In the fields of architecture and civil engineering, construction is a process that consists of the building or assembling of infrastructure. Far from being a single activity, large scale construction is a feat of human multitasking...
is the process of strengthening and stabilizing the foundation
A foundation is the lowest and supporting layer of a structure. Foundations are generally divided into two categories: shallow foundations and deep foundations.-Shallow foundations:...
of an existing building
In architecture, construction, engineering, real estate development and technology the word building may refer to one of the following:...
or other structure
Structure is a fundamental, tangible or intangible notion referring to the recognition, observation, nature, and permanence of patterns and relationships of entities. This notion may itself be an object, such as a built structure, or an attribute, such as the structure of society...
. Underpinning may be necessary for a variety of reasons:
- The original foundation is simply not strong or stable enough.
- The usage of the structure has changed.
- The properties of the soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
supporting the foundation may have changed (possibly through subsidence
Subsidence is the motion of a surface as it shifts downward relative to a datum such as sea-level. The opposite of subsidence is uplift, which results in an increase in elevation...
) or were mischaracterized during design.
- The construction of nearby structures necessitates the excavation
Earthworks are engineering works created through the moving or processing of quantities of soil or unformed rock.- Civil engineering use :Typical earthworks include roads, railway beds, causeways, dams, levees, canals, and berms...
of soil supporting existing foundations.
- It is more economical, due to land price or otherwise, to work on the present structure's foundation than to build a new one.
Underpinning is accomplished by extending the foundation in depth or in breadth so it either rests on a more supportive soil stratum
In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...
or distributes its load across a greater area. Use of micropiles http://www.ismicropiles.org
and jet grouting are common methods in underpinning. An alternative to underpinning is the strengthening of the soil by the introduction of a grout
Grout is a construction material used to embed rebars in masonry walls, connect sections of pre-cast concrete, fill voids, and seal joints . Grout is generally composed of a mixture of water, cement, sand, often color tint, and sometimes fine gravel...
. All of these processes are generally expensive and elaborate.
Underpinning may be necessary where P class (problem) soils in certain areas of the site are encountered.
Through semantic change
Semantic change, also known as semantic shift or semantic progression describes the evolution of word usage — usually to the point that the modern meaning is radically different from the original usage. In diachronic linguistics, semantic change is a change in one of the meanings of a word...
the word underpinning has become to encompass all abstract
Abstraction is a process by which higher concepts are derived from the usage and classification of literal concepts, first principles, or other methods....
concepts that serve as a foundation.
Mass Concrete Underpinning
Also known as 'traditional underpinning,' the mass concrete underpinning method is nearly 100 years in age, and the protocol has not changed since. This underpinning method strengthens an existing structure's foundation by digging boxes by hand underneath and sequentially pouring concrete in a strategic order. The final result is basically a foundation built underneath the existing foundation. This underpinning method is generally applied when the existing foundation is at a shallow depth, however, the method still works very well even at fifty feet deep. The method has not changed since its inception with its use of utilitarian tools such as shovels and post hole diggers. Heavy machinery is not called for in this method due to the tight nature of the boxes being dug. There are several advantages to using this method of underpinning including the simplicity of the engineering, the low cost of labor to produce the result, and the continuity of the structure's uses during construction.
Beam and base underpinning
The beam and base method of underpinning is a more technically advanced adaptation of traditional mass concrete underpinning. A reinforced concrete beam is constructed below, above or in replacement of the existing footing. The beam then transfers the load of the building to mass concrete bases, which are constructed at designed strategic locations. Base sizes and depths are dependent upon the prevailing ground conditions. Beam design is dependent upon the configuration of the building and the applied loads. Anti-heave precautions are often incorporated in schemes where potential expansion of clay soils may occur.
Mini-piles have the greatest value where ground conditions are very variable, where access is restrictive, where environmental pollution aspects are significant, and where structural movements in service must be minimal. Mini-piled underpinning is generally used when the loads from the foundations need to be transferred to stable soils at considerable depths - usually in excess of 5.0 metres. Mini-piles may either be augured or driven steel cased, and are normally between 150mm and 300mm in diameter. Structural engineers will use rigs which are specifically designed to operate in environments with restricted headroom and limited space, and can gain access through a regular domestic doorway. They are capable of constructing piles to depths of up to 15 metres. The technique of minipiling was first applied in Italy in 1952, and has gone through a plethora of different names, reflecting worldwide acceptance and expiration of the original patents.
The relatively small diameter of mini-piles is extremely distinctive of this type of underpinning and generally uses anchoring or tie backs into an existing structure or rock. Conventional drilling and grouting methods are used for this method of underpinning. These mini-piles have a high slenderness ratio, feature substantial steel reinforcing elements and can sustain axial loading in both senses. The working loads of mini-piles can sustain up to 1,000kN loads.
In comparison to Mass Concrete Underpinning, the engineering aspect of mini-piles is a bit more involved, including rudimentary engineering mechanics such as statics and strength of materials. These mini-piles must be designed to work in tension and compression, depending on the orientation and application of the design. In detail, attention with design must be paid analytically to settlement, bursting, buckling, cracking, and interface consideration, whereas, from a practical viewpoint, corrosion resistance, and compatibility with the existing ground and structure must be regarded.
Mini-piled underpinning schemes
Mini-piled underpinning schemes include pile and beam, cantilever pile-caps and piled raft systems. Cantilevered pile-caps are usually used to try and avoid disturbing the inside of a building and require the construction of tension and compression piles to each cap. These are normally linked by a beam. The pile and beam system usually involves constructing pairs of piles on either side of the wall and linking them with a pile cap to support the wall. Again, the pile caps are usually linked by reinforced concrete beams to support the entire length of the wall. Piled raft underpinning systems are commonly used when an entire building needs to be underpinned. The internal floors are completely removed, a grid of piles is installed and a reinforced concrete raft is then constructed over the complete floor level, picking up and fully supporting all external and internal walls.