Underpinning has a way of entering a conversation too early. A crack appears, someone mentions piers, and before long a homeowner is weighing a substantial project without being sure the house is still moving. This entry is meant to slow that down a little.
Underpinning means extending a foundation’s support down to soil or rock that can carry the load better than what the footing sits on now. Done for the right reasons, it can stabilize a settling foundation. Done for the wrong ones, it adds cost and disruption without addressing why the house moved.
The order of questions
Before any pier is discussed, a careful evaluation tends to work through four questions, in this order:
- Is the foundation actually moving, or did it move once and stop?
- If it’s moving, what is driving it: water, soil, slope, or something else?
- Can the cause be corrected without deepening the foundation?
- If not, what kind of deeper support suits this ground?
Most of the value lies in the first two. Our entry on reading cracks in Del Mar covers how cracks and floor measurements help answer the first.
Three Del Mar settings, three different conversations
Del Mar is small, but the ground under it isn’t uniform. The City’s 2023 hazard mitigation annex, prepared with the County, rates expansive soils as a limited, occasional hazard here. The same annex notes that the City applies State and local permit rules to new housing in the places its Community Plan identifies as having the greatest potential for ground failure: along the coastal bluffs and sea cliffs, on Torrey Sandstone where existing slopes exceed 25 percent, and on terrace escarpments of colluvium and slope wash in Crest Canyon and on the northeast side of the Del Mar Hills.
For underpinning, that sorts homes roughly into three groups.
Hillside and mesa homes. Many sit on sandstone or on cut-and-fill pads. Settlement here, when it happens, often traces to fill placed during grading or to water softening soil near a footing. Our entry on Torrey Sandstone describes how that rock behaves.
Canyon-edge homes. Along Crest Canyon and similar escarpments, the material near the surface may be colluvium and slope wash, which is looser and more sensitive to water than the rock beneath. Here, deep support has to reach past that layer, and the slope itself becomes part of the analysis.
Low-lying homes. Near the river and the beach, the question shifts toward floodplain rules and softer soils. Underpinning is possible, but it sits inside a different set of regulations, discussed below.
Establishing what the soil is
The state residential code gives the building official room to ask for information. Under Section R401.4, where accepted soil data indicate that expansive, compressible, shifting or other questionable soils are likely to be present, the building official decides whether a soil test is required at that location.
In practice, underpinning on anything beyond a very simple case usually involves a geotechnical investigation. Borings or other exploration tell the designer how deep competent material lies, which in turn shapes the choice of pier and its depth. Guessing at that depth is how projects end up with piers that stop short of where they should.
Pier families, in brief
There are several broad families of underpinning, each suited to different ground and loads. A short orientation:
Helical piers. Steel shafts with helix-shaped plates are rotated into the soil. Their capacity is commonly correlated with installation torque. ICC Evaluation Service publishes acceptance criteria for helical pile systems, AC358, which manufacturers use to obtain evaluation reports. Asking whether a proposed system has such a report is reasonable.
Push piers. Steel sections are driven downward using the weight of the structure as reaction. They depend on the building having enough weight to advance them, which makes them more common under heavier walls.
Drilled, cast-in-place piers. Holes are drilled and filled with reinforced concrete, sometimes tied to new grade beams. They are often discussed where loads are larger or where the ground calls for a stiffer element, such as near a slope.
Mass concrete underpinning. Sections of soil beneath an existing footing are excavated in sequence and replaced with concrete. It is older and labor-intensive, and is suited mostly to modest depths.
Which family is appropriate depends on the soil profile, the loads, access and the slope. That determination belongs to the design, and we coordinate with licensed engineers as needed so the work matches what the investigation found.
Near the bluffs
On bluff-top lots, underpinning enters the territory of coastal review. Any work that adds or deepens foundations near a bluff edge may draw questions about bluff stability, setbacks and whether the work could function as shoreline protection. Outcomes and timelines for those reviews vary and are decided by the agencies involved; no one can promise them in advance. Our page on bluff and hillside homes describes how we approach work in that setting.
In the floodplain
The City’s FEMA page explains that a new Flood Insurance Rate Map took effect on December 21, 2019, and that the annex points to the City’s Floodplain Overlay Zone for development in the Special Flood Hazard Area. For a home there, foundation work can interact with floodplain rules in ways that matter for scope, permits and insurance. It is worth checking the property’s flood zone before planning anything structural. Our page on North Beach floodplain foundations goes into this in more depth.
Permits and documentation
Del Mar’s Building Services page notes that building permit applications and plan checks are submitted electronically. Underpinning generally involves plans, and often a soils report, so expect the documentation to be prepared before work begins rather than after. Keeping the final plans, the soils report and the inspection record together will matter at resale and for any future work.
Water, before and after
Even well-designed underpinning can be undermined, figuratively, by the same water that caused the trouble. Correcting drainage, irrigation and downspouts is usually part of any sensible plan. Sometimes it is the whole plan.
Considerations that shape scope
There are no prices here. The factors that most influence an underpinning project in Del Mar:
- whether movement is ongoing, confirmed by monitoring
- the soil profile and depth to competent material
- the pier type suited to that profile and the loads
- access around and under the house
- proximity to a bluff, canyon edge or slope
- floodplain status and related requirements
- permits, plans, inspections and any coastal review
A closing note
Underpinning can be the right answer. It should be the conclusion of an investigation, not the start of one. If you are wondering whether your home’s foundation needs deeper support, request a Free Quote. We work throughout Del Mar, from the beach to the hills. See also our pages on foundation settlement repair and foundation repair.