Underpinning

Last updated

In construction or renovation, underpinning is the process of strengthening the foundation of an existing building or other structure. Underpinning may be necessary for a variety of reasons:

Contents

Underpinning may be accomplished by extending the foundation in depth or breadth so it either rests on a more supportive soil stratum or distributes its load across a greater area. Use of micropiles [1] and jet grouting are common methods in underpinning.

Underpinning may be necessary where P class (problem) soils in certain areas of the site are encountered.

Through semantic change the word underpinning has evolved to encompass all abstract concepts that serve as a foundation.

Mass concrete underpinning

Mass concrete underpinning is one of the simplest forms of remedial underpinning at shallow depths. This type of underpinning is done by excavating "bays" along and under the existing foundation and filling them with mass concrete. It is sometimes called a "traditional" method to distinguish it from other types of underpinning like piling and needling. The latter often require underpinning specialists and may use proprietary underpinning systems. Mass concrete underpinning work is performed in compliance with the Party Wall Act (in the UK) using plans that are designed with engineering calculations to plan a sequence of excavating bays along and underneath the existing foundation without damaging existing walls. In some cases walls have collapsed because lateral support was inadequate leading to disputes among contractors, subcontractors and architects about where the responsibility lay for the mistake. [2] [3] [4]

Mass concrete underpinning is commonly used when permanent support is needed to comply with the Party Wall Act of 1996 in the construction of a new basement during a restoration, rehabilitation or redevelopment. [5]

In the United Kingdom most subsidence claims are for buildings at least 40 years old with shallow strip foundations. This is one of the most common types of foundations suffering from subsidence related damage and according to the Building Research Establishment subsidence database, mass concrete underpinning was the most common underpinning and was often applied only to part of a building. If the soils have a low bearing capacity partial underpinning may increase the risk of differential settlement and localized settlement due to additional load on the soil. [6]

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-piled underpinning

Mini-piles have the greatest use where ground conditions are 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 m (16 ft). Mini-piles may either be augured or driven steel cased, and are normally between 150 mm (5.9 in) and 300 mm (12 in) 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 m (49 ft). The technique of minipiling was first applied in Italy in 1952, and has gone through many different names, reflecting worldwide acceptance and expiration of the original patents. [7]
The relatively small diameter of mini-piles is 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. [7] The working loads of mini-piles can sustain up to 1,000 kN (100 long tons-force; 110 short tons-force) loads.
In comparison to Mass Concrete Underpinning, the engineering aspect of mini-piles is somewhat 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 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. 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.

Chemical grout

One of the uses of the soil improvement technique of pressure grouting is foundation underpinning especially during excavations like subway constructions. [8] This technique has been used in municipal development projects of significant scope in the United States. One of the largest chemical grouting projects was the extension of the Pittsburgh Light Rail Transit subway when the foundations of six large buildings needed to be protected from ground movement and related building settlement. [9]

Pressure grouting can be low pressure or high pressure. "Jet grouting" is a general term used for high pressure grouting where the high pressure air, water and cementing grout are injected into the ground at high velocity. This can be doing with single tube systems to mix the grout with in situ soil to form a grouted "column" in the ground. Double tube systems and triple tube systems using air and water remove some of the soil to create larger grouted bulbs or columns. [8]

Low pressure chemical injection grouting is used to underpin structures in sandy soils. [10]

Related Research Articles

<span class="mw-page-title-main">Grout</span> Building material

Grout is a dense fluid that hardens used to fill gaps or as reinforcement in existing structures. Grout is generally a mixture of water, cement, and sand, and is employed in pressure grouting, embedding rebar in masonry walls, connecting sections of precast concrete, filling voids, and sealing joints such as those between tiles. Common uses for grout in the household include filling in tiles of shower floors and kitchen tiles. It is often color tinted when it has to be kept visible and sometimes includes fine gravel when being used to fill large spaces. Unlike other structural pastes such as plaster or joint compound, correctly mixed and applied grout forms a water-resistant seal.

<span class="mw-page-title-main">Retaining wall</span> Artificial wall used for supporting soil between two different elevations

Retaining walls are relatively rigid walls used for supporting soil laterally so that it can be retained at different levels on the two sides. Retaining walls are structures designed to restrain soil to a slope that it would not naturally keep to. They are used to bound soils between two different elevations often in areas of inconveniently steep terrain in areas where the landscape needs to be shaped severely and engineered for more specific purposes like hillside farming or roadway overpasses. A retaining wall that retains soil on the backside and water on the frontside is called a seawall or a bulkhead.

<span class="mw-page-title-main">Foundation (engineering)</span> Lowest and supporting layer of a structure

In engineering, a foundation is the element of a structure which connects it to the ground or more rarely, water, transferring loads from the structure to the ground. Foundations are generally considered either shallow or deep. Foundation engineering is the application of soil mechanics and rock mechanics in the design of foundation elements of structures.

<span class="mw-page-title-main">Seismic retrofit</span> Modification of existing structures to make them more resistant to seismic activity

Seismic retrofitting is the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes. With better understanding of seismic demand on structures and with recent experiences with large earthquakes near urban centers, the need of seismic retrofitting is well acknowledged. Prior to the introduction of modern seismic codes in the late 1960s for developed countries and late 1970s for many other parts of the world, many structures were designed without adequate detailing and reinforcement for seismic protection. In view of the imminent problem, various research work has been carried out. State-of-the-art technical guidelines for seismic assessment, retrofit and rehabilitation have been published around the world – such as the ASCE-SEI 41 and the New Zealand Society for Earthquake Engineering (NZSEE)'s guidelines. These codes must be regularly updated; the 1994 Northridge earthquake brought to light the brittleness of welded steel frames, for example.

Shoring is the process of temporarily supporting a building, vessel, structure, or trench with shores (props) when in danger of collapse or during repairs or alterations. Shoring comes from shore, a timber or metal prop. Shoring may be vertical, angled, or horizontal.

<span class="mw-page-title-main">Lilla Bommen (building)</span> Office in Gothenburg, Sweden

The Lilla Bommen named after the surrounding location of Lilla Bommen, commonly referred to as Läppstiftet , is an 86 m, 22-floor building housing office, networking and restaurant spaces in Gothenburg, Sweden. The building’s distinct post-modern architectural style, popularly referred to as “The Lipstick” due to its distinctive red-and-white colour schematic, was erected on the banks of Göta River in 1989 by Skanska Property West AB. The 32000 sqm office space which houses over 900 office workers, is a popular tourist destination for its top-floor lookout, Götheborgs Utkiken and restaurants such as the Restuarang Läppstiftet, has contributed to the landmark status of the building in the Central Gothenburg skyline.

<span class="mw-page-title-main">Pile driver</span> Heavy equipment

A pile driver is a heavy-duty tool used to drive piles into soil to build piers, bridges, cofferdams, and other "pole" supported structures, and patterns of pilings as part of permanent deep foundations for buildings or other structures. Pilings may be made of wood, solid steel, or tubular steel, and may be driven entirely underwater/underground, or remain partially aboveground as elements of a finished structure.

<span class="mw-page-title-main">Deep foundation</span> Type of foundation

A deep foundation is a type of foundation that transfers building loads to the earth farther down from the surface than a shallow foundation does to a subsurface layer or a range of depths. A pile or piling is a vertical structural element of a deep foundation, driven or drilled deep into the ground at the building site.

<span class="mw-page-title-main">Shallow foundation</span> Type of building foundation

A shallow foundation is a type of building foundation that transfers structural load to the earth very near to the surface, rather than to a subsurface layer or a range of depths, as does a deep foundation. Customarily, a shallow foundation is considered as such when the width of the entire foundation is greater than its depth. In comparison to deep foundations, shallow foundations are less technical, thus making them more economical and the most widely used for relatively light structures.

Landslide mitigation refers to several human-made activities on slopes with the goal of lessening the effect of landslides. Landslides can be triggered by many, sometimes concomitant causes. In addition to shallow erosion or reduction of shear strength caused by seasonal rainfall, landslides may be triggered by anthropic activities, such as adding excessive weight above the slope, digging at mid-slope or at the foot of the slope. Often, individual phenomena join to generate instability over time, which often does not allow a reconstruction of the evolution of a particular landslide. Therefore, landslide hazard mitigation measures are not generally classified according to the phenomenon that might cause a landslide. Instead, they are classified by the sort of slope stabilization method used:

<span class="mw-page-title-main">Tieback (geotechnical)</span>

A tieback is a structural element installed in soil or rock to transfer applied tensile load into the ground. Typically in the form of a horizontal wire or rod, or a helical anchor, a tieback is commonly used along with other retaining systems to provide additional stability to cantilevered retaining walls. With one end of the tieback secured to the wall, the other end is anchored to a stable structure, such as a concrete deadman which has been driven into the ground or anchored into earth with sufficient resistance. The tieback-deadman structure resists forces that would otherwise cause the wall to lean, as for example, when a seawall is pushed seaward by water trapped on the landward side after a heavy rain.

Deep Foundations Institute (DFI) is an international association of contractors, engineers, manufacturers, suppliers, academics and owners in the deep foundations industry.

<span class="mw-page-title-main">Pile cap</span>

A pile cap is a thick concrete mat that rests on concrete or timber piles that have been driven into soft or unstable ground to provide a suitable stable foundation. It usually forms part of the deep foundation of a building, typically a multi-story building, structure or support base for heavy equipment, or of a bridge. The cast concrete pile cap distributes the load of the building into the piles. A similar structure to a pile cap is a "raft", which is a concrete foundation floor resting directly onto soft soil which may be liable to subsidence.

Ground–structure interaction (SSI) consists of the interaction between soil (ground) and a structure built upon it. It is primarily an exchange of mutual stress, whereby the movement of the ground-structure system is influenced by both the type of ground and the type of structure. This is especially applicable to areas of seismic activity. Various combinations of soil and structure can either amplify or diminish movement and subsequent damage. A building on stiff ground rather than deformable ground will tend to suffer greater damage. A second interaction effect, tied to mechanical properties of soil, is the sinking of foundations, worsened by a seismic event. This phenomenon is called soil liquefaction.

<span class="mw-page-title-main">Screw piles</span> Construction component used for foundations

Screw piles, sometimes referred to as screw-piles, screw piers, screw anchors, screw foundations, ground screws, helical piles, helical piers, or helical anchors are a steel screw-in piling and ground anchoring system used for building deep foundations. Screw piles are typically manufactured from high-strength steel using varying sizes of tubular hollow sections for the pile or anchors shaft.

<span class="mw-page-title-main">Pressure grouting</span>

Pressure grouting or jet grouting involves injecting a grout material into otherwise inaccessible but interconnected pore or void space of which neither the configuration or volume are known, and is often referred to simply as grouting.

<span class="mw-page-title-main">Tripod (foundation)</span> Three-legged foundation for wind turbines, especially offshore structures

The tripod is a type of foundation for offshore wind turbines. The tripod is generally more expensive than other types of foundation. However, for large turbines and higher water depth, the cost disadvantage might be compensated when durability is also taken into account.

<span class="mw-page-title-main">Static load testing</span>

Static load testing is an in situ type of load testing used in geotechnical investigation to determine the bearing capacity of deep foundations prior to the construction of a building. It differs from the statnamic load test and dynamic load testing in that the pressure applied to the pile is slower. Static load testings are performed in order to measure a design's axial tension or axial compression. It can also be used to measure its deflected shape under lateral load

Marine construction is the process of building structures in or adjacent to large bodies of water, usually the sea. These structures can be built for a variety of purposes, including transportation, energy production, and recreation. Marine construction can involve the use of a variety of building materials, predominantly steel and concrete. Some examples of marine structures include ships, offshore platforms, moorings, pipelines, cables, wharves, bridges, tunnels, breakwaters and docks. Marine construction may require diving work, but professional diving is expensive and dangerous, and may involve relatively high risk, and the types of tools and equipment that can both function underwater and be safely used by divers are limited. Remotely operated underwater vehicles (ROVs) and other types of submersible equipment are a lower risk alternative, but they are also expensive and limited in applications, so when reasonably practicable, most underwater construction involves either removing the water from the building site by dewatering behind a cofferdam or inside a caisson, or prefabrication of structural units off-site with mainly assembly and installation done on-site.

References

  1. "International Society for Micropiles". International Society for Micropiles. Retrieved 16 December 2012.
  2. Gwynne, Anthony (2013). Guide to Building Control For Domestic Buildings. Wiley-Blackwell. ISBN   9780470657539.
  3. The JCT 05 Standard Building Contract: Law and Administration. Butterworth-Heinemann. 2009. p. 88. ISBN   9781856176293.
  4. McGuinness, John (2008). The Law and Management of Building Subcontracts. Wiley. p. 155. ISBN   9780470759752.
  5. Douglas, James (2006). Building Adaptation. Elsevier. p. 288. ISBN   9780750666671.
  6. Problematic Soils: Proceedings of the symposium held at the Nottingham Trent University School of Property and Construction. Thomas Telford Publishing. 2001. p. 65. ISBN   0727730436.
  7. 1 2 "We Fix Foundation – Foundation & Structural Repair Company/Contractor". We Fix Foundation. Retrieved 2017-09-18.
  8. 1 2 Chu, Jian; Indraratna, Buddhima (2005). Ground Improvement: Case Histories. Elsevier. pp. 357–358. ISBN   0080446337.
  9. "Underpinning with Chemical Grout". Transportation Research Board (National Academics of Sciences Engineering and Medicine.
  10. Hayward Baker, Wallace (1983). "Design and Control of Chemical Grouting". Bureau of Transportation Statistics. Department of Transportation. One of the fundamental questions that must be asked when grouting is first considered is whether is ground is groutable...For very low permeability sands, the injection rate at permissible pressures may be so slow that grouting becomes unfeasible. Thus, chemical grouting is recommended only in predominantly sandy materials with less than 25% silts and clays.