SHORING AND UNDERPINNING IN CONSTRUCTION: OVERCOME SITE RISKS
Because the urban environment is always changing at an unfathomable pace and there is a scarcity of suitable land on which to erect new structures, shoring and underpinning in building construction will continue.
As a result, existing structures are demolished to make room for new constructions or renovated to match the building’s new function. When this happens, the new building’s footings, basements, and/or excavations may extend farther below the older building’s footings and perimeter walls, possibly requiring shoring and underpinning of the older structures in order to achieve stability during the excavation of the new building or to avoid collapse caused by vibrations during the construction of the new building structure.
Shoring and Underpinning
What is Shoring
Shoring in building construction is the process or technique of temporarily supporting a structure so that remedial or alteration works can go on in the construction in order to prevent the partial or total collapse of the structure.
Purpose of shoring and underpinning in building construction
Shoring is very important in construction and it can become necessary but it is not limited to the following:
Worthy of consideration while Shoring
- The lines of action of overturning forces in floors and roofs, the forces in walls, and the action of the shores must all meet at a single place in order to maintain balance.
- Shoring should be strong enough to withstand the active forces while remaining cost-effective.
- Internally or externally or on both sides, shoring can provide temporary support to unstable buildings.
Types of shoring and shoring procedure in building construction
i.) Raking shores (inclined shores)
When one or more timbers lean between the face of the structure to be supported and the ground, it is called a raking shore.
Raking shores are commonly employed when external support is required, Raking shores are normally inclined at an angle of 60 to 75 degrees between the wall and the ground to give the most effective support with the support of a wall plate to increase the support area.
A raking shore consists of the following members:
- Rakers or inclined member
- Sole plate
- Wall plate
The following considerations should be critically made while using the raking shores:
- Rakers should be 45 degrees in the ground. However, the angle might be anything between 45 and 75 degrees.
- The length of the raker can be shortened for high buildings by using rider rakers.
- At regular intervals, rakers should be adequately braced.
- The size of the rakers will be determined by the expected pressure from the wall.
- It is ideal that, at floor level, the raker’s centerline and the wall should meet.
- To cover a longer length of the bar, shoring can be placed at 3 to 4.5m intervals.
- The sole plate should be correctly implanted in the ground on an incline, with the appropriate section and size.
- Wedges should not be installed on sole plates since they are prone to giving way whenever they are subjected to vibrations.
ii.) Flying shores (horizontal shores)
Flying shores are a temporary support system for the adjoining walls of two buildings while the intermediate structure is being demolished and rebuilt.
This category includes all forms of systems for supporting an unsafe construction where the shores do not touch the ground (hence the name “flying”). The flying shore, like the raking shore, is also made up of wall plates, needles, cleats, horizontal struts (which are sometimes called horizontal shores. This is where the a.k.a came from), and inclined struts that are organized in various ways based on the situation.
The wall plates are generally put against the wall and attached to it in this way. Between the wall plates is a horizontal strut that is supported by a needle and cleat system. The needle at the top and the straining parts at the bottom, support the inclined struts.
The straining piece is attached to the horizontal shore and is also known as the straining sill. The straining piece’s width is the same as that of the struts. When the distance between the walls (to be strutted apart) is wide, a horizontal shore is inadequate to serve the function of flying shore. A trussed framework of members is required instead.
iii.) Dead or vertical shores
When the bottom section of a wall is removed for the purpose of establishing an opening in the wall or to replace a damaged load-bearing wall in a structure, the system of shoring known as the dead shore is employed to provide vertical support to walls, roofs, floors, and other structures.
The dead shore is made up of a series of beams and supports that supports the weight of the structure above and transmit it to the ground via a solid foundation below.
When openings in the wall are required, openings are created at a height that allows for the introduction of the beam or girder that will be permanently installed to support the weight of the structure above.
The distance between the openings is determined by the kind of brickwork but it generally ranges between 1.2m and 1.8m in diameter. Beams called needles are inserted into the holes and supported on each side of the wall by vertical supports called dead shores. The needles can be made of wood or steel and have a large enough section to support the load.
When the needle and supports are in place, the dead shores stand apart from the wall on both sides to allow for working space.
The props are held in place by folding wedges at their bases, while the connection between the prop and the needle is held in place by dogs. All doors, windows, and other openings are carefully strutted before the dismantling operation begins.
They are independently supported to ease the strain of the floors and roof above. When wall cutting, vibrations, and shocks are unavoidable; as a precaution, raking shores are sometimes built before the wall cutting activity begins.
Other types of shoring
- Hydraulic Shoring
Hydraulic shores use hydraulic pistons that may be pumped outward until they press up against the sidewalls. They’re commonly used in combination with steel plate or plywood, which is between 25 -30mm thick (for plywood) or a pretty heavy Finland Form (FINFORM) 20 -25mm thick Finland Form (FINFORM).
- Beams and Plate Shoring
Beams and plate steel beams are inserted in between I-beams that have been driven into the ground. Soldier boarding is a similar technique that uses wood planks. Hydraulics are often fast and easier to use; the other options are typically used for longer-term projects or bigger excavations.
History of underpinning
Underpinning dates back to long before the 15th century. When bridges, tunnels, buildings, and residences are observed to be weak, it has long been a custom to utilize underpinnings to reinforce them.
The tale of the historic Hotel Commodore is a more recent example of underpinning.
The Hotel Commodore, which opened in 1917, was built directly above (and hence subsidized by) the Grand Central Station underground system.
The Hotel Commodore had a major renovation in 1979 before reopening as the Grand Hyatt on November 1, 2021.
This refurbishment was extensive, necessitating the opening of space on the subway mezzanine level (middle floor) to enhance headroom and improve passenger outflow.
This necessitated a lot of underpinning and a lot of load transfer from the hotel above via temporary support, sequencing, and jacking.
In a nutshell, underpinning enables current architectural wonders.
And the unfortunate part is that you never see it when it’s done because the technology is entirely underground.
Underpinning definition and meaning
Underpinning is a delicate method of construction for strengthening an old foundation or burying a new stronger foundation deeper beneath the existing foundation in order to prevent a structure from further failing structure or collapsing.
Underpinning is necessary when it is noticed that the foundation of a structure is failing or becoming unstable.
Purpose of Underpinning
Underpinning may be required in construction for a variety of reasons, including:
- Earth movement or poor soil qualities may have caused the foundations of the present building to shift. This might be due to natural disasters such as earthquakes, floods, and droughts, among other things.
- More building levels added, either above or below ground level, requiring existing foundations to be upgraded to withstand the increased load as against what was initially calculated for by the civil engineer.
- Structures built around the site shifting the soil’s stability.
- In a case where the strength and stability of the original foundation is deemed to be inadequate.
- Where there is a change from the initial intended design of the structure to something else (mostly, now carrying more load)
- When there is a need to construct a basement in a part or in the entire existing structure, underpinning is required.
- To strengthen the settled foundation that may be caused by cracks in the wall.
- When a structure with a deep foundation is to be built next to another one with shallow foundations. In this situation, shoring and underpinning should be used to reinforce the shallow foundations.
Preparation and/or precaution before underpinning
- Because the underpinning process is an art rather than a science, it should be applied according to the circumstance
- Before underpinning, all poor masonry work such as joints and cracks should be repaired,
- Necessary shoring and proper strutting should be completed so that the existing structure is safe.
- The current structure(s) should be thoroughly investigated, and the best underpinning approach should be used.
- Before underpinning begins, necessary repairs such as crack grouting and rod insertion between walls should be completed.
Major types of underpinning
The major types of underpinning are:
1. Mass concrete underpinning
2. Beam and base underpinning
3. Mini piled underpinning
Methods of underpinning
The following are methods of underpinning.
a.) Pit method of underpinning (mass concrete underpinning method)
A trench/ pit is dug to reveal the foundation to be renovated. The existing foundation is either totally demolished or strengthened appropriately by building a new foundation below the existing one.
The length of the foundation to be underpinned is split into smaller portions of 1.2 to 1.5 m in length. A hole is drilled in the wall above the plinth level for each portion, and a needle beam is put into the hole (the beam may be made of timber or steel). Bearing plates are then used to hold the brickwork above the needle.
On both sides of the wall, wooden supports and screw jacks support the needle into position. A new foundation is constructed once the foundation pit has been dug to the required level.
Process of underpinning while using the pit method:
- In the first round, alternate portions are taken up first to be worked upon. After that, the remaining intermediate portions are tackled. Just one portion should be completed at a time.
- Only once the foundation has reached full strength should all the arrangements, such as needle beams, etc be gradually removed.
- If the new foundation is deeper, the foundation trench may be properly timbered (trenching)
- When working on a long wall that extends in both directions, it’s best to start from the middle.
- The needle holes initially drilled and other brickwork deficiencies should be filled with cement mortar and covered up.
Raking shores comes in very handy to support the wall for which underpinning is to be done i.e. if the wall is weak. Shoring and underpinning combined are sometimes necessary for the pit method.
b.) Pile method
It entails stabilizing the structure on newly constructed piles of various sorts without the need for excavation.
It is also known as or called needle and pile underpinning
Piles are driven on opposite sides of the wall that supports the weak foundation. A needle or pin enters the wall, which is then attached to the piles. These needles have the effect of pile caps. This approach can be used to alleviate soil settlement caused by waterlogging or clayey nature or weak bearing strata.
c.) Cantilever Needle Beam Method of Underpinning
The cantilever pit technique of underpinning, as shown below, is an expansion of the pit method.
This method can be used for underpinning if the foundation only has to be extended to one side and the plan features a stronger inner column.
Advantages of Cantilever Needle Beam Method
It is quicker than the usual procedure.
Only one side has access.
It has the capacity to lift heavy loads
It has the capacity to lift heavy loads
Disadvantages of cantilever Needle Beam Method:
When the existing foundation is deep, digging is discovered to be uneconomical.
The application of needle beams is limited due to access restrictions.
d.) Pier and beam method of underpinning
This method combines the pit and pile methods by excavating below the existing foundation and building a structure for the new foundation.
Piers are placed beneath the foundations of structures, filled with concrete, and then wedged to transmit the weight to a new pier in this type of underpinning. This approach works well on dry soils. When laying the sheeting for pier underpinning, extreme caution must be exercised to avoid ground loss; otherwise, the building structure would sink.
The smallest underlying pits that can be used to offer working space for workers is 1m x 1.3m. The pits have indeed been dug to sufficiently strong strata.
When the underlying ground has water-bearing strata, piles are jacked into the earth with a casing for underpinning the structure.
e.) Jack pile underpinning
When typical underpinning is uneconomical due to the depth of required bearing capability level of the subsoil, jack pile underpinning is used. The major benefit of Jack Pile is that it is vibration-free and flexible due to its ability to adjust the pile depth to meet the subsoil conditions.
The existing foundation is spread over the heads of the pile caps that are cast in onto the Jack pile heads once the hydraulic jacks have been removed.
This procedure makes a sound foundation.
Generally, shoring and underpinning work hand in hand when an existing structure is to be demolished or strengthened.
Difference between Shoring and Underpinning
- `Shoring is used mainly for wall repair or demolition or opening while Underpinning is mainly used for foundation repair or strengthening
- Shoring provides temporary support while underpinning provides permanent support
- Shoring is used to temporarily support the floors or roofs connected to a defective wall that is to be demolished or reconstructed while underpinning is used to provide permanent support to a defective foundation that needs to be replaced, strengthened, or stabilized
- Shoring is intangible of the loads on the wall. It is majorly done to repair a wall or prevent it from collapsing while underpinning is done so as to help in carrying heavier loads
- Shoring can also double as a platform for walking on heights while underpinning is for one use only. Strengthening or stabilizing a foundation
We can avoid collapse, loss of lives, and properties by first taking note of foundation behavior and then adopting the most suitable type/ method of shoring and underpinning. If you need an expert assessment, there are a number of specialists engineers across the globe in shoring and underpinning.
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Originally published at https://baseandgrounds.com on December 12, 2021.