A hillside house starts to show it. A crack opens over the garage. The downhill bedroom floor tips a little more each year. Someone mentions helical piers.
It’s a fair thing to ask about. Helical piers are one of the more common ways to support a settling foundation. But on a Poway slope, the answer depends on what’s under the house. This post explains what helical piers are, what an engineer looks at, and why they fit some Poway lots and not others.
What a helical pier is
A helical pier is a steel shaft with one or more round steel plates welded to it. The plates are shaped like a screw thread. A machine turns the shaft into the ground, like a screw into wood.
When the pier reaches soil that can carry the load, a bracket connects it to the footing. The house weight then goes down the shaft to the plates, and out into firmer ground below.
There’s no concrete to cure. Installation can be fairly quick. The equipment can be small. That’s why they’re popular for houses where big rigs can’t reach.
How capacity is figured
Helical piers aren’t sized by guesswork. ICC Evaluation Service publishes acceptance criteria, AC358, for helical pile systems. Under those criteria, the allowable capacity is the lowest of several values: the bracket, the shaft, the helix plates, and the soil.
That last one matters most on a hillside. Soil capacity is set either by a site-specific design or by a correlation with the torque measured as the pier is turned in. The installer records torque as each pier goes down. The engineer uses that record to confirm the pier reached the expected capacity.
The California Building Code’s chapter on soils and foundations sets the wider rules for deep foundations, including when a soils investigation is required. On a slope with signs of movement, an engineer will usually want to know what the soil is doing before choosing any pier.
Poway ground is not one thing
The California Geological Survey’s map pamphlet for this part of the county shows a mix of ground under Poway. There are granitic rocks that weather into decomposed granite. There are older sedimentary units with clay. And there’s plenty of fill placed when pads and roads were graded.
Each of those behaves differently with a helical pier.
- Fill and clay over firmer ground. This is the classic case. A pier can be turned through soft or settling material until it reaches denser soil below. Many tract and custom pads in Poway fit this pattern.
- Decomposed granite. DG can range from loose and sandy to very dense. Piers may reach good torque at shallow depth in dense DG.
- Boulders and shallow rock. Granite boulders are common on Poway hillsides. A helical plate can’t screw through a boulder. If the pier hits rock early, it may stop short of where the design needs it. That’s one reason an engineer may choose a different system.
Our post on granite boulders and rockfall has more on how boulders show up around Poway.
Slopes add their own questions
On flat ground, a pier mostly resists downward load. On a slope, the engineer also thinks about the ground moving sideways or downhill.
- Is the slope itself stable, or is only the house settling?
- Is fill creeping downhill, taking footings with it?
- Will slender shafts need extra bracing or a different layout?
- Does the repair need a retaining structure or drainage work along with piers?
If the slope is moving, piers under the house won’t fix the slope. They may only be one part of a larger plan. That’s why a soils report and an engineer’s design come first. See our hillside foundation repair page for how those pieces fit together.
Corrosion and steel in the ground
Helical piers are steel. Steel in soil can corrode. AC358 notes that certain soils are outside its scope. It lists soils with low resistivity, low pH, high organic content and high sulfate levels as corrosive conditions it doesn’t cover.
Most Poway soils aren’t extreme. But where a geotechnical report flags corrosive soil, the engineer has to address it. That might mean coated steel, a different pier, or extra testing.
Access on a big lot
Poway lots are often large and steep. Access can be a bigger issue than the pier itself.
- Can equipment reach the downhill side of the house?
- Is there room to work without cutting into the slope?
- Will the work change drainage?
The City’s grading permit page says a grading permit is needed for excavations two feet or deeper with slopes steeper than 2:1, for moving 50 cubic yards or more, and for changing drainage patterns. Most pier jobs are smaller than that. But if access requires cutting a path or a pad, it’s worth checking early.
Helical piers versus other options
Helical piers are one choice. Others include push piers, drilled concrete piers, and grout or other soil treatments. Each has its own place.
- Push piers rely on the house weight to drive them. Lighter houses may not have enough weight.
- Drilled piers can go through rock but need bigger equipment and concrete.
- Soil treatment works in some soils and not others.
Our post on underpinning in Poway compares methods in more depth.
Paperwork worth keeping
If helical piers are installed, ask for the records and keep them with the house file:
- The engineer’s design and any soils report it relied on.
- The pier layout plan, showing where each pier went.
- The installation log, with depth and final torque for each pier.
- Permit and inspection sign-offs.
These records matter later. A future buyer, an insurer, or an engineer looking at the house years from now will want to know what’s under it. Without the log, nobody can tell how deep the piers went or what they reached.
What to expect from an evaluation
A visit starts with the house. We look at cracks, floors, doors and the slope around the foundation. We note drainage and signs of slope movement. If piers look possible, the next step is usually engineering. Where design is needed, we coordinate with licensed engineers as needed.
We work across all of Poway, from the valley floor to the hills. If your hillside house is showing signs of settlement, request a Free Quote and we’ll set up a time to look.