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CITY PROCEDURE
Procedure Title: Trenches and Excavating
Procedure Number:
HUR 130-006
Reference:
Occupational Health & Safety Act
Date Originated:
January 30, 2009
Date Revised:
Approval: Chief A minis i
Officer
Point of Contact: Coordinator, Health & Safety
Procedure Objective:
To establish procedures and standards for excavations and trenches that protect the
safety of employees and that comply with the Occupational Health & Safety Act and the
Construction Project Regulation.
Index
01 Definitions
02 Procedures
Appendix 1 Table — Excavation Shoring and Timbering (Metric Sizes)
01 Definitions
01.01 Engineered Support System - An excavation or trench shoring
system, designed for a specific project or location, assembled in
place and which cannot be moved as a unit.
01.02 Hydraulic Support System - A system capable of being moved as
a unit, designed to resist the earth pressure from the walls of an
excavation by applying a hydraulic counter pressure through the
struts.
01.03 Prefabricated Support System - A trench box, trench shield or
similar structure, composed of pieces connected to each other and
capable of being moved as a unit, and designed to resist the
pressure from the walls of an excavation but does not include a
hydraulic support system.
01.04 Pressure - In relation to a wall of an excavation, means the lateral
pressure of the earth on the wall calculated in accordance with
generally accepted engineering principles and includes hydrostatic
pressure and pressure due to surcharge.
01.05 Sheathing — Shoring that is placed up against the walls of an
excavation to directly resist the pressure exerted from the walls of
the excavation.
01.06 Soil Type — Refers to the consistency, density, hardness and
moisture levels of soil, and is classified by visual and physical
examination of the soil, at the walls of the excavation; and within a
horizontal distance from each wall equal to the depth of the
excavation measured away from the excavation.
01.07 Strut — Shoring that directly resists pressure from a wale.
01.08 Wale - Shoring that is placed against the sheathing to directly resist
the pressure from the sheathing.
02 Procedures
02.01 Working Alone
No employee may perform work in a trench unless another
employee is working above ground in close proximity to the trench
or close to means to access the trench.
02.02 Locating and Marking Services
Before an excavation is begun, gas, electrical and other services in
and near the area to be excavated must be accurately located and
marked. If a service may pose a hazard, the service must be shut
off and disconnected.
The owner of the service should be requested to locate and mark
the service. If a service may pose a hazard and cannot be shut off
or disconnected, the owner of the service shall be requested to
supervise the uncovering of the service during the excavation.
Pipes, conduits and cables for gas, electrical and other services in
an excavation shall be supported to prevent their failure or
breakage.
Procedure Title: Trenches and Excavating Page 2 of 9
Procedure Number: HUR 130-006
02.03 General Requirements
Every excavation that a worker may be required to enter shall be
kept reasonably free of water.
An excavation in which a worker may work shall have a clear work
space of at least 45 centimetres between the wall of the excavation
and any formwork or masonry or similar wall.
The walls of an excavation shall be stripped of loose rock or other
material that may slide, roll or fall upon a worker.
The walls of an excavation cut in rock shall be supported by rock
anchors or wire mesh if support is necessary to prevent the spalling
of loose rock.
A level area extending at least one metre from the upper edge of
each wall of an excavation shall be kept clear of equipment,
excavated soil, rock and construction material.
The stability of a wall of an excavation shall be maintained where it
may be affected by stockpiling excavated soil or rock or
construction materials.
No person shall operate a vehicle or other machine and no vehicle
or other machine shall be located in such a way as to affect the
stability of a wall of an, excavation.
If a person could fall into an excavation that is more than 2.4
metres deep, a barrier at least 1.1 metres high shall be provided at
the top of every wall of the excavation that is not sloped.
02.04 Soil Types
Soil is to be classified in one of the four types described below.
Prior to commencing excavation or trench work, the written opinion
of a geotechnical engineer must be obtained to determine the soil
type unless the support system to be used is sufficient as though
the soil type is deemed to be Type 4 soil.
02.05 Type 1 Soil:
a) is hard, very dense and only able to be penetrated with difficulty
by a small sharp object;
Procedure Title: Trenches and Excavating Page 3 of 9
Procedure Number: HUR 130-006
b) has a low natural moisture content and a high degree of internal
strength;
c) has no signs of water seepage; and
d) can be excavated only by mechanical equipment.
02.06 Type 2 Soil:
a) is very stiff, dense and can be penetrated with moderate
difficulty by a small sharp object;
b) has a low to medium natural moisture content and a medium
degree of internal strength; and
c) has a damp appearance after it is excavated.
02.07 Type 3 Soil
a) is stiff to firm and compact to loose in consistency or is
previously -excavated soil;
b) exhibits signs of surface cracking;
c) exhibits signs of water seepage;
d) if it is dry, may run easily into a well-defined conical pile; and
e) has a low degree of internal strength.
02.08 Type 4 Soil:
a) is soft to very soft and very loose in consistency, very sensitive
and upon disturbance is significantly reduced in natural
strength;
b) runs easily or flows, unless it is completely supported before
excavating procedures;
c) has almost no internal strength;
d) is wet or muddy; and
e) exerts substantial fluid pressure on its supporting system.
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Procedure Number: HUR 130-006
02.09 Support Systems
Where No Support System is Required:
The walls of an excavation do not require a support system if:
a) the excavation is less than 1.2 metres deep;
b) no worker is required to enter it;
c) it is cut in sound and stable rock;
d) it is made in Type 1 or Type 2 soil and whose walls are sloped
to within 1.2 metres of its bottom with a slope having a minimum
gradient of one horizontal to one vertical;
e) it is made in Type 3 soil and whose walls are sloped from its
bottom with a slope having a minimum gradient of one
horizontal to one vertical;
f) it is made in Type 4 soil and whose walls are sloped from its
bottom with a slope having a minimum gradient of three
horizontal to one vertical; or
g) it is not a trench and is not made in Type 4 soil and a
professional engineer has given a written opinion that the walls
of the specific excavation are sufficiently stable that no worker
will be endangered if no support system is used.
02.10 Types and Uses of Support Systems
In all other cases, a support system is required and must be
constructed with a type of support system that is appropriate in the
specific circumstances, as described below.
In the case of any prefabricated support system, hydraulic support
system or engineered support system, the system must be
designed by a professional engineer. The system must be
constructed, installed, used and maintained in a manner consistent
with the design specifications.
Before the system is used, two copies of the design drawings and
specifications must be delivered to the closest Ministry of Labour
office.
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Procedure Number: HUR 130-006
A support system for the walls of an excavation shall be installed,
progressively in an excavation in Type 1, 2 or 3 soil, and in
advance of an excavation in Type 4 soil, if practicable.
A support system for the walls of an excavation shall provide
continuous support for it. No support system for the walls of an
excavation shall be removed until immediately before the
excavation is backfilled. A competent person shall supervise the
removal of a support system for the walls of an excavation.
If a support system is used for the walls of an excavation, a ladder
for access to or egress from the excavation shall be placed within
the area protected by the support system.
A support system for the walls of an excavation shall extend at
least 0.3 metres above the top of the excavation unless otherwise
permitted or required by this section.
If an excavation is located where there is vehicular or pedestrian
traffic and if the excavation will be covered when work on or in it is
not in progress, the support system for the walls of the excavation
shall extend at least to the top of the excavation.
02.11 Timbering and Shoring
Timbering and shoring may be used if no hydrostatic pressure is
present in the soil and if the specifications as to its use are
consistent with the table at the end of this procedure.
Sheathing, struts and wales used should be made of sound
Number 1 Grade spruce.
Sheathing must be placed against the side of the excavation so that
it is vertical, must be secured in place by wales, and must be driven
into the soil and firmly secured in place if the excavation is made in
Type 3 or Type 4 soil.
Struts must be placed in the excavation so they are horizontal and
at right angles to wales, cut to proper length and held in place by at
least two wedges driven between the strut and the wales, and must
be cleated using cleats that extend over the top of the strut and rest
on the wales or that are attached securely to the wales by spikes or
bolts.
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Procedure Number: HUR 130-006
Wales must be placed in the excavation so that they are parallel to
the bottom of the excavation and supported by either cleats
secured to the sheathing or posts set on the wale below it (or if it is
the lowest wale, on the bottom of the excavation).
02.12 Prefabricated Support System
A prefabricated support system should never be used in type 4 soil.
It may be used for repairing underground pipe breaks if:
a) it has four side walls;
b) is designed for a maximum depth of 3.6 metres;
c) is not used ata depth greater than 3.6 metres;
d) is designed to resist all hydrostatic and earth pressures found in
type 4 and type 4 soils;
e) is installed so as to extend to the bottom of the excavation;
f) is installed so that the walls of the system touch the walls of the
excavation; and
g) is not pulled forward after being installed in the excavation.
The space between the walls of a prefabricated support system and
the walls of the excavation must be restricted to the minimum
clearance required for the forward progression of the support
system.
02.13 Hydraulic Support System
A hydraulic support system should never be used in type 4 soil.
It may be used for repairing underground pipe breaks if:
a) it has four side walls;
b) is designed for a maximum depth of 3.6 metres;
c) is not used at a depth greater than 3. 6 metres;
d) is designed to resist all hydrostatic and earth pressures found in
type 4 and type 4 soils;
Procedure Title: Trenches and Excavating Page 7 of 9
Procedure Number: HUR 130-006
e) is installed so as to extend to the bottom of the excavation;
f) is installed so that the walls of the system touch the walls of the
excavation; and
g) is not pulled forward after being installed in the excavation.
02.13 Engineered Support System
Where the excavation is a trench and the depth exceeds six metres
or the width exceeds 3.6 metres, the support system shall consist
of an engineered support system designed for the specific location
and project.
Procedure Title: Trenches and Excavating Page 8 of 9
Procedure Number: HUR 130-006
APPENDIX 1
TABLE
EXCAVATION SHORING AND TIMBERING (METRIC SIZES)
Column 1
Column 2
Column 3
Column 4
Excavation
Depth
Soil
Type
Sheathing
Struts
Width of Excavation at
Strut Location
Strut Spacing
Wales
1.8 m to 3.6
m
Up to 1.8 m
Vertical
Horizontal
3.Omor
less
1
50mmx200
mm at 1.2 m
o/c
200 mm x
200mm
150 mm x
150mm
1.2 m
* 2.4 m
*200 mm x
200mm
2
50mmx200
mm at 1.2 m
o/c
200mmx
200 mm
150mmx
150 mm
1.2 m
* 2.4 m
*200mmx
200 mm
3
50 mm x 200
mm at 10 mm
gap
200 mm x
200 mm
200 mm x
200 mm
1.2 m
2.4 m
250 mm x
250 mm
4
75 mm x 200
mm at 10 mm
gap
250 mm x
250 mm
200 mm x
200 mm
1.2 m
2.4 m
300 mm x
300 mm
Over3.Om
to 4.5 m
1
50mmx200
mm with 10
mm gap
200 mm x
200 mm
150 mm x
150 mm
1.2 m
2.4 m
200 mm x
200 mm
2
50 mm x 200
mm with 10
mm gap
200 mm x
200 mm
200 mm x
200 mm
1.2 m
2.4 m
250 mm x
250 mm
3
50 mm x 200
mm with 10
mm gap
250 mm x
250 mm
250 mm x
250 mm
1.2 m
2.4 m
250 mm x
250 mm
Over 3.0 m
to 4.0 m
4
75 mm x 200
mm with 10
mm gap
300 mm x
300 mm
300 mm x
300 mm
1.2 m
2.4 m
300 mm x
300 mm
Over 4.5 m
to 6.0 m
1
50 mm x 200
mm with 10
mm gap
200 mm x
200 mm
200 mm x
200 mm
1.2 m
2.4 m
200 mm x
200 mm
2
50 mm x 200
mm with 10
mm gap
250 mm x
250 mm
250 mm x .
250 mm
1.2 m
2.4 m
250 mm x
250 mm
3
50 mm x 200
mm with 10
mm gap
300 mm x
300 mm
300 mm x
300 mm
1.2 m
2.4 m
300 mm x
300 mm
Procedure Title: Trenches and Excavating
Procedure Number: HUR 130-006
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