The Basics of Underpinning

A structure is only as strong as its foundation, and sometimes underpinning may be needed. 

Underpinning is one process of deep foundation, which occurs when a type of foundation is placed at a greater depth below the ground surface and transfers structure loads to the earth at depth.

With underpinning, a new foundation is installed under an existing one as a means of providing additional support to the structure.

When a Structure Needs Underpinning 

“Regardless of why underpinning may be needed, the process is critical to maintaining the integrity of any structure. A building cannot exist without a strong foundation,” said Dan Connolly, owner of Connolly Engineering.

Underpinning may be necessary for a variety of reasons including:

• The original foundation isn’t strong or stable enough
• Usage of the structure has changed 
• To increase the capacity of an existing structure (i.e. floors are being added to upper stories)  
• The soil supporting the foundation has changed in some way (i.e. due to moisture, damage, etc.)
• Properties of the soil were misunderstood during the original design and construction of the foundation 
• A natural disaster such as an earthquake, flood or drought have caused the structure to shift or become unstable

In dense metropolitan areas like New York City, underpinning is used during the construction of adjoining or adjacent structures that call for the removal or excavation of the soil surrounding nearby properties.

How The Underpinning Process Works

The structural configuration of the building to be underpinned plays an essential role in the design of the underpinning.

The majority of the issues that arise with underpinning occur during the underpinning of load-bearing unreinforced masonry buildings. Quite often, these older buildings have never been explicitly designed to sustain horizontal loads. If horizontal loads are applied perpendicular to the face of a masonry wall (out of plane loads), the wall’s response is weak. Pin underpinning has the potential to introduce such out-of-plane loads.

It is now a code requirement in New York City to understand the potential response of the underpinned building to a lateral load.

Some Key Elements of Underpinning

To repair, strengthen, or increase the depth of an existing foundation, the soil from beneath an existing foundation is excavated or removed. In doing so, the footing is often lowered so that the foundation can rest on more supportive soil.

Underpinning must be done in controlled stages – called “pins” – to avoid the risk of overly disrupting the foundation. Failure to approach the process too abruptly could lead to structural failure. By underpinning through these stages, it can then be determined by studying the soil composition how much weight the structure can bear.

Steps for designing a pin underpinning:

1. Determine soil bearing capacity and other properties.
2. Determine condition of the existing building to be underpinned, including the potential response of the existing building. To assess the condition of a building, engineers look for building leans, wall cracks, wood deterioration, evidence of foundation settlement, and eroded mortar joints.
3. Determine vertical loads on existing foundations.
4. Evaluate the dimension of the pin (for each phase).
5. Determine the structural model of the underpinned structure that satisfies the known building and soil conditions.
6. Verify strength, sliding, and overturning for each element at each phase, including soil carrying capacity.