Construction Engineering I

Unit 1: Structures
 Unit 1: Structures
 1.1 Earthworks
 1.2 Buildings
 1.3 Dams
 Unit 1: Structures
 1.1 Earthworks
• Introduction
• Site Clearance
• Site Setting
• Foundations
• Excavations
• Timbering
• Excavation Basements
• Construction Plants
 1.2 Buildings

 Unit 1: Structures
 1.1 Earthworks
 1.2 Buildings
• Introduction
• Structures
• Structural Forms
• Scaffolding
• Formwork
• Concrete Compaction
 1.3 Dams
 Unit 1: Structures
 1.1 Earthworks
 1.2 Buildings
 1.3 Dams
• Introduction
• Feasibility Study
• Type of Dams
• Embankment Dam Design
• Concrete Dam Design
• Dam Construction
• Water Conservation
 Introduction
Earthworks involves preparing the site for construction of
structures

Earthworks is defined in different ways:

• Geotechnical Engineers: dealing with soils in general are

concerned with the engineering behaviour of earth materials.

• Structural Engineers and Building contractor will interpret

earthwork with reference to foundations.

• Roads Engineers will look at the general earthwork in relation to

road layerworks.
 Introduction
Earthworks involves preparing the site for construction of
structures

• Before Earthworks
-> Site boundaries
 Site clearance

• Before construction –
cleared of vegetation,
bushes, trees, topsoil,
disused buildings (any
unwanted material)

• Mechanically or Manually

 Construction plant
 Setting out of site

Setting out involves:

• Setting out of site boundary lines
• Setting out of building/structure
boundary lines
• Datum level (Set fixed point)needs to
be set.
 Setting out of site
DEFINITIONS
 Bearing capacity: safe load per unit area that the ground can carry
(kN/m2)

 Bearing pressure: pressure produced by applied load (kN/m2)

 Settlement: vertical ground movement due to applied load

 Backfill: excavated material re-used for filling purpose

 Self-weight loads: structure load

 Imposed loads: external load

 Differential settlement: uneven settlement

 Self-weight loads
Self-weight loads are all the permanent components of a structure – for example,
foundations, walls, columns, beams and slabs.
 Soil bearing capacity and
nature

The nature and bearing capacity of the soil can be determined by:

 Trial holes and trial pits

 Boreholes and core analysis
 Local knowledge
 Soil testing methods like the California bearing ratio (CBR), nuclear
testing methods, etc.
 In situ field tests:
• Plate loading tests,
• Standard penetration tests,
• Dynamic cone penetrometer (DCP), etc.
 Laboratory testing - the triaxial test.
 Foundations

Foundations are built below the ground and form the

base for any structure such as buildings, bridges and
dams.
 Their function is to distribute the load of the top
structure into the soil.

Two categories of foundation:

 Shallow foundations
 Deep foundations
Foundations
 Foundations

Shallow foundations transfer the loads of the

structure to the soil at a point near to the ground floor
of the building.
 Classified as less than 1.5 m deep

Deep foundations transfer the load of the structure

to the soil some distance below the ground floor of
the building.
 Classified as being deeper than 3.0 m.
 Foundation Types
Strip Foundations: used where light loads need support – for example, in
supporting walls.
 A reinforced concrete strip foundation can be used for heavier loads

Pad or isolated foundations:

 Used in portal frame
construction and for
columns in multi-storey
structures and bridges
 Foundation Types
Raft foundations:
 Used where the soil has a low bearing capacity or undergoes differential settlement
 where light to medium loads are supported.
 Used for buildings with basements.
 Generally have a larger surface area over which to spread the load.
 Foundation Types
Pile foundations:
• Used for structures where the loads have to be transmitted to a point some distance
below ground level.
• Used where ground
conditions are poor or the
bearing capacity is very
low.
• Piles are driven down to a
level where conditions are
more suitable.
• Able to carry much higher
loads then other types of
foundations. E.g.,Bridge
foundations
 Foundation Types
Caissons: are box-like structures that can be sunk through ground or in water
to install foundations or similar structures below the water line or water table.
Foundations in water or through
unstable shifting soil to depths
greater than 25m.

Types of caissons used in

constructing foundations
 Box caissons
 Open caissons
 Pneumatic caissons.
 Foundations
Two categories of foundation:
 Shallow foundations
 Deep foundations
 Choosing foundation types

The choice and design of foundations depends on:

Total load of the building (self-weight & imposed loads)

Nature & bearing capacity of the soil

Settlement of the soil

 Choosing foundation types

 Bearing capacity is the safe load per unit area that the ground can
carry, measured in kN/m2.
 Excavations
 Before foundations can be laid, a trench of the required depth and width
needs to be excavated. On small contracts this is still carried out by hand,
but on large works it is often more economical to use some form of
mechanical excavator

Manual Mechanical
 Timbering
Timbering refers to the temporary supports for the sides of excavations. It
is also called planking, strutting, shuttering and shoring.

The supports are used to:

 Prevent the sides collapsing
 Prevent the inflow of loose material
 Prevent damage to the adjoining property
 Keep the excavation open by acting as a retaining wall to the sides of the
trench
 Keep the workers in the trench safe from collapsing soil.

 Not required for trenches less than 1.5 m deep, if the soil is stable enough to
support itself.
 Timbering
Timbering refers to the temporary supports for the sides of excavations. It
is also called planking, strutting, shuttering and shoring.

Factors affecting type and amount of timbering:

 Nature of the soil (hard, firm, loose, sandy, clayey, etc)

 Depth of excavation

 Presence of ground water

 Proximity of buildings
 Weather conditions
 Duration of the operation
 Timbering - Terminology
 Poling boards are 1.0 to 1.5 m in length and vary in cross-section from
175 × 38 mm to 225 × 50 mm. They are placed vertically and support
the soil at the sides of the excavation.
 Timbering - Terminology
 Walings are longitudinal members running the length of the trench and are
used to support the poling boards.
 Timbering - Terminology

 Struts are square timbers (100 × 100 mm or 150 × 150 mm) that are generally
used to support the walings. They are placed approximately 2.0 m centre-to-
centre to allow for adequate working space between them.
 Timbering - Terminology

 Sheeting consists of horizontal boards abutting one another to provide a

continuous barrier when excavating in loose soil.
 Timbering - Terminology

 Runners are poling boards in continuous formation with tapered base edges. A
common size is 225 × 50 mm. They are suitable for use in loose or waterlogged
soils.

 In case of soft ground runner system

is provided.
 The system is similar to vertical
sheeting
 Timbering - Terminology

 Puncheons are vertical supports that are wedged between the walings at or near
the ends of struts. They are necessary in deep excavations to prevent the walings
from dropping/falling off.
 Trench excavation safety

Causes of collapses
 Materials too close to the sides. When working in a trench, the materials
required (concrete, pipes, stones, etc.) are practically on the edge of the
trench. This could result in the trench caving in if there are no supports.
 Unstable subsoil material (subsoil refers to the soil below the
topsoil).
 Variations in the nature of the soil, for example pockets of sand.
 Changes in the moisture content of the soil. Drying out or rain
may result in a change or breakdown of the soil strength.
 Vibrations from compaction plant or passing vehicles in the
vicinity of the excavated trench.
 The soil is unable to support its own weight.
 Unstable nature of the soil. When excavating on or near the site of
a previous excavation, the unstable nature of the soil could also
result in collapse
 Trench excavation regulations

Trench and excavation safety; and

https://sheqafrica.com/excavation-safety/

Excavations: A guide to safe work practices

http://www.labour.gov.za/DOL/downloads/legislation/
regulations/occupational-health-and-safety/Regulation%20-
%201031%20-%20OHS%20-%20General%20Safety
%20Regulations.doc
 EXCAVATING BASEMENTS

 Most multi-storey structures require basements.

Four basic methods

 Perimeter trench –used in weak soil- to provide for firm
base-blinding layer
 Raking struts – used where base is excavated in
sections. But firm subsoil must be present for use of
raking
 Cofferdams – is an enclosure beneath the water
constructed to allow water to be displaced by pumping
for the purpose of creating a dry work environment.
 Diaphragm walls- used as retaining wall to large masses
of soil constructed using bentonite slurry.
 Retaining Walls
 Normal soil has an angle (to the vertical) at which it can support
itself without collapse – this angle is commonly referred to as the
‘angle of repose’.
 To assist the soil and make it stable, a retaining wall is
constructed.
 The basic function of a retaining wall is to retain soil at an angle
greater than it would naturally assume, usually at a vertical or near
vertical position.
Retaining walls are designed to ensure:
 Overturning does not occur
 Sliding does not occur
 The soil on which the wall rests is not overloaded
 The materials used in construction are not overstressed.
 Retaining Walls
When designing a retaining wall several factors needs to be taken into consideration:

 The nature and type of soil

 The height of the water table
 Subsoil water movements
 The type of wall
 The materials used in the wall’s construction.

Reinforced Earth
 Refers to the strengthening of soil used as a fill material through the
addition of strong tensile reinforcement in the form of strips.
 Strength is obtained through the generation of frictional forces between the
soil and the reinforcement.
 Compacted layers with reinforcing strips between the layers.
 Types of Retaining Walls

Gravity retaining walls

• Also called mass retaining
walls and rely on their own
own mass and the friction on
the underside of the wall to
overcome the tendency to
slide or overturn
 Types of Retaining Walls

Cantilever walls
Two forms are considered
• A base with a large heel
so that the mass of the
earth above can be added
to the mass of the wall for
design purposes
• A cantilever wall with a
large toe, or even both.
 Types of Retaining Walls

Precast concrete retaining

structures
• The units come in various shapes
and sizes but mostly are 600 mm wide
and made from high grade concrete
• They are erected on foundations and rely on
their own mass, interlocking ability and
friction within the soil mass to perform
their retaining function.
 Types of Retaining Walls

Concrete block retaining

walls (CRBs)
• This is a more cost-effective solution
And is a reasonably versatile structures
compared to reinforced concrete walls
or gabion structures.
 Construction plant
Construction Plant

Excavation Plant Earth-moving Plant Compaction Plant

Skimmer Bulldozer Static weight roller

Face shovel Angledozer Vibratory roller
Backactor Scrapers Pneumatic roller
Dragline Graders Mini vibrating roller
Multi-purpose Excavator Tractor shovel Vibrating plate
Trencher Front-end loader Impact plates
 Excavation Plant

Factors that affect the choice plant to be used:

Site conditions
What needs to be done on site
Volume of work to be done
Buying v/s hiring

• Final choice of the plant to use is left to the site supervisor

 Excavation Plant

Skimmer
• Consist of a bucket
sliding along
horizontal jib.

• Used for shallow

excavation usually
up to a depth of 500
mm
 Excavation Plant
Face shovel:

• Used to excavate hard-soil and soft-rock. It

can double as loading & excavating machine
into the face of sloped surface.

Backactor:

• It is mainly used for excavation of

basements, trenches and pits. It is used in
excavation of loose material, to discharge
material the bucket is raised over haul truck
and emptied through open front end.
 Excavation Plant
Dragline:
• It only be used to excavate loose & soft
soil below the level of the machine.
 Excavation Plant
Trencher:

• A trencher is a construction
equipment used to dig trenches,
typically for laying pipes or cable,
or for drainage. Trenchers may
range in size from walk-behind
models, to attachments for a
skid loader or tractor, to very
heavy tracked
engineering vehicles.

Portable trencher
 Earth-moving Plant
Bulldozer :

 The dozers are used

strip topsoil, they can be
used to clear the site for
construction and to
level the surface in road
construction.
 The tractor is mounted
on tracks rather than on
wheels but the problem
is that the machine
needs to be transported
from site to site.
 Earth-moving Plant
Tractor shovel:

• Used to load loose material on

haul trucks

Front-end loader:

• Popular loading machine on site.

• It is fast, can easily manoeuvre in tight areas
• Does not need transportation from site to
site.
 Earth-moving Plant
Scrapers
• Used to excavate and transport
materials where surface stripping,
site levelling and cut and fill
fill activities are planned,
especially where large volume of
of soil is involved

Grader
• Used to create the final finishing
surfaces by cutting or grading the soil
until the required levels are obtained.

• It can also grade inclined surfaces by

moving the blade away from the body
to the desired gradient.
 Compaction Plant
Compaction in construction
 Compaction is the process by which volume of air in a material is
reduced by using external forces to reorient aggregate particles into a
more closely spaced arrangement
 This reduction of air volume in a mixture produces a corresponding
increase in the material unit weight, or density
 Increasing the density of soil, along with its side effects of increased
strength and decreased permeability, is usually desirable in earthwork
construction and below building foundations.
 Compaction is accomplished by use of heavy equipment. In sands and
gravels, the equipment usually vibrates, to cause re-orientation of the
soil particles into a denser configuration.
 In silts and clays, a sheepsfoot roller is frequently used, to create small
zones of intense shearing, which drives air out of the soil.
Compaction….
Compaction Plant
END