College of North West London presentation; for the National Diploma in Construction

1. SOIL INVESTIGATION &
FOUNDATION DESIGN
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2. FOUNDATIONS ON POOR SOIL
 There are 2 ways to improve the strength
of the ground;
 To excavate the ground until ground
capacity is reached.
 Vibro- compaction of the soil- a vibrating
poker is used to make the ground more
dense.
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3. SUBSOIL SHRINKAGE
 Shrinkage in the soil is caused by a number
of factors;
 Extreme seasonal change
 Vegetation
 Trees
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4. Ground Heave
 This is caused in a number of ways;
 Water freezing in the ground which
results in the expansion of the ground
 A high water table
 The recent removal of trees or vegetation
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5. Excavations
 Trenches over 1m deep require temporary
support
 To stop trench collapse, timber supports-
shores or planks- are used to retain the
earth
 On bigger projects sheet piling is used
 All trenches should be checked with a CAT
scanner to detect an services
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6. Foundation Design
 The design of foundations is covered by
document ‘A’ under Building Regs.
 Strip foundations for example require a
minimum of 750mm in clay soils or 450mm
in others.
 Most councils choose to go to 1m deep
 Other factors include ‘dead’ and imposed
loads such as wind, snow, floors etc.
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7. Foundation Design
 When designing a foundation, Engineers
concentrate on the unit of force (Kilonewtons )
rather than weight.
 1 tonne is equal to 10 kilonewton
 An average building load to a house is 120 tonnes
which exerts a force of 1200 kn to the ground
 This number is then divided into the perimeter
length of the building eg. 30m which is then
divided into 1200kn. This formula will show how
much each metre run is carrying eg. 40kn
 Depending on the soil, a design on the foundation
can be made
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8. Old Foundations
 Prior to the 1875 Public Health Act most
houses were built on a wide brickwork base
which ‘corbelled in’ just before ground
level.
 Brickwork was built in sand and lime to
allow for settlement in the ground
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9. PILING
 Piling is used to transfer the load of the
building through weak or unstable soil to
ground of higher load bearing capacity
 Vertical concrete piles are poured into the
ground by a crane mounted auger machine
which removes the soil and injects
concrete down a hollow stem.
 A horizontal beam is then connected to the
to the top of the piles
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10. Strip Foundations
 This is a strip of concrete which is under
all load bearing walls. The strip width and
depth depends on the building load and
nature of the ground
 Strip foundations can be ‘stepped up’ with
sloping ground
 Thickness of slab must NOT be less than
the ‘toe’ of the slab
 The most economical option for builders
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11. Raft Foundations
 This is a slab of concrete which supports the
building over a large area.
 Tends to be used on ground of low bearing
capacity eg soft clays, loose sands etc.
 Where differential movements are expected
 Where subsidence due to mining is a possibility
 No trenching required
 Cheap and easy to construct
 Less interference with sub soil water movement
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12. Pad Foundations
 Pad foundations are used to support an individual
point load such as that due to a structural
column. They may be circular, square or
rectangular. They usually consist of a block or
slab of uniform thickness, but they may be
stepped or hunched if they are required to
spread the load from a heavy column. Pad
foundations are usually shallow, but deep pad
foundations can also be used.

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13. Summary
 Strip foundations are suitable on;
 Rock, gravel, dense sand or stiff clay
 On soft clay or soft sandy clay, wider strip
foundations are required
 Piled foundations are suitable for;
 Shrinkable clays
 Where the water table is high and where a firm
layer of ground is at a suitable depth
 Where a firm layer of ground is at a considerable
depth
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14. Summary
 Raft foundations are suitable on;
 Grounds of low bearing capacity such as
soft clay or silt
 In mining areas where subsidence is a risk
 On deep areas of fill where piling would be
uneconomic
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