Soil and Foundation

Construction Technology & Materials 1

BRE 261 Lecture

Dr. Hung-Lin CHI

 Announcements
• The make-up class on this Thursday is cancelled. We will have it
today during lecture session, right after today’s content been taught!
– In the end of this tutorial, you need to do a group discussion exercise
– Send your answer back to me through email
• Quiz for construction technology part will be held in Week 11
tutorial session, instead of Week 10
– The topic for tutorial session of Week 10 will be “Roof & Roofing” only
– 10 questions (9 multiple choice, 1 short description)
– Close book
– Cover the content from Week 6 (except those taught in the lecture session of
Week 6) to Week 10
– 1 hours
 Expected Learning Outcomes
• Possess a knowledge of functional requirements of foundation
structure and give preliminary appraisal to the performances
• Get to know relate basic soil and ground characteristics of
constructing foundations
• Have the basic extant of knowledge on the classification of different
foundation types
 What is a Foundation?
• Foundation is building structure designed to transmit loads to the
ground so that any movements of the foundation are limited and
thus will not adversely affect the functional requirements of the
building or neighboring buildings/ground
 What is Ground?
• Ground is the term used for the earth’s surface, which varies in
composition within the following five groups:
– Rocks
– Non-cohesive soils
– Cohesive soils
– Peat and organic soils
– Made-up ground and fill
 Compositions of Ground
• Rocks include the hard, rigid, strongly cemented geological deposits
• Soils include comparatively soft, loose, uncemented geological
deposits
 Ground Stability
• Unlike rocks, soils, made-up ground and fill are compacted under
the compression of the loads of buildings on foundations
• Movement of the foundations may be caused by the load of the
building on the ground and/ or by movements of the ground that
are independent of the load applied to the building
• The applied load of buildings on foundations may cause settlement
either through the compression of soil below foundations or
because of shear failure due to overloading
 Ground Stability
• Settlement movements on non-cohesive soils take place as the
building is erected and this settlement is described as immediate
settlement
• On cohesive soils settlement is a gradual process as water, or water
and air, are expelled from pores in the soil. This consolidation
settlement may continue for several years after completion of the
building
 Movements of the Foundation
• Movements of the foundation independent of the applied loads of
buildings are due to seasonal changes or the effects of vegetation,
which lead to shrinking or swelling of clay soils, frost heave, changes
in ground water level and changes in the ground due to natural or
artificial causes

frost heave
 Differential Settlement
• Differential settlement may occur if the building loads are not
properly distributed and foundations not designed and constructed
correctly
 Rocks
• Rocks may be classified as sedimentary, metamorphic and igneous
according to their geological formation

Group Rock Type

Sedimentary Sandstones
Some hard shales
Limestones
Metamorphic Some hard shales
Slates
Schists
Gneisses
Igneous Granite
Dolerite
Basalt
Bearing Capacity of Rocks and Soils
 Rocks
• Hard igneous and gneissic rocks, in sound condition, have so high an
allowable bearing pressure that there is little likelihood of
foundation failure
• Hard limestones and hard sandstones are, when massively bedded,
stronger than good quality concrete and it is rare that their full
bearing capacity is utilized. Where water containing dissolved
carbon dioxide runs over the face of limestone, the limestone may
also dissolve into the solution
 Soils
• Soils are commonly classified as non-cohesive or cohesive as the
grains in the former show a marked tendency to be separate
whereas the grains of the latter have a marked tendency to adhere
to each other. These characteristics affect the behavior of the soils
under the load of buildings
• Characteristics of a soil that affect its behavior as a foundation are
compressibility, cohesion of particles, internal friction and
permeability
• It is convenient to compare the characteristics and behavior of
clean sand, which is a coarse-grained non-cohesive soil, with clay,
which is a fine-grained cohesive soil, as supporting to buildings
 Compressibility
• For soils easy to be compressed, e.g., sand, the settlement of the
structure is very likely to occur throughout the course of
construction and at the time when the final structure is erected,
further compression is not likely to happen, unless super-imposed
loading is excessive
• Whereas for soil like clay, though it is very compressible, because of
its impermeability, compression takes place in a very slow rate, and
hence results in gradual settlement after completion of the
structure
 Cohesion of Particles
• The cohesion of soil is also called plasticity
• For soil like sand which cohesion among particles is negligible, when
load is imposed onto the soil, the soil is compressed quickly with
little disturbance of surrounding soil
• However, for soil like clay having high plasticity, when load is applied
onto the soil, it will sink gradually and because of the plasticity of
the soil, heave on the surrounding ground may occur
Cohesion of Particles
 Internal Friction
• For coarse particles like sand, there exists stronger friction in
resisting displacement and re-arrangement of the soil particles than
fine particles (e.g., clay) having less internal friction among particles
• Together with the high plasticity of clay, shear failures is likely to
occur on clay soil as the imposed load is excessive. Such kind of soil
failure is also called plastic failure
• Owing to the plastic nature of clay, shear failure under load of a
building may take place along several strata simultaneously with
consequent heaving of the surrounding soil
Internal Friction

Shear failure
 Permeability
• For soils having higher permeability imply that the ground can be
consolidated in a quicker rate resulting from the expulsion of the
water underground
• For soil of low permeability, the dissipation of water takes longer
time, and so as the consolidation of the ground

Soil Permeability Coefficient Relative

(k) (cm/sec) Permeability
Coarse Gravel Exceeds 10-1 High

Clean Sand 10-1 to 10-3 Medium

Dirty Sand 10-3 to 10-5 Low

Silt 10-5 to 10-7 Very Low

Clay Less than 10-7 Impervious

 Soil and Foundation
• From the various soil properties as mentioned, it is observed that
soil of nature resemble to clay is likely to cause more problems on
the substructure, and more careful design to the foundation is
essential
• It is cleared that settlement of soil because of imposed load is
inevitable
• Therefore, a proper design of foundation has to achieve the aim of
spreading the loading pattern to the soil in a more even manner so
as to prevent differential settlement which is most harmful to the
structure
 Functional Requirements of Foundation
• The primary functional requirements of
foundations are strength and stability
• To comply with building regulations the
combined dead, imposed and wind loads
of the building should be safely transmitted
to the ground without causing movement
of the ground that may impair the stability
of any part of another building
 Functional Requirements of Foundation
• Foundations should be designed so that
the combined loads from the building are
spread over an area of the ground capable
of sustaining the loads without undue
movement
• The foundation should also be constructed
so that ground movement caused by
swelling, shrinking or freezing of the subsoil,
landslip or subsidence will not impair the
stability of any part of the building
 Bearing Pressures
• The intensity of pressure on subsoil is not uniform across the width
or length of a foundation and decreases with depth below the
foundation
• In order to determine the probable behavior of a soil under
foundations, the engineer needs to know the intensity of pressure
on the subsoil at various depths
• This is determined by Boussinesq’s equation
for the stress at any point below the surface
of an elastic body, and in practice is a
reasonable approximation to the actual
stress in soil
Boussinesq’s Equation
Bulbs of Pressure Diagram

Pressure by point load

at different depths
 Bulbs of Pressure Diagram
• The practical use of
bulbs of pressure
diagrams is to check
that at any point in the
subsoil, under a
foundation, the unit
pressure does not
exceed the allowable
bearing pressure of the
soil

Bulbs of pressure under a strip foundation

 Bearing Capacity
• Bearing capacity refers to the strength of soil or rock to withstand
loading without causing shear failure. The bearing capacity of soil is
usually expressed in the following ways:
– Ultimate Bearing Capacity: Ultimate bearing capacity is the minimum
gross pressure intensity at the base of the foundation at which the soil fails in
shear
– Safe Bearing Capacity: Ultimate bearing capacity divided by a suitable
factor of safety is known as safe bearing capacity. The value of factor of safety
starts from 1.5 and may be up to 5. Such factor of safety is to act as a safety
measure to prevent soil from being overload, and create a buffer at which the
structure can remain safe even the soil properties deteriorate due to various
factors after the structure is erected
 Factors Affecting Bearing Capacity
• The determination of bearing capacity depends upon the following
major factors:
– Type of soil and its physical properties (density, shear strength, etc.)
– Position of water table
– Amount of allowable / anticipated total and differential settlement
– Physical features of the foundation, including:
• Type of foundation
• Size of foundation
• Shape of foundation
• Depth of foundation below ground level
• Rigidity of the structure
 Foundation Design
• A rational design of the foundation is based upon the bearing
capacity of the soil
• Results from site investigation report is a vital source of providing
information for foundation design
• To achieve proper design to the foundation, it is necessary to have
understandings on how soil reacts with loading
• In order to balance the loading, it can be achieved by either careful
design of the substructure or by re-arrangement of the loading
pattern of the overall structure
 Foundation Design
• In the case where differential settlements cannot be eliminated or
minimized by the substructure itself, then the whole structure must
respond to this by either providing flexible construction joints or
by dividing long span structure into rational "units", with each unit
having their own individual substructure systems
 Foundation Design Procedure
• The main objective of foundation design is to ensure that the
structural loads are transmitted to the subsoil safely, economically
and without any unacceptable movement during and after the
construction of the structure
• In order to achieve these objectives, one must have a rational
procedure in designing foundation
 Foundation Design Procedure
1. Assessment of site conditions in the context of the site and soil
investigation reports
2. Calculation of the anticipated structural loadings
3. Choosing the appropriate foundation type with throughout
considerations to the following points:
– Soil conditions (shear strength, compressibility, settlements, etc.)
– Types of structure (R.C., steel, cross walls, columns, beams & slabs, etc.)
– Rigidity of the structure (flexible vs. rigid)
– Structural Loadings (dead load, super-imposed load)
– Economic factors (efficiency in utilization of resources, etc.)
– Time factor with related to the contract period
– Construction problems (resources availability, safety, etc.)

(Continuous…)
 Foundation Design Procedure
4. Sizing the chosen foundation type(s) in the context of loadings,
ground bearing capacity, likely movements and physical constraints
of the construction site
 Foundation Structures
• The most common type of foundation structures can be classified
into the following categories:
– Strip foundation
– Pad foundation
– Raft foundation
– Pile foundation
– Hand dug caisson
– Basement
 Strip Foundations
• Strip foundations consist of a continuous, longitudinal strip of
concrete designed to spread the load from uniformly loaded walls
of brick, masonry or concrete to a sufficient area of subsoil
• The spread of the strip depends on foundation loads and the
bearing capacity and shear strength of the subsoil
• The thickness of the foundation depends on the strength of the
foundation material
Strip Foundations
Strip Foundations
 Strip Foundations
• Strip foundation is suitable for low rise load bearing wall type of
buildings
• However, in Hong Kong, though there aren't many projects which
are low-rise in nature, strip foundation has been commonly used as
a base for non-structural walls at ground level of high rise structure
• The common material used in Hong Kong for strip foundations is
reinforced concrete
Strip Foundations (Video)
 Pad Foundations
• Pad foundation is mainly for carrying point loading (column)
• However, as pad foundation doesn't penetrate deep into the soil, its
ability to take up loading is restricted to low-rise structure only
Pad Foundations
 Pad Foundations
• In the situation where columns are placed too close that
constructing individual pads for corresponding columns are
becoming uneconomical, continuous strip or combined pad instead
of individual pads foundation may be employed
• There exists different configuration of combined pads, but they all
follow the same principle of spreading the load effectively
• This is achieved by positioning the centre of gravity of the load in
line with the centre of gravity of the combined pad foundations
Pad Foundations
Pad Foundations (Video)
 Raft Foundations
• For soil of bearing capacity incapable of taking concentrated loading
imposed by other types of foundation structure, or in the situation
where spacing of concentrated load is too close together in
forming individual pad foundations, raft foundation may be
employed
• Raft foundation is fundamentally a large combined slab foundation
designed to cover the whole or a large part of area of the site so as
to spread the load more evenly to the soil, rather than deeper
foundation types (piling, caisson, etc.) so as to save the cost in term
of money, resources and time
• Apart from this, as raft foundation distributes the loading to the soil
in a more or less consistent manner, the chance of differential
settlement is hence reduced
Raft Foundations
 Raft Foundations
• Raft foundation is constructed in massively reinforced concrete in
considerable thickness
• It must be borne in mind that thick concrete is always a problem in
the construction process, as well as economic consideration
• Therefore, in case where the required thickness of the raft exceeds
the economical and technical threshold, other form of raft
foundations may be considered
• Apart from solid slab raft foundation, the other types of rafts are:
– Beam and Slab Raft
– Cellular Raft
 Raft Foundations
• The need of beam and slab or cellular raft rather than slab raft
occurs when the loading and requirement on the rigidity of the
structure necessities a slab raft of thickness greater than 300mm, as
it is cheaper and cause fewer problems in the course of
construction
• The principle of beam and slab and cellular raft is similar. They make
use of beams forming grid structure to stiffen the slab raft. Such
grid beams can be facing either to the soil or upright. In the case of
upright beams raft, it is usually incorporated with precast ground
floor design
Beam and Slab Raft Foundation
 Beam and Slab Raft Foundation

Grid beams facing either to the soil

Grid beams facing upright

Beam and Slab Raft Foundation (Video)
 Cellular Raft Foundation (Buoyant Raft)
• Where differential settlements are likely to be significant and the
foundations have to support considerable loads, the great rigidity of
the monolithically cast reinforced concrete cellular raft is an
advantage
• This type of raft consists of top and bottom slabs separated by and
reinforced with vertical cross ribs in both directions
• The monolithically cast reinforced concrete cellular raft has great
rigidity and spreads foundation loads over the whole area of the
substructure to reduce consolidation settlement and avoid
differential settlement
Cellular Raft Foundation
 Cellular Raft Foundation
• A cellular raft is also used when deep basements are constructed
to reduce settlement by utilizing the overburden pressure that
occurs in deep excavations
• This negative or upward pressure occurs in the bed of deep
excavations in the form of an upward heave of the subsoil caused
by the removal of the overburden, which is taken out by excavation
• This often quite considerable upward heave can be utilized to
counteract consolidation settlement caused by the load of the
building and so reduce overall settlement
Cellular Raft Foundation
 Pile Foundation
• Pile foundations can be defined as series of columns constructed or
inserted into grounds of considerable depth, usually up to the level
where firm soil strata capable of carrying load
• The major reasons of adopting pile foundation over other
foundation designs are:
– There exist no suitable soil strata to take up load at reasonable depth
– The applied load is uneven so that it makes the use of raft foundation not
feasible
– Firm strata exists at a low level beyond the economical use of raft foundation
– High water table which give rise to high operational cost on dewatering
– Presence of highly compressible or decayed subsoil which is not suitable for
foundation to sit on
– Subsoil may be subjected to moisture movement or plastic failure
Pile Foundation
 Pile Foundation
• There exist numerous types of pile foundations being in use
• Normally they are classified in two ways; one in term of the pattern
of loading taking, and the other in term of the way to construct the
pile
• Under the first way of classification, that is, in term of the loading
taking pattern, pile foundations can be divided in two categories:
– Friction Pile: which makes use of the skin friction between the soils and the
subsoil to disturb the load
– End Bearing Pile: the pile carries the load through the weak strata and
transfer it to a firm stratum on which its end rest on
Pile Foundation
 Pile Foundation
• Under the second way of classification, that is, in term of the way in
constructing the piles, piles foundation can also be classified into
two main categories as well:
– Replacement Piles
– Displacement Piles
 Replacement Piles
• Holes are preformed on the ground up to the level which the piles
have to rest on
• In other words, soil has been taken away before the piles are
constructed
• The diameter of replacement (bored) piles ranges from 200mm
(mini piles) to 3m
 Replacement Piles
• A wide variety of machines are available in the market to construct
replacement piles of different sizes and depths
• In order to prevent soil from collapse during the course of
excavation which may affect the integrity and performance of the
pile, precautionary measures like steel casing and/or the use of
special fluid (bentonite) are employed
Continuous Flight Auger Pile
Large Diameter Bored Pile
Replacement Piles (Video)
 Displacement Piles
• Preformed piles are driven into the ground up to the required
depth
• In other words, the subsoil is penetrated by the piles and soil is
displaced as the piles drive through
• Steel H-section Pile is the most commonly used displacement piles
system in Hong Kong
• It is noisy and caused a lot pollution issues on noise and are
subjected to stringent regulations on operation as a result of this
• However, it offers advantages on time required to drive the pile
into position and is widely used in Hong Kong
Displacement Piles
Continuous Helical Displacements Pile
Displacement Piles (Video)
 Caisson (Hand-dug)
• In Hong Kong, caisson is usually referred to hand dug caisson
• The principle of caisson is similar to replacement piles, except it is
in a much larger diameter. In extreme case, the diameter of the
caisson may be up to 3m
• The construction sequences of caisson, explained in the simplest
term, is to excavate holes, which will be filled by reinforced
concrete, with the casing to the holes to be constructed at the time
when excavation reaches a certain depth
• A hand-dug caisson is a cylindrical shaft formed in the ground, with
openings at both top and bottom. The shaft is excavated in stages
by hand and the wall of the shaft is lined with 75 to 100mm thick
in-situ concrete as excavation progresses. If the water table is high,
the thickness of lining may be increased to 150 or 200mm at depths
of 30m
Caisson (Hand-dug)
 Basement
• Basement can be seen as a further extension of the raft foundation
concept and in certain extent similar to cellular raft foundation
• The major difference is that basement usually refers to
underground structure, usually multi-storey in existence, with
objectives in providing enclosed area underground capable of
accommodating human activities to be carried out
• Whereas in cellular raft, the prime concern is to have economical
design on foundations, and any further applications of the cells are
merely secondary advantage of cellular raft foundation
 Basement
• Basement can help in supporting load by the ways of acting as a
kind of deep raft foundation, though basement always has its own
foundation structure (pile, caisson, etc.)
• Apart from this, because of vast void areas created, provided that
water table is high (above the basement level) and consistent, the
buoyancy of the basement can help in taking up loadings
• However, this must be subjected to extremely careful examinations,
and waterproofing of the interior of the basement is always a major
problem in maintaining basement structure
 Summary
• Definition of ground and foundation
• Characteristics of a soil
 Settlement
 Compressibility, Cohesion of particles, Internal friction and Permeability

• Functional requirement of foundation

• Bearing pressures
 Boussinesq’s Equation

• Foundation structures
 Strip foundation
 Pad foundation
 Raft foundation
 Pile foundation
 Hand dug caisson
 Basement
Thank you for your attention!

Hung-Lin CHI, PhD

ZS706
hung-lin.chi@polyu.edu.hk