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
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| Source: http://www.mason-mason.co.uk/ |
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. Freeedictionary.com 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.
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| Underpinning Raft before concrete is poured. Source: http://www.larsenpiling.com/ |
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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Good and interesting presentation.
ReplyDeleteVery helpful. Thanks for sharing!
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