91
g/solar • Posted by u/roof_mount_ray •

Ground mount on expansive clay — two years of movement and what I would do differently Design

24kW ground mount, driven pier foundation, installed two years ago on a site with expansive clay that I did not adequately account for.

What happened: seasonal movement of roughly an inch and a half of heave and settlement across the array. Not uniform — the north rows moved more than the south. Two years in, module clamps have loosened on about a dozen modules and one rail joint has a visible gap.

Nothing has failed and production is normal. But this array is going to need attention every few years for its whole life and it should not.

What I would do differently:

Get a geotech report. I skipped it because it was a residential ground mount and the cost felt disproportionate. It was maybe $1,800. The remediation is going to cost more than that.

Drive deeper than the frost line. Frost depth is the number everybody uses and on expansive clay it is the wrong number entirely. Active zone depth for moisture change is what matters and it can be substantially deeper.

Helical piers, not driven. Helicals resist uplift, which is the direction clay heave actually pushes.

Allow for differential movement in the racking. Slip joints at rail splices instead of rigid connections.

Posting this because ground mounts get treated as the easy version of solar and the foundation is a real engineering problem that I treated as a detail.

💬 5 replies

Log in or sign up to leave a reply.

u/millwright_mo •

Helical piers over driven on any expansive or soft soil, and I would go further — on clay specifically, get the installing torque logged. Helical capacity correlates with installation torque and it is the only in-situ verification you get. An installer who does not log torque has given you a pier of unknown capacity.

62
u/roof_mount_ray OP •

Torque logging is not something I knew was standard practice and it is obviously the right answer. Adding it as a spec requirement on the next one.

33
u/solar_steve •

The geotech point is the one to underline. $1,800 on a project that size is about 2% and it is the only way to know what you are building on. Everybody skips it on residential and it is the single most common source of ground mount problems.

Appreciate you posting this. Threads where somebody explains what went wrong are worth ten threads about what went right.

54
u/pipefitter_pat •

Expansive clay is a known problem in every other discipline that puts things in the ground. Plumbers deal with it constantly — it is why slab leaks cluster in certain regions. Foundation movement is a mature field with real answers, and solar as an industry mostly has not gone and read them yet.

41
u/estimator_erin •

Worth noting for anyone pricing these: a geotech report is also a liability document. If you build to a report and the soil does something unexpected, that is a different legal position than if you built to an assumption. The $1,800 buys engineering and it buys a defense.

29