Transcript Minimum width of strip foundations

Issues to be considered :foundations
Professor Chris Gorse and Ian Dickinson
These slides should be read in conjunction with Emmitt, S. and Gorse, C.
(2010) Barry’s Advanced Construction of Buildings. Oxford, Blackwell Publishing
Advanced Construction Technology
By Professor Chris Gorse & Ian Dickinson – licensed under the Creative Commons Attribution –
Non-Commercial – Share Alike License
http://creativecommons.org/licenses/by-nc-sa/2.5/
Position of loads on foundations
• Loads should be positions centrally on
foundations ensuring that loads are
distributed evenly across the whole
foundation area
Walls should be positioned centrally on the
strip foundation
Loads should be evenly distributed
to avoid uneven or differential
settlement
Uneven distribution of loads may
cause the foundation to crack
If the foundation exerts different
loads on the ground under the wall,
the foundation will settle into the
ground to different extents and the
foundation and wall may crack.
Causes of differential settlement
If one part of the building is loaded more than
another (e.g. heavy equipment) and the
foundations do not allow for this settlement may
occur in this area as the ground is overstress
Buildings or loads
in close proximity
Load bearing
strata
Where foundations are close
together the stress exerted on
the soil can overlap increasing
the force such that the ground
fails and settlement occurs
Weak strata
Where one part of the build
sits on firm ground and
another part rests on weak
or unstable strata the building
is likely to suffer differential
settlement.
Excessive
settlement may
fracture
services
Differential settlement due to different
foundation systems
If different foundations systems are used within
the same building, the buildings may need to be
separated to prevent cracking. Where different
foundation systems are used buildings will move
and settle differently. Where multiple foundation
systems are used buildings should be treated as
separate buildings and have movement joints.
Strip foundation and foundation design
• The following chart provides an extract
from the Building Regulations.
• Look at the different types of soil.
• Stronger soils can take higher loads (rock,
sand, gravel and stiff clay).
• Where soils are weaker (sandy soft clays)
the width of the foundation has to be
increased.
Allowable bearing pressure of soil and
ground
Bearing capacity (kN/m2)
Soil and ground classification
Rocks
Strong sandstone
Schists
Strong shale
Granular soils
Dense sand and gravel
Medium dense gravel
Loose sand and gravel
Compact sand
Loose sand
Cohesive soils
Stiff boulder clay
Stiff clay
Firm clay
Soft clay and silt
4000
3000
2000
> 600
200 – 600
< 200
> 300
< 100
300 – 600
150 – 300
75 – 150
< 75
Adapted from BS 8004;1986
Avoiding shear failure
Strip foundation
P should be less than
depth T to avoid shear
P
Load
distribution angle 45o
T
Forces outside the compressive zone
• As the compressive force can only be contained
within a 45 degree angle, un-reinforced
foundations that are wider than that
accommodated within the compressive zones
will suffer shear failure.
• If the width of the foundation is too wide and not
sufficiently deep tensile forces will be exerted on
the foundation. Concrete is poor in tension and
will crack.
Shear failure
Strip foundation
The foundation fails
where tension is exerted
on the concrete
P
If P is greater than T then
the foundation may shear at
45o reducing the width of
the foundation and bearing
area.
T
Shear failure
angle 45o
Following the shear failure
the load is concentrated on
a smaller area, the ground
may consolidate under the
increased load.
Strip foundation – deep formation level
Built up
with
trench
blocks
Built up
with
concrete
Area above main
foundation filled with
concrete
Area above main
foundation filled with
trench blocks
Use of trench blocks below ground
Trench blocks used to build up to ground
level
Foundation
depths
To avoid frost heave
foundations should be
constructed at least
450mm below the surface.
The depth of the foundation will depend on
the type of soil, distance from the tree and
water demand of the tree. Water demand
is dependant on the height and type of tree.
E.g. for a 20m high Oak tree in high
shrinkage soil, 10m from the face of the
foundation, the foundation should be at
least 2.50m deep
Min. 450
mm
Shrinkable soils (Clay)
Min. 750
should be at least 750mm
mm
deep for clays with a low
potential shrinkage and 1m
where there is high
potential shrinkage.
High shrinkage soils close to
mature tree
1m – 3.43m deep
Precautions against heave
Backfill
Where the ground is susceptible to
heave precautions should be taken.
Voids should be left below floor
slabs and compressible material
should provide a barrier between
the soil and foundations
Compressible material or void
former to the inside face and
underside of the ground beam, this
prevents the beam lifting or moving
when the clay expands
Void 125 –300mm
Slip liner can be
used around pile
Precaution against heave
Where the ground is susceptible to heave
precautions should be taken.
Void 125 –300mm
Backfill
Voids should be left below floor slabs and
compressible material should provide a
barrier between the soil and foundations
Where trench fill is greater than 1.5m deep
a compressible material or void former
should be used against the inside face of the
foundation, positioned in accordance with
manufactures instructions
Prevents the foundation being pushed
outwards
500mm
Backfill