3.1 State TWO methods that are used to prevent inhaling dangerous fumes in a manhole - NSC Civil Technology Civil Services - Question 3 - 2021 - Paper 1

1. The slope of the trench: It is crucial to determine the slope to ensure proper drainage and flow of sewage.
2. Invert levels: These must be correctly set out and verified to ensure that the pipes are at the appropriate depth and slope.

The member is called a strut, which helps maintain space between the poling boards, ensuring stability during excavation.

The type of manhole shown in FIGURE 3.4 is a concrete ring manhole.

One necessary accessory is a manhole cover.

1. Excavate the hole: Begin by excavating the site, ensuring it is at least 300 mm wider than the manhole.
2. Allow for depth: Ensure there is extra depth for the thickness of the base, below the invert level of the drain.
3. Cast the concrete base: Once the excavation is complete, pour and set the concrete to form the base of the manhole.

The slope of cuttings can be vertical if the ground conditions are sound, specifically if the soil is intact rock.

1. Steepness of slopes: The slope of cuttings in excavations shall not be steeper than 1:2 to prevent collapsing.
2. Water control: The excavation must be sloped so that no water ponds within 3 m of the toe of the cutting.

1. Determine the area of walls: The centre line length is 6.22 m, and the height is 1.2 m which gives an area of approximately 7.46 m².
2. Calculate bricks per area: At 50 bricks per m², the total required is: $ext{Total bricks} = ext{Area} imes ext{Bricks per m²} = 7.46 imes 50 = 373$ Thus, we would need approximately 373 bricks to construct the manhole.