Grade 10 building construction Building Construction Processes – Foundations Notes

1. Describe types of foundations used in building construction.
2. Set out a strip foundation from working drawings.
3. Prepare trenches for construction of a strip foundation.
4. Lay a strip foundation for a building.
5. Acknowledge the importance of foundations in a building.

1. What is a foundation?

A foundation is the lowest part of a building that transfers the load of the structure safely to the ground. It prevents settlement, gives stability and spreads loads to a wide area of soil.

2. Types of foundations (simple explanation)

• Strip foundation — continuous concrete footing under loadbearing walls (very common for houses and small buildings).
• Pad (isolated) foundation — square or rectangular pads under single columns.
• Raft (mat) foundation — a large slab covering whole building footprint (used when soil is weak).
• Pile foundation — long columns (piles) driven or bored into deep strong soil (used where topsoil is too weak).
• Stepped foundation — used on sloping ground to keep levels consistent.

(In Kenya small houses commonly use strip foundations on firm soils; in expansive soils or near water a structural engineer may recommend raft or piles.)

3. Strip foundation — simple cross-section (visual)

Note: Exact width and depth are given by the structural engineer or working drawings according to load and soil strength.

4. Setting out a strip foundation from working drawings (step-by-step)

1. Read the working drawing: note foundation lines, dimensions, scale (e.g., 1:100) and reference points (grid lines).
2. Transfer dimensions to site: choose a fixed reference (corner, boundary peg, grid line) and measure using tapes with the drawing scale.
3. Place batter boards or pegs beyond the foundation edges to keep lines clear.
4. Stretch mason's line between pegs to mark the outside edges of the strip foundation.
5. Check squareness using the 3-4-5 method (or measure diagonals) to ensure corners are right angles.
6. Mark the centreline of walls if required, and transfer levels using a spirit level or dumpy level to get correct top of foundation level.
7. Confirm all measurements with the supervisor/teacher before excavation begins.

Tip: Always keep a copy of the working drawing on site and confirm dimensions with the engineer for any doubt.

5. Preparing trenches for a strip foundation

1. Clear topsoil, vegetation and rubbish from the strip area.
2. Excavate to the required depth and width shown on the drawing. Width must allow for the footing and working space.
3. Observe safe slopes or use shoring if trenches are deep — do not let sides collapse. Typical safe slope depends on soil – ask a teacher/engineer.
4. Remove loose material and soften spots; if soft, dig deeper until firm bearing is reached or consult engineer.
5. Level and compact the trench bottom. If required, lay 50–100 mm blinding concrete (lean mix) to provide a clean working surface.
6. Ensure trenches keep away from drains, trees and service lines, and that neighbours' foundations are not undermined.

Safety: Wear boots, gloves and helmet. Keep people away from trench edges and do not allow children near deep excavations.

6. Laying a strip foundation (simple practical guide)

1. Check the trench: line, width, level and compacted bottom must match drawings.
2. Place blinding concrete if required (thin lean concrete to prevent contamination of main concrete).
3. If required by design, place reinforcement bars (steel) on chairs to keep them the correct distance above the blinding.
4. Build simple formwork (shuttering) if the design needs neat sides; otherwise trim edges with timber or batter boards.
5. Prepare and mix concrete: use the mix specified by the engineer. A commonly used nominal mix for small works is 1:2:4 (cement:sand:coarse aggregate) — but always follow the design instructions.
6. Pour concrete into the trench evenly, avoid cold joints; compact by tamping or using a poker to eliminate air pockets.
7. Finish the top surface to the correct level and allow to set.
8. Cure the concrete by keeping it moist for at least 7 days (longer is better — 28 days gives full strength). Protect from sun and heavy rain.

Quality checks: Check dimension, level, reinforcement position, and ensure no contamination (soil, water) during pouring.

7. Importance of foundations (short points)

• Transfer building loads safely to the ground to prevent excessive settlement.
• Provide stability against wind and lateral forces.
• Prevent differential settlement that can crack walls and floors.
• Protect the structure from soil movement (e.g., swelling/shrinking soils).
• Ensure durability — a good foundation keeps the building safe for many years.

8. Simple checklist for practical class (teacher/learner)

• Have the working drawing and measurements available.
• Set out lines and check squareness before digging.
• Excavate to correct depth and compact bottom.
• Place blinding, reinforcement and formwork as required.
• Use correct concrete mix and compact properly.
• Cure concrete and record the start and finish of curing.
• Observe safety and keep site tidy.

9. Suggested learning experiences (activities suitable for 15-year-olds in Kenya)

1. Classroom: Read simple working drawings of a small house. Identify foundation lines and write down dimensions.
2. Field practical (school compound): Set out a mock strip foundation using pegs and mason's line (use stakes and string, no deep digging). Practice squareness using the 3-4-5 method.
3. Excavation demo: Teacher demonstrates safe shallow trench excavation, clearing topsoil, and measuring depth (students observe and take notes).
4. Mixing and pouring demo: Small-scale demonstration with a shallow mock trench or mould. Students mix small quantities of concrete and pour into a mould to see compaction and finishing.
5. Site visit: Short visit to a nearby small house construction site to see foundations being laid. Prepare checklist for what to observe (PPE, set-out, reinforcement, compaction, curing).
6. Group project: In groups, draw a simple plan and calculate lengths of strip foundation needed; present findings to class.
7. Safety and environmental activity: Discuss how to protect trees, avoid soil erosion and dispose of waste responsibly on site.

Assessment ideas: Short quiz on types of foundations; practical mark for correct set-out; peer assessment during group project.

10. Short glossary

Working drawing

A detailed drawing showing dimensions and instructions for construction.

Blinding concrete

A thin, lean concrete layer to create a clean, level surface before reinforcement and main concrete.

Reinforcement

Steel bars used to strengthen concrete against tension (if required by design).

Squareness (3-4-5)

A simple method using lengths in a 3:4:5 ratio to form right angles when setting out.