earthworks in buildings.pdf
TRANSCRIPT
-
1
Introduction:
A building is a man-made structure with a roof and walls standing more or less permanently in one place,
such as a house or factory. Buildings come in a variety of shapes, sizes and functions, and have been
adapted throughout history for a wide number of factors, from building materials available, to weather
conditions, to land prices, ground conditions, specific uses and aesthetic reasons. To better understand the
term building compare the list of nonbuilding structures.
Earthworks are engineering works created through moving or processing of part of earth surface involving
the quantities of soils, rocks and other sediments.
If we apply the earthworks in building construction there are three involved Components:
Foundation
Water System (such as groundwater, drainage etc.)
Basement
-
2
Earthworks in Buildings
Components of the Building:
1. Superstructure
Construction above the basement or foundation
supported by an infrastructure which in turn is
supported by the substructure.
2. Substructure
Basic framework or foundation that supports a
superstructure, and is supported by an
infrastructure.
3. Foundation
The foundations of the building transfer the
weight of the building to the ground. While
'foundation' is a general word, normally, every
building has a number of individual
foundations, commonly called footings.
-
3
Process and parts of Building Construction ( Superstructure and Substructure)
-
4
-
5
Earthworks in buildings
Foundation
Major component of building earthworks. The structure, that transmits the load of the building to the soil.
Choosing a kind of foundation depends on:
the ground conditions the groundwater conditions the site, the environment (the buildings nearby) the structure of our building
Requirements:
structural requirements: safe, be able to carry the load of the building constructional requirements: schedule, minimal resources, minimal cost
Foundation Components
-
6
Steps in establishing Foundation
1. Site Preparation
Remove trees and any debris Remove top soil (4-6 below surface)
2. Site Layout
Ensure lot lines are known & setbacks are complete Layout building perimeter Use batter boards Establish building corners & building perimeter Use surveying instruments
3. Excavation Excavate foundation along line created by batter boards
Excavate remainder of soil inside perimeter
Dont excavate inside soil if slab on grade
If deep foundation, taper edges to prevent collapse
If soil unstable, or very deep - use shoring
4. Pour Footings
Construct formwork (if required)
Install reinforcement (rebar) for footings
(protrudes above footing to tie-into foundation wall)
Pour concrete footings
Smooth / finish surface
-
7
Types of Foundation:
1. Shallow Foundation
also called spread footings or open footings. The 'open' refers to the fact that the foundations
are made by first excavating all the earth till the bottom of the footing, and then constructing
the footing.
During the early stages of work, the entire footing is visible to the eye, and is therefore called
an open foundation.
The idea is that each footing takes the concentrated load of the column and spreads it out over
a large area, so that the actual weight on the soil does not exceed the safe bearing capacity of
the soil.
Several kinds of shallow footings
a. Individual footings
Individual footings are one of the most simple and common types of foundations. These are used
when the load of the building is carried by columns. Usually, each column will have its own footing.
The footing is just a square or rectangular pad of concrete on which the column sits. To get a very
rough idea of the size of the footing, the engineer will take the total load on the column and divide it
by the safe bearing capacity (SBC) of the soil.
-
8
c. Raft Foundations Raft Foundations, also called Mat Foundations, are most often used when basements are to be constructed. In a raft, the entire basement floor slab acts as the foundation; the weight of the building is spread evenly over the entire footprint of the building. It is called a raft because the building is like a vessel that 'floats' in a sea of soil.
Mat Foundations are used where the soil is week, and therefore building loads have to be spread over a large area, or where columns are closely spaced, which means that if individual footings were used, they would touch each other.
b. Strip footings
Strip footings are commonly found in
load-bearing masonry construction,
and act as a long strip that supports
the weight of an entire wall. These
are used where the building loads are
carried by entire walls rather than
isolated columns, such as in older
buildings made of masonry.
-
9
2. Deep Foundations
The load-bearing layer is in deeper location
The loads of the building are too heavy
Kinds of Deep Foundation
a. Pile Foundation
A pile is basically a long cylinder of a strong material such as concrete that is pushed into the
ground so that structures can be supported on top of it.
Pile Materials
Steel; H- piles, Steel pipe
Concrete; Site cast or Precast
Wood; Timber
Composite
Pile foundations are used in the following
situations:
1. When there is a layer of weak soil at the surface.
This layer cannot support the weight of the
building, so the loads of the building have to
bypass this layer and be transferred to the layer
of stronger soil or rock that is below the weak
layer.
2. When a building has very heavy, concentrated
loads, such as in a high rise structure.
Pile foundations are capable of taking higher
loads than spread footings.
Top View of the Materials
-
10
There are two types of pile foundations, each of which works in its own way.
Pile Construction:
End Bearing Piles In end bearing piles, the bottom end of the pile rests on a layer of especially strong soil or rock. The load of the building is transferred through the pile onto the strong layer. In a sense, this pile acts like a column. The key principle is that the bottom end rests on the surface which is the intersection of a weak and strong layer. The load therefore bypasses the weak layer and is safely transferred to the strong layer. Friction Piles Friction piles work on a different principle. The pile transfers the load of the building to the soil across the full height of the pile, by friction. In other words, the entire surface of the pile, which is cylindrical in shape, works to transfer the forces to the soil.
A. CFA technology
Drilling continuously until planned depth
(using guiding tube if necessary) Placing the concrete and removing
the drill Placing reinforcement (vibration)
(CFA= Continuous Flight Auger)
-
11
b. Slurry walls
A deep, narrow trench filled with
concrete (and reinforcement)
Functions
Retaining wall during excavation
(can be watertight)
Foundation
Wall of the basement
B. Soil-Mec technology
Boring until planned depth (using a guiding tube)
Using bentonite mud (slurry) under the groundwater level
Placing reinforcement Placing the concrete and removing the
guiding tube
C.Franki technology (bulb pile or compacted concrete
pile)
Filling concrete in a steel pipe (creating a plug)
Pushing down the pipe using a heavy drop hammer
Fastening the pipe and creating the foot Placing the concrete and compacting while
removing the pipe (reinforcing)
-
12
Excavation
In general, excavation means to loosen and .take out materials leaving space above or below ground.
Sometimes in civil engineering term earthwork is used which include backfilling with new or original
materials to voids, spreading and levelling over an area.
Excavation and earthmoving plants
Advantages of using mechanical plant in excavation :
a) work done quicker,
b) avoid dangerous condition of work by human workers, say, existence of ground water or collapse of soil,
c) achieve greater depth,
d) use fewer manpower and work done in lower cost (for larger scale work only)
Disadvantages
a) involve larger running and maintenance costs,
b) require a larger operating area,
c) access provision to working area,
d) less flexible in work planning,
e) idling time increase cost of work
Excavation in most situations nowadays is done by mechanical means. However, the exact method to be adopted still depends upon a number of factors:
1. Nature of subsoil affect type of machine used and the necessity of soil protection.
2. Size of excavation affect type of machine used and method to excavate.
3. Scale of work large volume of excavation may involve complicated phasing arrangement and work
planning
-
13
4. Ground water condition affect degree of protection (watertight sheet piling or dewatering may
required.)
5. Surrounding condition impose certain restrictions and precautions (eg. diversion of a government
drain, or underpinning work to the nearby building foundation)
Deep excavation
Deep excavation, unlike a shallow one, often requires to protect the sides of cut using
suitable support. Besides, the problem of ground water cannot be avoided. There are methods
to overcome this, such as:
1. Dumpling method
This is used where there are buildings or street in the proximity. The method is to construct a
series of retaining wall in trench, section by section, around the site perimeter ,leaving a
centre Called "dumpling"
2. Diaphragm walling
This method need to construct a R.C. retaining wall along the area of work. Because the wall
is designed to reach very great depth, mechanical excavating method is employed. Typical
sequence of work includes:
a. Construct a guide wall guide wall is two parallel concrete beams running as a guide to the
clamshell which is used for the excavation of the diaphragm wall.
b. Excavation for the diaphragm wall In normal soil conditions excavation is done using a
clamshell or grab suspended by cables to a crane. The grab can easily chisel boulder in soil
due to its weight.
-
14
c. Excavation support excavation for the diaphragm wall produces a vertical strip in soil which can
collapse easily. Bentonite slurry is used to protect the sides of soil. Bontonite is a naturally
occurring clay which, when added to water, forms an impervious cake-like slurry with very large
viscosity. The slurry will produce a great lateral pressure sufficient enough to retain the vertical soil.
d. Reinforcement reinforcement is inserted in form of a steel cage, but may require to lap and
extend to the required length.
e. Concreting - concreting is done using tremie. As Concrete being poured down, bontonite will be
displaced due to its density is lower than concrete. Bontonite is then collected and reuse. Usually
compaction for concrete is not required for the weight of the bontonite will drive most of the air
voids in concrete.
Joining design for the diaphragm wall Diaphragm walling cannot be constructed continually for a
very long section due to tremendous soil pressure. The wall is usually constructed in alternative
section. Two stop end tubes will be placed at the ends of the excavated trench before concreting. The
tubes are withdrawn at the same time of concreting so that a semi-circular end section is formed. Wall
sections of this type are built alternatively leaving an intermediate section in between .
-
15
Cofferdams
A cofferdam may be defined as a temporary box structure constructed in earth or water to exclude soil or
water from a construction area, such as for foundation or basement works.
Use of cofferdam suitable for excavation of larger scale can be of :
a) Sheet pile cofferdam Also known as single skin cofferdam. Interlocking type steel sheet pile is used
and can use for excavation up to 15m. Sheet pile in this case acts as a cantilever member to support the
soil therefore adequate depth of pi le or suitable toe treatment may be required. In addition, cofferdams are
need to be braced and strutted or anchored using tie rods or ground anchors.
b) Double skin cofferdam This works similarly like the sheet pile to form a diaphragm. However, the
diaphragm is double-skinned using two parallel rows of sheet pile with a filling material placed in the void
between. This creates somewhat a gravity retaining structure and increase the ability to counteract the soil
behind. However, more working space is required.
-
16
Sheet Steel Piling
Steel, amongst other materials such as timber, is most effective to be used as sheet pile due to its high
tensile as well as their interlocking ability. It can be used as timbering to excavation in soft and/or
waterlogged soils especially in congested site where there is no enough space for complicated shoring.
Ground anchor
Ground anchor is basically a pre-stressing tendon embedded and anchored into soil or rock to provide
resistance to structural movements by a tying back" principle.
Common applications are :
1. General slope stabilization
2. Tying back/stabilizing a retaining structure
3. Tying back/stabilizing for diaphragm walls, but for a temporary nature during excavation
4. Tying back the entire building from up possible uplifting
Ground anchor can be classified into:
1. Rock anchor for anchorage in rock
2. Injection anchor suitable for most cohesive and non-cohesive soils
Method to form a ground anchor
A hole is predrilled on soil or rock in position carefully calculated. For rock anchor, an anchor bar with
expanded sleeves at the end is inserted into the hole. A dense high strength grout is injected over a
required length to develop sufficient resistance to hold the bar when it is stressed. Stressing is by hydraulic
mean and when the stress is developed, the head of the bar is hold by an end plate and nut.
For injection anchor, a hole should be bored usually with an expanded end to increase anchorage ability.
The pre-stressing bar is placed into the bore hole and pressure grouted over the anchorage length.
-
17
Earthwork in Basements:
CONSTRUCITON OF BASEMENT USING TRADITIONAL METHODS
Construction of basement is difficult for it must be carried out below deep ground in adverse condition such
as existence of ground water, muddiness or limited working space. Besides, works are needed to be done
amidst layers of props, struts, walings and shores, which cannot be removed until the permanent works are
completed and capable of carrying the final loads. For each case of basement construction, the method of
soil support, sub-soil condition, structure of the basement as well as the layout requirement of the entire
building must be taken into consideration before designing the method of works.
-
18
Method of constructing ordinary basement
One of the most effective methods to construct ordinary basement is by the use of diaphragm wall or sheet
pile wall (cut-off) which serves as a retaining structure during excavation and as the sides of the basement
walls. When the central soil is removed during excavation, the cut-off wall should be properly supported for
works. Below are some method suggested.
1. Use of lattice beams
A series of lattice beams or steel trusses
are installed so that they span between the
top of opposite diaphragm walls enabling
them to act as propped cantilevers. The russ
trusses can be removed after the internal
floors have been constructed and
receiving all the lateral forces from soil.
2. Use of Ground Anchors
Diaphragm walls are exposed by carrying out
the excavation in stages and ground anchors
are provided to stabilize the walls as the works
proceeds. This method is most effective for
basement of very large span or without
intermediate floors as lateral support
Basement Excavation Support using Lattice Truss
Basement Excavation Support using Ground Anchors
-
19
3. Construct floor slab as support floor slab as support (top-down method)
After the perimeter diaphragm walls
have been constructed, the ground
floor slab and beams are cast
providing tip edge lateral support to
the walls. An opening is left in the slab
for labours, material or plant as access
to continue excavation to the lower
stages. This is repeated until the
required depth is reached.
4. cast the centre basement slab to support struts
Centre area between the diaphragm walls
can be excavated leaving an earth berm
around the perimeter to support the walls
whilst the lowest basement floor in centre
can be constructed. Slots to accommodate raking struts acting
between the wall face and the floor slab
are cut into the berm. Final excavation and
construction of the remaining of the
basement can take place in stages around
the raking struts.
Basement Excavation Support using Ground floor slab
(top-down method)
Basement Excavation Support using Shore or Strut
from a Central Basement Slab cast in advance phases
-
20
Waterproofing the basement
A water-tighted basement wall is an essential element to waterproof a basement. |However, due to the
basement walls are often constructed under complicated phases to match with the excavation sequences
and this may increase the possibility of leaking, therefore, careful construction joining design is essential to
ensure the basement structure is perfectly water-proved. Very often the providing of water stops into these
joints is helpful. However, the most widely used method to water-proof a basement is to provide a cavity to
the wall of the basement (by building a skin wall to the sides). The ground water leaks into the basement
can then be collected through concealed channel to a sump pit and remove by pumps.
Earthworks in a Water System
The spacing between drains will vary between 10m for
clay soils to 50m for sand. Subsurface drains are usually
formed from buttjoined clay pipes laid in narrow
trenches. In cases where it is desirable to catch water
running on the surface, the trench is back-filled nearly to
the top with rubble either continuously along the trench
or in pockets. A trench filled with rubble or broken stone
will provide passage for water and is effective in dealing
with flows on the surface. Pipes and trenches belonging
to the main site drainage system may cause uneven
settling if allowed to pass close to or under buildings.
Where needed a separate drain, that surround the
building and installed not deeper than the footing, is
used to drain the foundation trench.
-
21
References
http://cee.engr.ucdavis.edu/faculty/boulanger/
http://en.wikipedia.org/wiki/Foundation_(engineering)
http://www.foundationengineering.info/photo_galleries/08/footing_construction/
www.wikipedia.org
http://www.wikihow.com/ How to build a concrete foundation in 7 steps/
Civil Engineering Construction by J.M. Antill, Paul Ryan and G.R. Easton (McGraw Hill 1988)
Civil Engineering Technology by B.G. Fletcher and S.A. Lavan; (Butterworths 1982);
Introduction to Civil Engineering Construction by Roy Holmes (College of Estate Management, 1996)