The planning stage of setting-out process requires the assemblage of materials and instrument required for the process. The following equipment/instrument are required for setting-out.
a. Steel tapes: these are more suitable type of setting out instrument because they are not subject to the same degree of stretching as the fibrous or nylon tape. However, the accuracy of the setting-out depends on the condition of the tape and expertise of the user.
b. Levels: automatic levels are usually more commonly used because they produce better reliable results.
c. Theodolites: these are used in surveying to determine vertical and horizontal angles by utilizing a tiny telescope that may move within the horizontal and vertical planes.
d. Total station: a total station combines the functionality of theodolite with that of level and electronic distance meter (EDM) such that it can determine angles, levels and distances and also have the capacity to store this information to be later transferred to computer for further processing.
e. Autoplumb: these are used to set out vertical lines especially in high rise buildings.
f. Ranging poles: ranging poles of length 2 m or 2.5 m are commonly used for alignment in setting-out long straight lines.
g. Profiles: a profile is a wooden stake to which cross-piece in constating coluored steps is nailed. For sewer works, sight rails which are goal post type of profiles are preferred. The profiles are erected over the offset pegs in order to remain clear of excavation.
h. Travellers: they are mobile profile boards used in conjunction with sight rails. The length of the traveler is usually 2.0 m, equals the sight rail in height and the length must be kept in multiples of 0.25 m.
i. Corner profile: they are constructed from stout wooden stakes to which wooden boards are securely nailed.
j. Setting-out pegs: these are marks used to define ground points usually 40- or 50-mm square wooden pegs about 0.5 or 0.75 m long with a point cut at one end. Pegs are commonly wooden and of usual precise sizes, 50 mm x 50 mm x 500 mm in soft ground and 25 mm x 25 mm x 300 mm in hard ground. Pegs should be colour coded with paint. Centerline pegs are coloured white, offset pegs are yellow while level pegs are blue.
They are placed reasonably accurately in position, then the precise point required is marked by a nail hammered into the legs leaving about 10 mm projecting. Reference points and other important marks may be further secured by scooping a shallow trench around the peg and gently heaping concrete around it to keep it firm. If necessary, a light railing may be erected around it to warn lorries and other vehicles to keep clear.
Permanent marks, though not often used due to time and cost involved are constructed from concrete blocks set up to 1 m into the ground and built up to any required height. A brass plate with a drilled centre-mark or a pipe or other devices is set into the concrete to define the precise point.
Principles of setting out
General: on any construction site, it is common practice to construct first the road ways and sewers in order to provide access to the site and main drainage of all buildings. This implies three different kinds of setting-out to be done for roadways, sewers and then buildings and these are what we want to discuss in this post on the processes to achieve that. All setting-out works usually follow common key stages. These are outlined below.
Key stages of setting-out
a. Establishment of baseline: this is the line first set out from which subsidiary pegs are placed to define the new construction. The baseline may be one of:
i. the centerline of new road, railway, pipeline, sewer etc.
ii. the building line for domestic and smaller constructions.
iii. the centerlines of columns and stanchions in large projects.
iv. a line parallel to the longest construction line in the works.
b. Subsidiary lines: these are set out from the baseline and pegged. The data should be figured dimensions from working drawings or calculated measurements based on plan information. Scaled dimensions usually give errors and not often used.
c. Establishment of reference marks: this is necessary because setting out pegs are usually destroyed in the course of construction. These are needed for that. They should be positioned such that:
i. they are not likely to be disturbed.
ii. relocation of major points from them can be easily done.
iii. if they may be destroyed, alternative marks can be established to replace them.
This is a plan that shows the position and description of all pegs set out. The description should include the linear dimensions between pegs and angles between them, or the bearings of them. Lines used for setting out and referencing must be recorded as well. It is basically necessary for all types of work except minor works. The reduced levels of any pegs or bench-marks to be used for vertical height control must also be known.
In general, four types of plan are required for any construction from the beginning to the end.
1. Survey plan: this is prepared as a result of the original site survey.
2. The working plan or site plan: this is produced by the designs and show the location and form of the new construction sited on the survey plan.
3. The setting-out plan: this shows the relation between the recovery pegs and the pegs defining the position of the new works.
4. Record plans/as-built/as-laid plans (for pipelines): this records the deviations from the original design and shows the new works as finally constructed. Sometimes, it is just only revisions of the working plan.
How to place level pegs to define working plan
1. Hold the staff where the peg is to be placed and raise or lower it until the calculated ready is obtained. This indicates the approximate peg projection above the ground.
2. Hammer the peg down almost to the required level with frequent checks on the staff reading until it is accurately placed.
3. If it is hammered too far by mistake, it cannot be lifted as it will lose stability. Place a nail on top and hammer down to define the reduced level then.
4. To save time in accurately positioning the top of the peg, it may be left projecting above the required level. Then, hold the staff against the peg until the calculated reading is obtained and then draw a line across one side of the peg to define the level usually as an arrow with chalked level for clarity. For example, assuming a peg is required to define a 5 m level. A level is set up and a back sight,, say 1.490 is observed to a temporary benchmark (TBM) with a reduced level (RL) of say 4.042 m. the height of instrument is then 4.042 + 1.490 = 5.532 m. Thus, when a reading on a staff of 0.532 m is taken, its base will be at the required level of 5.000 m
Setting-out roads and railways
Setting-out of roads involves three processes combining initial horizontal control, vertical control and establishment of reference marks.
Horizontal control – the initial setting-out process involves physically pegging on the ground the centerlines of the road or railway. The setting-out processes for both roads and railways are similar, except for technical differences in the actual construction works. The baseline being the centerline of the new works, it is usually defined in one of three ways by the designer, and the setting-out process for each of the following ways that the centerline has been established must be considered;
a. Plan location only
b. Centre-line straights ground marked
c. Co-ordinated centerline
Once the centerline has been aligned or ranged by one of the three methods above, the pegging of the line takes place with pegs being placed along the line at continuous changes at intervals of 20 or 25 m. Pegs are numbered in 100-m units. Thus,
Chainage + Origin = 0 + 00
Chainage + 1st peg = 0 + 25
Chainage + 2nd peg = 0 + 50
Chainage + 3rd peg = 0 + 75
Chainage + 4th peg = 1+ 00
Additional intermediate pegs are placed at bridge centres, intersections and at the beginning and end of transitional and circular curves. The first stage of the work is complete when all centerline pegs on straights and curves have been pegged and checked. The next stage of the work consists of pegging the limits of the work area. The limits include:
a. Fencing limits – these are pegged first as the contractor must provide at least temporary fencing to secure right of way and prevent trespass claims from landowners.
b. Top soil strip limits – pegs are used to guide removal of top soils often within fencing limits.
Vertical control (Levelling) – vertical control is required at all stages of the work. A series of temporary benchmark (TBM) must be established along the route at about 300 m to provide level control. They are placed close to fence limits to avoid damage by machinery and surrounded by concrete. Cross-section levels would also be provided to enable computation of earth work quantities.
Reference marks – lastly, reference marks would be established usually at tangent points, road intersections, bridge control points, sufficient intervisible centre-line pegs etc. The marks must be fixed clear of plants and close to the fence line. Reference marks should be made in such a way that they are recoverable even after destruction.
Summarily, in setting-out roads and railway works, pegs are first established at intervals say 20 – 25 m intervals along the centerline of the route. These pegs are consecutively numbered from the commencement of the routes for easy identification of any change along the route, while the offset pegs are positioned left and right of the centerline pegs at about 3 – 5 m. There may also be need to set-out curves such as simple horizontal curves, compound curves, transition curves and reverse curves using predetermined curve parameters.
Video: Setting out of roads.
Setting-out drainages (sewers and drains)
Drains and sewers are laid to gradient such that the flow of water allows the drain to be self-cleansing. The old rule of thumb which states that 100-mm, 150-mm and 225-mm drains were laid to falls of 1:40, 1:60 and 1:90 respectively is satisfactory for drains where the water flow is fairly low. In combined drains that contain both foul and storm water and where the quantity of flow is greater, shallower gradients are used. Sewers are thus laid to shallower gradients generally than drains (see Table 1) for cleansing gradients for average flow. This is also more economical and less excavation would be required.
Table 1: Cleansing gradients for average flow
|Quantity of flow (m3/s)||Gradient||Pipe diameter (mm)|
|0.003||1/80||100 or 150|
|0.004||1/100||100 or 150|
|0.005||1/120||150 or 225|
|0.006||1/150||150 or 225|
|0.007||1/160||150 or 225|
|0.008||1/175||225 or 300|
|0.009||1/180||225 or 300|
|0.010||1/220||225 or 300|
Note: too large drains are usually non-self-cleansing
The first process in setting out drains is to set out the baselines which is usually the centerline of the sewer. Following a plan showing the lines of existing and proposed sewers, the lines of the new sewers are then marked on the ground by placing a peg at each manhole or inspection chamber position obtained by setting-out scaled or recorded dimensions from the plan. Manholes will be built at the following positions:
a. At the maximum of every 100 m or straight runs.
b. At all changes of direction.
c. At all sewer junctions.
d. At the head of each sewer or drain.
e. At interceptors near junctions of drains and sewers (this should be within 12 m of the junction).
Once the position is located, line pegs are placed every 20 or 30 m along the length lines as a guide to the excavation. After the pegging, longitudinal sections are run and sections are also drawn indicating the ground profile and any features or other services crossing the path of the new sewer line to provide all information necessary for the drainage scheme design and so that the depth of excavation and any problems peculiar to the site may be evaluated and costed. From the longitudinal section, working profiles are prepared showing the gradient of drain and sewer runs and invert levels (level of the bottom of the inside of any pipe or channel or (in manhole, the level of the channel floor of the outgoing pipe) of all manholes and junctions. After pegging, the position of manholes and inspection chambers which are required at every change of direction or gradient in all drainage runs (except very large diameter pipes) should be established. The limits of these manholes should be marked with pegs for manual excavation or set with cutting blades where machines are used. There should be proper control for position, levels and excavation works. After excavation, pegs are referenced to mark the final manhole positions.
Laying of pipes
Drains and sewer pipes are laid from the lower end of the trench with sockets facing uphill. The concrete bases of the manholes are first poured and the beds for the pipes are laid. The lower manhole channels are bedded down and the upper manhole channels temporarily laid. A side line is drawn between the two just clear of the flanges to keep the pipe-line straight. The gradient of the pipes may then be controlled by using one of the travelling rod, straight edge and boning rods.
a. In pipelaying stage, it is more helpful and productive to fix a level peg near to each machine so that operatives can transfer the invert level to the trench bottom easily without continuous references to the sight rail or travelers.
b. Pipes are often laid first unjointed, in the trench and the whole set is adjusted for line and gradient, jointing being carried out afterwards.
c. Sideline are preferably used instead of topline to control both gradient and line because even taut line can sag, resulting in a dip in the pipeline.
The first stage is setting out building is setting out the baseline. The baseline is usually the building line which is a line related to the physical features of the site. The building line is the line of the front face of the building. Its position may be defined on the working plan by measurements from the following:
a. Property boundary line.
b. The edge of the road kerb, and
c. The centerline of the road.
Where the three features above are not present, competent authorities should be consulted to agree on building line that would not cause dispute in the future.
The building line is first ranged by eye and pegs are placed at the two first corners of the outer face of the proposed building. After confirming the building lines, nails on pegs are used to define the points. On large sites where it may be difficult to do this manually or where obstructions exist, coordinate calculations using sophisticated survey equipment like theodolite, total station etc. can be used to do the setting out.
The subsidiary lines are set out from the base lines using instrument like theodolite or total station or manual methods using either of 3-4-5 taped triangle, builder’s square, optical square or site square. After setting out these lines, pegs are placed with nails on top and measurements are taken between them and the diagonals for confirmation. In a rectangle, the two diagonals must be equal before the pegs can be positioned firm. After pegging the main outline, other offshoot or recesses are also checked and pegged.
The next is to establish reference system using profile boards because pegs would be destroyed during excavation. Profile boards (see Figure 2) are constructed of 150- or 200-mm by 25-mm boards or 50 mm x 50 mm purlins supported on 50-mm square posts hammered firmly into the ground well clear of the working area (clearance should not be less than 1 m). It is better to place the boards at very high level like DPC or finished floor level to help keep the tape and rope horizontal during measurements.
After erecting the profile boards, full foundation width is marked on the board and two lines strung between these points define the width of the foundation trench to be dug. Once the trench has been started the lines are removed. Alternatively, the lines may be temporarily defined along the ground by means of strips of line or sand to guide excavation.
After the foundations are laid, the line defining the building face are again strung, the building corners are plumbed down to the foundations and the brickwork/blockwork or formwork correctly positioned.
Vertical alignment of the structures may be achieved by the use of either;
- Centre-line axis method using theodolite
- Plumb-bob method
- Optical plumbing method
- Theodolite plumbing method
- Laser level
In setting out buildings, there must be adequate control for position, size, shape, level and verticality during construction. Level is determined by establishing a datum (Bench Mark) on site and then by transferring levels to the profiles from which construction work can be carried out without constantly requiring the use of instruments.
Reference points should be used to control the position, shape and size of the buildings.
Video 1: SETTING OUT OF BUILDINGS 1.
Video 2: SETTING OUT OF BUILDINGS 2.
The procedure of setting out buildings can be summarized as follows:
- Set out corner pegs from the data using 3-4-5 or builder’s square method.
- Check diagonals (for large buildings, use instrument instead).
- Compare position with check dimensions.
- Set out stakes for profiles and specify offset.
- Check level from the nearest Temporary Bench Mark (TBM) or standard floor level (SFL).
- Set out profiles’ boards in relation to SFL.
- Using string lines, transfer line of the corner pegs to the profiles.
- Show transverse length of the foundation excavation
- Mark foundation limit on profile boards.
- Recover corner pegs for re-use after checking.
For further reading, consult
Land Surveying by Ramsay J.P. Wilson. M & E Handbooks
And other suitable land surveying handbooks