Issues to be considered :foundations

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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
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Commercial


Share Alike License

http://creativecommons.org/licenses/by
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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

Buildings or loads
in close proximity




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


Load bearing
strata


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


Where foundations are close
together the stress exerted on
the soil can overlap increasing
the force such that the ground
fails and settlement occurs


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


Soil and ground classification


Bearing capacity (kN/m
2
)

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

T

P should be less than
depth T to avoid shear




















Load
distribution
-

angle 45
o


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

Shear failure
angle 45
o


P

T

If P is greater than T then
the foundation may shear at
45
o

reducing the width of
the foundation and bearing
area.


Following the shear failure
the load is concentrated on
a smaller area, the ground
may consolidate under the
increased load.


The foundation fails
where tension is exerted
on the concrete


Strip foundation


deep formation level

Area above main
foundation filled with
concrete


Area above main
foundation filled with
trench blocks


Built up
with
concrete


Built up
with
trench
blocks


Use of trench blocks below ground

Trench blocks used to build up to ground
level

Foundation

depths

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


To avoid frost heave
foundations should be
constructed at least
450mm below the surface.





Shrinkable soils (Clay)
should be at least 750mm
deep for clays with a low
potential shrinkage and 1m
where there is high
potential shrinkage.

Min. 450
mm

Min. 750
mm

High shrinkage soils close to
mature tree


1m


3.43m deep

Precautions against heave

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

Backfill

Slip liner can be
used around pile

Precaution against heave

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


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

Void 125

300mm

Backfill

Backfill

500mm