People ask us about “south Poway” like it’s one neighborhood. It isn’t. The City’s Safety Element says land uses in the southern portion of the city are mainly industrial, commercial and open space. Most of the homes near that end of town are on the southwest side. And the southwest side is where Poway’s geology gets unusual.
That’s what this post is about. Not a sales pitch for any one street. Just what’s under these tracts, what tends to go wrong, and what you can check yourself.
Poway grew in tracts
A quick bit of history. The City’s history page says Poway Valley Homes opened the first subdivision in the late 1950s. Lake Poway came in 1971. The city incorporated in December 1980. A lot of Poway’s housing went in as graded tracts over those decades. Graded means cut and fill. Some lots sit on native ground. Some sit on fill pushed in from somewhere nearby. Often a single lot has some of each.
That matters more on the southwest side than almost anywhere else in town. Here’s why.
Three units you won’t find much elsewhere
The Safety Element describes the city’s geologic units. Two of them, the Mission Valley Formation and the Pomerado Conglomerate, underlie only a small part of Poway. Both are in the southwest portion of the city.
- Mission Valley Formation. A soft, friable, light silver-gray sandstone. Fine to medium grained. Rich in quartz. It also has lenses and tongues of claystone that locally make up about 20 percent of the section.
- Pomerado Conglomerate. A massive cobble conglomerate. Geologically it matches the Stadium Conglomerate, the unit full of the well-known rounded “Poway” cobbles.
- Friars Formation. More widespread. Lagoonal and nonmarine sandstones with expansive clays. It shows up near these units and across other parts of town.
Short version: sandstone that crumbles, cobble beds that are hard to dig, and clay layers inside both.
The real issue is the fill
Native sandstone and cobbles usually aren’t the problem. The fill is.
The Safety Element puts it directly. Fill soils derived from the Friars and Mission Valley formations are the main materials involved in fill settlement and expansive soil problems in Poway. It also says that historically, the Friars and Mission Valley formations have been involved in numerous landslides.
So when a tract was graded here, the fill under a pad may have come from those same units. If it was placed and compacted well, it can perform fine for decades. If it wasn’t, or if it gets wet year after year, it can settle or swell.
One caution. We’re not saying any particular street has a problem. Most homes on the southwest side do just fine. A geologic map or a soils report for your lot is what tells you which unit, and how much fill, you’re actually sitting on.
Red flags on a southwest pad
Here’s what we’d watch for on a southwest Poway tract home:
- Cracks in the slab or garage floor that keep getting wider.
- A diagonal crack running up from a door or window corner.
- Doors that stick in one season and swing free in another.
- A gap opening between the patio slab and the house.
- A back yard that slopes toward the house after years of landscaping.
- A retaining wall at the back of the pad that leans or has stair-step cracks.
One of these alone isn’t cause for alarm. Several together, on a fill lot, are worth checking.
Water is the trigger
Clay-rich fill doesn’t move much when it stays at a steady moisture. It moves when it gets wetter or drier. The Safety Element lists saturation from over-irrigation, leaky pools and pipes among the triggers for slope trouble in Poway.
The residential code says ground around a house should fall at least 6 inches in the first 10 feet away from the foundation. Where that can’t happen, swales or drains should carry water away. Check yours.
Do:
- Keep downspouts discharging well away from the house.
- Water deep and less often, not a little every day.
- Fix leaks in pools, spas and irrigation lines promptly.
- Keep the soil level along the slab steady, not swamped then baked.
Don’t:
- Pile planters and mulch against the stucco.
- Let a sprinkler spray the foundation.
- Cut into a slope at the back of the pad without checking the grading rules.
- Patch a growing crack and forget about it.
Before you dig
Planning a pool, a new wall or a bigger patio? The City of Poway’s grading page says a grading permit is needed in advance for excavations 2 feet or deeper with slopes steeper than 2:1, for moving 50 cubic yards or more, and for changing drainage patterns. Retaining walls, pools and patio covers need building permits. There are exemptions for minor landscaping on developed lots, but they come with conditions, including staying 10 feet from structures and not altering drainage.
On a fill lot, that permit step is a good thing. It often brings a soils engineer into the picture before something gets cut.
Find your paperwork
Many Poway tracts have an as-graded report on file. It shows where fill was placed and how deep. It’s one of the most useful documents you can have. Our as-graded report post explains how to find one and what to look for. If your home dates from the 1970s, our 1970s tract checklist covers age-related items too.
For a deeper look at clay fill in town, see our post on Friars clay fill settlement.
How we size it up
We walk the outside first. Grade, drainage, walls, flatwork. Then the inside. Floor levels, crack patterns, doors. We look at whatever records you have. If the signs point to fill settlement or swelling clay, we explain the options in plain terms. That might be drainage work, slab repair, or underpinning. The code says foundations on expansive soil need to be designed for it, so for anything structural we coordinate with licensed engineers as needed.
Our pages on fill settlement repair, expansive soil repair and foundation inspection have more detail.
We serve all of Poway, from the southwest tracts to Green Valley and the estate lots up north. If something on your pad has started moving, ask for a Free Quote.