Excavation, Trenching, and Shoring

Chapter Introduction

Excavation, trenching, and shoring are among the most dangerous areas of the Construction Craft Worker trade. In Canada, trench collapses cause serious injuries and preventable deaths every year. This chapter covers all the theoretical and practical knowledge required for the Red Seal exam, including soil classification, slope angles, lateral pressure calculations, shoring systems, and federal regulatory requirements.

Fundamental Definitions

Excavation

An excavation is any hole, cavity, trench, or depression created by the removal of earth, rock, or other materials. Distinctions include:

Open excavation: large surface area, generally for foundations or infrastructure.

Trench: narrow excavation where the depth exceeds the width, with a maximum width of 4.5 metres.

Shaft: vertical excavation of circular or rectangular cross-section, with depth significant relative to its cross-section.

Trench

A trench is defined as an excavation where the depth is greater than the width, and the width does not exceed 4.5 metres. This distinction is crucial because the protection methods differ depending on whether it is a trench or an open excavation.

Shoring and Bracing

Shoring is the set of temporary supports (braces, planks, jacks) installed to prevent the collapse of excavation walls. Bracing specifically refers to the horizontal elements that hold the walls in place. Shielding is a prefabricated system that protects workers without necessarily supporting the walls.

Soil Classification According to the Canada Occupational Health and Safety Regulations

Classification System (Types I to IV)

The Canada Occupational Health and Safety Regulations (COHSR) classify soils into four types based on their cohesion and stability:

Soil Type	Characteristics	Unconfined Compressive Strength (kPa)	Natural Angle of Repose
Type I	Stable rock, very resistant cohesive soil	> 200 kPa	90° (vertical)
Type II	Moderately resistant cohesive soil, stiff clay	100–200 kPa	53° (3/4:1)
Type III	Cohesive soil with low resistance, dense sand, gravel	50–100 kPa	45° (1:1)
Type IV	Loose granular soil, loose sand, saturated soil	< 50 kPa	34° (1.5:1)

Soil Identification Methods

Visual and manual identification is essential. For each soil type, you must know:

Ball test: press a handful of moist soil in your hand. If it holds its shape without cracking → cohesive soil (Type II or better).

Ribbon test: roll a moist soil sample into a thread. A 2.5 cm ribbon without breaking indicates plastic clay (Type II).

Penetration test: push your thumb into a sample. Easy penetration → Type III or IV.

Shake test: place a ball of saturated soil in your palm and shake. Water appearing on the surface → granular soil (Type IV).

Factors Modifying Classification

Classification must be adjusted based on:

Water content: saturated soil can lose up to 50% of its strength.

Vibrations: heavy machinery traffic, blasting, compaction.

Adjacent loads: buildings, walls, equipment.

Site history: disturbed soils, previous excavations, cracked pipelines.

Slope Angles and Safe Slopes

Principle of the Natural Angle of Repose

The natural angle of repose is the maximum angle at which soil remains stable without support. This angle varies depending on soil type and moisture condition. For excavations less than 1.2 metres deep, no sloping is required if the soil is stable.

Table of Maximum Allowable Slopes

Soil Type	Horizontal:Vertical Slope	Angle from Horizontal
Type I (rock)	Vertical (0:1)	90°
Type II (stiff clay)	0.75:1	53°
Type III (dense sand)	1:1	45°
Type IV (loose soil)	1.5:1	34°

Calculating the Width of the Work Area

For a trench of depth H and a slope angle of x:1, the additional width on each side is H × x.

Example: Trench 3 metres deep in Type III soil (slope 1:1).

Additional width per side: 3 m × 1 = 3 metres.

Total width at the top = width at the bottom + 2 × 3 m = width at the bottom + 6 metres.

The 1.2-Metre Rule

Any excavation more than 1.2 metres deep must be protected by sloping, shoring, or shielding, unless it is entirely in stable rock. This rule applies to all excavations where workers must enter.

Lateral Earth Pressures

Rankine Theory

The lateral pressure exerted by soil on a shoring system is calculated according to Rankine theory:

P = K × γ × H

Where:

P = total lateral pressure (kN/m²)

K = active earth pressure coefficient

γ = unit weight of soil (kN/m³)

H = depth of excavation (m)

Pressure Coefficients

Soil Type	Unit Weight γ (kN/m³)	Coefficient K
Dry sand	16–18	0.33
Saturated sand	19–21	0.50
Stiff clay	18–20	0.50–0.70
Gravel	17–19	0.30
Fill material	15–17	0.60

Calculation Example

Given: 4-metre trench in dry sand (γ = 17 kN/m³, K = 0.33).

P = 0.33 × 17 × 4 = 22.44 kN/m²

This pressure is the lateral pressure at the bottom of the trench. The pressure varies linearly from zero at the surface to the maximum value at the bottom. The resultant (total force) is applied at one-third of the height from the bottom:

F = (P × H) / 2 = (22.44 × 4) / 2 = 44.88 kN per linear metre of wall

Safety Factor

Shoring systems must be designed with a minimum safety factor of 2.0 relative to the theoretical calculated pressure. This factor accounts for uncertainties in soil classification, vibrations, and variations in water content.

Shoring and Shielding Systems

Braces and Struts

Braces are horizontal elements (generally wood or steel) that bear against vertical planks to hold the walls in place. They are spaced vertically and horizontally according to depth and soil type.

Maximum brace spacing by depth:

Trench Depth	Maximum Vertical Spacing	Maximum Horizontal Spacing
1.2 to 3 m	1.2 m	1.8 m
3 to 4.5 m	1.0 m	1.5 m
4.5 to 6 m	0.9 m	1.2 m

Support Planks (Timber)

Vertical planks (or timbers) are placed against the walls and held in place by braces. Their minimum thickness depends on depth:

Up to 3 m: 50 mm thick planks.

3 to 4.5 m: 64 mm thick planks.

Over 4.5 m: 75 mm or thicker planks, or an engineered system.

Prefabricated Shielding (Trench Boxes)

Trench boxes are prefabricated metal structures, installed by crane or excavator. They protect workers but do not necessarily support the walls. Key points:

The box must extend at least 0.3 metres above the adjacent ground level.

The gap between the box and the wall must be backfilled to prevent material falls.

The box must never be used as a work platform.

The maximum depth of use is specified by the manufacturer and must not be exceeded.

Hydraulic Systems

Hydraulic braces are quick-acting jacks that adjust to the trench width. They are installed from the surface, eliminating the need to descend into the trench for installation. Advantages:

Fast and safe installation.

Precise pressure adjustment.

Reusable on multiple job sites.

Access Ladders

Any trench more than 1.2 metres deep must have a ladder or other safe means of access, located within 8 metres of any worker. The ladder must:

Extend at least 1 metre above the edge of the trench.

Be securely fastened.

Be inspected before each use.

Inspection and Monitoring Procedures

Inspection Before Each Work Shift

A competent supervisor must inspect the excavation before each work shift and after any event that could affect its stability (rain, frost, vibration, blasting). The inspection covers:

90.Cracks in the walls or adjacent ground.

91.Water seepage or water accumulation at the bottom.

92.Settlement of the excavation edge.

93.Condition of braces: deformation, looseness, corrosion.

94.Condition of planks: cracks, rot, splitting.

95.Access: ladders in place and secure.

96.Spoil pile area: excavated materials more than 1 metre from the edge.

Continuous Monitoring

During work, the supervisor must:

Observe signs of soil movement (progressive cracking, bulging).

Check for the absence of infiltration water.

Ensure that loads (excavator, trucks) remain at a distance.

Evacuate the trench immediately if any sign of instability appears.

Inspection Records

Inspection results must be documented in writing and kept on site. The record must include:

Date and time of inspection.

Name of the inspector.

Weather conditions.

Findings and corrective measures.

Supervisor's signature.

Specific Safety Rules

Spoil Pile Area (Berm)

Excavated materials, tools, and equipment must be placed at least 1 metre from the edge of the excavation. This distance is measured horizontally from the top edge. For deep excavations (> 4.5 m), this distance must be increased to 1.5 metres.

Access and Movement

Workers must never cross a trench on unsecured planks.

Walkways with guardrails are required if crossing is necessary.

Heavy machinery must remain at least 1 metre from the edge, or wheel stops must be installed.

Hazardous Atmospheres

Deep trenches (> 1.2 m) in fill areas or near gas pipelines may contain hazardous atmospheres. Requirements:

Atmospheric testing before entry if there is a risk of gas (H₂S, CO, CH₄) or oxygen deficiency (O₂ < 19.5% or > 23%).

Forced ventilation if necessary.

Appropriate respiratory protection.

Surface supervisor with a means of communication.

Working Near Utilities

Before any excavation, it is mandatory to:

125.Locate utilities (call the location service: "Call Before You Dig").

126.Mark the position of lines with standardized colour codes.

127.Hand-excavate within 600 mm of an identified line.

128.Support exposed lines to prevent breakage.

Practical Calculations for the Exam

Calculating Excavation Volume

Volume = (Width at bottom + Width at top) / 2 × Depth × Length

Example: Trench 1 m wide at the bottom, 3 m deep, 1:1 slope, 20 m long.

Width at top = 1 + 2 × (3 × 1) = 7 m.

Volume = (1 + 7) / 2 × 3 × 20 = 4 × 3 × 20 = 240 m³.

Calculating Bulking

Bulking is the increase in volume of excavated soil relative to its in-place volume. It varies by soil type:

Soil Type	Bulking (%)	Settlement After Compaction (%)
Sand	10–15	5–10
Gravel	12–18	8–12
Clay	20–30	10–15
Rock	30–50	15–25
Topsoil	25–30	10–15

Example: Excavation of 100 m³ of clay (25% bulking).

Bulked volume = 100 × 1.25 = 125 m³.

Volume after compaction = 100 × 1.10 = 110 m³ (10% settlement).

Calculating Brace Load Capacity

The load on a brace is calculated as follows:

Load = Lateral pressure × Vertical spacing × Horizontal spacing

Example: Lateral pressure of 22 kN/m², braces spaced 1.2 m vertically and 1.8 m horizontally.

Load = 22 × 1.2 × 1.8 = 47.5 kN.

With a safety factor of 2: required capacity = 95 kN.

Canadian Regulatory Requirements

Canada Occupational Health and Safety Regulations (COHSR)

The COHSR, adopted under the Canada Labour Code, applies to work sites under federal jurisdiction. Relevant sections include:

Section 13.2: Definition of excavation and trench.

Section 13.3: Obligation to slope, shore, or shield any excavation over 1.2 m.

Section 13.4: Inspection by a competent person before each work shift.

Section 13.5: Minimum distance of 1 m for excavated materials.

Section 13.6: Access requirements (ladders within 8 m).

Section 13.7: Protection against falling materials.

Section 13.8: Hazardous atmospheres and ventilation.

Applicable CSA Standards

CSA Z1006: Management of health and safety in construction work sites.

CSA S269.2: Fall protection equipment.

CSA Z150: Safety of mobile cranes (applicable to trench box installation).

Construction Safety Code (CSTC)

The Construction Safety Code (part of the COHSR) specifies detailed technical requirements for excavations, including brace spacing tables and minimum plank thicknesses.

Common Pitfalls to Avoid

164.Confusing soil types: Type I is stable rock, not hard clay. Type IV is the most unstable soil, not the strongest.

165.Forgetting the safety factor of 2: Pressure calculations must be multiplied by 2 to size the braces.

166.Neglecting the effect of water: Saturated soil can lose 50% of its strength. Water must be controlled by pumping or drainage before entry.

167.Ignoring the 1-metre distance: Excavated materials must be at least 1 metre from the edge, not 0.5 metres.

168.Using the trench box as a ladder: Shielding protects but does not support the walls. Never climb on a trench box.

169.Forgetting the ladder: Any trench over 1.2 m must have a ladder within 8 metres of each worker.

170.Confusing bulking and settlement: Bulking increases volume, settlement decreases it. The percentages are not interchangeable.

171.Hand-excavating within 600 mm of a utility: The safe distance is 600 mm, not 300 mm.

172.Not inspecting after rain: An inspection is mandatory after any event that could affect stability.

173.Forgetting the inspection record: Inspections must be documented, not just performed verbally.

Summary

A trench is a narrow excavation (width ≤ 4.5 m) where the depth exceeds the width.

Soils are classified into four types (I to IV) based on strength and cohesion.

Any excavation over 1.2 metres must be sloped, shored, or shielded.

Maximum slopes range from vertical (Type I) to 1.5:1 (Type IV).

Lateral pressure is calculated using the formula P = K × γ × H, with a safety factor of 2.

Braces are spaced according to depth: 1.2 m vertically for trenches 1.2 to 3 m deep.

Excavated materials must be 1 metre from the edge.

A ladder must be available within 8 metres of any worker.

Inspection by a competent supervisor is mandatory before each work shift and after any major event.

Utilities must be located before excavation, with hand-excavation within 600 mm.

The COHSR (Sections 13.2 to 13.8) governs all these requirements at the federal level.

Volume calculations must account for bulking (increase) and settlement (decrease).

Exam Tips

Memorize the soil type table and their corresponding slope angles.

Redo the example calculations several times until the method becomes automatic.

Pay special attention to questions about distances (1 m, 1.2 m, 600 mm, 8 m).

For classification questions, first identify whether the soil is cohesive or granular, then its strength.

Questions about inspections often focus on frequency (before each shift, after rain) and qualification (competent person).

When in doubt on a safety question, choose the most cautious answer — the exam always favours maximum safety.

Excavation, Trenching, and Shoring