Del Mar’s ground changes character over very short distances. Along the bluffs and up the hill, houses stand on sandstone and claystone that are tens of millions of years old. Down by the river mouth, along North Beach and around the fairgrounds, the ground is a different thing entirely: deposits laid down by the river and the lagoon in geologically recent time.
Our earlier entry on the river and lagoon dealt with water on the surface. This one sets the water aside and looks at the soil itself, because it explains much of what owners on the valley floor see in their floors and walls.
How the regional map describes the valley floor
The California Geological Survey’s pamphlet for the San Diego 30’ x 60’ quadrangle describes the units in plain if technical language. Young alluvial flood-plain deposits are described as poorly consolidated, poorly sorted and permeable, made of sandy, silty or clay-bearing material. Paralic estuarine deposits, the kind that form at the edges of lagoons, are described as unconsolidated and mostly fine sand and clay.
Three words carry most of the meaning for a homeowner. Unconsolidated: the grains have not been compressed or cemented into rock. Poorly sorted: coarse and fine material are mixed, often in layers. Permeable: water moves through the coarser layers readily.
Compare that with the older formations on the hill. The same pamphlet describes the Delmar Formation as sandy claystone interbedded with sandstone. It is not hard rock, but it has been buried and compressed for a very long time. The contrast between the two is the whole story of this entry.
Why young deposits settle
A foundation pushes down on whatever is beneath it. Rock and old, dense soil carry that load with little change. Young, loose deposits behave differently in three ways.
They compress. A new load, such as an addition, a heavy fill, or a second story, can squeeze the soft layers and the building moves down, slowly and sometimes unevenly.
They vary. A pocket of soft clay beside a lens of sand means one corner of a house can rest on very different ground from another. Uneven settlement, not uniform settlement, is what cracks walls.
They respond to water. Clay-bearing layers swell when wet and shrink when dry, and the water table near a river mouth rises and falls with seasons and tides.
The question of liquefaction
Loose, saturated sand can lose strength during strong shaking. That is liquefaction, and it is the reason geologists pay close attention to river valleys.
The County’s hazard mitigation annex for Del Mar notes that its earthquake loss estimates include liquefaction. Across the city line, the City of San Diego’s Seismic Safety Study map legend for the adjacent area assigns its highest liquefaction category to areas with shallow groundwater, major drainages and hydraulic fills, and a lower category to areas with fluctuating groundwater and minor drainages. That map covers the neighboring city, not Del Mar parcels, and no general map can say what is under a particular lot. It does show how geologists think about valley floors like this one.
A site-specific geotechnical report answers the question for a given property. Under section R401.4 of the residential code, where data indicate that expansive, compressible or shifting soils are likely, the building official determines whether to require a soil test for that location.
Where the City’s own documents point
Del Mar’s annex to the County plan maps flood-prone areas in North Beach, the San Dieguito River Valley and the State Fairgrounds, and it notes that the southern section of the fairgrounds lies in the AE flood zone. The City’s FEMA flood map page is the place to check a particular address. We mention flood zones here only because they trace the same ground: the low areas built from river and lagoon deposits.
The annex also rates expansive soils for the city and lists ground-failure concerns elsewhere, on the bluffs, on steep Torrey Sandstone slopes, and in the colluvium of Crest Canyon. Those are different problems on different ground, covered in our note on Torrey Sandstone.
What owners on the valley floor tend to notice
The signs are the familiar ones, though the pattern is telling:
- Floors that slope toward one side or one corner of the house.
- Diagonal cracks at door and window corners, wider at one end.
- Gaps opening between a slab and an attached patio or garage.
- Doors that bind, then free up, as the seasons change.
- Additions that have moved relative to the original house.
On older raised floors, the slope often appears first. Our entry on sloping floors sets out how to tell old slope from new.
Records worth gathering
For a house on young deposits, the most valuable documents are usually:
- Any soils or geotechnical report from original construction or later work.
- Permits and plans for additions and foundation repairs.
- Elevation or survey data, especially any with dates.
- Notes on past water: leaks, ponding, high groundwater in crawl spaces.
A report written for one lot is still useful to a neighbor as background, though never as a substitute for their own.
When to ask for a closer look
An evaluation earns its keep where cracks are growing, where floors are changing, or where an addition or remodel is planned on soft ground. A foundation inspection records what is visible. Our page on North Beach floodplain foundations describes the approaches common on this ground, and foundation settlement repair covers how settlement is addressed. Where the soil itself is the question, we coordinate with licensed engineers as needed, and a geotechnical engineer usually leads.
Factors that bear on the work
We do not quote prices in a journal entry. The considerations are:
- Depth to firmer ground, which governs how far any support must reach.
- Groundwater level and access for equipment.
- Whether the house is on a slab or a raised floor.
- The extent of movement and how much of the house it touches.
- Coastal and floodplain review that may apply in this part of the city.
A closing thought
Ground like this is not a reason for alarm. Many valley-floor houses have stood for decades. It is a reason to keep records, to watch for change, and to know what the house is standing on.
Owners anywhere in Del Mar who want a considered look can request a Free Quote.