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TIMBERING IN TRENCHES
When the depth of trench is large, or when the sub-soil is loose, the
sides of the trench may cave in. The problem can be solved by adopting
a suitable method of timbering. Timbering of trenches, sometimes also
known as shoring consists of providing timber planks or boards and
struts to give temporary support to the sides of the trench. Timbering of
deep trenches can be done with the help of the following methods:
1. Stay bracing.
2. Box sheeting
3. Vertical sheeting
4. Runner system
5. Sheet piling.
1. Stay bracing. This method (Fig. 2.31) is used for supporting the
sides or a bench excavated in fairly firm soil, when the depth of
excavation does not exceed about 2 metres. The method consists of
placing vertical sheets (called sheathing) or polling boards opposite
each other against the two walls of the trench and holding them in
position by one or two rows of struts. The sheets are placed at an
interval of 2 to 4 metres and generally, they extend to the full height of
the trench. The polling boards may have width of about 200 mm and
thickness of 44 to 50 mm. The struts may have size 1OO x 100 mm
for trench up to 2 m width and 200 x 200 mm for trench up to 4 m
FIG. 2.31 STAY BRACING.
2. Box sheeting. This method is adopted in loose soils, when the depth
of excavation does not exceed 4 metres. Fig. 2.32 (a) shows the box like
structure, consisting of vertical sheets placed very near to each other
(some times touching each other) and keeping them in position by
longitudinal rows (usually two) of Wales. Struts are then provided across
Another system of box sheeting, shown in Fig. 2.32(b), is adopted for
very loose soils. In this system, the sheeting is provided longitudinally,
and they are supported by vertical Wales and horizontal struts [Fig.
2.32 (b)]. If the height is more, braces are also provided along with
3. Vertical sheeting. This system is adopted for deep trenches (up to
10 m depth) in soft ground. The method is similar to the box sheeting
[Fig. 2.32 (a)] except that the excavation is carried out in stages and at
the end of each stage, an offset is provided, so that the width of the
trench goes on decreasing as the depth increases. Each stage is limited
to about 3 m in height and the offset may vary from 25 to 50 cm per
stage. For each stage, separate vertical sheeting, supported by
horizontal wailings and struts are provided (Fig. 2.33).
4. Runner system. This system is used in extremely loose and soft
ground, which needs immediate support as excavation progresses. The
system is similar to vertical sheeting of box system, except that in the
place of vertical sheeting, runners, made of long thick wooden sheets
or planks with iron shoe at the ends, are provided. Wales and struts are
provided as usual (Fig. 2.34). These runners are driven about 30 cm in
advance of the progress of the work, by hammering
5. Sheet piling. This method is adopted when (i) soil to be excavated
is soft or loose (ii) depth of excavation is large (iii) width of trench is
also large and (iv) there is sub-soil water. Sheet piles are designed to
resist lateral earth pressure. These are driven in the ground by
mechanical means (pile driving equipment). They can be used for
excavating to a very large depth.
Shoring is the construction of a temporary structure to support
temporarily an unsafe structure. These support walls laterally. They can be
used under the following circumstances:
•When walls bulge out
•When walls crack due to unequal settlement of foundation and repairs are
to be carried out to the cracked wall.
•When an adjacent structure needs pulling down.
•When openings are to be newly made or enlarged in a wall.
Types of shoring
In this method, inclined members known as rakers are used to give
lateral supports to walls. A raking shore consists of the following
•Rakers or inclined member
It is a system of providing temporary supports to the party walls of the
two buildings where the intermediate building is to be pulled down and
rebuilt. All types of arrangements of supporting the unsafe structure in
which the shores do not reach the ground come under this category.
They flying shore consists of wall plates, needles, cleats, horizontal
struts (commonly known as horizontal shores) and inclined struts
arranged in different forms which varies with the situation. In this
system also the wall plates are placed against the wall and secured to it.
A horizontal strut is placed between the wall plates and is supported by
a system of needle and cleats. The inclined struts are supported by the
needle at their top and by straining pieces at their feet. The straining
piece is also known as straining sill and is spiked to the horizontal
shore. The width of straining piece is the same as that of the strut.
This is the system of shoring which is used to render vertical support
to walls and roofs, floors, etc when the lower part of a wall has been
removed for the purpose of providing an opening in the wall or to
rebuild a defective load bearing wall in a structure. The dead shore
consists of an arrangement of beams and posts which are required to
support the weight of the structure above and transfer same to the
ground on firm foundation below.
When opening in the wall are to be made, holes are cut in the wall at
such a height as to allow sufficient space for insertion of the beam or
girder that will be provided permanently to carry the weight of the
structure above. Distance at which the holes are cut depends upon the
type of masonry and it varies from 1.2m to 1.8m centre. Beams called
needles are placed in the holes and are supported by vertical props
called dead shores at their ends on either side of the wall. The needles
may be of timber or steel and are of sufficient section to carry the load
The method of underpinning help to strengthen the foundation of
an existing building or any other infrastructure. These involve
installation of permanent or temporary support to an already held
foundation so that additional depth and bearing capacity is achieved.
The process of providing a new foundation below an existing
foundation of an existing foundation, without endangering the
stability of an existing structure.
1. Mass Concrete Underpinning Method (Pit
•Mass concrete underpinning method is the traditional method of
underpinning, as it has been followed by centuries. The method
involves extending the old foundation till it reaches a stable
•The soil below the existing foundation is excavated in a controlled
manner through stages or pins. When strata suitable is reached, the
excavation is filled with concrete and kept for curing, before next
•In order to transfer the load from old foundation to new one, a
new pin is provided by means of placing dry sand-cement pack.
This is a low-cost method suitable for the shallow foundation.
Pile Method of Underpinning
In this method, piles are driven on adjacent sides of the wall that
supports the weak foundation. A needle or pin penetrates through the
wall that is in turn connected to the piles as shown in figure-3.
These needles behave like pile caps. Settlement in soil due to water
clogging or clayey nature can be treated by this method.
Underpinning By Cantilever Needle Beam Method
Figure-2 represents the arrangement of cantilever pit method of
underpinning, which is an extension of pit method. If the foundation
has to be extended only to one side and the plan possess a stronger
interior column, this method can be used for underpinning.
Advantages of Cantilever Needle Beam Method:
•Faster than traditional method
•One side access only
•High load carrying capability
•Digging found uneconomical when existing foundation is deep
•Constraint in access restricts the use of needle beams
When wall or column or other structural member of building
exceeds 1.5m, a temporary structures are needed to support the
platform over which workmen can work and material can kept.
These temporary structures constructed very close to wall, is in the
form of timber or steel framework, called Scaffolding.
1. Single scaffolding or brick layers scaffolding
2. Double scaffolding or mason’s scaffolding
3. Cantilever or needle scaffolding
4. Suspended scaffolding
5. Trestle scaffolding
6. Steel scaffolding
7. Patented scaffolding
TYPES OF SCAFFOLDING
1. SINGLE SCAFFOLDING OR BRICK
Single scaffolding is generally used for brick masonry and is also called
as brick layer’s scaffolding. Single scaffolding consists of standards,
ledgers, putlogs etc., which is parallel to the wall at a distance of about
1.2 m. Distance between the standards is about 2 to 2.5 m. Ledgers
connect the standards at vertical interval of 1.2 to 1.5 m. Putlogs are
taken out from the hole left in the wall to one end of the ledgers.
Putlogs are placed at an interval of 1.2 to 1.5 m.
2. DOUBLE SCAFFOLDING OR
Double Scaffolding is generally used for stone masonry so, it is also
called as mason’s scaffolding. In stone walls, it is hard to make holes in
the wall to support putlogs. So, two rows of scaffolding is constructed
to make it strong. The first row is 20 – 30 cm away from the wall and
the other one is 1m away from the first row. Then putlogs are placed
which are supported by the both frames. To make it more strong
rakers and cross braces are provided. This is also called as independent
3. CANTILEVER OR NEEDLE
This a type of scaffolding in which the standards are supported on series
of needles and these needles are taken out through holes in the wall. This
is called single frame type scaffolding. In the other type needles are
strutted inside the floors through the openings and this is called
independent or double frame type scaffolding. Care should be taken while
construction of cantilever scaffolding.
Generally cantilever scaffoldings are used under conditions such
•When the ground does not having the capacity to support standards.
•When the Ground near the wall is to be free from traffic.
•When upper part of the wall is under construction.
Light weight scaffolding used for repair works like painting,
The working platform is suspended from roofs by means of
wire ropes or chains.
Platform can be raised or lowered at any desired level.
This is also used for repair works inside the room, up to height
The working platform is supported on the top of movable
contrivances like tripods, ladders may be mounted on wheels.
Wooden members are replaced by steel tubes and rope lashings are
replaced by steel couplets or fittings.
Such scaffolding can be erected and dismantled rapidly.
It has greater strength, greater durability and higher fire resistance.
Can be used for stone and brick masonry construction.
Market available scaffoldings.
Equipped with special couplings, frames.
Working platform is supported on brackets which can be adjusted
at any suitable height