Site Preparation and Setting Out – JSS2 Basic Technology Lesson Note (Third Term)

Site preparation and setting out is the process of clearing a building site and marking the position of the walls and the width of the excavation on the ground before digging begins.

Behavioural objectives

By the end of the lesson, students should be able to:

1. explain what site preparation and setting out means;
2. list the tools used for site preparation and setting out;
3. describe how the centre lines and corner points are set out;
4. explain why the centre points are marked outside the excavation;
5. describe how the width of the excavation is marked on the ground.

What is site preparation?

Site preparation is the work done to make a piece of land ready for building.

• Clearing: remove bush, trees, stumps, rubbish and loose stones from the plot.
• Removing topsoil: scrape off the loose, plant-filled top layer from the building area.
• Levelling: even out high and low spots so that measuring is easier.
• Checking the plan: confirm the plot boundaries and the building position shown on the drawing.

What is setting out?

Setting out is the transfer of the building plan from paper to the ground using pegs, lines and measurements.

• It fixes the exact position of every wall.
• It makes sure the corners are square (at right angles).
• It shows the workers where to dig the trenches.

Key terms in setting out

What tools are used for site preparation and setting out?

The tools are used to clear the land, measure distances, mark points and show lines clearly.

• Cutlass and hoe: clear bush and loosen the soil.
• Shovel and spade: remove soil and cut a furrow along marked lines.
• Measuring tape: measures lengths and diagonals.
• Pegs: mark corner points and centre points.
• Hammer or mallet: drives the pegs into the ground.
• Nails: fixed on top of pegs to show the exact point.
• Thread or rope (line): stretched between pegs to show straight lines.
• Lime: a white powder used to mark lines on the ground.
• Builder’s square: checks that a corner is a right angle.

How is a building site prepared and set out?

A site is set out by fixing the centre lines and corner points, checking them, then marking the excavation width.

1. Clear and level the site.
2. Calculate the centre line of the walls from the plan.
3. Set out the centre lines of the rooms by making perpendiculars in the ratio 3:4:5.
4. Mark the corner points (for example a, b, c, d, e, f and g) with pegs. Fix a nail on top of each peg.
5. Check the corner points by measuring the diagonals (for example ac, bd, cf and eg). Diagonals of a true rectangle are equal.
6. Extend the centre lines and mark the centre points about 2 m outside the edge of the excavation. These points are called A1, A2, B1, B2 and so on.
7. Fix sit-out pegs at these outside points.
8. Stretch thread or rope between the pegs to show the centre lines clearly.
9. Mark the width of the excavation from the plan, using thread and pegs at the right positions.
10. Mark the excavation width on the ground with lime or with a furrow cut with spades.

What is the 3:4:5 method?

The 3:4:5 method is a way of making a right angle on the ground using a triangle with sides in the ratio 3:4:5.

1. Fix a peg at the corner where the right angle is needed.
2. Measure 3 units along one line from the corner and mark it.
3. Measure 4 units along the other line from the corner and mark it.
4. Measure the distance between the two marks.
5. If the distance is exactly 5 units, the corner is a right angle.
6. If it is not 5 units, adjust the second line until it is.

• Units: the units can be metres, or multiples such as 6, 8 and 10 m for a bigger building.

Why are the centre points marked outside the excavation?

The centre points are marked about 2 m outside the excavation so that the marks are not lost during digging.

• Digging removes pegs: the original points a, b, c, d, e, f and g may be dug away.
• Points can be restored: the workers can stretch a line between the outside points and find the lost centre points again.
• Checking during work: the lines can be used to check the trench and the walls as work goes on.

How is the width of the excavation marked on the ground?

The width of the excavation is marked by taking the width from the plan and showing its edges with thread, pegs and lime or a furrow.

1. Read the trench width from the plan details.
2. Measure out from the centre line to mark the trench edges, usually half the width on each side.
3. Drive pegs at the right positions and stretch thread between them.
4. Sprinkle lime along the thread, or cut a furrow with spades.
5. Remove the thread before digging starts, since the outside points still hold the centre lines.

What if the plan is complicated?

For a zigzag plan, the centre pegs are placed at suitable positions chosen according to site conditions.

• Zigzag walls: use more pegs, so each bend in the wall has its own marked point.
• Uneven ground or obstacles: place pegs where they will not be disturbed, such as away from trees and paths.

Evaluation

1. What is site preparation?
2. List 5 tools used for site preparation and setting out.
3. Why are the centre points marked about 2 m away from the outer edge of the excavation?
4. How does the 3:4:5 method make a right angle on the ground?

Assignment

Explain how the width of the excavation is marked on the ground.