Foundation Settlement
Foundation settlement is the shifting of the
foundation (and the structure built upon it) into
the soil. This can cause damage to the structure.
Whether the soil is moist or dry is central to
predicting the amount of settlement to expect in a
given foundation. Areas with moist soils will have
more foundation settlement than dry areas. The
idea is that as water is squeezed out from the
soil, the structure will shift according to the empty
spaces the water left. The more water, the more
shift.
Settlement
Immediate or Elastic Settlement: Occurs immediately after the construction. This
is computed using elasticity theory (Important for Granular soil)
Primary Consolidation: Due to gradual dissipation of pore pressure induced by
external loading and consequently expulsion of water from the soil mass, hence
volume change. (Important for Inorganic clays)
Secondary Consolidation: Occurs at constant effective stress with volume
change due to rearrangement of particles. (Important for Organic soils)
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Types of Foundation Settlement
1- Immediate Settlement or elastic Settlement
Immediate settlement concerns the initial pressure on the soil under
and surrounding the foundation. It is "immediate" because it occurs
during and right after construction. It has nothing to do with water
displacement, but is merely caused by the weight of the structure. In
terms of building foundations, immediate settlement is relatively
easy to predict and measure. In many cases, given the nature of the
soil, foundations are constructed with the ability to withstand a
certain amount of shift without damage. Damage usually occurs
only in the long term, as the shift slowly continues over time.
• Immediate settlement takes place as the load is applied or within a
time period of about 7 days.
• Predominates in cohesion less soils and unsaturated clay
• Immediate settlement analysis are used for all fine-grained soils
including silts and clays with a degree of saturation < 90% and for all
coarse grained soils with large co-efficient of permeability (say
above 10.2 m/s)
2- Consolidation
• Consolidation settlement is distinguished from immediate settlement
both by the duration of the settlement and by displacement of water.
Consolidation is the more worrisome form of settlement because it is
difficult to predict over months or years. Consolidation settlement is
the settling of a foundation, over time, due to pressure exerted by
the structure and squeezes out the water content of the soil, thus
compressing it. Expulsion of moisture from the soil usually is a longterm process.
• Consolidation settlements are time dependent and take months to
years to develop. The leaning tower of Pisa in Italy has been
undergoing consolidation settlement for over 700 years. The lean is
caused by consolidation settlement being greater on one side. This,
however, is an extreme case. The principal settlements for most
projects occur in 3 to 10 years.
• Dominates in saturated/nearly saturated fine grained soils where
consolidation theory applies.
Here we are interested to estimate both consolidation settlement
and how long a time it will take or most of the settlement to occur.
The Leaning Tower of Pisa is the bell tower
of the Cathedral. Its construction was
commenced in 1173 and contiued haltingly
over a period of 200 years! The tower
began “leaning” soon after construction
began in 1173. The inclination of the tower
is attributed to the non-uniform, spongelike saturated clay soil on which the
foundation of the tower rests. The softer
area within this strata has settled more
causing the tilt.
Several engineers have proposed plans to
“straighten” the tower. However, with its
800+ years of “leaning” history, locals do
not want the tower to be straightened.
Every few years some form of restoration
is performed to ensure that the tower does
not become unstable or collapse.
3- Primary and Secondary Consolidation
Consolidation settlement has two components,
primary and secondary. The former deals
explicitly with the settlement caused by soil
moisture displacement, and the latter deals with
the elastic settlement after all movable water has
been squeezed out of the soil. Primary
consolidation is the most significant and
potentially harmful of the two. Primary
consolidation takes quite a bit of time, from
weeks to years. Secondary consolidation is the
quicker result of primary consolidation. Once
primary has been completed, and all movable
water has been moved, secondary kicks in.
Secondary consolidation occurs immediately
after primary, and takes far less time to
complete.
After secondary consolidation is complete,
the structure remains in its permanent
position. As a result, many builders advise
residents in new homes to avoid repairing
any settlement damage until secondary
consolidation is complete, which is
normally after two years at most.
• Occurs under constant effective stress due
to continuous rearrangement of clay
particles into a more stable configuration.
• Predominates in highly plastic clays and
organic clays.
Uniform Settlement
Total settlement refers to the uniform
settlement of the entire structure and
occurs due to weight of the structure and
imposed loads.
When all points settle with equal amount,
the settlement is called uniforms
Definition of Differential
Settlement
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Differential settlement refers to the the unequal
settling of a building's piers or foundation that
can result in damage to the structure. The
damage occurs when the foundation sinks in
different areas at different times.
Differential or uneven settlement can occur if
the loads on the structure are unevenly
distrbuted, variations in the soil properties or
due to construction related variations.
Settlement Limits
Total settlement is the magnitude of downward movement.
Differential settlement is non-uniform settlement. It is "the
difference of settlement between various locations of the
structure. Angular distortion between two points under a
structure is equal, to the differential settlement between the
points divided by the distance between them.
Theoretically speaking, no damage will be done to a structure
if it settles uniformly as a whole regardless of how large the
settlement may be. The only damage would be to the
connections of the underground utility lines. However, when
the settlement is non-uniform (differential), as is always the
case, damage may be caused to the structure.
Settlement Limits
• The tolerable, settlements of different structures, vary considerably.
Simple-span frames can take considerably greater distortion than
rigid frames. A fixed-end arch would suffer greatly if the abutments
settle or rotate. For road embankments, storage silos and tanks a
settlement of 300mm - 600mm may be acceptable, but for machine
foundations the settlement may be limited to 5mm -30mm. Different
types of construction materials can withstand different degrees of
distortion. For example, sheet metal wall panels do not show
distress as readily as brick masonry.
• To reduce differential settlement, the designer may limit the total
settlement and use the following equation for the calculation of the
differential settlement:
• (ΔHdiff) max = ½ ΔHtotal
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Guidelines to limiting values are suggested by a number of sources, but
following routine limits appear to be conventionally acceptable
(Skempton and Mac Donald, 1956)
Sands
Maximum total settlement = 40 mm for isolated footings = 40 to 65 mm
for rafts
Maximum differential settlement between adjacent columns = 25 mm
Clays
Maximum total settlement = 65 mm for isolated footings = 65 to 100 mm
for rafts
Maximum differential settlement between adjacent columns = 40 mm.
The differential settlement may also be evaluated in terms of the angular
distortion given by: (ΔHdiff) = Δ/L
Where Δ = relative settlement between the two points and L = Horizontal
distance between the two points. Based on a large number of settlement
observations and performance of structures, the suggested limits for
tolerable differential settlements are show in table below.
Angular distortion
Type of limit and structure
1/150
Structure damage of general
buildings expected
1/250
Tilting of high rigid buildings
may be visible
Cracking in panel walls expected
Difficulties with overhead cranes
Limit for buildings in which cracking
is not permissible
Overstressing of structural frames
with diagonals
Difficulty with machinery sensitive
to settlement
1/300
1/500
1/600
1/750
Causes
Differential settlement is primarily due to the
condition of the soil upon which the structure
sits. Soil has the capacity to expand or contract
based upon the temperature or weather
conditions. It can also shift or wash away due to
poor drainage, heavy rainfall, soil drying
unevenly, or changes in the water table.
Effects
The settlement causes cracks in a
structure's foundation, slab or
supporting piers. These cracks lead to
cracks in the building's interior walls and
uneven settling of the building's doors,
windows and trim.
Prevention and Solution
The best way to prevent damage from
differential settlement is to thoroughly analyze
the soil and make necessary amendments
before construction begins. It may be
necessary to reinforce the structure's piers or
foundations if a problem occurs after the
building has been constructed.
Signs of Foundation Settlement
When the foundation to your home or office
building has settled, it means that there has
been movement of the building below the
position in which it was built. The most common
reasons for changes in the foundation include
shifting soil, changes in the soil or even
changes in the moisture. Most houses
experience some settling after construction, but
these are normally not significant. Knowing
signs of foundation settlement can help you
address the issue in your home or avoid
purchasing a home with severe foundation
movement.
Cracks
Settlement cracks often occur in the foundation or the house slab or
on the ceilings and walls. Small cracks are often because of minor
foundation settlement or even because of expansion and contraction
of the settlement. Large cracks, however, can represent a more
significant concern. Cracks can warrant concern if they exceed onequarter of an inch in width.
Windows, Doors and Plumbing
Foundation settlement may make it difficult to open or
close doors and windows because the door frame does
not fit the door and causes some sticking. Gaps between
the windows and doors with the frame may also occur. A
door or window may not close all the way. Plumbing
lines can also be affected by a poor foundation as well
as mechanical equipment.
Examples of prediction of
settlement
1- Tilted Chimney
A tilted chimney is a sign that the
foundation of a building or home has
settled; however, it can be difficult to
recognize unless it is severe. A slight
shift in the chimney may not be as
noticeable, but it is a sign of a
shifting or settling foundation.
2- Loose Nails and Wallpaper
When nails begin to loosen or wallpaper
starts to separate from the walls, it can be a
sign of a foundation problem. Nails and
wallpaper shift if the walls move and they do
not bind or attach to the wall any more.
Bending or bowing walls can be a sign of the
age of the home, settling of the foundation,
deterioration of the home or shrinkage.
3-Slanted flooring
Slanted or sloped flooring may be a
result of foundation settlement in a
specific area of the building's
foundation or other structural issues,
such as rotting or termite damage. In
some cases, the foundation causing
the sloping floor may need to be jacked
up so that new footers can be installed.
How to Repair Cracks in
Concrete Pavements
It is not possible to pour concrete that will not
develop cracks or incur damage during use.
Many factors that contribute to cracks and
damage can be avoided when the concrete is
prepared and poured. Proper subgrade
preparation and support, providing adequate
expansion joints and correctly consolidating,
finishing and curing the concrete can prevent
most damage. Even so, cracks can occur and
they need to be repaired before they become
worse.
Instructions For Shallow Cracks
1 -Gauge the depth of the crack to determine whether it is less than a
1/2-inch deep. If it is, wear safety goggles, work gloves and a dust
mask to clean the crack using a stiff wire brush and whisk broom,
then clean the area around the crack.
2 -Paint the crack with a latex bonding agent, available at home
improvement stores, to improve the bond when the mortar mix is
applied.
3 -Fill the crack with concrete repair mortar mix, available at home
improvement stores, while the latex bonding agent is still wet. Mix
and apply the mortar according to the label directions on the mix.
4 -Allow the patch to dry and settle for 24 hours. Apply more mortar mix
if necessary and trowel smooth.
5 -Paint the patch and the area immediately surrounding it with a waterbased polyurethane sealer, available at home improvement stores,
to protect the patch from water seepage and staining.
Instructions For Deep Cracks
6- Wear safety goggles, work gloves and a dust mask to clean the
crack. If it is more than 1/2-inch deep and 1 inch wide, use a hammer
and chisel to remove loose chunks of concrete. Be sure to remove all
weak areas around the crack that are crumbling or in danger of
breaking off.
7- Use a stiff wire brush to abrade the crack and sweep it out thoroughly
to remove all debris. Paint the crack with a water-based polyurethane
sealer, available at home improvement stores.
8- Fill the crack with concrete repair mortar mix, according to label
directions, while the sealer is still wet. Smooth with a trowel and let it
set for 24 hours. Add more filler, if needed.
9- Paint the patch area and the area immediately surrounding it with a
water-based polyurethane sealer.