Sunday, September 14, 2014

What is Underpinning? - Part 2 - Other Methods

Underpinning is often considered as a solution for subsidence however the need to stabilise ground could warrant underpinning for other reasons therefore underpinning is not a solution exclusive to subsidence

In last week’s article (Link) I discussed traditional underpinning and explained that this is often a solution to ground movement (subsidence), which will stabilise a building or structure in the event that movement is continuing.   I also explained that dealing with ground movement is not always disruptive or expensive however professional advice is always recommended to ensure that an appropriate remediation method is adopted.  In this week’s article I will consider a number of different methods of underpinning as alternatives to traditional underpinning that may be selected to suit individual circumstances.

Underpinning is often considered as a solution for subsidence however the need to stabilise ground could warrant underpinning for other reasons therefore underpinning is not a solution exclusive to subsidence.  For example, the use of a building may change which would add load.  The original foundation design may not have been designed for this additional load, this could warrant underpinning.  Adding an additional storey/s to a building would be a good example of this.  Also, the construction of nearby structures may warrant additional support to an existing foundation for which underpinning may be selected.  For whatever reason underpinning is used it is important that the correct method is selected.  Traditional underpinning was discussed in last week’s article however this week will I discuss needle beam underpinning, cantilever beam underpinning and an underpinning raft.

Needle Beam Underpinning – This method of underpinning stabilises existing foundations with the installation of concrete ‘needle beams’ supported by concrete mini-piles.  The amount, distance between needles and diameter of the mini piles will be determined by the design. Concrete mini-piles can be either cast in-situ (this is where wet concrete is used with steel reinforcement) or pre-cast (made off site a delivered as a dry solid concrete component).  A small hole or pocket is broken out below ground level and just above the existing foundation as detailed on the image at the left.  Concrete mini piles are then installed into the ground adjacent to the newly made hole. We will consider cantilever beams in a moment, however a common method of installing needle beam underpinning is to install a concrete mini pile either side of the newly formed hole, one inside the property and one on the outside.
The concrete mini piles, once installed will have short steel reinforcement bars projecting at the top.  These are referred to as starter bars and allow further steel reinforcement to be attached which will connect each of the concrete mini piles to each other as they pass through the hole/pocket that has been made in the wall.  The next stage is to provide temporary timber formwork around the reinforcement bars to accommodate and hold the wet concrete until it has cured (hardened).  Once the formwork is complete the wet concrete is poured and allowed to cure, after which the timber formwork is removed, leaving behind a single solid concrete beam supported off the concrete mini piles.  The hole/pocket, which the new concrete beam now passes through, is made good with the installation of ‘packers’ to fill any gap between the top of the new concrete needle beam and the underside of wall that is being supported.  Packers will usually take the form of a masonry material or possibly materials such as slate to ensure that any voids are completely filled so that the wall has a solid support.

Needle beam underpinning is used where traditional underpinning is not appropriate due to the existing foundations being too deep, or good bearing strata is so deep that it is uneconomical to dig (depths greater than 1.5m). Concrete mini piles are typically installed in pairs at 1.0m-1.5m intervals and approximately 1.0m-1.5m apart, although this can vary with design.  The advantages of this system include suitability for restricted access, the needle beams can be constructed at a higher level if the existing foundations are too deep, it is often faster than traditional underpinning, it is more economical at greater depths, the system has a high load capacity and there is less disruption and spoil produced compared to traditional underpinning.

Cantilever Beam Underpinning – Firstly it is worth clarifying the term cantilever. provide the following definition; ‘A projecting structure, such as a beam, that is supported at one end and carries a load at the other end or along its length’ This method of underpinning will stabilise a wall foundation either internally or externally however it does not require support on both the internal and external side of the wall.  Basically all of the support is provided at just one side of the wall with the load supported off a concrete cantilever beam which passes through the wall in the same manner as the needle beam method previously described; i.e. with pockets cut into the wall and a beam cast through the wall with the use of mini-piles, reinforcement, formwork and concrete which links the two mini piles. The image above shows that two mini-piles are installed, one is a compression pile (taking downward force) and the other is tension pile (resisting uplift).   

Many of the advantages of cantilever beam underpinning compared to traditional underpinning are the same as needle beam underpinning described above in terms of speed, more economical at greater depth etc.  A further significant advantage of cantilever beam underpinning is where access is particularly restricted as the mini-piles are cast from just one side of a wall/structure.

Underpinning Raft – Of all methods of underpinning described the installation of an underpinning raft is by far the most disruptive and expensive as it can stabilise walls and foundations for a whole building. Mini-piles are installed within a property and capped with an integral reinforced concrete raft.  The diagram on the right shows that needle beams project from the slab into the walls below ground level. This system is used where whole rooms or whole structures are to be underpinned as opposed to individual walls or parts of a building.  Although more expensive than other methods of underpinning an advantage is that a new integral floor slab is provided at the same time as stabilising a building. The image below shows an underpinning raft just prior to concrete being poured.

Underpinning Raft before concrete is poured. Source:
This week's article has discussed a number of alternatives to traditional underpinning as ways of stabilising walls, buildings or structures.  Professional advice should always be sought to ensure an appropriate method of stabilising is selected. This and last week’s article provide a short introduction into to some of the commonly used methods of stabilising ground, structures and buildings however there are many other ways that stabilisation can be achieved.  This is something I will no doubt discuss in a future article.

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