Timbering of excavation Q1

(a) When contractors are given permission to start construction work, site works will begin to be implemented.

Briefly explain the following site work operations:

(i) Clearing the site
(ii) Setting out the building
(iii) Establishing a datum level (12 marks)

(b) With the use of diagrams, explain the method of timbering of excavation for the following nature of soil:

(i) Firm soil
(ii) Moderately firm soil
(iii) Loose soil (13 marks)

(25 marks, 2016 Q1)

A.
a)
(i) Clearing the site

For construction, site clearing is mainly land clearing for erecting building later on.

Land clearing is the process of removing trees, stumps, brush, stones and other obstacles from an area as required to increase the size of the land base to provide land for building construction. The newly cleared land should be ready for setting up parameter for building plan and eventually piling works.

(ii) Setting out the building

A building is set out in order to clearly define the outline of the excavation and the centre line of the walls, so that construction can be carried out exactly according to the plan. The centre line method of setting out is generally preferred and adopted.

1. From the plan (fig 1), the centre line of the walls are calculated. Then the centre lines of the rooms are set out by setting perpendiculars in the ratio 3:4:5. Suppose the corner points are a, b, c, d, e, f and g which are marked by pegs with nails on top.

2. The setting of the corner point is checked according to diagonals ac, bd, cf and eg.

3. During excavation, the centre points a, b, c, d, e, f, g may be removed. Therefore the centre lines are extended and the centre points are marked about 2m away from the outer edge of excavation. Thus the points A1, A2, B1, B2 and like wise, are marked outside the trench. Centre line are shown clearly by stretching thread or rope. The centre points fixed 2m away from the excavation are marked with sit out pegs.

4. From the plan details, the width of excavation to be done is also marked by thread with pegs at appropriate positions.

5. The excavation width is then marked by lime or by with furrow with spade.

6. If the plan is much to complicated and follows a zigzag pattern, then the centre pegs are kept at suitable positions according to site conditions.

Ref:
https://theconstructor.org/tips/setting-out-a-building-plan-on-ground/5897/

(iii) Establishing a datum level

From Encyclopedia.com, a dictionary of earth science, it is defined as:

"A surface or level which is regarded as a base from which other levels can be counted (i.e. a datum). For example, sea level is often used as a datum level against which the height of land and depth of the sea bed are measured."

In construction, a datum point is a known point of reference on the basis of which further measurements or analysis can be made. The point can be based on the finished floor level, an existing building or a benchmark.

When based on a benchmark, a datum point refers to a position above sea level. Benchmarks form a network of reference points from which a datum can be adopted. These are utilized in the construction of new houses, roads or stone pillars. A datum point can also be used in modeling geometry, where it’s possible to add several points on a model.