Topic
Understand foundation design and construction
Today’s Objective
1. Understand the impact that soil conditions have on foundation
design
Unit 6: Building Technology in Construction
Foundation Design: Soil Condition
Two main soil characteristics affect the choice of foundation:1.Maximum loads that can be carried without failure
2.Excessive settlement
Key term: Settlement
The way the soil reacts to having a load placed upon it – usually the soil ‘sinks’ a
little due to the extra weight placed upon it, although this settlement should not
affect the building a great deal.
Key term: Compressibility
The ability of soil to compress and withstand a load imposed on it.
Unit 6: Building Technology in Construction
Foundation Design: Soil Movement
.
Unit 6: Building Technology in Construction
Foundation Design: Soil Condition
How many different types of soils can you list?
• Rock
• Gravel
• Sands
• Silt
• Chalk
• Clay
• Peat
Which do you think will be the best material on which to put a foundation?
Unit 6: Building Technology in Construction
Site Investigation: Ground Investigation
1. Rock
Hard solid formations, often impervious, such as granite or sandstone. Form
excellent strata on which to build.
2. Granular- Sand/gravel: non-cohesive granular material.
Can have good ground bearing capacity if well compacted. Good water dissipation
and run off, and often very stable.
3. Cohesive-Clay
Good ground bearing, with impervious characteristics preventing water from
passing between strata. Often found at depths close to surface of the earth making
it economical, and reasonable, on which to build low-rise structures.
4. Organic-Peat
Soft, permeable and unsuitable surface for foundations. Can indicate the presence
of water table and groundwater conditions that can make the ground unstable.
5. Filled or made ground
Good quality, clean, hard-core rubble. Graded, selected and compacted in layers to
engineer’s levels means that ground stabilisation is possible with this method.
Unit 6: Building Technology in Construction
Site Investigation: Ground Investigation
Unit 6: Building Technology in Construction
Foundation Design: Soil Condition
Poor Soil
• Eg. Peat, silt, soft alluvial, filled ground
• Poor load bearing and settlement characteristics
• There are techniques that can be used to stabilise the ground
and improve the load bearing capacity
• Normally the unsuiable soil is excavated and the founations
are sited on a better load bearing ground at a lower level
Non cohesive soil
• Eg Sand/gravel
• Stronger and higher bearing capacity and low compressibility compared
to cohesive soil
• Settlement occurs instantaneously
• May be suitable for any foundation type.
Unit 6: Building Technology in Construction
Foundation Design: Soil Condition
Cohesive soil
• Eg. Clay
• Reasonable bearing capacity
• Prone to long term settlement
• Sensitive to changes in moisture content, prone to swelling/shrinkage
Rock – Strongest material on which to put a foundation
• Solid rock only needs to be levelled (and it can take some
doing) for most house foundations.
• A thin layer of blinding can then be laid to provide a level base
for bricklayers.
• Cavities and fissures may need filling.
• There is a potential problem of differential movement if rock
and other materials are supporting the foundation. For
example, a predominantly clay subsoil, with rocky outcrops,
could pose problems.
Unit 6: Building Technology in Construction
Foundation Design: Soil Condition
Contaminated Soil
List some examples of contaminated land
• Landfill sites
• Gasworks
• Sewage farms and works
• Scrap yards
• Industrial sites
• Primary concern is health
• Some aggressive contaminants can damage buried services and structures
Common approaches to contaminated land
• Removal –costly and disposal needs to be made to licenced tips. Permanent.
• Capping –Sealing of material by a clean material 1m deep. More Economic.
Unit 6: Building Technology in Construction
Foundation Design: Soil Condition
Gravel – Non-cohesive soil
Field identification
Field assessment of structure &
strength
Possible foundation problems
Gravels
Particle size 2mm to 60mm
Loose, easily removed by shovel
Some dry strength indicates presence 50mm square stakes can be well driven
of clay
into the ground
Loss of fine particles in water bearing
ground
Unit 6: Building Technology in Construction
Foundation Design: Soil Condition
Field identification
Field assessment of structure &
strength
Possible foundation problems
Sands
Particle size 0.06mm to 2mm - when
dry, clean sands break down. Sand
particles are just visible to the naked
eye and feel gritty between the
fingers.
Frost heave , especially in fine sands.
Compact, requires pick for excavation.
Excavation below water table can wash
50mm square stakes will only penetrate a
out sand and cause runs and local
little way
collapse.
Unit 6: Building Technology in Construction
Foundation Design: Soil Condition
Chalk
White and thus readily identifiable
Plastic - shattered, damp and slightly
compressible or crumbly.
Solid - needing a pick for removal
Frost heave is a risk in chalk, floor slabs
are particularly at risk if constructed in
cold weather. Swallow holes can also
form.
Unit 6: Building Technology in Construction
Foundation Design: Soil Condition
Field identification
Field assessment of structure &
strength
Possible foundation problems
Silts
Particle size 0.002mm to 0.06mm - not
normally distinguishable to the naked
eye. Silts feel slightly gritty; moist
A soft silt can easily be moulded with the
lumps can be moulded with the
fingers; a firm silt requires strong finger
fingers but not rolled into threads.
pressure.
Shaking most lumps in the hand
brings water to the surface. Silts also
dry rapidly.
Frost heave , especially in fine sands.
Excavation below water table can wash
out sand and cause runs and local
collapse.
Unit 6: Building Technology in Construction
Foundation Design: Soil Condition
Field identification
Field assessment of structure &
strength
Possible foundation problems
Clays
Smooth and plastic to the touch.
Sticky when moist. Hold together
when dry. Wet lumps immersed in
water soften without disintegrating
Soft clays either uniform or show
horizontal laminations.
Harder clays often fissured, the
fissures opening slightly when the
overburden is removed or a vertical
surface is revealed, eg, by a trial pit.
5 types:

very soft - exudes between fingers
when squeezed

soft - easily moulded with the
fingers

firm - moulded with strong finger
pressure

stiff - cannot be moulded by the
fingers

hard - brittle or tough
Shrinkage and swelling caused by
vegetation and long term settlement by
consolidation. Some clays contain
sulfates which can attack concrete.
Drainage can be poor.
Movement down slopes, most soft clays
lose strength when disturbed.
Unit 6: Building Technology in Construction
Foundation Design: Soil Condition
Field identification
Field assessment of structure &
strength
Possible foundation problems
Peat
Fibrous, black or brown; often smelly,
Very low bearing capacity, peats shrink
and very compressible and water
Soft - very compressible, firm - compact and swell considerably, high
retentive.
compressibility leads to high settlement.
Unit 6: Building Technology in Construction
Foundation Design: Soil Condition
Fill
Field identification
Field assessment of structure &
strength
Possible foundation problems
Fill
Miscellaneous material including
rubble, mineral waste, previously
excavated material, decaying wood
and other building debris.
To be avoided unless carefully
compacted. May contain chemicals which
either burn or are toxic.
Unit 6: Building Technology in Construction
Foundation Design: Soil Classification
Complete Worksheet
Unit 6: Building Technology in Construction
Foundation Design: Soil Movement
.
Foundations need to be designed with sufficient density to resist what
forces in the ground?
• Acidic attack/corrosion through nitrates – all foundations should use
acid and sulphate resistant concrete
• Rise in water level can cause float
• Lack of water can cause shrinkage
• Ground heave –water level freezing – compressible materials are
placed in excavations and foundations ton enable the ground to take up
this differential movement without affecting the structure.
• Differential Settlement – caused by buildings exerting differing levels of
point loads on the earth at their foundations. Flexible joints and
opportunities for building to accept movement will be incorporated into
the design.
Unit 6: Building Technology in Construction
Foundation Design: Soil Movement
Key Terms
Settlement:The vertical downward displacement at the base of a foundation or structure due
to soil consolidation, normally caused by the load applied by the structure.
Shrinkage:Soil shrinkage is the reduction of water content which causes a change in
volume of soil. It is most evident in clay soils during long periods of dry weather,
compounded by moisture abstraction from vegetation.
.Heave:Soil expansion due to the increase of water content which causes a change in
volume of soil. It occurs during wet weather and is compounded by previous
removal of moisture-dependent trees that would otherwise effect some drainage
and balance to subsoil conditions.
Unit 6: Building Technology in Construction
Foundation Design: Soil Movement
Trees
~ damage to foundations.
.
Unit 6: Building Technology in Construction
Foundation Design: Soil Movement
Foundations on poor ground
What are the options?
1. Excavate the poor soil away until ground of good bearing capacity is
found
2. Improve the soil so it can accept the load of the building
How can the soil be improved?
.
• Vibro-compaction of poor soil
• Vibro-replacement of poor soil
• Dynamic compaction of poor granular soil
Unit 6: Building Technology in Construction
Foundation Materials
What are the key requirements of a foundation?
A foundation must be constructed of a durable material
of adequate strength.
Experience has shown that the most suitable material is
concrete.
What is concrete?
Cement
Aggregate
Water
Concrete is a mixture of cement, aggregates and water in
controlled proportions.
Unit 6: Building Technology in Construction
Foundation Materials
Cement is manufactured from clay
and chalk and is the matrix or binder
of the concrete mix. Cement powder
can be supplied in 50kg air-tight
sealed bags which require a damp
free store, or alternatively it can be
delivered in bulk by a tanker and
pumped into a 12 to 50 tonne storage
silo.
Unit 6: Building Technology in Construction
Foundation Materials
What is aggregate?
Aggregates can be either natural
rock which has disintegrated or
crushed stone or gravel.
• Coarse aggregate is defined
as a material which is retained
on a 5mm sieve.
• Fine aggregate is defined as a
material which passes a 5mm
sieve.
Unit 6: Building Technology in Construction
Foundation Materials
Mixes are expressed as a ratio e.g. 1:3:6 /
20mm which is 1 part cement, 3 parts of
fine aggregate, 6 parts of coarse aggregate
with 20mm maximum size of coarse
aggregate for the mix.
Water must be of
drinking water
quality.
Water is added to start the chemical
reaction and to give the mix workability.
• The amount of water used is called the
Water/Cement Ratio and is usually
about 0.4 to 0.5.
• Too much water will produce a weak
concrete of low strength whereas too
little water will produce a concrete mix of
low and inadequate workability.
Unit 6: Building Technology in Construction
Foundation Materials
Compete worksheet
Unit 6: Building Technology in Construction
Next Lesson
You can now complete P3 of your assignment.
Criteria P3 &
Edexcel Guidance
You need to describe the principles of foundation design. This should convey an
understanding of how subsoil conditions influence foundation design and knowledge of
different foundation types.
Tutor Guidance
1.
2.
a)
b)
c)
Explain the principles of foundation design, how foundations spread the load of the
building over the ground the materials and techniques used in the construction of the
substructure for low-rise domestic and commercial buildings.
Describe how different subsoil conditions influence the following foundation design
Strip Foundation
Piled Foundation
Raft Foundation.
Unit 6: Building Technology in Construction