A common application of underpinning is maintaining the stability of the foundations for adjacent structures during excavation on urban sites. If the excavation is deeper than the foundation depth for an adjacent structure, then the soil below the foundation could fail as the excavation proceeds, potentially causing the structure to collapse. Increasing the depth of an existing basement or cellar often requires underpinning to avoid similar foundation failures.
In these circumstances, concrete pier underpinning is typically employed. This technique became popular during subways construction in New York early in the twentieth century. The piers are constructed in a carefully staged sequence whereby a sheeted hand-excavated pit is advanced to the required depyh below a short segment of the existing foundation and is filled with concrete upon completion to create a pier. After load is transferred to the new pier, work can begin on an adjacent pier. Work continues until the entire foundation is resupported below the bottom of the excavation for the new structure. Then, excavation for the new structure can begin. In addition to transmitting loads to below the bottom of the excavation, the underpinning retains soil below the structure preventing soil below the building from being undermined.
While pier underpinning is common and well understood, it is a relatively expensive and high-risk type of construction. Sometimes adverse subsurface or site conditions, including access constraints, make pier underpinning difficult. This sometimes leads project stakeholders to seek alternatives. So what are the alternatives?
One alternative that I have seen too many times is no underpinning. This sometimes occurs when a project owner or constructor fails to obtain an access agreement with an abutting owner and attempts to excavate without underpinning so as to avoid delays. In addition, sometimes the project team expanding the basement of an existing building will fail to understand that they must underpin adjacent buildings, as well as their own. Failing to underpin is reckless, yet it is probably the most common type of underpinning failure I have investigated.
There are, however, safe ways to avoid underpinning adjacent structures. The design of the building can set back the basement and associated excavation from adjacent structures. This leaves a partial basement and usually requires drilled piles supporting the above-ground portions of the building along the property lines.
Another approach to avoid underpinning is to use excavation support systems to retain soil below an adjacent building. This will often result in a small basement set-back because the excavation has to be installed near along the property line and takes up space. However, typical excavation support systems like soldier and pile and lagging walls and sheet piles are often too flexible to prevent soil movement below an adjacent building. If wall movement is not specifically accounted for in the design, as is often the case when contractors design excavation support, the risk of damage to the adjacent building can be high. Alternatively, a rigid excavation support method can be used. These systems are often used as permanent basement walls, reducing the loss of footprint compared to conventional excavation support and sometimes presenting schedule cost advantages.
Structural shoring can sometimes be used as an alternative to underpinning. This approach is most often used when construction must be performed below a structure on the same site. Rather than extend each foundation to a deeper level in segments, the whole building can be supported on beams and girders to transfer load to temporary footings, cribbing or piles. When the below ground work is complete, the existing structure loads can be transferred to new foundations. The excavation would have to be supported by other means.
Piles have been successfully used for excavation-related underpinning for over a century, but are now more commonly used to underpin foundation subject to movement due to soil settlement and other foundation failures. Historically, full-sized piles, sometimes installed in segments, were used to take foundation loads to firm soil or rock and support lateral soil loads from excavations. Some of these methods are archaic, but others, like segmental jacked piles, are still useful under specific conditions.
The types of piles commonly used for remedial underpinning, like micropiles and helical piles, are slender elements. When confined by soil they behave adequately. However, if the confinement of the soils is removed, as is the case when a structure is underpinned for adjacent excavation, the pile’s capacity is markedly diminished. If the same pile is also called on to resist lateral pressure from soil under the building, it’s gravity load-carrying capacity is further reduced. The means of connecting small-diameter piles to an existing structure may induce secondary forces that prevent the full capacity of the pile from being developed. Failure to account for these considerations can result in a hazardous design. There have been failures from the use of small-diameter piles from underpinning and for a time, helical piles were banned in New York City as a result.
Some of the methods that are thought of as alternatives to underpinning are underpinning by another name. For example, some projects have used jet grouting – a ground improvement method by which in-situ soils are mixed with cement – to stabilize the ground below adjacent structure foundations. The “soil-crete” formed by jet grouting forms a mass of material that can provide vertical support for the foundation and lateral support for the ground below the building during excavation, functioning in essentially the same way as pier underpinning. Jet grouting has the advantage that it can be used where high groundwater would make traditional underpinning more expensive and hazardous. However, the equipment required for batching and placing the grout tends to be large, expensive to operate and logistically challenging, particularly on small urban sites. Other grouting methods are available that have smaller footprints but are more limited in applicability.
An alternative of sorts to pier underpinning is intermittent pier underpinning. Individual underpinning piers are evenly spaced along the foundation. Instead of constructing secondary piers to form a continuous underpinning wall, lagging is installed between piers as the excavation proceeds. This method has limits to its applicability. The structure being underpinned has to be light enough that the reduction in bearing area is acceptable. Unless the piers are laterally braced, the effects of earth pressure must be small enough that the added mass of continuous piers is not necessary for stability. The foundation also needs to be able span between pier without being damaged or be supported by lintels installed between piers.
Why Use Pier Underpinning?
The hand-work associated with pier underpinning has significant cost and schedule impacts, yet it is usually the method of choice when a building to be underpinned is supported on continuous footings, the underpinning depth is less than a full story, soil is reasonably competent and groundwater is not present. The behavior of pier underpinning is relatively simple. Since it provides continuous support for the foundation, the permanent state of stress in the existing construction is relatively unchanged, reducing the potential for damage.
Pier underpinning requires care in design and construction, but the level of care is little different than the realistic alternatives. However, since cost is often the primary motivation to avoid pier underpinning, it may be more likely that the requisite care may not be provided when alternative methods are used because owners, designers and contractors fail to properly resource the design and oversight of the process. This ends in an often unwitting acceptance of risk disproportionate to the cost savings, which is a common attribute of underpinning failures.
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.