The foundation of a solar farm is the work package that weighs most on the schedule and one of those that most conditions the durability of the plant. A wrong choice here is not corrected later: it is paid for in weeks of delay, in cost overruns or in structures that move over the years.
In Spain, direct driving of metal profiles is the usual solution in ground-mounted photovoltaic plants. But it is not universal. There are soils where the pile does not go in, soils where it goes in but does not hold, and soils where a concrete foundation is still the right answer.
The choice between a driven foundation and a conventional one is not made by the budget. It is made by the ground, and the geotechnical study is what tells you.
What a driven foundation is in a solar plant
It consists of introducing galvanised steel profiles (C, omega, IPE or HEA sections depending on the structure manufacturer) directly into the ground with a pile driver, with no prior excavation and no concrete. The profile itself acts as the foundation element and as the leg of the structure.
The machine drives the profile by impact, vibration or pressure, controlling position, verticality and embedment depth. That depth is set by the structural calculation and later checked against field tests.
Current pile drivers handle profiles of up to 5 metres, in line with what the latest solar trackers require. That is why pile driving for photovoltaic plants has become the reference technique for plants of tens of megawatts.
Three variants depending on the ground
- Direct driving: the profile goes in with no preparation. It is the fastest and cheapest option when the ground allows it.
- Pre-drilling and driving: a smaller-diameter pilot hole is drilled and the profile is then driven. It solves hard soils, coarse gravels or cemented layers.
- Pre-drilling and grouting: the profile sits in a bore that is filled with grout or concrete. It is the boundary with conventional foundations and is used in rock or in soils with no lateral friction capacity.
What we mean by conventional foundation
We are talking about concrete footings or piles cast in situ, with excavation, reinforcement, pouring and curing. The steel structure is anchored to them with embedded plates or bolts.
It is the same principle as any building foundation, applied to thousands of support points spread over tens of hectares. That is where its problem lies: what in a building is fifty footings, in a solar farm is tens of thousands.
Its advantages are real. A concrete foundation is indifferent to soil type, takes high loads and does not depend on the lateral friction of the ground. So are its costs: more earthworks, more concrete logistics, curing times and a larger environmental footprint that later has to be decommissioned.
Five criteria for deciding the foundation of a solar farm
The ground rules
Direct driving works in cohesive and granular soils of medium consistency: clays, silts, compact sands, fine gravels. There the profile develops the lateral friction it needs to resist pull-out and lateral loads.
It starts to fail in three scenarios. Rock or cemented layers close to the surface, where the profile does not penetrate or buckles. Man-made fill and very soft soils, where it goes in but does not hold. And expansive soils, which push the profile with moisture cycles.
All of this is picked up by a well-designed geotechnical study, with boreholes and penetrometers distributed across the whole plant area, not just in one corner.
Field tests confirm or refute
The geotechnical study says what to expect. The tests say what really happens. Before starting production, a test-driving campaign is carried out in the representative areas of the farm.
- Pull-out tests: measure the vertical load the driven profile resists at design depth.
- Lateral load tests: reproduce wind thrust on the structure and check the deformation of the profile.
- Driving log: final depth, deviation, refusals and time per profile, which later defines the real site output.
A pull-out test that does not reach the design load is not solved by driving harder: it is solved by changing the profile, the depth or the foundation system in that area.
The schedule, with real data
Speed is the main argument for driving, but it should be handled with site numbers. A study published by pv magazine on real construction times of photovoltaic plants in Spain, with data from more than 40 farms of around 50 MW, puts pre-drilling and driving at 11.5 planned weeks against 16 actual weeks.
In other words: even the fast solution slips when the ground surprises. A concrete foundation on those same hectares would add excavation, pouring and curing on top of that baseline.
The structure that goes on top
Founding a fixed-tilt structure is not the same as founding a single-axis tracker. IDAE distinguishes between fixed and solar-tracking photovoltaic technologies, and that distinction reaches all the way down to the ground.
Trackers concentrate dynamic wind loads on the rotation axis and require very strict alignment tolerances between posts. Laser-guided driving meets them without difficulty; a poorly set-out concrete footing forces corrections with adjustment pieces that make assembly more expensive.
Sustainability and end of life
The service life of a solar farm is around 25-30 years, after which the land must be returned. The UNEF recommendations for ground-mounted photovoltaic plants call for minimising the use of concrete and restoring the original state of the land at the end of the installation.
A driven profile is extracted and recycled. A buried concrete footing is waste that must be demolished and removed. More and more developers and lenders are writing this criterion into the specification.
Mixed solutions: the farm is not homogeneous
In practice, few large farms have a single type of ground. The usual approach is to zone the foundation: direct driving over most of the area, pre-drilling on rocky rises and grouting at outcrops.
That zoning is decided at design stage with the geotechnical study and adjusted with the field tests. A contractor that only knows how to drive, or only knows how to pour, forces a single solution across the whole farm, and that almost always costs money somewhere.
The right question is not “driven or concrete”. It is “where driven, where pre-drilled and where concrete”, and what real output each zone delivers.
Who executes the foundation in a solar EPC project
In the chain of an EPC project, civil works and pile driving usually fall to a specialised subcontractor that provides machinery, field crews and experience in the different piling techniques in civil works.
The decisive criterion is in-house machinery. A contractor that rents its pile drivers depends on third-party availability precisely in the phase that sets the critical path. As an international construction company Site and Field, we carry out pile driving with our own fleet of pile drivers, integrating pre-drilling, extraction and re-driving whenever the ground requires it.
We deploy that same capability on photovoltaic projects in the United Kingdom, Denmark, Luxembourg and the United Arab Emirates through our local subsidiaries, adapting procedures and testing to the regulations of each country.
Frequently Asked Questions about driven foundations in solar farms
What is a driven foundation in a photovoltaic plant?
It is the system in which galvanised steel profiles are driven directly into the ground with a pile driver, with no excavation or concrete, and act both as the foundation and as the leg of the solar structure. The profile resists through lateral friction with the soil and its depth is defined by the structural calculation, checked against field tests.
When is pile driving preferable to a concrete foundation?
Driving is preferable in cohesive or granular soils of medium consistency, where the profile develops enough friction and production is fast. Concrete foundations are reserved for outcropping rock, fill, very soft soils or areas with special loads, where the driven profile does not penetrate or does not reach the design resistance.
What type of ground allows direct driving of profiles?
Clays, silts, compact sands and fine gravels without boulders allow direct driving. Hard or cemented ground requires prior pre-drilling, and rock or soils with no friction capacity require the bore to be filled with grout or concrete. Expansive and highly corrosive soils require specific analysis before deciding.
What role does the geotechnical study play in choosing the foundation?
The geotechnical study identifies the strata, their strength, the water table, and the expansiveness and corrosivity of the soil across the whole farm area. With that information the foundation is zoned and the design profile and depth are defined, which are then validated with test drives and with pull-out and lateral load tests.
Who carries out pile driving and civil works within a solar EPC project?
Usually a subcontractor specialised in civil works for photovoltaic plants that provides the pile drivers, the testing equipment and the experience in pre-drilling and re-driving. Having in-house machinery is the decisive criterion, because driving sets the critical path of the plant and any dependence on third parties translates into delays.
