Setting-out principles/calculation of cut/fill volume for buildings

Setting-out is often seen as a reverse process of surveying. Setting-out means transferring the building or structure design onto the land itself so that builders or site engineers can follow it during construction. It is needed to guide construction process and achieve accuracy.  To achieve this aim, the new structure set-out must be correct in all 3-dimensions both relatively and absolutely. That is, it must be of the same correct size, in the correct plan position and at the correct level. To minimize cost in setting out, once it is started, it must proceed quickly with little or no delay. Setting-out is required for building construction, road construction, bridge construction, dams, jetty construction and all types of construction works.

Points to be considered in setting out

The following points needs to be considered in setting out

1. All angles must be set-out using a correctly adjusted theodolite or total station
2. All distances should be set out using steel tape
3. Each design point should be set-out from at least 3 control points (3 stations)
4. To locate each design point, a large cross-section wooden peg should be drilled into the ground at the point and the exact design position marked on top of the peg.

Reference grid in setting out in engineering surveying

There are four (4) types of reference grid used in engineering surveying. These include:

1. The survey grid: This type of grid is drawn on the survey plan from the original traverse. The grid points have known Eastings and Northings which are usually determined from Global Positioning System (GPS)
2. Site grid:   This is the type of grid normally used by the designer. It is usually related in some way to the survey grid
3. Structure grid:     This is the type of grid usually established around a particular building or structure and it contains some building details like column position (Figure 1)
4. Secondary grid:   This type of grid is established inside the structure grid when it is no longer possible to use the structure grid (Figure 1)

Estimation of volume of cut from a structure grid

Immediately after setting-out necessary excavations follow for construction work. Prior to excavation, it is necessary to determine the volume of cut and/or fill required for the site. This however depends on the type of construction work going on. This volume of excavation is estimated from the reduced level of survey points. Figure 2 below shows the grid of reduced level taken on an existing ground at the produced site for basement. The surface level are shown beside each point. The grid surface are 10 m interval and the required excavation level is at reduced level of 12 m below datum. Calculate the volume of earth to be excavated.

Solution

1. Create a table (Table 1) with six columns as shown below
2. Arrange the ground points in the first column
3. Arrange the reduced level of ground points in the second column
4. Arrange the reduced level of excavation in the third column
5. Calculate depth of excavation in the fourth column.

Depth of excavation = (Reduced level of ground point – Reduced level of excavation)

6. Arrange the number of squares common to each point on the fifth column

7. Calculate the product in the sixth column

Product = (Depth of excavation x Number of squares common to each point)

8. Find the total of the product

Table 1; Estimation Table

Volume of cut = (Area of each grid x Sum of product)/Number of grid

Volume of cut = (10 x 10 x 49.14)/4 = 1229 m³