Trench Supports

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Trench Support Methods
A) Traditional Methods include the
following types:
i) Sheeting, waling and struts
ii) Soldier piles
iii) Staged trench excavation
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Trench Support Methods
B) Proprietary Support Systems developed by the relevant manufacturers for
more labour saving construction, as well as
more conscious desire for site safety.
They are grouped into three broad types:
i) hydraulically strutted shores and walings
ii) shields and boxes
iii) plate lining systems
2
Sheeting, walings and
Struts (Fig. G-1)
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pg
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Sheeting, walings and
Struts
Materials available
Sheeting:timber boards, steel trench sheets
Walings: timber, steel sections, aluminium
sections
Struts:
timber, steel trench struts(screw
type), hydraulic struts, heavy duty steel
struts, purpose made tubular struts
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Design Methods
CIRIA Report 97 (Ref.)
Use of Sheeting
Design of walings and Struts:
Either by Calculation Methods; or
by Quick Reference Tables (Table G)
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This method may not give the most
economical solution.
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Precautions in Design
1. The lateral earth pressure acting on
the trench support will tend to
increase with time for cohesive soils.
Increased pressure should be allowed
in the design for trenches which are
expected to be left open for a long
period
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Precautions in Design
2. To avoid any weakness or excessive pressure at
the cantilevered ends of the sheeting:
The first waling should be placed not lower than
500 mm below the ground surface;
b) The sheeting should be tied in at the base
c) The waling should extend approx. 300 mm beyond
the last strut to ensure even spread of strut load
d) Timber wedges driven between waling & sheeting
to ensure uniform support between the soil face
and the support system
a)
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Advantages of
Sheeting, Waling, Strut
Support System
convenient and versatile support system
components cheap and easily obtained
light and easily dismantled by operatives
flexible to accommodate changes in trench
profile, local obstruction, crossing services and
construction features e.g. wider excavation for
manholes
ensure safe working in all stages during hand
excavation
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Disadvantages of
Sheeting, Waling, Strut
Support System
trench walls left unsupported when full depth of
the trench is excavated by backhoe/backacters
wide variations in the design ,i.e. the
arrangement and spacing of struts and walings
which may require attention of more competent
site supervisors
Longer pipe lengths and/or plants installation
may be obstructed by the closely spaced struts
inside the trench of deeper depth in particular
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Soldier Piles
Materials
1. H-section soldier piles ( between
200x200 and 300x300 mm ), with or
without timber walings and
2. the sheeting
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Soldier Piles
Method
H-section piles driven at 2.5 metres
spacing to a depth below the trench
bottom prior to trench excavation
Timber boards wedged against the
inner flanges of the H-section piles or
the timber walings (Fig G-2)
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Staged Trench
Excavation
The excavation could be split in stages
for very deep trench.
Care must be taken not to disturb the
strutting of the upper stages and to
allow space excavation and lowering
pipes etc. between the more complex
pattern of struts.
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Proprietary Support
Systems
Hydraulically Strutted Shores and Walings
Method
Most systems available are aluminium made to
save weight.
The struts and palings are pre-assembled in
rectangular forms which can be lowered into the
trench by the excavator between previously
placed sheeting
..\Teaching_Notes\TEMP_WK\multimedia\Cofferdam-Box.jpg
..\Teaching_Notes\TEMP_WK\multimedia\Cofferdam-Drag Box.jpg
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Proprietary Support
Systems
Hydraulically Strutted Shores and Walings
Method (Cont’d)
The hydraulic struts are then expanded
by remote operation from ground level.
Frames up to 5 metres long walings
Struts could be used for trenches up to
5 metres wide.
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Hydraulically Strutted
Shores and Walings
Advantages
The operatives need not enter the
trench during strutting.
do not require continuous sheeting
(allow gap between sheetings for
crossing services)
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Shields and Boxes
Method
Simple two sided rectangular structures
placed in the trench to provide a safe
environment for operatives to work
Generally can be extended in width and
height.
Sometimes the walls can be jacked Vs trench
sides but for most occasions being a device
to protect worker s Vs trench collapse only.
..\Teaching_Notes\TEMP_WK\multimedia\Cofferdam-Shield.jpg (Fig
..\Teaching_Notes\TEMP_WK\multimedia\Cofferdam-Box1.jpg
G-3, G-4)
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Shields and Boxes
Advantages
The loose fit makes it easier to drag the
box (by the excavator) in progress with
the works along the trench.
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Shields and Boxes
Disadvantages
It is important to consider the loss of ground
between adjacent boxes.
Do not provide significant support to trench
sides
Inconvenient to use with high water tables or
where services cross the line of the trench.
Weight of the box/shield could be important
(could be up to 3 tonnes)
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Plate Lining Systems
(Fig G-5)..\Teaching_Notes\TEMP_WK\multimedia\Cofferdam-Platelining1.jpg
Method
The system consists of rectangular plate
supports with one/two adjustable struts at each
end.
The plates are supplied in vertical modules to
accommodate a wide range of trench depths and
to enable the upper section of the trench to be
supported as excavation proceeds.
Steel-made unit sized up to 5 metres long by 2.5
metres high.
Resisting external pressures up to 40 kn/sq.m.
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Plate Lining Systems
Disadvantages
Special care are required :
for setting out the line for the system
when dealing with crossing services and
manhole positions
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Working Clearances for
Trench Works
Trench Width (Fig G-6)
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to allow access for installing services,
space for any sumps required, and
sufficient access for compaction plant
during backfilling
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Recommendations
for Trench Widths
Pipe diameter
D (mm)
Minimum Width between Walings
Trench up to 3 m
deep
Trench 3 m to 6 m
deep
< 300
700
1000
300 - 850
D + 400
1000
> 850
D + 600
D + 1000
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Safety hazards in trench
excavation
Trench walls collapse
 Install trench support
 Avoid superficial load from
construction vehicles
Keep stockpiling at least 1.5 m from the
trench side
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Safety hazards in trench
excavation
Falling objects to trench operatives
Operatives to wear safety helmets
Install guard rails at both trench sides
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Safety hazards in trench
excavation
Misuse of lifting heavy objects by
excavators ( gravity fall in case of
hydraulic failure )
Check whether the machine has been
certified for lifting operations or
Use other suitable lifting plants
equipped with check valves
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Hazards from existing
underground services
Damage live public utilities e.g. water, gas,
electricity
..\Teaching_Notes\TEMP_WK\multimedia\Trenc
h_Services.jpg (Fig G-7)
Obtain the most updated location information
of the services from the relevant authorities
prior to excavation
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Hazards from existing
underground services
Locate ferrous and power cables by
electrolocation equipment
Pilot trench holes ahead of main
excavations
Crossing services to be supported
by hangers suspended at ground
surface or propping from beneath
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Hazards from existing
underground services
Ground movement resulted from trench
works can cause brittle cast iron pressure
pipes fracture and burst (sometimes some
time after reinstatement of the trench due to
adjusted movement of the adjoining soil)
May require replacement of existing services
by new installations through agreement with
the utility authorities
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Hazards to existing
buildings
Deep excavation may undermine
the foundations, settlement
damage or collapse of adjacent
buildings
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Hazards to existing
buildings
Backfill the trench with concrete up
to the level at which the perimeter
line of the supporting wedge cuts
the trench (foundation stress at a
spread of 45 degrees, and
additional 150 mm of concrete
above the 45 degree line are
assumed)
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Hazards to the public at
the surface
Danger to vehicles driving for carriageway
trenching works
Provide properly guard rail protection (at least
1 metre above ground), traffic signs, lighting
and illumination (at night) in accordance with
the appropriate traffic signs design manual.
The arrangement has to be agreed with the
highway authority and the police
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Site Safety Regulations
for Excavation Works
Under the Contraction Sites (Safety)
Regulations of Hong Kong, the
Contractor carrying out excavation work
shall be responsible for:
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Site Safety Regulations
for Excavation Works
(1) Safety of excavation:(a) erecting structures made by suitable
material in connection with the operations
of the work as soon as may be necessary
after their commencement so as to
prevent workmen from being endangered
by a fall or displacement of earth rock or
other materials adjacent to or from the
side of the excavation,
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Site Safety Regulations
for Excavation Works
(b) ensuring every part of the excavation to be
examined by a competent person at least
once per week after commencement of the
excavation work until it is completed. Such
a report will be made in approved forms
which include a statement that further
excavation is safe and secure. No further
work is allowed until the report is released.
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Site Safety Regulations
for Excavation Works
(2) Fencing of excavation wherever there is
risk that a person will fall a distance of more
than 2 metre by:
(a) erecting a suitable barrier as close as
practicable to the edge of the excavation
(b) the excavation is securely covered
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Site Safety Regulations
for Excavation Works
(3) No material is placed/stacked close to the
edge of excavation.
No load, plant is
placed/moved close to the edge of excavation
which is likely to cause the side of the
excavation to collapse.
(4) Whenever there is apparent danger to
persons employed in the excavation area
from rising water or from an irruption of other
material, adequate means of emergency
escape must provided for such persons.
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Site Safety Regulations
for Excavation Works
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