End of Unit Assignment - Site Surveying

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Lecturer’s Name:                 Tony Lomas

Submission Date:                 06/06/05

Candidate Number:                 0478764

End of Unit Assignment – Site Surveying

1.) Explain the procedures for setting out and levelling of foundations for steel framed and pre-cast concrete buildings.

Setting out involves the transfer of positions and levels of buildings and associated features from those dimensions and co-ordinates marked on a drawing or plan or computer printout to the ground by a variety of methods and instruments designed for that purpose. Precision is required in ensuring that the building and placement level match the position shown on the drawings relative to Ordnance survey references expressed by reference to longitude and latitude, plus established boundary points for the site; also ordnance bench marks (OBM) that indicate levels relative to ordnance datum i.e. mean sea level at Newlyn Cornwall.

OBM’s are frequently found chiselled into buildings and can be transferred to site with temporary benchmarks (TBM) suitably marked and protected from damage or disturbance. The purpose of setting out is to ensure correct positioning and dimensional control during construction. A good reference guide to the degree of accuracy required in setting out buildings is covered by BS5606: 1990 Accuracy in Building.  

Levelling is the establishment of a level height plane for the site from which measurements can be taken. This will be referenced from a datum known as a benchmark (BM), ideally linked to an OBM or a TBM.

Steel framed and pre-cast buildings have particular requirements for high accuracy in setting out as the limited tolerances available during the construction stages with much of the dimensional constraint applied off-site. In masonry built buildings a degree more tolerance is available due to the smaller components being assembled.

Setting out usually follows a set procedure albeit that most projects provide for a degree of uniqueness that requires skill and forethought to be applied before setting out proceeds.

A typical approach would include:

1.) An appraisal of the setting out documents to obtain the fundamental setting out data and this would include dimensions of buildings and boundaries also finished levels of the works.

2.) Site inspection would identify obstacles to setting out, ground conditions would help guide the type of markers to be used for pegs and site rails. Existing TBM could be confirmed.

3.) Verify works will match the planned use of the site by checking dimensions.

4.) Plan ahead by calculating any offsets required due to obstructions, level changes and other calculations needed during setting out that will improve site efficiency.

5.) Proceed to set out the plan of the works using tape to measure distance and theodolite for angles having first checked equipment for accuracy.

6.) It is usual to establish site lines and profiles and other markers outside of the area of construction to enable quick and easy reference during construction and avoid damage occasioned by the activities involved.

7.) Proceed to establish levels for the vertical dimensions of the works.

A grid is set up using a theodolite and marked out with marker pegs; it is essential that the theodolite is level. Calculation of diagonals by the means of triangulation is an effective means of checking the accuracy of the setting out. Triangulation calculates the length of lines by extrapolation from the angles between them, applying Pythagoras’ theorem. Following the completion of the grid; sight rails can be set up for the excavation, allowing the use of a traveller to assist in the levelling of the ground. Excavation would usually be done with machinery but may be done manually. Figure 1 illustrates how a grid appears once setting out is complete.

Figure 1: Surveying for Construction, Third Edition (Page 242)

2.) Explain the procedures for checking the verticality of the frame as above.

It is vital the frame of a building is perpendicular. Larger buildings require greater accuracy regarding verticality, as a minor error on the first storey can become a major error the higher up the building you get. A theodolite, a plumb-bob and other special instruments can be used to check for verticality.

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Theodolite Method

The theodolite is set up so that it is in line with the wall of the building usually at the lowest point visible; however you can use the highest point but any result taken from the top of the building will be the inverse of what it would be from the bottom. It is essential the theodolite is level for this task to work. You need to ensure the theodolite is fixed so it cannot be turned horizontally, you do however require it to move in a vertical plain as you are measuring for deviations regarding ...

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