Underpinning
Extending existing foundations to greater depth or capacity to arrest settlement.
Underpinning is the process of strengthening or extending an existing foundation to greater depth, increased capacity, or both. It is required when the original foundation is inadequate due to settlement, deterioration, change in loading, adjacent excavation, or the need to add basement space below an existing structure. Traditional underpinning involves excavating beneath the existing footing in sequential pits and casting new concrete to extend the foundation to a deeper bearing stratum. Modern underpinning methods include micropile underpinning, helical pier underpinning, jet grouting, and bracket pile systems that can be installed with minimal excavation and disruption. The choice of method depends on the cause of distress, required capacity increase, access constraints, and acceptable level of disruption to the structure and its occupants.
Technical Specifications
Common Applications
Installation Process
- 1Assess existing foundation condition through inspection pits and structural survey
- 2Design underpinning scheme (method, spacing, depth, sequence) with structural engineer
- 3Install monitoring points (crack gauges, level surveys) on existing structure
- 4Execute underpinning in planned sequence (never undermine more than 25% of wall length simultaneously)
- 5For traditional: excavate pit, pour concrete, allow cure, move to next pit
- 6For micropile/helical: drill or screw through or adjacent to existing footing, install bracket connection
- 7Transfer load from existing footing to new underpinning elements (jacking if needed)
- 8Monitor structure during and after load transfer; document final levels
Frequently Asked Questions
When is underpinning needed?
Underpinning is needed when: existing foundations have settled beyond acceptable limits, adjacent construction threatens foundation stability, building loads are increasing (additions, renovations), or new basement space is being created below an existing structure.
How much does underpinning cost for a house?
Residential underpinning typically costs $20,000–$80,000 depending on the number of support points needed, depth to bearing, access conditions, and method used. Simple helical pier underpinning (8–12 piers) may cost $15,000–$40,000.
How long does underpinning take?
A typical residential underpinning project takes 1–3 weeks. Helical pier underpinning can often be completed in 2–3 days. Traditional mass concrete underpinning takes longer due to sequential excavation and curing requirements.
Is underpinning disruptive to occupants?
Modern methods (micropile, helical pier) cause minimal disruption — occupants can often remain in the building. Traditional underpinning requires excavation around the perimeter and may require temporary relocation for safety.
Can underpinning fix a sinking foundation?
Yes — underpinning arrests further settlement by transferring loads to deeper, more competent soil or rock. In some cases, hydraulic jacking can lift the structure back toward its original elevation.
What causes foundations to need underpinning?
Common causes include: inadequate original design, soil consolidation or shrinkage, tree root desiccation, water table changes, adjacent excavation removing lateral support, increased building loads, and deterioration of original foundation materials.
Do I need an engineer for underpinning?
Yes — underpinning design requires a licensed structural or geotechnical engineer. They assess the cause of distress, design the underpinning scheme, specify monitoring requirements, and certify the completed work.
Will underpinning increase my property value?
Underpinning stabilizes the structure and prevents further damage, which protects property value. If underpinning adds usable basement space, it can significantly increase value. However, disclosure of foundation work is typically required at sale.