Trenching & Excavation Safety

Table of Contents

1. Purpose & Scope
2. Legal Requirements
3. Key Definitions
4. Soil Classification
5. Sloping & Benching
6. Shoring & Trench Boxes
7. Daily Inspection Requirements
8. Underground Utilities & Ontario One Call
9. Support for Adjacent Structures
10. Water Management
11. HCMI Excavation Procedures
12. Pre-Entry Checklist

1. Purpose & Scope

Excavation and trenching are among the most hazardous operations on any construction site. Trench collapses can occur without warning and are frequently fatal — a single cubic metre of soil weighs approximately 1,400 kg (3,000 lbs), enough to crush and suffocate a worker in seconds. On HCMI church construction projects, excavation work is performed for foundations, footings, below-grade mechanical rooms, storm water management systems, underground services (water, sanitary, storm), and utility connections.

This guide applies to all HCMI employees and sub-trades performing excavation or trenching work, or working in proximity to open excavations on HCMI-managed projects.

2. Legal Requirements

Excavation and trenching on Ontario construction projects is governed by O. Reg. 213/91 (Construction Projects), Part III — Excavations:

• s.222-228 — General Requirements: Soil classification, protection of workers in excavations, support systems, and daily inspection obligations.
• s.229-232 — Soil Types and Slopes: Prescriptive sloping angles based on soil classification.
• s.233-237 — Shoring and Bracing: Requirements for engineered and prescriptive shoring systems.
• s.238-242 — Tunnels, Shafts, and Cofferdams: Additional requirements for specialized underground construction.
• OHSA s.25(2)(h): General duty to take every precaution reasonable in the circumstances for worker protection.
• Ontario Underground Infrastructure Notification System Act, 2012: Mandatory locate requests through Ontario One Call before any excavation.

3. Key Definitions

• Excavation: Any hole, trench, tunnel, or underground opening made by removing earth or rock.
• Trench: An excavation that is deeper than its width at the bottom (typically a narrow, linear excavation for services or footings).
• Competent Person: A person who is qualified because of knowledge, training, and experience to organize and perform the work, is familiar with the OHSA and regulations, and has knowledge of any potential or actual danger to health or safety in the workplace (OHSA s.1).
• Shoring: A structure such as a system of timber or hydraulic supports that prevents the collapse of the walls of an excavation.
• Trench Box (Trench Shield): A pre-fabricated steel or aluminum structure placed in a trench to protect workers from wall collapse. It does not prevent collapse but shields workers if one occurs.

4. Soil Classification

O. Reg. 213/91 s.226 requires that a competent person classify the soil before any worker enters an excavation. Soil type determines the allowable slope angle and shoring requirements. Ontario regulation recognizes four soil types:

Type 1 — Hard or Stiff (Cohesive)

Dense, hard clay or cemented soils (hardpan). Compressive strength of 100 kPa or greater. This is the most stable soil type. Allowable vertical cut depth before sloping is required: up to 3 metres under specific conditions.

Type 2 — Medium (Cohesive)

Firm to stiff clay, silty clay, or glacial till. Compressive strength between 50 and 100 kPa. This is the most common soil type encountered on HCMI church projects across southern Ontario. Requires sloping or shoring for excavations deeper than 1.2 metres.

Type 3 — Soft (Cohesive) or Loose (Granular)

Soft clay, loose sand, gravel, or mixed granular soils. Compressive strength below 50 kPa for cohesive soils, or non-cohesive granular soils. These soils are prone to slumping and running. Requires wider slopes or stronger shoring systems.

Type 4 — Saturated or Running

Any soil that is saturated with water or that runs or flows freely when unsupported. This includes sand below the water table, quick clay, and any soil experiencing active groundwater seepage. Type 4 soils are the most hazardous and require the most conservative protection measures. Dewatering is typically necessary before work can proceed.

5. Sloping & Benching

Sloping means cutting the excavation walls back at an angle to prevent collapse. O. Reg. 213/91 s.229-232 prescribes maximum slope angles based on soil type:

• Type 1: Maximum slope of 1 horizontal to 1 vertical (1:1) — a 45-degree angle
• Type 2: Maximum slope of 1:1 (45 degrees)
• Type 3: Maximum slope of 1:1 (45 degrees) for depths up to 3 metres; flatter slopes required for deeper excavations as determined by a professional engineer
• Type 4: Must be determined by a professional engineer based on site-specific conditions. As a practical minimum, slopes of 3:1 or flatter are common.

Benching involves cutting the excavation wall into a series of horizontal steps. Benching is only suitable for cohesive soils (Types 1 and 2) and must comply with the same overall slope angle requirements. Benching is not permitted in granular or saturated soils (Types 3 and 4).

For excavations deeper than 6 metres, O. Reg. 213/91 s.230 requires that the support system be designed by a professional engineer regardless of soil type.

6. Shoring & Trench Boxes

Timber Shoring

Traditional shoring using timber uprights (sheeting), walers, and cross-braces (struts). The dimensions and spacing of timber shoring components are prescribed in O. Reg. 213/91 Table 1 based on soil type and excavation depth. Timber must be construction-grade lumber, free of defects that would reduce its load-bearing capacity.

Hydraulic Shoring

Aluminum or steel hydraulic shores that can be installed and adjusted quickly. These are commonly used on HCMI projects for service trenches up to 4 metres deep. Hydraulic shores must be rated for the expected soil loads and installed per manufacturer specifications.

Trench Boxes (Shields)

Pre-fabricated steel or aluminum boxes that are placed in the trench by a crane or excavator. Trench boxes protect workers from wall collapse but do not prevent the collapse itself. Key requirements:

• The box must extend at least 300 mm (12 inches) above the surrounding grade to protect against surface material falling into the trench.
• Workers must never enter the unprotected space between the end of the trench box and the trench wall.
• When stacking trench boxes for deep excavations, they must be connected per manufacturer instructions.
• The trench box must be moved and repositioned using equipment — never by workers standing in the trench.

7. Daily Inspection Requirements

O. Reg. 213/91 s.228(2) requires that a competent person inspect the excavation walls, support systems, and adjacent areas before any worker enters and at least once during each shift. Additional inspections are required after:

• Any rainfall or thaw event
• Blasting or heavy equipment vibration in the area
• Any change in conditions such as cracking, bulging, or slumping of the walls
• Any change in groundwater conditions

The inspection must verify:

• Soil conditions have not deteriorated since the last classification
• Shoring and bracing are intact, properly positioned, and showing no signs of failure
• Trench boxes are in position and adequately protecting the work area
• Spoil piles are at least 1 metre (3 feet) back from the edge of the excavation
• Adequate means of egress (ladder, ramp, stairs) within 7.5 metres of every worker in the excavation
• No cracks or tension lines parallel to the excavation edge on the surrounding ground
• No undermining of adjacent structures, roadways, or utilities

8. Underground Utilities & Ontario One Call

Before any excavation on an HCMI project, underground utility locates are mandatory under the Ontario Underground Infrastructure Notification System Act, 2012.

1. Contact Ontario One Call at 1-800-400-2255 or online at ontarioonecall.ca at least 5 full business days before the planned excavation start date.
2. Receive and review locate markings. Utility companies will mark the approximate location of their underground infrastructure using colour-coded paint or flags (red = electric, yellow = gas, blue = water, green = sewer, orange = telecom).
3. Hand-expose utilities. O. Reg. 213/91 s.228(3) requires that mechanical excavation within 1 metre of a known underground utility is prohibited. Expose utilities by hand digging, hydrovac, or air knife before using mechanical equipment near them.
4. Protect exposed utilities. Once exposed, support utilities to prevent damage from settlement or vibration. Do not use exposed pipes or cables as anchor points for shoring or bracing.
5. Document and photograph. Record the location and depth of all exposed utilities. This information is critical for as-built drawings and future maintenance.

9. Support for Adjacent Structures

O. Reg. 213/91 s.227 requires that excavations near existing structures be planned to prevent undermining or damage to those structures. On HCMI church projects, this is particularly relevant when:

• Excavating for a new addition adjacent to an existing church building
• Working near existing parking lots, retaining walls, or roadways
• Performing foundation work close to neighbouring properties

If the excavation extends below the footing level of an adjacent structure, underpinning or other structural support designed by a professional engineer is required. The engineer's drawings and specifications must be on-site and followed precisely. Monitor adjacent structures for signs of settlement, cracking, or movement throughout the excavation process.

10. Water Management

Water is one of the most dangerous factors in excavation work. Water in a trench destabilizes soil, increases lateral pressure on shoring, and can create Type 4 (running) soil conditions.

• Surface water diversion: Grade the surrounding area to direct surface runoff away from the excavation. Use berms or diversion ditches as needed.
• Dewatering: Use sump pumps or well points to lower the water table below the working level. Discharged water must comply with local municipal bylaws and Ministry of the Environment permit requirements.
• Monitoring: Check water levels in the excavation multiple times per shift. Rapidly rising water levels require immediate evacuation.
• Frozen soil caution: Do not rely on frozen soil to support excavation walls. When frozen soil thaws, it can collapse suddenly with no warning. On HCMI winter projects, treat frozen soil as Type 3 or Type 4 for sloping and shoring purposes.

11. HCMI Excavation Procedures

1. Pre-Excavation Planning: The HCMI site superintendent must complete the HCMI Excavation Planning Checklist before any digging begins. This includes confirming Ontario One Call locates, reviewing soil reports, identifying adjacent structures, and selecting appropriate protection methods.
2. Competent Person Assignment: A designated competent person must be on-site at all times during excavation work. This person is responsible for soil classification, selecting protection methods, conducting inspections, and authorizing entry into the excavation.
3. Means of Egress: A ladder, ramp, or stairway must be provided within 7.5 metres (25 feet) of every worker in an excavation deeper than 1.2 metres. HCMI policy requires ladders to extend at least 1 metre above the top of the excavation.
4. Equipment Setback: Heavy equipment must maintain a safe distance from the edge of excavations. As a guideline, equipment should be set back at least as far as the excavation is deep, or as determined by the competent person based on soil conditions.
5. Spoil Pile Management: Excavated material must be placed at least 1 metre from the edge of the excavation. On HCMI sites, spoil piles adjacent to pedestrian or vehicle traffic areas must be barricaded.
6. Barricading: All open excavations on HCMI sites must be barricaded with rigid barriers (not caution tape alone) and marked with signage. Excavations left open overnight must have continuous barricading and, where public access is possible, flashing warning lights.
7. Atmospheric Monitoring: If the excavation is deep enough to constitute a confined space (generally deeper than 1.2 metres with limited access), atmospheric monitoring for oxygen deficiency and hazardous gases must be performed before entry.

12. Pre-Entry Checklist

Complete before any worker enters an excavation deeper than 1.2 metres:

• Ontario One Call locates confirmed and markings visible on-site
• Soil classified by a competent person — type recorded
• Protection system in place (sloping, shoring, or trench box) appropriate for soil type and depth
• Adjacent structures assessed — no risk of undermining
• Spoil pile set back at least 1 metre from excavation edge
• Means of egress (ladder/ramp) within 7.5 metres of every worker position
• Barricades and signage in place around the excavation
• Heavy equipment operating at safe distance from edges
• Water conditions assessed — dewatering in operation if needed
• No cracks, slumping, or bulging visible in excavation walls
• Weather forecast reviewed — plan for rain or freeze/thaw events
• Competent person inspection documented for current shift
• Workers briefed on emergency egress routes and procedures
• Atmospheric testing completed (if confined space conditions exist)