Exercise 27. Look at the pictures that show surveyors of different historical periods. Compare and contrast these pictures.
Fig. 9. Surveying in 1950’s
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Fig.10. Modern Surveying
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Text C
Exercise 28. Read the text below. Match each part of the text (A–C) with the correct heading (1–3).
1. Excavation
2. Site Clearance
3. Setting out
EXCAVATION
| A. _____________________ Before the excavation for the proposed foundation is started, the site shall be cleared of vegetation, brushwood, stumps of trees etc. Roots of the trees shall be removed to at least 30 cm below the foundation level. The pits formed due to roots of trees, old foundations etc. shall be filled up with soil and compacted. |
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B. ______________________
A bench mark shall be established at the site by a masonry pillar and connected to the nearest standard bench mark. Levels of the site should be taken at 5 to 10 m intervals depending on the terrain and the importance of the building. The centre lines of the walls are marked by stretching strings across wooden pegs driven at the ends. The centre lines of the perpendicular walls are marked by setting out the right angle with steel tapes or preferably with a theodolite. The setting out of walls shall be facilitated by having a permanent row of pillars (not less than 25 cm side) parallel to aid at a suitable distance beyond the periphery of the building so that they do not foul with the excavation. The pillars shall be located at the junctions of the cross walls and the external wall and shall be bedded sufficiently deep so that they are not disturbed during excavation for foundation. The centre lines of the walls shall be extended and marked on the plastered tops of the pillars. The tops of the pillars may be kept at the same level, preferably the plinth level. In rectangular or square settings, the diagonals shall be checked to ensure accuracy of setting out.
C. __________________________
For small buildings, excavation is carried out manually by means of pick axes, crow bars. spades etc. In case of large buildings and deep excavation, mechanical earth cutting equipment can be used. For hard soils when the depth of excavation is less than 1.5 m, the sides of the trench do not need any external support. If the soil is loose or the excavation is deeper, some sort of shoring is required to support the sides from falling. Planking and strutting can be intermittent or continuous depending on the nature of soil and the depth of excavation. In the case of intermittent or “open” planking and strutting the entire sides of trenches are not covered. Vertical boards (known as poling boards) of size 250 x 40 mm of the required length can be placed with gaps of about 50 cm. These shall be kept apart by horizontal waling of strong timber section 100 x 100 mm at a minimum spacing of 1.2 m and strutted by a cross piece of 100 x 100 square or 100 mm diameter. In the case of soft soils continuous or “close” planking is adopted and the vertical boards are kept touching each other without any gap as shown.
Exercise 29. Is it true or false?
1. Before the excavation for the proposed foundation is started, the site shall be cleared of the ruins of other buildings.
2. Levels of the site should be taken at 20 m intervals depending on the terrain and the importance of the building.
3. The centre lines of the perpendicular walls are marked with steel tapes or preferably with a theodolite.
4. The pillars shall be located at the cross-sections of the cross walls.
5. The centre lines of the walls shall be reduced on the plastered tops of the pillars.
6. In rectangular or square settings, the diagonals shall be checked to ensure accuracy of setting out.