Pasadena Soil and Geology for Concrete Work

Pasadena Soil and Geology for Concrete

Pasadena spreads out from the foot of the San Gabriel Mountains, with the Arroyo Seco on its west side and the Raymond Fault crossing the area to the south, so the ground under a slab can change from one neighborhood to the next. This guide covers what is known about Pasadena geology, what isn't confirmed, and how to look up the soil under your own lot.

San Gabriel Mountains

Arroyo Seco

Raymond Fault

USDA Web Soil Survey

Last reviewed October 2026.

Pasadena geology at a glance

NASA's 3-D view of the area shows the setting plainly: the San Gabriel Mountains rise abruptly along the north, and Pasadena's street grid spreads south from their base. On the west, the Arroyo Seco cuts a wide, brushy channel through the city, crossed by the Colorado Street Bridge, a concrete arch bridge that opened in 1913, runs 1,467.5 feet and has been on the National Register of Historic Places since 1981.

Pasadena geology facts and where they come from

Feature	What is known	Source
San Gabriel Mountains	Rise along the north edge of the city	NASA 3-D perspective image
Arroyo Seco	Channel on the city's west side, spanned by the Colorado Street Bridge	Wikipedia
Colorado Street Bridge	Concrete arch bridge, opened 1913, 1,467.5 ft, National Register 1981	Wikipedia
Raymond Fault	Branches from the San Andreas in the San Gabriels; forms Raymond Hill	Wikipedia
Hillside soils, LA region	Expansive clays and old landslide debris common	constructlytools, 2026
Soil series under Pasadena	Not confirmed for this guide	Look up your lot on Web Soil Survey

Swipe the table sideways to see every column →

Colorado Street Bridge arches over the brushy Arroyo Seco in Pasadena — The Arroyo Seco below the Colorado Street Bridge, on Pasadena's west side.

The Raymond Fault

Wikipedia's entry on the Raymond Fault traces it from the San Gabriel Mountains, where its eastern end branches from the San Andreas Fault, west through the San Gabriel Valley and on through San Marino and South Pasadena, where it forms Raymond Hill, the hill the fault is named for. It is one of the reasons foundations and hillside walls in this area are engineered to California's seismic provisions rather than built by rule of thumb. This guide makes no claim about distances or hazard ratings for any particular lot; the California Geological Survey's maps and the engineer of record answer that.

Arches of the Colorado Street Bridge in Pasadena against a blue sky — The Colorado Street Bridge, Pasadena's concrete arch bridge over the Arroyo Seco.

What Pasadena soil means for concrete

Driveways, patios and walks

Flatwork fails from below more often than from above. Topsoil, roots, old fill and soft spots under a slab compress or move, and the concrete cracks over them. The fix is the same almost everywhere in Pasadena: strip soft and organic material, build a compacted base and slope the slab so water doesn't get under it.

Foundations and ADUs

A concrete foundation or an ADU foundation is designed from the soil, not the other way around. Where the ground is uncertain, the engineer asks for a soils report before sizing footings.

Hillside walls

On slopes, soil and water together decide whether a retaining wall stays put. constructlytools' Los Angeles guide (updated May 19, 2026) notes expansive clays and old landslide debris are common in the region's hillside soils, which is part of why Pasadena regulates hillside grading separately; see the hillside permit guide.

Concrete retaining wall stepping along a sloped lawn above a curb and gravel strip — A concrete wall stepping down a sloped lawn, with a gravel strip at its base.

How to look up the soil under your own lot

Open the USDA Web Soil Survey and start the free viewer.
Find your address and draw an area of interest around the lot.
Open the Soil Map tab to see which soil map units cover it.
Use the Soil Data Explorer for properties that matter to concrete, such as drainage class and shrink-swell behavior.
For a foundation or a wall, hand the result to the engineer; it is a starting point, not a soils report.

Pasadena Soil and Geology Questions

White Spanish-style house above a steep brushy hillside

A house set above a steep, brushy slope.

What type of soil is in Pasadena?

It varies by lot, and a specific soil series for Pasadena wasn't confirmed for this guide. The most reliable answer for your address is the USDA's free Web Soil Survey, which maps soil units and lists their properties, followed by a soils report from an engineer for any foundation or hillside wall.

Is Pasadena on a fault?

The Raymond Fault runs through the area. Wikipedia's entry traces it west from the San Gabriel Mountains, where it branches from the San Andreas Fault, through the San Gabriel Valley and on through San Marino and South Pasadena, where it forms Raymond Hill, the hill the fault is named for.

Do hillside lots in the Los Angeles area have problem soils?

Some do. constructlytools' 2026 Los Angeles guide notes that expansive clays and old landslide debris are common in the region's hillside soils, and prices a hillside soils engineer's report at $600 to $1,500.

How does soil affect a concrete driveway or patio?

Soft, organic or expansive soil under a slab moves or settles, and the slab cracks with it. The usual answer for flatwork is removing the soft material and building a compacted base; for foundations and walls, the engineer designs from a soils report.

Where can I check earthquake hazard zones for my address?

The California Geological Survey publishes the state's fault and seismic hazard zone maps. For a foundation or a hillside wall, the engineer of record accounts for them in the design.