Setting Out.
Surveying – produce a plan/map of a particular area.
Setting Out- begins in a plan and ends up with some particular project correctly positioned on the area.
It is important to realise that setting out is simply an application of surveying.
Definition- Setting out is the establishment of marks and lines to determine the position and level of the elements fro construction project so that works may proceed with reference to them.
Aims of Setting Out
The new structure must be correct in all three dimensions both relatively and absolutely.
i) i.e. at correct level, size, plan and position.
ii) Once setting out begins it must proceed swiftly with little delay so that costs can be minimised.
Techniques used to achieve these aims are based on three general principles.
a) Horizontal control points must be established within or near the design area.
b) Vertical control (bench marks) must be within or near the design area.
c) Accurate positioning techniques must be adopted to establish design points from horizontal and vertical control.
Important Considerations in Setting Out.
1. Recording and filling information, booking e.t.c for easy accuracy.
2. Care of instruments; check instruments before work commences and at regular intervals.
3. Maintaining accuracy, design points must be set from the control network and from other points to avoid cumulative errors.
4. Regular site inspections, to detect missing pegs i.e. a peg may be disturbed or replaced without the surveyor being informed, control should be permanently and clearly marked and protected.
5. Error detection- Apply independent checks *N.B- Nothing is gained hiding errors; therefore errors detected must be corrected at an early stages.
6. Communication on site- Lack of it cause errors, the surveyors need to understand what needs to be done before doing it.
Setting out in General
Methods used for setting out are generally simple but difficulties are often inaccurate there are no hard and fast rules but there are a few general factors that must be observed i.e. complete checking of your work.
Where closing errors cannot be determined, a peg must be co-ordinated from at least two points using independent data.
Setting Out of Detail Points.
Basic Tasks
i) Setting Out of a point
ii) Setting Out of a line
iii) Prolongation of a line
iv) Setting Out of Angles
v) Setting out of lines disturbed by obstacles.
The most commonly used methods are
a) Orthogonal co-ordinates (Offsets)
b) Polar co-ordinates.
c) Intersections (Angle of Intersection 30o <q<150o)
d) Linear Intersections (ties)
For Vertical Control the basic tasks are:
i) Transfer of height
ii) Setting out a horizontal line.
iii) Setting out a gradient line.
iv) Setting out a contour line.
The methods used are
a) Ordinary levelling
b) Projected heights (e.g. in Mining Surveying)
c) Trig Heighting
The Use of grids
Many Structures consist of steel reinforced steel columns supporting floor slabs. These columns are usually at right angles to each other. Setting out is generally facilitated by the use of a grid where the grid interactions define the positions of columns.
There are three main types of grids.
1) Survey Grid- this is the rectangular co-ordinate system on which the original topographic survey is carried out and plotted i.e. the National System or local system.
2) Site grid- this defines the position and direction of the main building lines of the project. In order to set out the sight grid it may be convenient to translate the coordinates of the sight grid to those of the survey grid.
Y =DY + Y1Cosq-X1Cosq
X= DX+X1Cosq+Y1Sinq
Where q = relative rotation of the two grids.
DY, DX = difference in Y and X of the respective grid origins.
Y1, X1 = Coordinates of the point on the sight grid.
Y,X – Coordinates of the point on the survey grid.
Structural Grid- This is used to locate the position of structural elements within the structure.
Controlling Verticality
We have three main methods.
Using a plumb-bob
In low-rise construction a heavy plumb-bob (5 to 10 kg) may be used as shown in Figure 10.13. If the external wall was perfectly vertical then, when the plumb-bob coincides with the centre of the peg, distance d at the top level would equal the offset distance of the peg at the base. This concept can be used internally as well as externally, provided that holes and openings are available.
Using a theodolite
If two centre-lines at right angles to each other are carried vertically up a structure as it is being built, accurate measurement can be taken off these lines and the structure as a whole will remain vertical. Where site conditions permit, the stations defining the 'base figure' (four per line) are placed in concrete well clear of construction (Figure 10.14(o)). Lines stretched between marks fixed from the pegs will allow offset measurements to locate the base of the structure. As the structure rises the marks can be transferred up onto the walls by theodolite, as shown in Figure
I0.14(b), and lines stretched between them. It is important that the transfer is carried out on both
faces of the instrument. Where the structure is circular in plan the centre may be established as in Figure 10.14(d) and the
radius swung out from a pipe fixed vertically at the centre. As the structure rises, the central pipe is extended by adding more lengths. Its verticality is checked by two theodolites (as in Figure W.14(b)) and its rigidity ensured by supports fixed to scaffolding.
The vertical pipe may be replaced by laser beam or autoplumb, but the laser would still need to be checked for verticality by theodolites.
Steel and concrete columns may also be checked for verticality using the theodolite. By string lining through the columns, positions A -A and B - B may be established for the theodolite (Figure 10.15); alternatively, appropriate offsets from the structural grid lines may be used. With instrument set up at A, the outside face of all the uprights should be visible. Now cut the outside edge of the upright at ground level with the vertical hair of the theodolite. Repeat at the top of the column. Now depress the telescope back to ground level and make a fine mark; the difference between the mark and the outside edge of the column is the amount by which the column is out of plumb. Repeat on the opposite face of the theodolite. The whole procedure is now carried out at B. If the difference exceeds the specified tolerances the column will need to be corrected.
Using optical plumbing
For high-rise building the instrument most commonly used is an autoplumb (Figure 10.16). This instrument provides a vertical line of sight to an accuracy of ±1 second of arc (1 mm in 200 m). Any deviation from the vertical can be quantified and corrected by rotating the instrument through 90° and observing in all four quadrants; the four marks obtained would give a square, the diagonals of which would intersect at the correct centre point.
A base figure is established at ground level from which fixing measurements may be taken. If this figure is carried vertically up the structure as work proceeds, then identical fixing measurements from the figure at all levels will ensure verticality of the structure (Figure 10.17).
To fix any point of the base figure on an upper floor, a Perspex target is set over the opening and the centre point fixed as above. Sometimes these targets have a grid etched on them to facilitate positioning of the marks.
The base figure can be projected as high as the eighth floor, at which stage the finishing trades enter and the openings are closed. In this case the uppermost figure is carefully referenced, the openings filled, and then the base figure re-established and projected upwards as before.
The shape of the base figure will depend upon the plan shape of the building. In the case of a long rectangular structure a simple base line may suffice but T shapes and Y shapes are also used
Controlling Grading Excavations
Excavations must be controlled in the construction of sewers, roads and pipelines. Site rails and travellers are used to control the gradient of excavation. Site rails consist of horizontal timber crosspiece nailed to a single upright or a pair of uprights.
Examples:
Used for road construction and small diameter pipes.
Used in building corners.
Used for construction of large diameter pipes.
A traveller is similar to a sight rail on singular support and is portable.
Portable and length determined for a particular project.
Traveller / Boning rod
Example: A drain is to be set out from the following information.
Length of drain AB: 150m Invert level @A = 64.35m
Gradient AB: 1 in 200 falling
Length of traveller: 2m
Calculate the staff reading necessary to locate the staff reading over AB if BS-1200m to BM with RL of 67.650m
1 in 200
BM 67.650
Height of sight rail at A = 66.35m
HPC = 68.850m
Staff reading at A = HPC – Height of sight rail.
= 2.50m
Height difference between A-B = 1/200 X 150
= 0.75m
Invert level at B = 63.60m
Staff reading at B = HPC- ( IL + Length of Traveller)
= 68.850 –(63.600+2.00)
= 68.850- 65.600
= 3.25.
Deformation Surveys
Relative Monitoring
Control stations are subject to deformation \ they must be checked by measuring distances between them.
Measure d1 @ time t1, measure again after some time t2. Building is monitored by checking target at A.
Absolute monitoring
Control stations are considered error free i.e. they are not subject to movement. Deformation can be checked by measuring changes in coordinates and distances.
Examples of structures.
1. Large dams
2. Tall multi storey buildings
3. Large Bridges.
Causes of Deformation
1. Slope instability
2. Seepage
3. Inadequate slope protection.
4. Floods/ water.
Planning the survey
The essential requirement of deformation survey is long term planning
Factors to consider.
a) Control points should be inter-visible from at least two other points.
b) They must be anchored on rock or at least they must be deep enough so that they are unaffected by surface movement.
c) They should be sufficiently marked and protected to prevent being damaged
For the control points, there is Horizontal control and vertical control.
The methods for each form of control is as follows:
a) Horizontal - Precise traversing
- Triangulation
b) Vertical control - Precise levelling
OBSERVATIONS
a) Theodolite Observation
At all stations, a minimum of two arcs must be taken. Whether fixing points by triangulation or traversing. The normal system of observation is employed i.e. closing on to the initial station at the end of each round.
ARC = FL and FR || FL= One round || FR=One Round.
Make horizontal observation when refraction is least ( early morning and late afternoon)
Vertical observations @ midday when vertical refraction is least.
Distance Measurement
Can be measured using E.D.M units. Measurement s must be made in both directions and a mean calculated. As with most E.D.M units the best weather conditions are when the day is cool. Measure in both directions (A to B and B to A). Apply the necessary correction after measurements.
Choice of Instruments.
The choice of instruments depend on the accuracy needed to detect deformation movement.
Theodolites
Where triangulation provide the main system of control i.e. for large dams a Wild T3 can provide the required accuracy. For small surveys, a universal single second theodolite is sufficient e.g. Wild T2. This can be used in towns where the main method of fixing points is by traversing.
These instruments must be tested for any residual errors.
Tapes
This should be made of high quality steel with millimetre graduation. They should be standardised and their weight per unit length known. If the quality of the tape is suspect, it must be checked against a standard tape.
Levels
Any modern precise automatic level can be used. An important feature in precise levelling is that invar staves with double scales are used (one scale on the left, one on the right). Levels should be tested for collimation error.
Computation Methods
The best method for computing deformation observation is by using least squares and this method is suitable if a computer is available. Whichever method is chosen, the same method should be used each time the points are re-fixed.