Geotechnical Engineering for Solar Foundation Design

PRI Virtual Event: Recording Now Available

Geotechnical investigation and pile testing and design are often overlooked yet extremely important steps in the development of a new solar project. With proper pre- and post-production testing and construction QA/QC, the lifetime of any civil or utility-scale energy project can double.

On Thursday September 10th, we held a virtual event on Geotechnical Engineering for Solar Foundation Design with Arash Yazdani, Director of Engineering Services, PRI Engineering and guest speaker Vishal Lala, Managing Director, Polar Racking Inc.

Quick Recap: Major Aspects of Geotech for Solar Foundation Design

Understanding the major aspects of geotechnical engineering for solar foundation design will help support better decision making both before and during construction.

The first major aspect of solar racking foundation design requires a review of the racking induced loads (such as wind, snow, and seismic loads) and the geotechnical design loads (such as frost design depth and adfreeze pressure).

The second major aspect is the onsite geotechnical investigation. This normally consists of a subsurface investigation which includes a preliminary site visit, test pit/borehole testing, and development of a subsurface profile for preliminary pile design.

Also involved in the geotechnical investigation is pre-production testing. Pre-production testing of the piles verifies design assumptions, assesses constructability, develops a better understanding of subsurface consistency, and reduces the risk during construction through optimizing design considerations.

Production testing differs from pre-production testing because it is the verification of the design during construction. It includes testing of the non-conforming piles, identification of deflection tolerances as required by the racking supplier for the solar project, and general project risk mitigation.

What’s Next?

PRI Engineering’s next webinar on October 8th (see details here) will cover more information about construction inspection services involved in production testing for solar racking foundation design. Follow us on social media to stay up to date with the latest news and events.

Transcription of Q&A*

How many test pits or boreholes are recommended for an area or pile or foundations?

That’s a very good question. What we typically recommend is we would have a standard number of test pits for a certain size of site. For example, let’s say you got like a 25-30-acre site, that’s going to be in the community solar size of project. We’d recommended about 20-25 test pits. You could maybe get that down to 15 test pits.

One thing I really want to stress here though is that we really got to let the ground do the talking for us. So, we might start with 15 test pits and if we’re finding extremely consistent conditions across the site, we’re probably good with 15 test pits.

But, if we find that there’s some very drastic changes spatially across the site and you’ve got different soil types, different ground conditions, different ground water conditions, and you’ve got different depths to bedrock – a lot of these factors have a major contribution to the final design. So, in those cases we may recommend to you to do another 10 test pits in this area over here because we found a very variable depth to bed rock. Or, it might make sense to do a couple more test pits here because we found an extremely soft clay layer which definitely needs to be analyzed a little bit closer from a foundation resilience standpoint.

So, hopefully to answer your question, like I said something in the 25-acre range we’d probably recommend something in the 15-20 test pits. However, I always like to look at projects on a case-by-case basis and if you have a project specifically you want us to take a look at feel free to send the details over to info@priengineering.com and either myself or someone on my team would be happy to send you a preliminary proposal on what could be completed.

What solutions do you recommend for rocky soil types?

So, I would say two things. Number one, a good foundation type for rocky soils is ground screws. If you remember the foundation slide presented, it showed what looked like a big wood screw. So, that’s a good pile for a rocky condition.

The other thing is, we had a really interesting site in Alberta where they were having zero success with helicals. They really wanted to do helicals but the helicals kept getting premature refusal at about four or five feet. In that case, what we did was we actually used a driven ground post. The benefits to that is you can use the energy that is being used to install the pile to the ground to displace the obstruction.

So, once again, if we’re talking about rocky ground – boulders and cobbles – driven piles and ground screws would be where I’d start. Of course, going back to our process, if you do the test pits ahead of time, I’d have it in the back of my head that we probably want to look at driven piles and ground screws. However, we want to assess that with test pits to determine the feasibility and then actually do the construction feasibility.

Do you have any experience working in Florida – high wind coastal solutions including carports, pavilions, others?

That’s a really good question. In short, yes, we can definitely help with high wind areas such as coastal zones in the Southern United States.

When you think about somebody from Canada that specializes in design for northern climates you probably don’t think of a project in Florida as something that would kind of suit our expertise. But one of the interesting things is that wind generates uplift, and back to one of my first slides during the presentation, frost is ultimately a very large uplift load.

So, from that standpoint, we do have a very good understanding of uplift and I can say right now we’re working on a project South of Florida where the reason we were selected as the designers for that project was because of our understanding of uplift design.

From a mechanism standpoint, wind is an uplift load like frost, but there is a bit of a difference between wind in the sense that it’s a transient load and it’s not a long-term sustained load like a frost load is. Frost load is going to act on the pile and it’s going to act over it for a long period of time.

In contrast, a wind load, whether you’re in Canada or the U.S., is well recognized as a gust load. When your uplift occurs and when the wind softens, that loading stops. But when you have frost and the ground freezes, it’s sustained over a long period. Fact of the matter is though, whether you’re holding something for 10-15 seconds or hours and days, the philosophy of the design is the same.

How do you recommend handling very small sites where the cost of geotech investigation can’t be justified?

I’ll be honest, this is one I’ve wrestled with on a multitude of projects. The only thing I can really recommend there is one of two things. You either need to have some background information that we can obviously reduce our design assumption risks or you need to have an installer that has a little bit of flexibility.

And what I mean by that is, three of the more common foundations are helical piles, ground screws, and driven piles. So, if you’ve got a smaller system and you know that you’re going to be using one of those three systems, it’s very beneficial if you can select a pile contractor that has the ability to maybe install a multitude of foundations with the same piece of equipment.

The equipment required to install a ground screw and a helical pile is more or less identical. You need a drive head which can be connected to an excavator or a bobcat which would then get connected to the hydraulic lines of the drive head and that would screw that foundation into the ground. There is also a hydraulic connection piece for a pile driver, and we’re currently working with a couple contractors to try and get them tooled up so that they have the capabilities of doing those different types of foundations installations.

So, if you do have a site that’s small and you really can’t justify doing geotech ahead of time, you can have this contractor come to site and he’s not going to basically back you into a corner where you need to make one type of foundation work.

I want to go back to an earlier point – we aren’t foundation agnostic. We don’t think that one foundation can resolve all your subsurface challenges. You need a multitude of foundations to combat different subsurface conditions. So obviously having a contractor that is going to work with you and be flexible to do those different types of installations is going to help.

How do you overcome high slope conditions? 40-degree hills for example.

Maybe now I can ask a follow up question. Is that slope a constant slope? And what I mean by that is, is it uniform 40-degrees from top to bottom or is there any undulations? I’m going to assume for the purpose of this question right now that it’s a uniform slope.

So everybody knows what happens when you take a 40-degree slope and you rotate it 40-degrees – it becomes flat. One of your larger challenges isn’t the racking system, isn’t the foundation system, it’s actually the installation. You need to be able to select an installation method that would be able to work on that 40-degree slope.

Now, that may require, depending on exactly how steep that slope is, what your foundation design is, what your pile orientation is and maybe doing some grading which I know is one of the words a lot of solar developers do not like to hear. Or it may involve shopping around and looking for a contractor that could install on that slope.

I can tell you that we’ve done projects up to 35 degrees maybe even 38 degrees so we’re not far off with 40. It’s definitely doable. As was mentioned in the chat, this is where the preliminary site visit really comes into play. Because if we come and do that preliminary site visit, that is one of the things that we are checking – your ground conditions and specifically the slope and topography.

If we know ahead of time that we are going to be dealing with a 40-degree slope, we will be keeping that in the back of our head, and we will be talking with the contractor on that project to make sure that the design we are proposing to install is feasible. And remember as part of the process, I recommended to do that pre-production testing program and we’re doing a construction feasibility. From that preliminary work, we are going to be able to get real life information, real life data, on how suitable is this foundation for that topography.

Now, let’s assume that the site in question is not uniform – so you’ve got undulating slopes. That one is a little tougher to combat, but one thing that we’ve recommended in the past – I mean obviously you can grade – but like I said, grading is very expensive and it creates a lot of other challenges with the project. There’s a lot of soil erosion control measures that need to go in place if you’re doing a major earthworks program. So, if you can avoid that, that’s great.

One thing we’ve done in the past is, instead of grading, we’ve upsized the size of your posts to accommodate a higher moment resistance. So if you can picture it, if your rack was one meter off the ground, but then, in order to accommodate the grade, maybe it needs to be two meters off the ground. All of a sudden you’re increasing the moment acting on that pile by two. So, in order to obviously increase your resistance, you’re going to have to play with the dimensions of that pipe or maybe increase the strength (there’s a lot of different things that could be done). We could let the steel work for us instead of having to do a grading program.

How do you ensure that the bearing capacity of a soil isn’t disturbed by pit excavation?

I just want to clarify that the question you’re asking is – is this specific to doing a geotech investigation using a test pit method and how are we ensuring that were getting in-situ undisturbed values?

I don’t know if you’re familiar with a dynamic probe test. It’s very similar to an SPT and we would actually do that test in undisturbed soil maybe just to the side of the test pit. A lot of times, we would do it at surface before we start the excavation. We’d excavate to a depth where it’s safe for the technician to enter, which is universally about 4 feet. In some cases, it may not be safe to enter a 4-foot excavation and our staff are trained to assess excavations before entering them but we’d also do an in-situ test at about 4-feet so that would give us about a 10-foot profile of in-situ shear strength at the site. And the key is that we are doing that in undisturbed soils, so it really is an in-situ representative test.

*Edited for clarity