FOUNDATIONS (Including Pilled)

Question 1

What is a Foundation?

Answer 1

The main function of a foundation is to safely
carry and transmit the loads of the building
(both dead and live loads) to the subsoil.

Question 2

What does a Foundation do?

Answer 2

• The foundation balances the building loads
  with the allowable bearing capacity of the
  local subsoil.
• Where the loads exceed the upward passive
  pressure of the ground the foundation has to
  increase the area over which the load is
  distributed.

Question 3

What is Snowshoe Application?

Answer 3

Increase the surface area and this decrease the pressure.

Question 4

What are the rock soils?

Answer 4

Igneous, Limestone/strong sandstone, Slate and Shale.

Question 5

What are cohesion-less soils?

Answer 5

Gravel/Sand and Sand.

Question 6

What are cohesive soils?

Answer 6

Clay and Peat/Fill.

Question 7

What foundation type suits Rock?

Answer 7

Possibly no foundation required; level-off rock and provide thin layer of concrete to provide even base for bricklayers. Subject, of course, to Building Control approval.

Question 8

What foundation type suits Compact sand, dense silty sands, hard sound chalk and gravels?

Answer 8

Strip foundations are the cheapest solution; either traditional strip or trench fill. Widths can normally be taken direct from Table in Building Regulations.

Question 9

What foundation type suits Uniform, firm and stiff clays - where vegetation is not significant?

Answer 9

Strip foundations are the cheapest solution; either traditional strip or trench fill. Widths can normally be taken direct from Table in Building Regulations.

Question 10

What foundation type suits Uniform, firm and stiff clays - where there are existing or new trees.

Answer 10

Reinforced concrete piles (usually bored or augured) with suspended floor.

Traditional strip or trench-fill - if trees are well away from foundations – usually at least a distance equal to mature tree height.

Question 11

What foundation type suits Uniform, firm and stiff clays - where trees have been removed in
recent months?

Answer 11

Trench fill with compressible layer one side (to minimise effect of heave) may be an option.

Suspended floors required.

Reinforced concrete piles (usually bored or augered) with collapsible layer underground beam.

The NHBC suggest concrete piers may also be an option - again with collapsible layer under
beam.

Question 12

What foundation type suits Soft clay, soft silty clay and soft sandy clay.

Answer 12

Reinforced wide strip if bearing capacity is sufficient Stiff reinforced rafts.

Consider piling through soft ground to firmer stratum - if one exists.

Question 13

What foundation type suits Peat or filled ground?

Answer 13

Pile through peat or fill to firmer ground - if it exists.

Stiff raft may be an option depending on nature and thickness of fill/peat.

Or, of course, avoid site.

Question 14

What foundation type suits Mining areas or areas where subsidence is a known problem?

Answer 14

Reinforced raft foundations.

Question 15

What shallow foundation are used for low load bearing buildings?

Answer 15

Strip foundations, raft foundations and pad foundations.

Question 16

What deep foundation is used for high load bearing buildings?

Answer 16

Pile foundations.

Question 17

What is a Greenfield site?

Answer 17

A Greenfield Site is a site that has not been developed on previously.

Question 18

What is a Brownfield site?

Answer 18

A Brownfield site is a site that has been developed on previously and is thus considered contaminated.

Question 19

What is the process of forming a strip foundation?

Answer 19

Stripping the top layer of soil, excavating to reduced level into the sub soil, pouring concrete, setting the concrete.

Question 20

What determines strip foundation width?

Answer 20

• The width of simple strip foundations depends
  on the nature of the ground; in particular its
  allowable bearing pressure.
• the load of the building.
• the need for working space (for bricklayers
  etc.).

Question 21

What determines a strip foundation depth?

Answer 21

The foundation depth will be the greatest of the
following:

• a depth to the selected bearing stratum
• a depth usually not less than 1000mm in clays
  subject to seasonal movement (see B. Regs -
  Section on Foundations).
• a depth possibly greater than 1000mm where
  trees are present or have been felled.
• in sands, chalks and other frost susceptible soils
  a depth below the zone of frost action. This may
  be 450mm or so, possibly more in upland areas
  or areas subject to long periods of frost.
• a depth sufficient to minimise load on adjacent
  services

Question 22

What are the advantages of strip foundations?

Answer 22

Shallow foundations (>3m) therefore little excavation needed.

Economically cheap due to the narrow, shallow design.

Little or no impact on neighbouring properties.

Question 23

What are the disadvantages of strip foundations?

Answer 23

Limited load carrying ability due to foundation depths and design, therefore only suited to small/medium developments.

Not ideal for framed construction.

Weak against uplift forces, wind forces and earthquake forces.

Weak in stratum of loose sand or gravel.

Question 24

When are strip foundations required?

Answer 24

In ground of medium to good bearing stratum, on domestic scale developments, underneath loadbearing walls, separating walls, chimneys,
piers and internal loadbearing walls.

To carry light loads.

Question 25

What is a Stepped Foundation?

Answer 25

A Stepped Foundation is a strip foundation with added elevation steps.

Each step in the foundation should be no higher than the thickness of the foundation.

Question 26

What are Stepped Foundations used for?

Answer 26

In supporting structures that are on an incline and require an adaptable foundation that will be able to accommodate for said incline.

Question 27

What is a Raft Foundation?

Answer 27

• A Raft foundation is a large combined thick
  slab designed to seat and support the whole
  or a large part of a structure.

Question 28

What is a Raft Foundation used for?

Answer 28

• A raft is usually used when subsoil is weak
  or unstable. They are also used where strip
  foundations would cover more than 50% of
  the ground area beneath the building.
• or where columns are closely located and
  with diverged loadings. By doing so,
  differential settlement can be avoided.

Question 29

How is a Raft Foundation Implemented?

Answer 29

Rafts are mainly used in the UK as foundations for
commercial building but can also be used for
residential buildings.

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.

The foundation may stiffened by ribs or beams
built in during construction which will add extra
strength and rigidity.

Question 30

What are the variants of a raft foundation?

Answer 30

Spread foundation, Piled-raft foundation and Pile foundation.

Question 31

What are important consideration associated with a raft foundation?

Answer 31

• Here are important considerations if you’re thinking about using a raft foundation:
• The raft design usually has an ‘edge beam’ formed by a cage of steel reinforcement, which will need to be carefully assembled on site. Internal stiffening beams are sometimes needed too. These beams transfer the building loads through the rest of the slab and then evenly over the ground.
• A site investigation is usually needed for the engineer to understand what the ground is like. Very poor ground may mean you need a different solution such as piles.
• The raft design will normally need a hardcore sub-base to level the ground. This stone should be mechanically compacted.
• You must ensure steel reinforcement is lapped at least 450mm (for both mesh and mild steel bars) and provided with 40mm concrete cover. ‘Treading in’ the
  reinforcement during the concrete pour isn’t a suitable means of placing reinforcement - use the special ‘chairs’ or ‘soldiers’ to support the mesh.
• The edge of the raft must be carefully detailed for the damp course and membrane position, which may need a ‘step’ forming in the concrete at the raft edge.
• Insulation is usually placed over the top of the raft - take care to avoid cold
  bridging at the junctions with external walls

Question 32

What are the advantages of a Raft Foundation?

Answer 32

Compared to pilling, relatively cheap due to the combined use of the foundation as the floor.

Easy to build.

Shallow depth of foundation means little excavation.

Can cope with poor/mixed ground conditions.

Question 33

What are the disadvantages of a raft foundation?

Answer 33

Weak when supporting point loads, specific treatment required.

Susceptible to edge erosion and frost attack.

Has to be designed (from first principles) by an Engineer.

Question 34

When are Raft Foundations required?

Answer 34

Lightweight structures on poor ground with low bearing capacity.

Used in areas with mixed bearing capacity usually filled ground. Differential settlement or subsidence due to mining is likely.

Used where traditional foundations would cover more
than 50% of the footprint.

Question 35

What are Pad Foundations?

Answer 35

Pad foundations are rectangular or circular pads used to support localised loads such as columns.

Question 36

What are continuous Pad Foundations?

Answer 36

Continuous pad exists when pads and the columns
they support are fairly closely spaced. Extending
the reinforcing between pads ensures longitudinal
stiffness. This technique helps in resisting the
differential settlement.

Question 37

What are Pad and Ground beams?

Answer 37

Pad and ground beam comprises smaller
isolated pads are connected by ground beams
to provide structural rigidity.

The technique helps in improving the integrity
along with balancing the eccentric loads.

Question 38

What are the advantages of a Pad Foundation?

Answer 38

Shallow foundation.

Requires little excavation.

Can be designed to accommodate tight sites.

Economic due to control of foundation size.

Reinforcement for tension and shear can be added.

Concrete mix can use SRPC in place of OPC.

Question 39

What are the disadvantages of a Pad Foundation?

Answer 39

Foundation size can be a very large to cope with high point loads.

Limited foundation suitability to point loads of framed buildings.

Separate foundations make this design weak against differential settlement that may affect the building.

Deep excavations for foundations would require
support to prevent caving in. Weak against uplift forces, wind forces and earthquake forces.

Question 40

When are Pad Foundations required?

Answer 40

Ideal foundation for point loads from framed buildings when bearing capacity of ground is suitable a shallow depths.

Question 41

What is a Pile Foundation?

Answer 41

A piled foundation is a form of foundation using slender elements to transfer the loads of a structure down to a firm soil stratum with sufficient load-bearing capacity.

Question 42

When are Pile Foundations used?

Answer 42

Heavy point loads exceed soil bearing capacity.

High water tables that are capable of flushing subsoils, which leads to the risk of erosion and/or sink holes.

Low bearing capacity subsoils which could include wet clay or loose and unstable sand.

Unconsolidated filled materials, artificially imported, which are very difficult to properly compress.

Naturally formed and highly compressible subsoils, like peat.

Question 43

How are Pile Foundations selected?

Answer 43

• Safety – both during construction and in use, along with a consideration of potential reuse or removal.
• Requirements of the proposed structure and applied
  loadings.
• The ground conditions.
• Environmental factors – including nuisance, effect on
  wildlife and habitat, noise and vibration, waste products (spoil), carbon reduction etc.
• Access to and location of site.
• Proximity to existing assets (e.g. utilities, roads railways), in particular when the works are adjacent to operational railways.
• Proximity to sensitive structures.
• Cost.

Question 44

How are Piles classified?

Answer 44

Piles may be classified and described in a
number of ways:

By the way they transmit the load (EB/SF).

By the material they are made from (concrete, steel very occasionally timber).

By construction method (displacement/replacement).

Question 45

How are pile loads transferred into the ground?

Answer 45

End-bearing –load is support by resting onto a firm
stratum such as bed-rock or stratum of subsoil with
the required bearing capacity. An end-bearing pile is
designed to transfer the bulk of its load directly to the
base or toe of the pile once a solid layer is penetrated

Skin friction –load is support by the frictional
resistance created between the contact surface of the
pile and the embracing soil. A Friction pile transfers
its load along the entire shaft using friction between
the pile and soil to achieve the required load when
a solid layer cannot be penetrated.

Question 46

What are replacement or bored piles?

Answer 46

• Bored types physically remove the subsoil spoil,
  creating a cylindrical tube into which concrete can be
  poured, so the technique is effectively replacing
  subsoil in favour of concrete.
• It is often used on sites that are in close proximity to
  other buildings as the excavation process is more
  controlled, not so noisy and causes much less ground
  vibration.
• Boring is frequently used for friction piles as this
  method suits cohesive and firm soils.
• However, it’s an expensive option because of the
  various on-site processes involved.
• Friction piles could need temporary casings,
  excavation, management of the spoil, concrete
  pouring, rebar cage production, insertion etc, plus all
  of the associated equipment required for handling
  each process.

Question 47

What are displacement or bored piles?

Answer 47

• The alternative to boring would be displacement piles, which are driven into the ground with hydraulic rams and hammers. |
• There is no excavated material to manage and
  instead the pile displaces the subsoil as it travels
  below ground. Some pros consider this to be a
  cheaper option and with guaranteed results, as the
  piles are manufactured off-site to predetermined
  levels of quality.
• For shallow depths, piles could be timber (much of
  Venice is founded on timber piles) but concrete or
  steel are more likely, certainly for deeper depths.
• Concrete piles are manufactured in specific lengths
  (generally up to around 10m), with the bottom one
  fitted with a spiked cap in concrete or steel.
• Sections are then jointed together with a splicing
  collar until the full predetermined depth is reached.

Question 48

What are displacement or driven piles?

Answer 48

• Alternatively, some depths may be determined as
  sufficient enough when the effort required to drive
  the pile becomes too hard, confirming that the
  bearing capacity of the subsoil had been reached.
• The piles are driven by percussion action, which
  sets the speed and frequency of weighted blows to
  the pile top (this must be protected by a helmet to
  preserve its integrity). They can also be vibrated into
  the ground.
• Steel piles come in a variety of sizes; some are
  cylindrical and these will usually be filled with
  concrete. Others could be H section (like a universal
  column) or of a box shape. This method of piling is
  well suited for foundations in non-cohesive soils,
  ground with a high water table and for soils that
  contain contaminates.

Question 49

What is Sheet Piling?

Answer 49

• Sheet piles, a type of driven pile, are constructed with a series of interlocking steel sheets. They can be used to create permanent or temporary retaining walls on construction sites where large excavations are required.
• This method of piling can be very cost effective
  on sites where temporary soil retention is necessary as the sheets can removed and used again on different sites once the ground works have been completed.

Question 50

What is Mini Piling?

Answer 50

• Mini piles, also known as micro piles, typically
  range in sizes from 100 mm - 400 mm in
  diameter. What distinguishes mini piles from
  other pile types, however, has more to do with
  how and where they are installed than just the
  pile size itself.
• In locations with restricted access smaller sized
  piling rigs have been required in order to install
  piles on sites where traditional piling rigs could
  not operate. This opens up many options for
  commercial & domestic development in the city.

Question 51

What are Contiguous Piles?

Answer 51

Closely spaced bored piles can be used to
form a retaining wall, perhaps for the
construction of a deep basement or a cut
and cover tunnel.

The piles may be constructed so that they
virtually touch each other (contiguous)
The gaps between the piles can be
grouted to form a watertight retaining wall.

Question 52

What is a Secant Pile?

Answer 52

Secant pile walls are formed by constructing intersecting reinforced concrete piles.

The secant piles are reinforced with either
steel rebar or with steel beams. Primary piles are installed first with secondary piles constructed in between primary piles once the latter gain sufficient strength.

Pile overlap is typically in the order of 80mm.

Question 53

What are the advantages of Pile Foundations?

Answer 53

Can transfer load in variable ground conditions and transfer loads to deep bearing stratum.

Suitable to tight sites.

Made off site and quality maintained due to factory
production.

There is no evidence that Steel piles are susceptible to
corrosion by the action of sulphur reducing bacteria (<0.03mm corrosion per annum).

No excavation required and No need to support excavated holes. Suited to framed construction.

Question 54

Recollect key construction points.

Answer 54

• Concrete burns the skin, so always wear protective gloves and footwear.
• Ensure the base of any foundation trench is clean, level, dry and free of any loose material. Trench sides should be cut square with the base. The ideal situation is to excavate the trenches and pour the concrete the same day. Special mixes that flow around whole channel are available, which do away with barrowing.
• Foundations poured in winter conditions must be done following strict guidelines and should be covered over.
• It is best to consult a weather forecast for possible frost conditions because overnight frost can ruin concrete.
• If you decide to use a concrete pump to place concrete, give the suppliers plenty of warning of your intention.
• Locate your nearest hire depot for urgent requirements such a water pump in the event of flooding.

Question 55

What is effect of trees and vegetation?

Answer 55

This shrinkage and expansion may cause
movement, which may lead to subsidence. In some circumstances trees can make the situation worse. The Which? consumer organisation estimates that nearly 70 per cent of subsidence claims are due to trees sucking moisture from the soil near buildings.

But put this into context. In 90 per cent of cases offending tree(s) were within 16ft of the building
and in 99 per cent of cases subsidence was on
‘‘shrinkable’’ soils. So if your soil is relatively free
draining, or you plant outside the ‘‘danger zone’’,
then a tree is unlikely to cause subsidence.