Excavation

Trenching fatalities are mainly caused by cave-ins. Death occurs by suffocation or crushing when a worker is buried by falling soil. The terms “trench” and “excavation” are sometime interchanged; however, it is important to note the distinction between the two. An excavation is a hole left in the ground as the result of removing material, and a trench is an excavation in which the depth exceeds the width.

The Excavations section of the Construction Regulation defines the various types of soils and specifies the type of shoring and timbering to be used for each. [O. Reg. 213/91, s. 222] It also spells out the requirements for trench support systems that must be designed by an engineer.

Types of Soil

The Construction Regulation sets out four soil types. If you are unsure about the soil type, have the soil tested to confirm the type. [O. Reg. 213/91, s. 226]

Type 1 soil:

is hard, very dense and only able to be penetrated with difficulty by a small sharp object,
has a low natural moisture content and a high degree of internal strength,
has no signs of water seepage, and
can be excavated only by mechanical equipment.

Type 2 soil:

is very stiff, dense and can be penetrated with moderate difficulty by a small sharp object,
has a low to medium natural moisture content and a medium degree of internal strength, and
has a damp appearance after it is excavated.

Type 3 soil:

is previously excavated soil, or
soil that is stiff to firm or compact to loose in consistency and has one or more of the following characteristics:
- it exhibits signs of surface cracking.
- it exhibits signs of water seepage.
- if it is dry, it may run easily into a well-defined conical pile.
- it has a low degree of internal strength.

Type 4 soil:

is soft to very soft and very loose in consistency, very sensitive and upon disturbance is significantly reduced in natural strength
runs easily or flows, unless it is completely supported before excavating procedures,
has almost no internal strength,
is wet or muddy, and
exerts substantial fluid pressure on its supporting system.

The type of soil determines the strength and stability of trench walls. When excavating, the type of soil must be determined by visually and physically examining at the walls and within a horizontal distance from each wall equal to the depth of the excavation measured away from the excavation. This requirement means if an excavation is 3 metres (10ft) you would have determine the soil type 3 metres (10ft) away from the excavation (wall). When classifying the soil type, it is not uncommon for more than one type of soil at a single location. For classifying purposes, soil should be classified as the highest numerical class (most protective). [O. Reg. 213/91, s. 227]

Excavation - General Requirements

No one may enter an excavation that does not comply with this legislation. When people are working in a trench, another worker must be working above ground close to the trench or its access points. [O. Reg. 213/91, s. 224] [O. Reg. 213/91, s. 225]

Before beginning an excavation, employers and workers must make sure that all gas, electrical and other services in and near the area to be excavated are located, marked and shut down if they are a danger during excavation. In Ontario, the Ontario1Call service can help. If a service cannot be shut off or disconnected, the owners of the service must supervise its uncovering during the excavation. Pipes, conduits and cables for gas, electrical and other services must be supported to prevent failure or breakage. [O. Reg. 213/91, s. 228]

When excavations could affect the stability of a nearby building or structure, constructors must make sure an engineer prepares written precautionary procedures and that these precautions are put in place. [O. Reg. 213/91, s. 229]

Excavations that workers might enter must be reasonably free of water. Workers in an excavation must have a clear work space of at least 450 millimetres between the wall of the excavation and any formwork, masonry or similar wall. The walls must be stripped of loose rock or other material that may slide, roll or fall upon a worker. When support is needed to prevent the spilling of loose rock, walls must be supported by rock anchors or wire mesh while cut out of the rock. [O. Reg. 213/91, s. 230] [O. Reg. 213/91, s. 231] [O. Reg. 213/91, s. 232]

A level area extending at least 1 metre from the upper edge of each wall of an excavation must be kept clear of equipment, excavated soil, rock, and construction material. The stability of excavation walls is to be maintained where it could be affected by stockpiling. Vehicles must be operated so that they do not interfere with the stability of an excavation’s walls. If excavations are more than 2.4 metres deep and people nearby could fall in, a barrier at least 1.1 metres high must be installed at the top of every wall that is not sloped (as described in clauses 234 (2) (e), (f) and (g) [O. Reg. 213/91, s. 234]). [O. Reg. 213/91, s. 233]

Support Systems

Excavation walls must have a support system that complies with the regulation. [O. Reg. 213/91, s. 234] The support system must provide continuous support for the excavation’s walls and may not be removed until immediately before the excavation is backfilled. A competent person must supervise this removal. [O. Reg. 213/91, s. 239] When there is a support system in place, ladders and any exit points must be protected by the support system. [O. Reg. 213/91, s. 240]

Support systems for the walls must be installed progressively in an excavation in Type 1, 2 or 3 soil and in advance of excavations in Type 4 soil when practical. [O. Reg. 213/91, s. 239] Unless otherwise allowed, support systems must extend at least 0.3 metres above the top of the excavation. [O. Reg. 213/91, s. 241] Where there is foot or car traffic and the excavation is covered when not being worked on, the support system must extend at least to the top of the excavation. [O. Reg. 213/91, s. 241]

Sloped upper portions of the excavation walls must be sloped for the type of soil present (as described above or in clauses 234(2)(e), (f) and (g) [O. Reg. 213/91, s. 234]). Lower portions that are vertical or near vertical, must be supported by a system which extends at least 0.5 metres above the vertical walls. [O. Reg. 213/91, s. 234] [O. Reg. 213/91, s. 241]

Before a support system is used, the constructor must submit 2 copies of the design drawings and specifications to the office of the Ministry of Labour nearest to the project. [O. Reg. 213/91, s. 237(3)]

When a trench’s depth is larger than 6 metres or the width is larger than 3.6 metres, there must be an engineered support system designed for the location and project. [O. Reg. 213/91, s. 235]

A support system must be:

must follow the timbering and shoring specifications laid out in the table specified in the regulation [O. Reg. 213/91, s. 238]
made with timbering and shoring that meets the requirements, if no hydrostatic pressure is present in the soil, and if the width and depth of the excavation are equal to or less than the width and depth indicated in the table specified in the regulation [O. Reg. 213/91, s. 238]
a prefabricated support system that complies with the legislation,
a hydraulic support system that complies with the legislation, or
an engineered support system that complies with section the legislation.

Support systems are not required for exactions: [O. Reg. 213/91, s. 234]

that are less than 1.2 metres deep,
where workers are not required to enter,
that are not trenches and where workers are not required to be closer to the walls than the height of those walls,
that are cut in sound and stable rock,
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,
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,
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
that are not trenches and not made in Type 4 soil and where an engineer has given a written opinion that the walls of the excavation are sufficiently stable that no worker will be endangered if without a support system is in place. Note for these projects, the engineer’s opinion must be kept on site and must include:
- the specific project and its location
- any specific condition for which the opinion applies, and
- the frequency of inspections

Prefabricated, hydraulic or engineered support systems must be designed by an engineer and be constructed, installed, used, and repaired according to their design drawings and specifications. These drawings and other related documents must be kept on site while the system is in place. Any variations must be approved in writing by an engineer. If the type of soil is different than what system was designed for, an engineer must modify the drawings and specifications conditions or approve the system for use under the actual soil conditions. [O. Reg. 213/91, s. 236]

The system design must specify the size of the system and its type and grade of materials, the maximum depth and the types of soil for which it was designed, how to install and remove the system and proper location of the system in an excavation including the maximum allowable clearance between the walls of the support system and the excavation’s walls. [O. Reg. 213/91, s. 236]

With a hydraulic support system, the designs must indicate its minimum working pressure and outline the personal protective equipment required to protect a worker if a loss of hydraulic pressure occurs. [O. Reg. 213/91, s. 236(4)]

A prefabricated or hydraulic support system may be used for repairing underground pipe breaks if the system: [O. Reg. 213/91, s. 237]

meets the general requirements,
has four side walls,
is designed for a maximum depth of 3.6 metres,
is not used at a greater depth than 3.6 metres,
is designed to resist all hydrostatic and earth pressures found in type 3 and type 4 soils,
is installed so as to extend to the bottom of the excavation,
is installed so that the walls of the system touch the walls of the excavation, and
is not pulled forward after being installed in the excavation.

In addition: [O. Reg. 213/91, s. 237(1)]

prefabricated or hydraulic support systems cannot be used in type 4 soil,
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, and
the walls of a hydraulic support system must touch the walls of the excavation.

Sheathing must be: [O. Reg. 213/91, s. 238(3)]

made of sound Number 1 Grade spruce and,
- placed vertically against the side of the excavation
- secured in place by wales, and
- driven into the soil and firmly secured in place if the excavation is made in Type 3 or 4 soil.

Struts must be: [O. Reg. 213/91, s. 238(4)]

made of sound number 1 structural grade spruce and,
- placed horizontally in the excavation and at right angles to the wales, and
- cut to the proper length and held in place by at least two wedges driven between the strut and the wales, and
- cleated with 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.

Wales must be: [O. Reg. 213/91, s. 238(5)]

made of sound number 1 structural grade spruce and,
- placed, in the excavation, parallel to the bottom, or proposed bottom, of the excavation, and
- supported by either cleats secured to the sheathing or posts set on the wale next below it or, if it is the lowest wale, on the bottom of the excavation.

Metal trench-jacks or trench-braces

A metal trench-jack or trench-brace can be used instead of a timber strut: [O. Reg. 213/91, s. 242]

if the allowable working load of the trench-jack or trench-brace is equal to or greater than that of the timber strut, and
if the size of the replaced timber strut is shown on the trench-jack or trench-brace.

An engineer is required to determine the allowable working load of a metal trench-jack. The allowable load must be legibly cast or stamped on the trench-jack or trench-brace.

Metal trench-jacks or trench-braces cannot be extended beyond the length used to establish the maximum allowable working loads.

Every metal trench-jack or trench-brace: [O. Reg. 213/91, s. 242]

must be placed against the wales in such a way that the load from the wales is applied axially to the trench-jack or trench-brace, and
must be adequately supported so that it does not move out of position.

Construction Projects

O. Reg. 213/91

Part III EXCAVATIONS

Section 222

222. In this Part,

"engineered support system" means an excavation or trench shoring system, designed for a specific project or location, assembled in place and which cannot be moved as a unit;

"hydraulic support system" means a system capable of being moved as a unit, designed to resist the pressure from the walls of an excavation by applying a hydraulic counter-pressure through the struts;

"prefabricated support system" means a trench box, trench shield or similar structure, composed of members 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;

"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.

[O. Reg. 142/17, s. 25]

Section 224

224. No person shall enter or be permitted to enter an excavation that does not comply with this Part.

Section 225

225. Work shall not be performed in a trench unless another worker is working above ground in close proximity to the trench or to the means of access to it.

Section 226

226. (1) For the purposes of this Part, soil shall be classified as Type 1, 2, 3 or 4 in accordance with the descriptions set out in this section.

(2) Type 1 soil,

(a) is hard, very dense and only able to be penetrated with difficulty by a small sharp object;

(b) has a low natural moisture content and a high degree of internal strength;

(d) can be excavated only by mechanical equipment.

(3) 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

(4) Type 3 soil is,

(a) previously excavated soil; or

(b) soil that is stiff to firm or compact to loose in consistency and has one or more of the following characteristics:

(i) It exhibits signs of surface cracking.

(ii) It exhibits signs of water seepage.

(iii) If it is dry, it may run easily into a well-defined conical pile.

(iv) It has a low degree of internal strength.

(5) 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;

(d) is wet or muddy; and

(e) exerts substantial fluid pressure on its supporting system.

[O. Reg. 345/15, s. 24]

Section 227

227. (1) The type of soil in which an excavation is made shall be determined by visual and physical examination of the soil,

(a) at the walls of the excavation; and

(b) within a horizontal distance from each wall equal to the depth of the excavation measured away from the excavation.

(2) The soil in which an excavation is made shall be classified as the type described in section 226 that the soil most closely resembles.

(3) If an excavation contains more than one type of soil, the soil shall be classified as the type with the highest number as described in section 226 among the types present.

Section 228

228. (1) Before an excavation is begun,

(a) the employer excavating shall ensure that all gas, electrical and other services in and near the area to be excavated are located and marked;

(b) the employer and worker locating and marking the services described in clause (a) shall ensure that they are accurately located and marked; and

(2) If a service may pose a hazard and it cannot be shut off or disconnected, the owner of the service shall be requested to supervise the uncovering of the service during the excavation.

(3) Pipes, conduits and cables for gas, electrical and other services in an excavation shall be supported to prevent their failure or breakage.

[O. Reg. 443/09, s. 6]

Section 229

229. (1) If an excavation may affect the stability of an adjacent building or structure, the constructor shall take precautions to prevent damage to the adjacent building or structure.

(2) An engineer shall specify in writing the precautions required under subsection (1).

(3) Such precautions as the engineer specifies shall be taken.

[O. Reg. 375/22, s. 5]

Section 230

230. Every excavation that a worker may be required to enter shall be kept reasonably free of water.

Section 231

231. An excavation in which a worker may work shall have a clear work space of at least 450 millimetres between the wall of the excavation and any formwork or masonry or similar wall.

Section 232

232. (1) The walls of an excavation shall be stripped of loose rock or other material that may slide, roll or fall upon a worker.

(2) 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.

Section 233

233. (1) 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.

(2) 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.

(3) 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.

(4) 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 as described in clauses 234(2)(e), (f) and (g).

Section 234

234. (1) The walls of an excavation shall be supported by a support system that complies with sections 235, 236, 237, 238, 239 and 241.

(2) Subsection (1) does not apply with respect to an excavation,

(a) that is less than 1.2 metres deep;

(b) that no worker is required to enter;

(c) that is not a trench and with respect to which no worker is required to be closer to a wall than the height of the wall;

(d) that is cut in sound and stable rock;

(e) made in Type 1 or Type 2 soil and whose walls are sloped to 1.2 metres or less from its bottom with a slope having a minimum gradient of one vertical to one horizontal;

(f) made in Type 3 soil and whose walls are sloped from its bottom with a slope having a minimum gradient of one vertical to one horizontal;

(g) made in Type 4 soil and whose walls are sloped from its bottom with a slope having a minimum gradient of one vertical to three horizontal; or

(h) that is not a trench and is not made in Type 4 soil and with respect to which an engineer has given a written opinion that the walls of the excavation are sufficiently stable that no worker will be endangered if no support system is used.

(3) The opinion in clause (2)(h) shall include details of,

(a) the specific project and the location thereon;

(b) any specific condition for which the opinion applies; and

(4) The constructor shall keep on the project a copy of every opinion given by an engineer for the purpose of clause (2)(h) while the project is in progress.

(5) The engineer who gives an opinion described in clause (2)(h), or a competent worker designated by him or her, shall inspect the excavation to which the opinion relates as frequently as the opinion specifies.

[O. Reg. 142/17, s. 26; 375/22, s. 5]

Section 235

235. (1) Subject to subsection (2), a support system shall consist of,

(a) timbering and shoring that meets the requirements of subsection 238(2), if no hydrostatic pressure is present in the soil, and if the width and depth of the excavation are equal to or less than the width and depth indicated in the Table to section 238;

(b) a prefabricated support system that complies with sections 236 and 237;

(d) an engineered support system that complies with section 236.

(2) 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.

[O. Reg. 631/94, s. 7]

Section 236

236. (1) Every prefabricated, hydraulic or engineered support system shall be designed by an engineer.

(2) Every prefabricated, hydraulic or engineered support system shall be constructed, installed, used and maintained in accordance with its design drawings and specifications.

(3) The design drawings and specifications for a prefabricated, hydraulic or an engineered support system,

(a) shall indicate the size of the system and the type and grade of materials of which it is to be made;

(b) shall indicate the maximum depth and the types of soil for which it is designed;

(c) shall indicate the proper positioning of the system in the excavation, including the maximum allowable clearance between the walls of the support system and the walls of the excavation; and

(d) shall indicate how to install and remove the system.

(e) Repealed. [O. Reg. 85/04, s. 21]

(4) In addition to the requirements of subsection (3), the design drawings and specifications for a hydraulic support system,

(a) shall indicate the minimum working pressure required for the system; and

(b) shall require the use of a device to ensure the protection of workers if a loss of hydraulic pressure occurs in the system.

(5) Before a variation from the design drawings and specifications for a prefabricated, hydraulic or an engineered support system is permitted, the variation shall be approved in writing by an engineer.

(6) If the soil conditions on a project differ from those assumed by the engineer in designing a prefabricated, hydraulic or an engineered support system, an engineer shall modify the design drawings and specifications for the actual soil conditions or shall approve the support system for use in the actual soil conditions.

(7) The constructor shall keep the design drawings and specifications for a prefabricated, hydraulic or an engineered support system at a project while the system is on the project.

(8) Repealed. [O. Reg. 443/09, s. 7]

[O. Reg. 145/00, s. 32; 85/04, s. 21; 443/09, s. 7; 375/22, s. 5]

Section 237

237. (1) Subject to subsection (2),

(a) no prefabricated or hydraulic support system shall be used in type 4 soil;

(b) the space between the walls of a prefabricated support system and the walls of the excavation shall be restricted to the minimum clearance required for the forward progression of the support system; and

(2) A prefabricated or hydraulic support system may be used for repairing underground pipe breaks in the system,

(a) meets the requirements of section 236;

(b) has four side walls;

(d) is not used at a greater depth than 3.6 metres;

(e) is designed to resist all hydrostatic and earth pressures found in type 3 and type 4 soils;

(f) is installed so as to extend to the bottom of the excavation;

(g) is installed so that the walls of the system touch the walls of the excavation; and

(h) is not being pulled forward after being installed in the excavation.

(3) Before a support system is used as described in subsection (2), the constructor shall submit two copies of its design drawings and specifications to the office of the Ministry of Labour nearest to the project.

[O. Reg. 631/94, s. 8]

Section 238

238. (1) In this section,

"cleat" means a member of shoring that directly resists the downward movement of a wale or strut;

"o/c" means the maximum distance measured from the centre of one member of sheathing, wale or strut to the centre of the adjacent member of sheathing, wale or strut;

"post" means a vertical member of shoring that acts as a spacer between the wales;

"10 millimetres gap" means that the space between two adjacent members of sheathing is a maximum of ten millimetres.

(2) Timbering and shoring referred to in clause 235 (1) (a) for the walls of an excavation with a depth described in Column 1 of the Table to this section and located in a soil type described in Column 2 of the Table shall meet the corresponding specifications set out in Columns 3 to 8 of the Table.

(3) Every piece of sheathing referred to in the Table to this section shall be made of sound Number 1 Grade spruce and,

(a) shall be placed against the side of the excavation so that it is vertical;

(b) shall be secured in place by wales; and

(4) Every strut referred to in the Table to this section shall be made of sound Number 1 structural Grade spruce and,

(a) shall be placed in the excavation so that it is horizontal and at right angles to the wales;

(b) shall be cut to the proper length and held in place by at least two wedges driven between the strut and the wales; and

(c) shall be cleated with 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.

(5) Every wale referred to in the Table to this section shall be made of sound Number 1 structural Grade spruce and,

(a) shall be placed in the excavation so that it is parallel to the bottom, or proposed bottom, of the excavation; and

(b) shall be supported by either cleats secured to the sheathing or posts set on the wale next below it or, if it is the lowest wale, on the bottom of the excavation.

EXCAVATION SHORING AND TIMBERING

Item	Column 1	Column 2	Column 3	Column 4	Column 5	Column 6	Column 7	Column 8
	Excavation Depth	Soil Type	Sheathing	Strut width where width of excavation at strut location is 1.8 to 3.6 metres	Strut width where width of excavation at strut location is up to 1.8 metres	Vertical strut spacing	Horizontal strut spacing	Wales
1.	3.0 m or less	1	50 mm x 200 mm at 1.2 m o/c	200 mm x 200 mm	150 mm x 150 mm	1.2 m	*2.4 m	*200 mm x 200 mm
2.	3.0 m or less	2	50 mm x 200 mm at 1.2 m o/c	200 mm x 200 mm	150 mm x 150 mm	1.2 m	*2.4 m	*200 mm x 200 mm
3.	3.0 m or less	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.	3.0 m or less	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
5.	Over 3.0 m to 4.5 m	1	50 mm x 200 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
6.	Over 3.0 m to 4.5 m	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
7.	Over 3.0 m to 4.5 m	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
8.	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
9.	Over 3.0 m to 4.5 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
10.	Over 4.5 m to 6.0 m	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
11.	Over 4.5 m to 6.0 m	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

* Note: For excavations to 3 m deep in soil types 1 and 2, the wales can be omitted if the struts are used at 1.2 m horizontal spacings.

[O. Reg. 631/94, s. 9; 345/15, s. 25]

Section 239

239. (1) A support system for the walls of an excavation shall be installed,

(a) progressively in an excavation in Type 1, 2 or 3 soil; and

(b) in advance of an excavation in Type 4 soil, if practicable.

(2) A support system for the walls of an excavation shall provide continuous support for it.

(3) No support system for the walls of an excavation shall be removed until immediately before the excavation is backfilled.

(4) A competent person shall supervise the removal of a support system for the walls of an excavation.

Section 240

240. 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.

Section 241

241. (1) 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.

(2) 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.

(3) If the upper portion of the walls of an excavation are sloped for the soil types as described in clauses 234(2)(e), (f) and (g) and the lower portion of the walls are vertical or near vertical, the walls shall be supported by a support system which extends at least 0.5 metres above the vertical walls.

Section 242

242. (1) A metal trench-jack or trench-brace may be used in place of a timber strut,

(a) if the allowable working load of the trench-jack or trench- brace is equal to or greater than that of the timber strut; and

(b) if the size of the replaced timber strut is shown on the trench-jack or trench-brace.

(2) The allowable working load of a metal trench-jack or trench- brace shall be determined by an engineer in accordance with good engineering practice and shall be legibly cast or stamped on the trench-jack or trench-brace.

(3) No metal trench-jack or trench-brace shall be extended beyond the length used to establish its maximum allowable working load.

(4) Every metal trench-jack or trench-brace, when it is used,

(a) shall be placed against the wales in such a way that the load from the wales is applied axially to the trench-jack or trench-brace; and

(b) shall be adequately supported so that it does not move out of position.