Get The Lowdown on Underpinning
Posted on Wednesday, May 29th, 2013<< Return to GeoBlog Page
Of the many underpinning methods available, which is the right one for your project? Determining the one that is best for the situation depends on the answers to a number of questions. What are the subsurface conditions? Are there boulders present? Do you need to prevent potential settlement or arrest actual settlement? Are there worksite access or headroom challenges or restrictions? If the structure has settled, do you need to lift and relevel it as well as prevent further settlement? Does an occupied facility need to continue to function during underpinning work? Are cost and scheduling considerations an issue? Often, more than one method is technically viable.
Lack of adjacent space and/or cost considerations can drive the decision to expand existing business premises by building up rather than out, imposing additional structural loads on the existing foundation system. Additional loading can also come in the form of new heavy manufacturing equipment installation. In either case, consideration must be given at the design stage to the impact on the existing foundation system. Vertical and/or inclined micropiles can be installed to provide the additional foundation support needed to withstand increased structural loading. Micropile foundation systems can be installed in confined space and low headroom situations, often without disrupting normal facility operations.
Excavation Below Existing Foundation Level
New construction shoehorned in between older buildings often involves excavation below adjacent existing footings for basement level installations. If those footings are shallow, or their condition uncertain, then the foundations must be extended to below the level of the new adjacent structure. Several methods are available to provide such support. Jet grouting has the advantage of providing both underpinning and excavation support in one operation, if required. Micropile foundation systems can readily be installed within low-headroom situations such as an existing basement level, or through existing footings, to underpin the building. Bracket piles mounted to the underside of the footing are commonly used in conjunction with earth retention work to support and/or stabilize existing foundations adjacent to a proposed excavation. When new construction abuts tight against an existing building, conventional concrete (pit) underpinning may be the only viable approach.
Differential settlement is typically due to uneven consolidation of loose foundation soils and can be addressed in several ways. Ground improvement, while not considered true underpinning since there is no direct physical connection with the footings, nevertheless serves a similar purpose in controlling settlement. Such methods include compaction grouting to densify and improve the bearing capacity of the foundation material, or permeation and fracture grouting, both of which reinforce the soil mass. Structural underpinning methods using deep foundation elements such as micropiles or helical piles bypass the unsuitable material to terminate in a competent stratum. Under certain conditions, it may also be possible to raise or re-level the structure and restore the original foundation level.