Raft foundations are concrete slabs that spread the load of a building evenly over a large area. They are often used when soil conditions are unstable or over 50% of the ground would need strip footings. There are different types including solid slab rafts, slab beam rafts, cellular rafts, piled rafts, and balancing rafts. Raft foundations are cheaper and easier to install than traditional footings and help reduce differential settlement. The main disadvantage is risk of edge erosion if not properly constructed.

1. Design of raft foundation
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2. introduction
raft foundation
:- Raft foundations (sometimes known as Mat Foundations)
are a large concrete slab which can support a number of
columns and walls. The slab is spread out under the entire
building or at least a large part of it which lowers the contact
pressure compared to the traditionally used strip or trench
footings.Because of the speed and volume of houses required
after the second world war,
.
Raft Foundations – How to make raft foundations are made

3. :- The raft foundation was cheaper, easier to install and most importantly, did
not require as much excavation as the usual strip foundations.
When the Building Regulations were introduced in 1965 there were no generic
rules for raft foundations as there were for strip foundations.
This meant that to use a raft foundation, it had to be designed and approved by
Building Control. This made the entire operation much more difficult and time
consuming so raft foundations became less widely used almost overnight.
raft foundation

4. Raft foundation used
raft foundation
:- Rafts are most often used these days when the strata
is unstable or (because of this) a normal strip foundation
would cover more than 50% of the ground area beneath
the building. There are also situations (usually in areas
where mining has occurred) where there may be areas
of movement in the strata.
They are much more commonly used in the construction
of commercial building in the UK that they are for
domestic homes, but can be used very successful in both
situations. To understand when it is better to use raft
foundations, you need to understand how they work.

5. raft foundation
Raft
foundation
used

6. Raft foundation work
raft foundation
:- A raft foundation spreads the weight of the building over
the whole ground floor area of that building. The raft is laid
on a hardcore, or scalping bed and usually thickened at the
edges, especially in very poor ground. Rafts are most suitable
when the ground is of good load bearing capacity and little
work is required to get a solid foundation.
:- Raft Foundations are built is this
following steps.
1. The soil removed down to correct
depth.
2. The foundation bed is then compacted
by ramming.

7. raft foundation
3. Lay reinforcement on spacers over the foundation bed.
4. Pour the concrete over the reinforcement.
:- The foundation may stiffened by ribs or beams built in
during construction which will add extra strength and
rigidity.

8. Advantages and Disadvantages of Raft Foundations.
raft foundation
:- Raft foundations tend to be cheaper and quicker to use than
traditional footings. There are a number of reasons why this is
the case:
1. The foundation and floor slab is combined, which saves
time and materials.
2. Less excavation is required.
:- Other reasons that make raft foundations preferable to
footings are due to their engineering benefits. They are
ideal for poor ground condition where normal footings
would not cope well as they cannot spread the load as
effectively.

9. raft foundation
:- Related to this is that raft foundations can reduce differential
settlement, where settlement occurs at different rates across the ground
surface of the building, which reduces cracking and other more serious
problems.
The main disadvantage is that they can prone to edge erosion if they are
not treated properly. They are not effective is the load of the building is
going to be focused on a single point, although this is rare in domestic
construction, so this isn’t generally of concern.

10. Types of raft foundation
raft foundation
:- There are five main types of raft foundation:
1. Solid slab rafts.
2. Slab beam rafts.
3. Cellular rafts.
4. Piled raft foundation.
5. Balancing (or floating) rafts.
:- The first type – solid slab rafts – can be further divided as flat
mats, wide toe rafts, blanket rafts and slip plane rafts.

11. Solid slab rafts
raft foundation
1. Flat Raft Mat :- Flat raft mats are typically used for small
buildings where it’s possible to position columns at regular
distances across the footprint of the building, to provide equal
stress distribution on the foundation. Steel mesh is typically
used at the bottom (in the column-foundation connection
proximity) and the top (mid-span between columns) of the
concrete mat, to provide resistance in both directions.

12. Solid slab rafts
raft foundation
2. Wide Toe Raft :-
A wide toe raft is used when the ground conditions mean that
to provide the necessary load support, a flat raft concrete slab
would have to be extremely thick, which would make it very
expensive. Instead, the slab is formed with a deeper reinforced
‘toe’ which extends to support the external side of a cavity
wall.

13. Solid slab rafts
raft foundation
3. Blanket Raft :- Where the build site includes small areas of
weaker soil or different soil types, a blanket raft may offer the
best solution. Here, a stone ‘blanket’ is laid on the prepared
ground in layers, building it up to the required level, before
the concrete raft is poured on. The raft foundation and the
stone blanket work together to provide an even load support,
despite the areas of weakness within the footprint of the
build.

14. Solid slab rafts
raft foundation
4. Slip Plane Raft :- Slipplaneraftsinvolve a preliminary layer of sand, which is
laid over a slightly larger surface than the required raft foundation, with the gap
around the raft filled with packed material.

15. Slab beam rafts
raft foundation
:-This type of raft foundation is typically used in
a build that involves column loads that are not
evenly distributed across the footprint of the
building. Reinforced columns are sited on top of
reinforced ground beams, and this provides the
necessary load bearing capacity.

16. Cellular rafts
raft foundation
:- This type of raft foundation comprises two concrete
slabs, which lock together via ground beams. Cellular
raft foundations are exceptionally rigid, and so they
are most suited for ground that is likely to settle
unevenly, or where very heavy loads are anticipated.
Cellular rafts can also be used where significant
ground heave is expected to be a problem.

17. Piled raft foundation
raft foundation
:- Piled raft foundations are less common in residential building projects, and are more
often seen in larger, high-rise developments as well as in structures that require a single
foundation element, such as silos, tanks and chimneys. Whereas a standard raft
foundation involves a concrete slab that sits on compressed ground, the slab in a piled
raft sits instead on a number of much deeper pile foundations that are bored deep
enough to reach a suitably stiff soil layer. These piles are long, thin footings made of
reinforced concrete or steel, and they are spaced evenly across the site. The raft then
‘floats’ on top of these pilings.

18. raft foundation
:- Piled raft foundations are
typically used when the water table
is close to the ground surface or
when the minimisation of
settlement is extremely important
for the serviceability of the
superstructure

19. Balancing rafts
raft foundation
:- Balancing rafts or floating foundations are increasingly used in cases where the reduction
of soil settlements is of absolute importance, as well as in projects where the soil bearing
capacity is extremely low. They are created through excavation of the soil in a way that the
weight of the removed soil (including the water weight) balances (entirely or partially) the
weight of the superstructure plus the foundation. This solution might not be always cost-
effective as it involves significant amounts of earthworks as well as a lot of work related to the
construction of retaining structures (permanent or temporary) prior and during the
excavation, to reach the depth of construction of the foundation.

20. raft foundation
-This is especially so when the water table is close to the ground surface.
:-Floating rafts are very popular when the
excavation works must be performed within an
urban environment with very strict
specifications on the minimisation of
settlements of neighbouring buildings. One
good example of this is for the construction of
large underground car parks in urban
environment.

21. raft foundation