic lateral loads, and long-term settlement all pass through one critical interface — the foundation.

The mistake many project teams make is selecting a foundation type first based on cost or familiarity, then hoping the soil cooperates. This works on homogeneous, well-documented sites. On real-world ground — laterite in West Africa, expansive black cotton soil in South Asia, loose alluvial sand in river deltas — it leads to foundation pullout failures, differential settlement, and expensive rework mid-construction.

The correct sequence is:

1. Conduct geotechnical investigation
2. Classify soil and determine bearing parameters
3. Run pile test (specifically C-post pull-out) at representative embedment depths
4. Select foundation type based on data
5. Confirm with structural analysis (SAP2000 or equivalent)

A good investigation follows a site-specific test plan scaled to site size, surficial geology and topography — not a one-size-fits-all checklist.

Soil Classification: What the Geo Report Actually Tells You

The USCS Classification System (ASTM D2487)

The most widely used soil classification system in geotechnical engineering is the Unified Soil Classification System (USCS), standardized under ASTM D2487. It divides soils into two broad families:

Coarse-grained soils (more than 50% retained on a No. 200 sieve):

• GW / GP — Well-graded or poorly-graded gravel
• SW / SP — Well-graded or poorly-graded sand
• GM / SM — Silty gravel or silty sand
• GC / SC — Clayey gravel or clayey sand

Fine-grained soils (50% or more passing a No. 200 sieve):

• ML / MH — Low or high plasticity silt
• CL / CH — Low or high plasticity clay
• OL / OH / PT — Organic soils and peat (problematic for any pile foundation)

What This Classification Means for Foundation Choice

Key Bearing Capacity Values by Soil Type

Ultimate bearing capacity (qu) quantifies how much load per unit area the soil can support before shear failure. It depends on the size and depth of the footing as well as on the soil, so the geotechnical engineer calculates it for each foundation; what codes publish for early screening are presumptive allowable bearing pressures, which already include a safety margin and sit well below ultimate values (IBC 2021 Table 1806.2, reproduced below with values converted from psf). For solar mounting foundations, engineers care about two load types: axial compression (dead load) and tension / pull-out (wind uplift, which is the governing load case for most utility-scale ground-mount systems).

Important: These are code values for shallow footings in compression; higher values need site data, and they say nothing about uplift. Site-specific pull-out testing always governs pile design.

The 5 Foundation Types Used in Solar Mounting

1. Driven C-Post (C-Channel Pile)

Best for: Dense sand (SW/GW), medium-stiff clay (CL), sandy loam (SM/SC) Governing check: shaft friction in uplift, confirmed by pull-out test

The C-post is a cold-formed steel channel section — typically 100×50×3.0 mm or 120×50×3.0 mm — driven vertically into the soil using a hydraulic hammer or vibratory driver. It is the most common and cost-efficient foundation for utility-scale ground-mount solar in favorable soil conditions.

• 100×50×3.0C: lighter, faster to drive, lower steel tonnage per MW
• 120×50×3.0C: wider flange means higher passive soil resistance and greater pull-out capacity

Embedment is set from the pull-out test: shallower in dense sand or stiff clay, deeper in softer ground, within the 1.5–3.5 m range the pull-out tests below cover.

2. Helical / Ground Screw Pile

Best for: Loose to medium sand (SP/SM), soft to medium clay (CL/ML), mixed layered soils Governing check: helix bearing in uplift and compression, confirmed by installation torque and load test

Helical piles use one or more helix plates welded to a central steel shaft. Installation is torque-controlled, providing an indirect measure of soil resistance in real time.

3. Micropile (Drilled and Grouted)

Best for: Rock, hardpan, caliche, or dense cobble layers that cannot be penetrated by driven piles

Micropiles require drilling, inserting a steel tube or rebar cage, and grouting. Used in mountainous or rocky terrain for agrivoltaic or hillside ground-mount projects.

4. Concrete Spread / Strip Footing

Best for: Very soft or waterlogged soils, expansive clays (CH), high water table sites

When soil is too weak to transfer point loads from a pile, spread footings distribute structural load over a larger area.

5. Ballast / Non-penetrating Foundation

Best for: Impenetrable rock, contaminated land, rooftop carport extensions, sites where ground penetration is prohibited

Concrete ballast blocks or steel frames resist wind uplift through gravity. No excavation or soil testing required.

Critical Geotechnical Parameters for Racking Design

1. Soil Classification (ASTM D2487)

Request borehole or test-pit logs at enough locations to capture how the soil varies across the site — the geotechnical engineer sets the spacing from site size and geology — and take each one below the deepest planned pile tip.

2. Bearing Capacity and Skin Friction

The governing case for solar pile design is almost always tension/pull-out, not compression.

Ask for skin friction (shaft resistance) by soil layer as well as bearing values: uplift on a driven post is resisted almost entirely by friction along its embedded length.

3. Friction Angle (φ) and Cohesion (c)

• Friction angle (φ): Dominant in granular soils. Higher φ = greater passive resistance = better pull-out resistance.
• Cohesion (c): Dominant in clay soils. Sensitive to moisture — a site that tests well in dry season may have cohesion drop after rain.

4. SPT N-Value (ASTM D1586)

• N < 4: Very loose — pile penetration easy but pullout resistance very low
• N = 4–10: Loose — borderline for driven C-post; helical pile or deeper embedment likely needed
• N = 10–30: Medium dense — standard C-post often adequate; confirm embedment by pull-out test
• N = 30–50: Dense — good for C-post; check driving feasibility
• N > 50 or refusal: Very dense soil, cobbles or rock — predrilling may be required

5. Atterberg Limits

IBC 2021 §1803.5.3 identifies expansive soil from its plasticity index (15 or more), fines and clay content, and expansion index (above 20); the higher the PI, the greater the swell potential. In expansive clay the pile will experience cyclic uplift and compression as the clay swells and shrinks seasonally over a 25-year life.

6. Corrosion Parameters: Resistivity, pH, Chloride and Sulfate

Measure in-situ resistivity (ASTM G57) and, on samples, laboratory minimum resistivity (AASHTO T 288), pH (ASTM G51) and water-soluble chloride and sulfate. Published schemes draw the corrosive line in different places: in the NCHRP classification reproduced by the FHWA, soil below 2,000 Ω·cm is rated corrosive and below 700 Ω·cm very corrosive; FHWA/AASHTO criteria for non-aggressive fill are resistivity above 3,000 Ω·cm, pH 5–10, chlorides below 100 ppm and sulfates below 200 ppm; Caltrans treats a site as corrosive where chloride is 500 ppm or more, sulfate 1,500 ppm or more, or pH 5.5 or less. These limits refer to laboratory minimum resistivity, which can be lower than an in-situ reading. Name the scheme in the specification and use the results to set the post protection — ZAM-coated steel or hot-dip galvanizing, the coating mass and any sacrificial thickness.

For more on coating selection, see: HDG, ZAM, or Stainless — The Solar EPC Manager's Guide to Mounting Structure Coatings.

The test methods behind these limits are covered in Part 1: What the Ground Is Telling You: Soil Types and Geotechnical Tests Every Solar EPC Engineer Needs to Know.

7. Frost Penetration Depth

Pile tip must extend below the frost line. Take the design frost depth from the local building code or national frost-depth data — IBC 2021 §1809.5, for example, requires foundations to extend below the frost line of the locality — and check that the length below the frost line anchors the post against adfreeze uplift on the frozen section of the shaft.

The C-Post Pull-Out Test: How It Works and What Embedment Depth to Target

Why the Pull-Out Test Cannot Be Skipped

Wind uplift is the critical load for solar racking. Manufacturer datasheets give theoretical pull-out values based on soil assumptions — they are starting points, not site guarantees.

Test Setup (per ASTM D3689)

1. Drive sample piles at 1.5, 2.0, 2.5, 3.0, and 3.5 m embedment using the same driver and installation method planned for production
2. Apply tension load using a hydraulic jack reacting against a crossbeam
3. Measure displacement with dial gauges or LVDTs
4. Identify failure load against a displacement criterion set by the foundation engineer: ASTM D3689 does not fix one, so state it in the test specification
5. Apply the safety factor of the governing code: IBC 2021 §1810.3.3.1.5 limits allowable uplift to the ultimate capacity from ASTM D3689 tests divided by 2.0, or by 1.5 where uplift is due to wind or seismic load; limit-state codes such as EN 1997-1 use correlation and partial factors instead

Reading the Results

Plot load against displacement for each test pile and read the capacity at the agreed displacement criterion. Capacity rises with embedment, and the wider 120×50 section usually carries more than the 100×50 at the same depth, but the values belong to your site: a table of typical pull-out values from other ground cannot stand in for them. Test enough piles in each soil zone the geotechnical report identifies, and repeat in the wet season where cohesive soils are present.

When to Extend Embedment vs. Change Foundation Type

• Required capacity met at 3.0 m or shallower with C-post → continue with driven C-post
• 3.5 m embedment still falls short → evaluate helical pile or predrilling + grouting
• Driving refusal before target depth → predrill through hard layer, then drive or switch to micropile

Soil Type to Foundation Type: Decision Framework

Common Mistakes EPC Teams Make with Foundation Selection

1. No geo test at all. Cost is cited as the reason. Change orders after a foundation failure cost far more than the investigation would have.
2. Single test point for a large site. Soil variability across 5 MW can span two full rows in the bearing capacity table.
3. Pull-out capacity assumed from manufacturer table. These are handbook values in ideal soil — not your site.
4. Dry season test on a seasonally wet site. Cohesion drops dramatically when clay saturates.
5. Ignoring corrosion parameters. A C-post that meets structural requirements on day one but loses section faster than its corrosion allowance is still a failure.

Solaracks Foundation Support

Solaracks supports EPC teams from the geo report stage:

• Foundation type recommendation based on your soil classification and bearing capacity data
• Section selection (100×50×3.0C vs. 120×50×3.0C) and embedment depth confirmation from pull-out test results
• SAP2000 simulation where needed
• Static structural analysis with loads to the applicable standard (for example ASCE 7-22 or JIS C 8955:2017)
• All delivered within 24 hours of receiving your geo report

Send your geo report to info@solaracks.com or contact us through solaracks.com.

Sources: International Building Code 2021, Chapter 18 (§1803.5.3, Table 1806.2, §1809.5, §1810.3.3.1.5); ASTM D3689/D3689M, Deep Foundation Elements Under Static Axial Tensile Load; TxDOT Geotechnical Manual (LRFD), Table 4-1, SPT N and relative density; FHWA-NHI-09-087, Corrosion/Degradation of Soil Reinforcements (2009); Caltrans Corrosion Guidelines v3.2 (2025), Chapter 6; ASTM D2487; EN 1997-1.