Foundation Underpinning and Undermining
By: Richard E. Daniels, P.E.
Excavation next to or near a foundation and in particular near a wall requires special care.
Many structures, particularly residential row homes, were constructed on shallow foundation walls that were set at the frost depth. A shallow foundation wall exerts downward force in the form of a pressure bulb. The effects of this pressure bulb extend outward beyond the footprint of the wall and rely on the confining pressure of surrounding soil to maintain the structural integrity of the structure. Any excavation into this zone of influence can be problematic.
The Standard Handbook for Civil Engineers, Frederick S. Merritt, Editor explained, “Presumably, an excavation influences an existing substructure when a plane through the outermost foundations, on a 1-on-1 slope for sand or a 1-on-2 slope for unconsolidated silt or soft clay, penetrates the excavation [the pressure bulb].” In other words, the bearing pressure below a foundation is distributed through the soil within a wider width than the foundation itself. Further, “Loss of ground, even though very small, into an adjoining excavation may cause excessive settlement of existing foundations.” Therefore, when construction activities disturb the influenced subgrade soils, settlement and damage are likely to occur.
When a new structure is constructed near an existing structure or when a basement is added to an existing structure, the excavation can weaken and undermine an existing foundation and cause a collapse; the existing structure must be protected. Underpinning is a method of accomplishing this.
Underpinning is the process of deepening the foundation of an existing building to obtain adequate foundation support, typically by constructing a new reinforced concrete foundation underneath the existing foundation.
Underpinning is an engineered process. For certain foundations, a customary practice is a pattern of excavating 2-foot, 6-inch-wide sections below the existing foundation wall at 5-foot intervals. Most walls can reasonably be expected to safely bridge over these spaces. When the new underpinning concrete gains sufficient strength, generally 24 hours later, any gap between the new foundation wall section and the existing foundation wall is filled in with grout or mortar. The process is then repeated for a new series of 2-foot, 6-inch-wide sections, and finally repeated for the last series of sections.
Underpinning is a code requirement. The International Building Code (IBC) prohibits excavation that could reduce lateral support from any foundation or adjacent foundation without first underpinning or protecting the foundation against detrimental lateral or vertical movement, or both. Where underpinning is selected, the IBC requires underpinning to be installed in a sequential manner (see above) that protects the neighboring structure and the working construction site. Additionally, the IBC requires the owner or owner’s agent to apply to the building official for a permit and to submit construction documents that have been prepared by a registered design professional for review. Further, the construction work is subject to inspection by the building official.
More importantly, worker safety must be considered during underpinning work. Both the IBC and OSHA require excavation to be performed in a manner that does not endanger life or property. OSHA specifically prohibits excavation below the level of the base or footing of any foundation or retaining wall that could be reasonably expected to pose a hazard to employees, unless a support system, such as underpinning, shoring, or bracing is provided to ensure the safety of employees and the stability of the structure. Also, inspection of the excavation area by a competent person is required prior to the start of work and as needed throughout the shift. Where the competent person finds evidence of a situation that could result in a possible cave-in, indications of failure of protective systems, hazardous atmospheres, or other hazardous conditions, exposed employees shall be removed from the hazardous area until the necessary precautions have been taken to ensure their safety.
In summary, underpinning is specialized construction work that is a somewhat tedious and time-consuming. Nevertheless, properly engineered and executed underpinning is critical for the safety of both the workers and the structure, when excavating within the pressure bulb of an existing foundation. Also, supplementary conditions like construction bracing or shoring cannot be overlooked.
If you have a construction matter involving underpinning, the experts at CESI can help. Contact us to speak with the appropriate expert.