Grade 10 building construction Building Construction Processes – Timbering (10 lessons) Notes

Specific Learning Outcomes

1. Identify materials for timbering.
2. Describe the use of different types of timbering.
3. Illustrate timbering methods for different soils.
4. Perform timbering to a foundation trench (practical skill).
5. Value the importance of timbering to foundation trenches (safety and quality).

Overview

Timbering is the temporary support of excavations using timber. It keeps trench walls from collapsing while workers dig and build foundations. In Kenya, timbering is often used on small to medium sites where metal sheet piles are not available or are expensive. This unit has 10 lessons mixing theory and practical work.

Materials & Tools for Timbering (Outcome a)

Common materials:

• Treated timber planks (boards/lining) — processed pine or cypress from local suppliers.
• Struts/props (solid timber beams) — for bracing between trench walls.
• Walings/joists — horizontal members to hold sheeting in place.
• Soldier piles (vertical posts) — heavy posts driven at intervals.
• Nails, coach bolts, wire and metal straps.
• Treated plywood sheets (where finer sheeting is needed).
• Preservatives (creosote, CCA) or water-repellent treatment—use legal, approved products.

Tools and safety equipment:

• Hammer, saw (hand or power), spirit level, tape measure, square.
• Crowbar, chisel, shovel, spade.
• Hard hats, gloves, gumboots, eye protection, high-visibility vest.
• Tarpaulin, dewatering pump (for waterlogged trenches).

Types of Timbering and Uses (Outcome b)

• Plank Sheeting (Close sheeting): Vertical timber boards placed tightly together and held by walers and struts. Good for sandy soils and where shallow trenches are required.
• Box (or Crib) Timbering: Interlocking timber forming a box around workers. Useful for deep pits and where walls must stay clear of workers.
• Soldier Piles & Lagging: Rows of vertical posts (soldiers) with horizontal lagging between. Used where partial excavation and open access are needed.
• Bracing with Struts: Placing struts between opposite walls to hold walers in position.
• Shores/Props: Simple diagonal braces for short excavations or temporary support.

Timbering Methods for Different Soils (Outcome c)

General rule: The more loose or water-bearing the soil, the closer and stronger the timbering must be.

• Cohesive soils (clay): Less likely to collapse suddenly. Sheeting with spaced walers and struts is usually enough. Provide drainage to avoid softening.
• Granular soils (sand, gravel): High risk of sudden collapse — use close sheeting or soldier piles with close lagging and strong struts. Dewater if possible.
• Silty soils: Behaves between clay and sand — treat like sand if water present. Close sheeting and dewatering recommended.
• Waterlogged soils/high water table: Provide dewatering pumps and use continuous sheeting; consider steel sheet piles if timber will not hold.
• Rock or very stiff ground: Timbering may be minimal — benching or no support if stable; always inspect rock faces for loose blocks.

10-Lesson Plan (Each lesson ~40–60 minutes)

Step-by-step: Timbering a Simple Foundation Trench (Practical guide — Outcome d)

1. Plan & mark the trench — Check depth, width and locate utilities. Mark edges with pegs and stringlines.
2. Put on PPE — Hard hats, gloves, boots and eye protection are essential.
3. Start excavation — Excavate to the required depth in stages, not letting sides overhang.
4. Inspect soil — If soil is sandy or wet, prepare for closer sheeting and dewatering.
5. Install soldier posts or place sheeting — Place vertical posts at required spacing or fix boards tightly as lining.
6. Fix walers horizontally — Bolt or nail walers to the posts at two or more levels depending on depth.
7. Place struts/props — Install struts between opposite walers to resist inward pressure.
8. Check level & alignment — Use spirit level and measure; ensure safe working width inside trench.
9. Continue excavation inside the protected area — Remove material in controlled steps to the final depth.
10. Maintain & monitor — Check timbering every day and after heavy rain; pump water if levels rise.
11. Remove timbering safely — Only remove when concrete has gained sufficient strength (follow engineer’s instructions), and remove in sequence from top to bottom while supporting surrounding ground.

Teacher note: For school practicals, use a small model trench (0.5–1.0 m deep) and supervised use of hand tools. Always have a first-aid kit on site.

Importance of Timbering to Foundation Trenches (Outcome e)

• Protects workers from trench collapse — saves lives and reduces injuries.
• Keeps trench shape and dimensions correct for accurate foundation work.
• Prevents material loss into the trench and reduces rework and delays.
• Helps control groundwater and reduces water entry when combined with pumps.
• Improves site efficiency — allows safe and faster construction of foundations.

Suggested Learning Experiences

• Site visit to a nearby construction site (with permission) to observe timbering in use. Kenyan context: observe small house foundations or road culvert works.
• Practical group activity: build a model timbered trench using scrap timber, plywood and nails; measure and test stability.
• Class demonstration: teacher-led dewatering and pumping demo using a small pump.
• Role-play safety scenarios (what to do if trench sides show cracks or water rises).
• Use short local videos or photos (from reputable sources) showing timbering methods used in Kenya.
• Project: students design a timbering plan for a small house trench and present material list and cost estimate (simple).

Assessment & Success Criteria

Use a mix of formative and summative assessment:

• Formative: observations during practicals, checks on PPE and safe work, short quizzes after lessons.
• Summative: practical test (timber a model trench), written test (identify materials, choose timbering for soil), and group project presentation.
• Performance criteria: correct spacing, safe bracing, correct use of tools, no unsafe practices, ability to explain why chosen method suits the soil.

Safety & Kenyan Considerations

• Always follow the school's safety policy and the site supervisor’s instructions.
• Use treated timber from licensed suppliers; do not use protected or illegal timber species.
• Dispose of treated timber responsibly — follow local environmental rules and avoid open burning of treated wood.
• During rainy seasons or near rivers in Kenya, increase frequency of inspections and dewatering plans.

Quick Practical Checklist (for students)

• Mark trench and check for services before digging.
• PPE on before starting work.
• Use correct timber size for depth (ask teacher/engineer).
• Place walers at correct height and fit struts securely.
• Check levels and spacing frequently.
• Never work under unsupported overhangs; stop work if walls crack or bulge.