Soil Types – Understanding Your Ground Conditions
The Problem
Soil isn’t just dirt – it’s a complex and variable material that can either support an excavation or cause it to collapse without warning. Many excavation-related injuries and fatalities occur because soil conditions are underestimated. Just as you wouldn’t build a house without understanding its foundation, you shouldn’t work in a trench without knowing the soil type. Today, we’ll cover the importance of soil classification and how it relates directly to safety on site.
Identify the Hazards
- Soil Failure: Trench walls can collapse suddenly if the wrong protective system is used for the soil type.
- Changing Conditions: Soil stability can be affected by rain, vibrations, or water seepage—sometimes in just a few hours.
- Worker Exposure: Being in or near an unsupported or unstable excavation places workers at serious risk of entrapment or injury.
Safety Practices and Preventative Measures
Understand the Types of Soil
Soils are classified by stability:
- Stable Rock: Solid and unbroken. Rare and often weakened by excavation techniques.
- Type A (Most Stable): High clay content, cohesive, and hard to break.
Examples: Clay, silty clay, sandy clay.
Cannot be Type A if water, fissures, or prior disturbance are present. - Type B: Softer than Type A. May include previously disturbed soils and angular gravel.
- Type C (Least Stable): Loose materials such as sand, gravel, or soils with water present.
Competent Person’s Role
Only a Competent Person can classify soil. Their responsibilities include:
- Testing each layer or area of soil.
- Selecting the correct protective system (e.g., sloping, benching, shoring, or shielding).
- Monitoring for changing conditions such as water ingress or vibration.
- Making final decisions on whether work can proceed safely.
Adjust for Multiple Soil Types
- Sites often contain more than one soil type at different depths or areas.
- Each location must be individually tested and classified.
- Pretective measures must match the least stable soil type present.
Stay Alert for Changing Conditions
- Rain, heavy machinery, or water seepage can instantly destabilise soil.
- If you see any sign of change, report it to the Competent Person immediately.
Real-Life Case or Scenario
On a trenching job, soil was classified as Type B. After unexpected rainfall, water began to seep in, quickly turning conditions into Type C. Workers continued without reporting the change. A collapse occurred, burying one team member to the waist. Quick thinking and a rescue team prevented tragedy, but the incident could have been easily avoided if the soil had been reassessed after the rain.
Employee Responsibilities
- Understand the basics of soil types and why classification matters.
- Follow the Competent Person’s instructions regarding trench protection.
- Stay alert for changes in soil or water conditions.
- Report any hazards immediately.
Employer Responsibilities
- Appoint a Competent Person to classify and monitor soil conditions.
- Provide the correct protective systems for each trench (e.g., sloping, shoring, shielding).
- Train all workers in excavation safety and soil hazard awareness.
Discussion & Questions
- “What types of soil do we usually see on our sites?”
- “How do weather events like heavy rain affect trench safety?”
- “Who is the Competent Person on this site, and how do we get in touch with them?”
- “What steps should we take if we spot water seepage or soil shifting in a trench?”
Encourage team members to share experiences and observations.\
Commitment/Action Plan
Starting today, let’s commit to monitoring soil conditions constantly. If you see any change in soil stability, don’t ignore it—report it to the Competent Person. And remember: never enter an unstable or unsupported trench, no matter how quick the job seems.
Wrap-Up
Soil conditions can change in an instant—and understanding them is key to keeping everyone safe. By staying informed, following the Competent Person’s guidance, and reporting changes early, we can prevent trench collapses and protect our team. Thanks for staying alert and making safety a priority.
Standards Overview
Under the Health and Safety at Work Act 2015 (HSWA) and WorkSafe NZ excavation safety guidance:
- A Competent Person must assess soil conditions and determine the appropriate trench protection.
- Protective systems such as sloping, benching, shoring, or shielding must be used based on soil stability.
- Soil must be reassessed when conditions change (e.g., rain, water seepage, nearby machinery).
Employers must ensure workers are trained and protective systems are in place and maintained.
Disclaimer
These toolbox talks are provided for informational and educational purposes only and are not a substitute for professional legal, health and safety, or regulatory advice.
The content reflects recognised best practices and general industry standards as at the date of publication. However, it may not be applicable to all workplace environments or specific operational contexts.
It is the responsibility of the Person Conducting a Business or Undertaking (PCBU), as well as officers, workers, and other duty holders under the Health and Safety at Work Act 2015 (HSWA), to ensure compliance with all relevant laws, regulations, and WorkSafe New Zealand guidance.
The authors, presenters, and distributors of this toolbox talk disclaim all liability for any loss, damage, or injury arising from the use or reliance on the information provided. Users should seek advice from a suitably qualified health and safety professional or legal advisor for guidance tailored to their specific circumstances.
Always follow your organisation’s policies, procedures, and safe work practices to manage risks and maintain a safe and healthy workplace.
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