soil investigation part2
TRANSCRIPT
SOIL INVESTIGATION &
FOUNDATION DESIGN
February, 2012John Fox, College of North West
London 1
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.
February, 2012John Fox, College of North West
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SUBSOIL SHRINKAGE Shrinkage in the soil is caused by a
number of factors; Extreme seasonal change Vegetation Trees
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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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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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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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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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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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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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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 buildersFebruary, 2012
John Fox, College of North West London 10
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
February, 2012John Fox, College of North West
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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.
February, 2012John Fox, College of North West
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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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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
February, 2012John Fox, College of North West
London 14