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Rock Street, San Francisco

Foundation Problems

Foundations are the systems which are treated as the
interface elements in order to transfer various loads from a superstructure to
the rock or underlying soil over a larger area at a decreased pressure. There
are other factors which need to be taken into consideration as they vary with
the site purpose and specific requirements for the site including the
foundation for the following:

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Expansive soils

Extreme climatic conditions

Special loading situations such as:




Damages to a foundation can be caused via several factors
which range from construction to the natural environment. Other than
difficulties which can be encountered during the construction process, the
primary problem is to provide a foundation which will not settle a dangerous amount,
which depends on the flexibility of the structure. The causes of settlement

Elastic deformation



Plastic flow or shear failure of soil

There are a number ways which settlements can be reduced
which include typical methods such as raft foundations and piling. However, the
method used in specific cases will vary depending on the cause of the
settlement. If the correct method is not used then the settlement is likely to
incur the opposite effect and increase rather than decrease as planned.


Drag-down and Heave

Drag-down and heave is the foundation failure which occurs
if the footing of the foundation is placed on compressible soil. New
settlements in plastic soils are commonly enforced with upwards movements and
heave away with some distance. In a case such as this, the failure in the
foundation will be the cause of the heave or the differential settlement of the
soil which is used to support the foundation.

A paper by Srivastava (2012) displayed that building structures
on expansive soils require specific construction techniques as counteractive
precautions for the foundations which include:

Compaction on the wet side of the optimum
moisture content

Not taking into consideration the active zone of
contraction and expansion by applying pile piers at volume changing zone

Techniques for stabilising soil

Use of plastic fabric under the foundation in
order to control soil moisture

Use of “waffle slabs” to manipulate the
direction of expansion


Load Transfer Failure

The primary objective of foundation is to transfer the load
on superstructure to the foundation soil in a wider area. It functions as the
interface between the superstructure (overground) and substructure
(underground). The footing size is determined so that the pressure is
distributing the pressure on the subsoil and is expected not to exceed the
allowable limit of the subsoil. There are uncertainties which are involved in
the geotechnical design and practice which include:

Natural heterogeneity or inherent variability

Measurement error due to:



Random testing errors

Uncertainty of model transformation due to
approximation present during empirical or theoretical models relating to
measured quantities to design parameters

In such cases, underpinning is the most helpful approach to
sorting the problems out. This approach includes strengthening and stabilising
the foundation of a building which already exists. This process is accomplished
by increasing the depth or the width of the foundation so that there is more
place for it to rest across a greater area. (Srivastava, 2017)

Lateral Loads

Lateral movement in soil exists when the standing side
support nearby the building is removed or if there is an excessive overburden
on backfill or lateral thrust on the rear end of the retaining wall.
Earthquakes can also cause lateral movements and cause structures to fail due
to the soil beneath the building and the foundation following through with
liquefaction. Although there aren’t any counteractive measures to overcome this
type of foundation failure but there are ways to prevent it such as:

Subsurface investigation

Analysis and design

Construction control and supervision


Water Level Fluctuation

It is necessary to take into consideration that both rising
and falling groundwater levels can affect the behaviour of soil. The rise in
water leads to a reduction in bearing capacity of the soil and also case ground
collapse or form sinkholes as a result of increased overburden effective
stress. This can occur naturally due to heavy rain but will have more effect
when human activity such as uncontrolled pumping and dewatering during
construction of water of deep basement. In order to avoid the destruction of
properties and contamination to groundwater, it is necessary that ground
behaviour is monitored regularly for chances of potential formation of


Foundation Related Issues


Pile Foundation

Pile heave occurs when the displacement of soil from pile
penetration results in movement in previously driven piles nearby the site. It
is a phenomenon which generally happens within insensitive clays which behave
as incompressible materials through the pile driving process. In these types of
soils, the elevation of the adjacent piles is typically monitored during
driving. In terms of expenses, pile heave is considered as a necessary factor
to take into consideration, due to redriving additional time and effort.

Any pile which is subject to drag-load has a load
distribution comprised of resistance from the positive shaft along the lower
part of the pile, in stability with the negative skin friction collected with
the top part of the pile. In order to reduce skin friction, bitumen can be used
which can also decrease the hear force between the soil and pipe surface.

Shallow Foundation and Tree Roots

Tree roots can cause structural damage as they grow and can
affect the shallow foundations of light structures. The soil underground is
displaced as the tree roots move through the ground. Although the tree roots
themselves do not directly cause damage to the buildings and their foundations,
an increase in displacement through the ground could compromise the soil
integrity of the building which it is distributed upon along with it supporting

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