by Richard J. Driscoll, P.E.
Underpinning is the term applied to a variety of methods used to re-support a structure’s foundation, usually to a deeper bearing depth. Generally, underpinning is performed if the foundation system for an existing structure is compromised or if foundation support needs to be transferred to a deeper level to allow work that would otherwise cause foundation movement and damage to the structure. The latter scenario is common when new buildings are constructed on urban sites adjacent to older buildings since modern building codes incentivize placing the first floor near grade and deeper basements, both increasing excavation depth. Underpinning is a potentially high-risk specialty construction type. Managing the risk associated with underpinning requires project stakeholders to understand these risks and proactively implement best practices in the design and construction of underpinning.
Underpinning can take a variety of forms. Pier underpinning is the most common and is constructed by hand-excavating pits below a footing, which are filled with concrete to form piers bearing at a deeper level. The piers are constructed in a staggered sequence, so most of the foundation is supported at any given time. Pier underpinning is typically used to transfer the support of a wall footing, or sometimes a column footing, to a deeper level. Needle beams, spanning between temporary foundations is a common alternative to pier underpinning. Historically, needle beams were “threaded” through a bearing wall penetration the wall and providing bearing between the beam and the wall. It is more common today to bolt a needle beam to each face of an element to better distribute loads. This approach is common for temporarily supporting columns while replacing the footing supporting the column.
Several pile systems can be used for underpinning, as well. Micropiles are commonly used for underpinning and are either drilled through existing foundations or attached to the existing structure with retrofit pile caps. In soft soils, segmental piles can be jacked in to bear on a more competent material, using the existing structure to resist the jacking forces. Other pile types can be attached to a structure using fabricated or manufactured brackets. In some cases, ground improvement methods, like jet grouting, can be used for underpinning.
Indifference to the risks related to underpinning, and abutting structures generally, is a too common attitude among project stakeholders and it is a routine source of damage claims by abutters to construction projects in urban areas. Like sidewalk sheds and excavation support, underpinning systems are protective measures used to protect workers, property and the public from construction hazards. As a result, underpinning is considered to be among the contractor’s “means and methods” and are often deprecated by the design team and building officials because they are temporary. Some geotechnical engineers take the attitude to “let the contractor figure it out” and do not provide underpinning recommendations. Structural engineers responsible for the design of typical building projects often provide no underpinning requirements on their drawings or provide even a basic performance specification in the construction contract documents. Typically, if there are no contract documents for underpinning, the design team will not review underpinning plans, since they lack expertise with these systems. Consequently, oversight of underpinning may be minimal until a problem occurs.
The lack of respect given to temporary structures in construction is of concern for all protective systems, but underpinning is different. Unlike roof protection and most excavation support systems, underpinning is not temporary. Once installed, an underpinning system is part of a structure’s foundation system, usually for the rest of the life of the building. Therefore, in addition to loads from any temporary stages, underpinning must provide a durable load path for all loads to which the underpinned structure is subjected, including wind and seismic loads.
Most underpinning systems are deceptively simple on paper; a new foundation element is added below an existing foundation and excavation can proceed without destabilizing the existing structure. Foundation support is transferred to a deeper level, which should be more competent. In reality, underpinning a building without causing damage requires consideration of the behavior of the existing structure, the sequence of construction and the time-dependent and sometimes complex load and support conditions that result from transferring the weight of the structure from one foundation system to another.
Underpinning construction subjects the underpinned structure to the risk of damage that is inherent to the methodology. The existing structure has to deform and the underpinning had to settle to transfer loads to the ground through the new foundation elements. Deformation and settlement will occur until equilibrium is reached, which depends on the competency of the bearing stratum and the quality of construction. If the movement of the structure is sufficiently large or nonuniform, then damage will occur. Since this effect is cumulative, the risk of damage is substantially increased when a staged underpinning system (i.e. underpinning of the underpinning) is used. Preloading the underpinning using jacking or wedging, will mitigate settlement. However, Preloading cannot eliminate settlement without the risk of lifting the structure, which can also be damaging.
Underpinning systems that involve replacing a continuous support condition with discrete elements, like piles and piers, often require non-trivial structural engineering as part of the design. The use of helical piles to underpin relatively light structures is a currently fashionable example, especially for remedial applications. Discrete elements replace distributed forces with concentrated forces. For example, supporting a continuous element, like a bearing wall, with intermittent piers or piles can induce tension into the wall that it is not designed to resist. This may require providing beams between the piers or piles that provide both adequate strength and stiffness to prevent damage to the existing construction.
Common pile underpinning systems result in the support provided by the piles being eccentric from the building loads, resulting in the application of bending to the piles, existing structure or both. In the case of strip footings and grade beams, this eccentricity can result in those elements being subjected to torsion. When pile underpinning is used during excavation, the stability of the piles must be considered, especially if the underpinning is used to resist earth pressure loads. New York City banned helical piles for a few years in part because of stability problems resulting from their use in underpinning. In addition, whenever discrete foundation elements are used for underpinning, connection design is critical.
The changing and sometimes complicated loads that are common in underpinning and the structural design problems that appear when detailing certain systems are not always understood by underpinning designers and contractors. In addition, the deceptive simplicity of underpinning systems, the deprecated status of underpinning as “means and methods” and the lack of scrutiny from design professionals and building officials provide low barriers to entry and incentives for commoditization of underpinning design. This will often take the form of an engineer-sealed, but entirely generic, design sketch, perhaps with no project-specific analysis as its basis for design. Low-cost underpinning design is attractive to inexperienced and unsophisticated contractors, who do not understand the risk they are assuming and view an engineer’s seal as a formality, rather than evidence of design being performed under the responsible charge of a qualified professional. A sealed, generic underpinning design can be used by a contractor as a “permission slip”providing cover for them to improvise the details of the underpinning in the field without having to pay for a properly engineered design. This practice is not uncommon and exacerbates the risks already inherent to underpinning.
Those who consider underpinning to be nothing more than “means and methods” would not take exception to a contractor buying or preparing a generic underpinning design and working out the details in the field. In this scenario, the contractor assumes the risk and reward associated with the underpinning design. If the underpinning does not perform adequately, the contractor and designer would be liable for damages. However, poorly designed or improvised underpinning increases the risks for the owners and residents of adjacent buildings, as well as for the project owner and design team, likely without the knowledge and consent of those parties. In the event of a failure, the abutters could lose value and use of their property. The project owner and design team would almost certainly be party to any resulting claims and incur defense costs, as well as consequential costs like construction delays, opportunity costs and intangible losses. These losses are not likely to be fully recoverable.
Managing the risk from underpinning requires that the potential hazards be taken seriously. Project owners need to understand that an underpinning failure can threaten the success of the project and, therefore, provide proper budgets to manage this risk in both the design and construction phases. Depending on the make-up of the design team, a specialty consultant may need to be retained for this. A proactive approach to managing the risk associated with underpinning might include the following:
- Assess the potential need for underpinning early in the design process. The design team should always be aware of adjacent structures that might be impacted by construction. Site reconnaissance, walk-through visits of adjacent properties, if permitted, and geotechnical exploration will typically provide adequate information to make an early assessment of the need for underpinning.
- Consider alternatives. In some cases, the use of a rigid, permanent excavation support system can provide cost and schedule benefits while eliminating the need for underpinning. Alternatively, when the type or condition of adjacent structures makes underpinning unfeasible, it may be necessary to eliminate below-ground space adjacent to the existing structure and support new building loads by cantilever or drilled deep foundations.
- If the need for underpinning is expected, provide contract provisions. Some jurisdictions require that construction documents provide the expected extent of underpinning and some design information, even if final design is left to the contractor. This is usually good practice and should include relevant performance standards. Informing the contractor of where underpinning is expected and what type of underpinning may be feasible for the site conditions, provides the contractor with a baseline. Providing this information reduces the budget risk associated with underpinning. A baseline underpinning plan along with a differing site clause allows the contractor to reduce contingencies in their bids, reducing the owner’s cost if underpinning goes as planned. It also reduces the risk that the contractor will fail to plan for underpinning or deliberately exclude underpinning to lower their bid. This could subject the owner to change orders or claims if underpinning is required or incentivize the contractor to take reckless risks to recover their schedule and budget, exposing the owner to third-party claims and delay-related costs. Providing the baseline narrows the expected range of underpinning-related costs for the owner.
- Review underpinning plans. The design professional of record should review a contractor-developed underpinning plan for the limited purpose of confirming conformance with the performance requirements provided in the contract. This provides an opportunity to confirm that the contractor has a complete and competent design that does not expose the project team or abutters to unreasonable risk. In addition, the owner of a building that is to be underpinned should require that underpinning plans be submitted to them for review by their independent consultant as part of any license agreement.
- Perform condition surveys and monitoring. To the extent permitted by the adjacent owner, any structure to be underpinned should be subject to a pre-construction condition surveys to identify and baseline existing conditions that may be mistaken for underpinning damage. A high-quality condition survey can provide documentation required to expeditiously resolve damage claims. In addition, the condition survey can be an opportunity to identify vulnerabilities that would present unacceptable hazards if underpinning was to proceed before damage is done. Performance monitoring should accompany any underpinning work to provide early warning of unexpected performance and context for understanding any damage that may appear.
- Require special inspection. While the inspection requirements in most building codes would suggest that underpinning should be subject to special inspection, not all jurisdiction require it specifically. However, the design professionals of record have the discretion to require inspection of underpinning, even if it is not required by the building official. This is good practice in all case where underpinning is to support permanent loads or is unusually complicated or risky.
As cities are revitalized and deified, the need for new buildings to adjoin existing construction will continue to necessitate underpinning. For these projects to be successful, project teams and building officials should require that best practices be proactively implemented in the design and construction of underpinning. For this to happen, underpinning will have to be considered as something more than contractor “means and methods” and given the respect and consideration that is commensurate with the risk involved. This includes recognizing that underpinning is a type of specialty construction that should be the domain of properly qualified designers and contractors.
The information and statements in this document are for information purposes only and do not comprise the professional advice of the author or create a professional relationship between reader and author.