CE1202 Geotechnical Properties & Construction Methods
EG5111 Advanced Technology, Planning & ProductionAdvanced
Foundations1 Piling - IntroductionWhat is piling?Piles can be made
from steel or timber although in most housing work piles are made
from insitu or pre-cast reinforced concrete. They are used either
to transmit loads from the building through soft or compressible
ground to firmer strata below (end bearing pile), or to distribute
loads into the subsoil along the length of the pile (friction
pile).In housing, a concrete beam across the top of the piles
distributes the load from the load bearing brickwork into the piles
themselves. In framed buildings the piles usually support concrete
or steel columns.
Why so popular?20 or 30 years ago piling was comparatively rare
for housing (other than medium and high rise flats). Since then,
several factors have led to an increase in the use of piled
foundations. These include: the increased pressure to re-develop
'brownfield' sites, where strip foundations may not always be
appropriate increased costs of 'carting away' and tipping surplus
excavation from foundation trenches (particularly in cities) the
development and easy availability of smaller piling rigs and piling
systems which are, nowadays, cost effective for house foundations
greater understanding of piling in general (partly through better
building education).
Factors affecting choiceThere are literally dozens of piling
companies in the UK each offering a number of different piling
systems. In many cases more than one piling system will suit a
particular set of circumstances. However, when choosing a piling
system there are four main criteria to consider: building load the
nature of the ground (ie, the subsoil) local environmental or
physical constraints (noise restrictions, height restrictions)
cost
What are piers?Pier foundations, sometimes called pad and stem,
are not dissimilar to end-bearing piles in their function. However,
their construction is very different. Piling is carried out from
the surface, by drilling or driving down into the ground. When
building piers, individual pits are usually excavated and then
backfilled once the piers have been constructed.
2 Piling - The ChoicePile typesThere are basically three types
of pile: driven piles which are pre-formed - usually steel or
pre-cast concrete (displacement) driven piles which are cast insitu
(displacement) bored or augered piles which are cast in situ
(replacement) In recent years a new piling system has been
introduced by Roger Bullivant. It's called a bored displacement
pile and is described later in this section. NB: a displacement
pile forces the ground out of the way as the pile is driven. A
replacement pile removes it first.
Piles can be supported through their end-bearing or through
friction (or a combination of the two) End bearing piles are
generally used where rock or dense granular material underlies a
softer stratum. Friction piles, where most of the support comes
from friction between the pile sides and the soil, are more likely
to be used in clay.
Reasons for choosing pilingThe decision to choose piled
foundations rather than strip or spread foundations may not always
be straightforward. However, if firm ground is some distance below
the surface (2 metres or more), piling may be more economical.
Other situations which might make piling a preferred choice
include: a high water table expensive cart-away costs (very high in
some of the larger cities where tips are not close) where soils
such as clay are likely to swell or shrink with changes in moisture
content where trenches are not very stable and are likely to
collapse
Choice of piling system In many cases more than one piling
system is appropriate for any given set of ground conditions.
Different piling contractors have their preferred systems, often
developed in-house over a number of years. Advice, therefore, from
two contractors may differ, yet both both may provide suitable
solutions. The list below identifies a number of factors to be
considered when choosing a piling system. We have tried to keep
this section a simple as possible; this web site is mainly
concerned with providing an illustrated introduction to house
construction - not ground engineering. Click here for a clip
showing pre-cast driven piles.
Some factors affecting choice... If the length of the piles is
known and the site is readily accessible with no noise or vibration
restrictions, driven pre-cast piles may be the most economical
choice. Where noise and vibration is a problem augered piles may be
the best solution. If piles are close to existing buildings the
risk of displacement piles causing soil heave must be taken into
account. Shell piles, (driven in sections) can be used where
lengths are uncertain and where 'waisting or necking' might occur.
On restricted sites there may be problems handling long pre-cast
piles and large piling rigs. Ground obstructions can be a problem
but steel piles can sometimes be driven through them. Bored or
augered piles are normally used for housing in clay soils subject
to shrinkage or swelling. In loose granular soils the act of
driving can actually help to compact the soil. In clays, driven
piles tend to 'whip' as they are driven - augered or bored piles
are often preferable.
3 Pre-cast Piles (with insitu ground beam)On this site, formerly
a power station and docks, the nature of the ground varied. In some
parts of the site, mostly away from the water's edge, strip
foundations were acceptable. In others, in particular along the
quayside, piles had to be used. Here, there was a sloping stratum
of rock (mudstone and sandstone) some 6 to 18 metres below the
ground's surface. The material above the rock included soft clay,
silt, fill, and a number of man made obstructions from previous use
of the site.
Pre-cast reinforced concrete piles were chosen for this site; it
was the engineer's view that these offered the fastest and cheapest
solution. The piles were driven into the ground by a crane mounted
drop hammer - vibration etc was not an issue as the site was fairly
isolated. A wooden insert under the driving head helped cushion the
piles and prevent cracking in the pile itself. Where obstructions
in the ground prevented the pre-cast piles from being driven into
the ground or forced them out of position, steel piles were driven
alongside.
In one or two places the piles had to be jointed (ie, to make
them longer). Have a close look at the photo on the left - the
metal plates on the pile ends were joined using steel pins. The
excess lengths of pile were removed by a hydraulic 'crusher'
mounted on the back of a JCB. Further 'trimming' was carried out by
compressed air tools to expose the steel bars in the piles.
The photo on the left shows a steel pile nearing its 'set'. This
had already been determined by the engineer, taking into account
the building load and the nature of the ground. If 10 hammer blows
produced downward movement of not more than 5mm in the pile the
'set' had been reached.
The plan on the left shows part of the pile cap and ground beam
layout. The reinforced ground beam was poured monolithically (ie,
at the same time - from the Greek meaning 'single stone') with the
pile caps and floor slab. In the right hand picture you can see the
exposed steel of the piles and some blockwork walls - these walls
are in fact formwork designed to contain the concrete when it is
poured.
The left hand photograph shows the view from the adjacent
scaffolding. The line of the ground beam can clearly be seen - the
intermittent projections or the pile caps. The two concrete
cylinders are large inspection chambers (to be topped with
cast-iron covers). The right-hand photograph was taken a few days
later and shows the steel in position, and ready for
concreting.
The concrete for the ground beam and pile caps was delivered to
site ready-mixed and placed with the help of a track mounted
excavator. To ensure the beam was dead level (for the steel framed
superstructure) a spinning laser ('Laserplane') provided a
benchmark- by deducting the reading on the staff from the known
height of the laser, the exact level, or height, of the beam can be
worked out.
4 Augered PilesContinuous Flight Auger
Augered piles (replacement piles) are suitable for many types of
ground, particularly clays. Piles can be formed up to 750mm in
diameter (depending on type of rig) with safe working loads up to
3500kN (350 tonnes). A 200 mm diameter pile has an SWL of about
600kN. Pile lengths of up to 25 metres can be constructed. The
insitu concrete pile is reinforced, the exact details of the
reinforcement will depend on the nature of the loading.A standard
detail comprises a single,centrally positioned, reinforcement bar;
reinforcement cages will be required to withstand horizontal or
bending moment loadings.
CFA piles are formed using hollow stem augers boring techniques
(in most cases this will produce spoil 'arisings' which require
disposal). Once the correct depth is reached, concrete is injected
down the hollow stem of the auger. As the auger is extracted, the
concrete fills the void, thus forming an insitu pile. The
reinforcement is positioned once the auger has been removed.
Sectional Flight Auger
In principle this system is the same as the one above. It
differs in that the augers are in short lengths (or sections) thus
permitting the rig to be used in confined spaces with limited
headroom, ie, inside buildings. Additional augers can be added to
achieve the correct depth. Solid stem or hollow stem augers can be
used. The former are easier and cheaper, but only if it is
considered that the bore will stay open at the correct cross
sectional area. Hollow stem augers are essential where the side
wall material is unstable and might collapse. Reinforcement is
usually installed immediately after concreting the bore. A starter
bar cage can also be installed in the top of the pile to connect to
the ground beam or pile cap above.
5 Continuous Helical Displacement Piles This piling system is
quite unusual in that it is a bored displacement pile; most bored
piles are replacement piles - in other words the ground is removed
before concreting takes place. The advantage of this pile is that
there is minimal 'cart away' and, unlike most displacement piles,
it is quite and vibration free.
On the left you can see a small part of a much larger drawing
showing the pile layout. The whole contract comprised nearly 1000
piles, all with a safe working load of 300kN (approx. 90 tonnes).
On the right you can see some of the more important notes which
qualify the piling plan.
In most ground conditions this is an ideal alternative to
continuous flight augers (CFA). During boring the ground is
compacted as the rotary head 'drills' into the ground. At the
appropriate depth concrete is pumped under pressure down the hollow
shaft to the boring head while the shaft is reverse rotated and
withdrawn from the bore.
Reinforcement in the form of a cage and/or single bar is lowered
into the bore when the concrete operation is complete.
Reinforcement projects from the top of the pile to form a strong
connection with a pile cap or ground beam.
Once the piles are complete the ground beam or pile caps can be
cast. The example shown here is a continuous ground beam. The photo
on the left shows the shallow excavation along the line of the
piles for the ground beam. On the right you can see the weak
concrete blinding (laid to form a clean, level working surface) and
the reinforcement cage partly in position.
Test piles are loaded to assess their loadbearing capability.
The test rig comprises three steel beams anchored to four deep
corner piles. A hydraulic jack loads the test pile under the centre
of the beam. On this site a safety factor of 2.5 was required. So,
if the design load is say, 300kN, the test pile should carry 750kN.
Click here to see a video clip
6 Insitu Ground BeamsThis page shows how a ground beam can be
formed. The first stage is to cut the piles to the right length and
dig a trench between them. This has been blinded with a thin layer
of concrete to provide a level, clean surface for the next stage -
building the reinforcement cages. Before constructing the
ground-beam cage the the integrity of the piles is checked.
The steel cage is made from a series of preformed and bent
reinforcement rods. the size of the rods is calculated by an
engineer. The ground beam, when it is complete, will take the loads
from the walls and distribute them into the piles (you can just see
the top of the pile in the right-hand photo). The load from the
beams is not carried by the ground below the beam.
The yellow plastic is a proprietary permanent formwork. This
prevents concrete from being wasted. On some sites this formwork,
or shuttering, is built in blockwork. If there is danger of ground
swell, for example in clay soils, a special type of formwork can be
used which incorporates a collapsible layer. This is often required
under the beam (in between the piles) as well as at the sides.
Click here for another example
7 Piers
8 Foundation Case StudiesCase Study One
Roger Bullivant Ltd provided its pre-cast house foundations
package, consisting of segmental pre-cast piles, pre-cast ground
beams and suspended pre-cast floors to two major house developers
on the same site in Cheddar, Somerset. The developments were
undertaken on a greenfield site which suffers from high ground
water table problems. The majority of ground levels needed to be
raised by as much as 750mm across the area. It was also anticipated
that localised flooding of the site during work would be
experienced. A solution to overcome all of these difficulties was
needed to enable work to proceed, as conventional foundations would
be difficult and costly to construct.The use of segmental pre-cast
piles, Tee-beam system and suspended pre-cast floors, helped
overcome the poor ground conditions. The segmental pre-cast piles
provided a cost effective solution which eliminated both excavation
and spoil disposal.
Sacrificial probe piles were installed prior to final
negotiations to determine the actual pile lengths so that a fixed
price package could be agreed. In total 560, 175 x 175mm square
segmental pre-cast piles were installed to a maximum depth of 7m.
Each pile was capable of carrying loads to 350kN and comprised 3m
and 4m segments with single T16 bar reinforcement. The piles were
cropped to the required level and either pre-cast caps or cast
in-situ concrete caps positioned on top. Approx. 2000 metres of
pre-cast Tee-beams were installed, together with 2500metres of
pre-cast beam joist floors.
Both projects were completed using purpose-built piling rigs,
designed to undertake work of this nature where piles ranging in
size from 150 200mm are required, with loads from 100 400 kN. The
rigs use 2 3 tonne hydraulic hammers to drive in the piles. As the
gross weight of each rig is only 17 tonnes, it can be mobilised
onto site without the need for police escorts or movement orders.
The size and weight of the rig also reduces on-site problems
resulting from noise and vibration.
Case Study Two
Roger Bullivant Ltd provided pre-cast concrete piling for the
construction of a new housing development in Burnham-on Sea,
Somerset. The project involved foundation piling on a section of a
new development of timber-framed homes. Three of the planned houses
presented a particular challenge, as they were situated in close
proximity to a hedgerow, with the added complication of a ditch
around the perimeter where the ground was soft and wet. The plots
were also very close to existing adjacent housing which was being
developed and therefore any work had to be carefully contained.The
contractor (RB) proposed the installation of pre-cast segmented
piles together with pre-cast Tee beams. The system offers a
cost-effective solution to installations in close proximity to
existing buildings or structures and, in addition, overcomes
problems of poor ground conditions without the need for soil
disposal or excavation.
The area of the site was prepared to a reduced level of 650mm
below floor slab level. Investigations of the soil revealed eight
distinct layers; these varied from topsoil, firm clay, clayey and
laminated silt of various types, down to fine-medium sand. This
dictated that piling would need to be to a depth of approx 17m. As
the ground compaction was good, no hardcore was needed for piling
platform.The segmental pre-cast concrete piles were installed with
a top-driven hydraulic hammer rig to depths ranging from 1 5.5 1
6.4m . The pre-cast piles were 250mm square capable of loadings up
to 600kN. The segments were each 4m with single bar reinforcement
to reduce costs and minimise wastage.
Case Study Three
This housing development was being constructed on the site of a
former sand quarry which had been filled before the Second World
War. A school was then built on the site, incorporating underground
air raid shelters constructed from very thick, high-density
concrete. The site over the disused shelters had subsequently been
developed by the addition of extra classrooms and playgrounds
etc.The exact location of the air raid shelters, the degree of fill
and precise depth of the underlying bedrock were all unknown. It
had been identified that on an adjacent piece of land, homes
constructed post-war had needed to be demolished because of major
subsidence.
The developers had already completed the first six houses on one
section of the site, using vibrated concrete columns and very deep
foundations. As these initial properties had sold immediately, they
were looking for an alternative solution that would speed up the
construction programme and also avoid the high cost of such deep
foundations. The solution comprised a combination of steel tubular
piles and pre-cast concrete piles both followed by pre-cast pile
caps, pre-cast tee-beams and pre-cast concrete floor slabs. The
combination of both pile types enabled the differing conditions on
the site to be accommodated, with only minimal need for spoil
removal. On the section of the site where it had been identified
that air raid shelters had not existed, the ground conditions were
sound and pre-cast concrete piles 200mm to a depth of 6m were used.
On the more difficult parts of the site, 170mm diameter steel piles
were used, installed down to 8m and in some cases down to 16m. The
use of steel piles ensured that they could be driven through any
concrete structures forming part of the old shelters. Where any
major obstructions were identified, local excavations were carried
out to assess and re-plan the piling as required.
Installation was followed by pile cropping which ensured a sound
connection between the pile and the pre-cast cap. Over 1500metres
of pre-cast reinforced concrete tee-beams were then installed
directly onto the pile caps to carry the wall and floor loads.
Case Study Four
This housing development is on a site formerly used as a tramway
depot. The site was bounded by two roads, a railway and a nearby
underground line. Vibration was perceived as a major constraint, as
was the need to minimise traffic, and keep noise to a minimum.
Continuous helical displacement piles were used and the whole
project (nearly 500 piles) was completed in four weeks.
The piles were founded into clayed silt at depths down to 18.50
metres. The piles had designed working loads of up to 300kN. The
first test pile was toed into boulder clay at a depth of 17.60
metres and gave a settlement of 15mm at 1200kN. A second test pile,
founded in clayed silt at a depth of 15.50 metres provided an
ultimate load of 600kN, giving a safety factor of 2.
9 Repair Systems - Roger Bullivant This repair system (shown in
isometric on the left, and in section on the right) is suitable for
many types of shallow foundation stabilisation, especially where
access is restricted. A series of piers, at centres of up to
1.5ometres, transfer the loads from the wall down to a firmer
stratum. Loadings of up to 100kN per metre can be achieved,
individual piers are generally rated at about 50kN. a 300mm by
300mm pocket in the brickwork is removed to make way for the
reinforced concrete 'knuckle'. This system is less disruptive than
traditional piling, it's ideal for restricted situations, and
possibly most important of all, it only requires access from one
side of the wall - occupants do not have to be moved out.
This system, the pier and beam system, is slightly more complex
than the one above and is used where lateral, as well as vertical,
restraint is required. A typical situation might be where poor
quality underground brickwork requires lateral restraint between
supports. The longitudinal ground beam can be seen in the graphic
on the left. As in the above example the piers can be driven or
augered.
This system requires the installation of pairs of piles, one in
compression and one in tension, as shown in the graphics. A
reinforced concrete beam on top of the piles cantilevers into the
wall to provide support. This piling repair method can be economic
where the bearing stratum is deeper than 1.5metres. Pile sizes
range from 90mm to 250mm diameter; the piles themselves can be
drilled, driven or augered. Again, the work can be carried out from
one side.
Angle piles can be single or double as shown in the left-hand
graphic. Piles are normally installed both sides of the foundation
although they can be installed from one side if there is suitable
lateral restraint. Permanently cased steel driven piles, or solid
or hollow stem augered piles, are then installed through the
pre-drilled hole with the casing terminated at the underside of the
existing foundation. The pile is then concreted and reinforced up
through the existing foundation. This is a fast piling system with
high load capability.
10 Rafts Raft foundations were sometimes used as far back as the
1920s and 1930s. This example is a house designed in 1936 - the
site was a drained marsh. In the 1940s and 1950s raft foundations
were quite common, particularly beneath the thousands of
prefabricated pre-cast concrete or steel buildings erected during
the years following the Second World War. Most of these houses were
built on good quality farm land where the soil was generally of
modest to high bearing capacity. Rafts (or foundation slabs as they
were sometimes called) were often used because they were relatively
cheap, easy to construct and did not require extensive excavation
(trenches were often dug by hand). In 1965 national Building
Regulations were introduced for the first time (London still had
its own building controls), but these did not contain any 'deemed
to satisfy' provisions for raft foundations (as they did for strip
foundations) - consequently each had to be engineer designed. As a
result they quickly fell out of favour.
In modern construction rafts tend to be used: Where the soil has
low load bearing capacity and varying compressibility. This might
include, loose sand, soft clays, fill, and alluvial soils (soils
comprising particles suspended in water and deposited over a flood
plain or river bed). Where pad or strip foundations would cover
more than 50% of the ground area below the building. Where
differential movements are expected. Where subsidence due to mining
is a possibility.
Flat slab rafts (right hand graphic) offer a number of
advantages over strip foundations, no trenching is required, they
are simple and quick to build, there is less interference with
subsoil water movement, and there are no risks to people working in
trenches. Detailing needs careful thought, - the example on the
right for instance may be subject to frost attack around the edges,
the edges themselves are exposed, and there is the risk of cold
bridging around the perimeter. They are generally suitable for good
soils of consistent bearing capacity.Flat slab rafts (ie no
perimeter or internal beams - see below) have been recommended in
some mining areas. These rafts will flex if ground movement is
considerable so the superstructure needs to be designed
accordingly.
Shallow rigid rafts for 1, 2 and 3 storey housing can be cheaper
than piles. On poor ground the raft must be stiff enough to prevent
excessive differential settlement. This usually requires perimeter
and internal ground beams to help stiffness and minimise distortion
of the superstructure.Some, overall, settlement of the house will
inevitably occur but differential settlement should be kept within
acceptable limits. In this country rafts have to be designed on a
one-by-one basis, in other words there are no 'deemed to satisfy'
provisions in the Building Regulations as there are with strip
foundations. In practice, engineers are advised to consider local
practice with regard to raft design. Some typical dimensions of the
various elements are shown in the graphic on the right (upper) The
lower graphic shows the nature of the perimeter and internal
beams.
On filled sites rafts can, depending on the fill depth, be a
cost effective alternative to piling. They can also be used on
sloping sites as an alternative to stepped strip foundations. A
well compacted (in shallow layers), graded granular fill can form a
suitable base. Designing the fill and the raft is obviously
specialist work and many speculative house builders would probably
prefer 'tried and tested' stepped strip foundations.
2007 University of the West of England, Bristolexcept where
acknowledged