What Is House Underpinning?
Underpinning is the process of strengthening and stabilizing the foundation of an existing building or home. Many of the houses that require underpinning have been built on marginal land that has insufficient bearing capacity to support the substantial weight of the structure.
In many areas the near surface soils consist of expansive clays that shrink and swell as their moisture content changes. As a result underpinning is required to extend the foundation support to depths that provide greater bearing capacity or needs to be widened so that the structure rests on a more supportive soil stratum or distributes its load across a greater area.
It generally means that temporary supports need to be installed under the house so that all or part of the existing foundation can be excavated and new foundations may be installed. This is usually done to provide vertical support not present in the current design of the home. When properly designed and installed, underpinning can provide the basis to lift the structure to a more acceptable elevation and provide the vertical support to prevent the underpinned area from settling.
How Can I Tell If I Need House Underpinning?
Underpinning a home is necessary when the original foundation is not strong enough, the soil under the foundation has changed, the usage of the building has changed, new buildings are being built nearby, another story is being added to the existing structure or when natural elements have caused damage to the structure such as an earthquake or storm.
What Are The Different Kinds of House Underpinning?
The process of underpinning includes the construction of footings, stem walls, driven piling or drilled piers. Use of micropiles and jet grouting are common methods used in underpinning as well. The three most commonly used methods of UnderPinning are Mass Concrete Underpinning, Beam and Base Underpinning and Mini-Piled Underpinning:
Mass Concrete Underpinning
The mass concrete method of underpinning strengthens an existing structure’s foundation by digging boxes by hand underneath then pouring concrete in a specific order that results in a foundation built underneath an existing foundation. The advantages to using this method of underpinning include the simplicity of design and reduced labor costs. It is also very easy to duplicate consistently.
Beam and Base Underpinning
Underpinning with the beam and base method is a more highly technical process. A reinforced concrete beam is constructed above, below or in replacement of the existing footing. The beam then transfers the load of the building to mass concrete bases that are constructed at strategically designated locations. Ground conditions are used in determining base sizes and depths while the design of the beam is based on the configuration of the building and the applied loads.
Mini-piles are used when ground conditions are variable, access is restrictive, environmental pollution is prevalent and when structural movements need to be minimal. Most commonly used when the loads from the foundations need to be transferred to stable soils at considerable depths. Conventional drilling and grouting methods are used for mini-pile underpinning.
Successful house underpinning requires proper design and installation but even the best laid plan may not ensure a permanent fix to the structure. When the underlying soil moves it usually affects the structure above it correspondingly. Maintaining a consistent soil moisture level around and under the foundation, particularly in areas with highly expansive clay soils or environmentally induced changes in moisture availability.
Whether the cause is poor pre-construction compaction, soil moisture changes, plumbing leaks, poor drainage, inadequate pier depth, etc. it is important to find an engineer with experience in the specific type of home construction. They will analyze the problem and make suggestions what underpinning methods will best address your home’s specific issues.