Underpinning Enables Excavation and Construction under Existing Buildings
The construction of a basement or a similar underground structure beneath a building with a shallow foundation requires the installation of a underpinning by a geotechnical services firm. Without this, a building can't preserve its stability and integrity as the floor is removed beneath its footings.
The addition of an underground structure increases a building's load upon the soil. Unfortunately, most foundations are only designed to accommodate the weight of the original building. Additional loads can cause buildings to sink or even collapse.
Furthermore, an underground structure can go beyond the building's existing shallow foundations and it may dig deeper or wider than the existing footings. In both cases, the shallow foundation can no longer fulfill its original function.
Structural underpinning can strengthen a building's foundation and become the basis for a new deeper foundation that can bear the additional weight. The expertise of geotechnical services firm is necessary to safely underpin the project, especially when the aboveground structures are still in use.
There are numerous underpinning methods that can be employed in such a project, from conventional concrete underpinning to micropiles. Recommending which would be the most appropriate for a particular project is the task of a geotechnical services firm. Project conditions and requirements are unique in every building, and only an expert with intensive knowledge of the different underpinning methods and a thorough understanding of the site's subsurface conditions can find an adequate solution.
In one similar construction involving Princeton's Jones Hall, geotechnical engineers decided to employ conventional concrete underpinning. The exterior wall was initially underpinned before working downwards. Temporary concrete footings were then placed several feet off the existing column and a prefabricated needle system was installed to take the load. The existing column and footing was then replaced by a new foundation that can accommodate the planned new basement
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