Setting Out of Some Common Engineering Works | Surveying

The setting out of some of the common engineering works such as buildings. Culverts, bridges, slope of earth work etc. are discussed.

Setting Out a Building:

The foundation plan of the building is usually supplied or it can be prepared from the given wall plan of the building and size of foundations for different walls (Figs 14.1 and 14.2). It is of little use to set the pegs or stakes at the exact position of each of the comers of the building as they would be dug out while excavating the foundations.

It is therefore advisable to first set out a reference rectangle either outside the limits of the excavation or along the centre lines of the outside walls of the building and then to locate each centre by means of co-ordinates with reference to the sides of this rectangle. Both the methods of forming reference rectangle and setting out the building are described below.

The equipment required for the job consists of:

(i) A 30 m steel tape,

(ii) Two metallic tapes (15 m or 30 m),

(iii) A long cord,

(iv) A plumb-bob.

(v) Stakes or pegs,

(vi) Nails, and

(vii) A hammer.

1. Setting out buildings by circumscribing rectangle (Fig. 14.3):

The reference rectangle set outside the limits of excavation 2 to 5 meres from the building line as shown is known as circumscribing rectangle. The reference pegs P, Q, R and S will remain undisturbed during excavation.

The co-ordinates of all the corners w.r.t. the sides of the reference rectangle are calculated and shown in a tabular form given below on the foundation plan.

Procedure:

Two stakes P and Q are accurately driven at the required distance apart (15.4 m for building in Fig. 14.1 and 14.2), the ends P and Q being represented by wire nails driven at the centre of the pegs Stretch a cord between P and Q. At P set out a perpendicular PS to PQ with a tape by 3, 4, 5 triangle method. Drive a stake at S, PS being the required distance (10.7 m). Check the diagonal QS.

Follow the similar procedure at Q to set the stake at R. Check the diagonal PR. The distance RS should be exactly equal to PQ. The rectangle PQRS is the reference rectangle and the comers of the foundation plan are fixed by measuring their coordinates from the sides of the reference rectangle e.g. corner 1 is fixed by measuring its co-ordinates x₁ and y₁ (2m, 2m) from PS and PQ respectively, and stake is driven into mark its exact position, when all the corners are staked, cord should be passed round the periphery of the figures 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 etc. and the outline of the foundation marked by spreading lime along these lines.

2. Setting out building by centre line rectangle (Fig 14.4):

The reference rectangle formed by centre lines of the outside walls of the building as shown is known as center line rectangle and the corners are located by measuring their co-ordinates with references to the side of this rectangles.

The stakes put in at PQRS will be lost during excavation, therefore, reference stakes should be established on the prolongation of the sides of this rectangle stakes should be established on the prolongation of the sides of this rectangle where they remain undisturbed say about 2m from the building line.

In case of precise working i.e. for important buildings, a theodolite should be used for setting our right angles.

Bench marks should be established in convenient positions away from the site of work so that they remain undisturbed until the work is completed.

Setting out a culvert:

A culvert is set out by locating the corners of the abutments and wing walls at the foundation by means of their co-ordinates with reference to the centre lines of a road and a stream (nalla) crossed. The centre lines of road and stream which cross each other are taken as axes of co-ordinates, thief origin being at the centre of the culvert.

The co-ordinates of different points are found from the foundation plan and indicated in the tabular form.

In fig. 14.5, PQ and RS are centre lines of road and the stream respectively passing through the centre O of the culvert and co-ordinates for the points, 1, 2, 3 etc are x₁ y₁, x₂y₂, x₃ y₃. etc.

Procedure:

(i) Derive a peg at 0 and set up a theodo lite over it. Fix points A, B, C, D etc. on line POQ by arrows. The cord passing through the eyes of the arrows will define the line PQ.

(ii) Set out the line RS at right angles to PQ and fix the points EFGH, IJKI, etc by arrows.

(iii) Stretch the cords along lines PQ and Rs. Set off the distances OA, OB, OC etc on PQ and OE, OF, OI, OJ etc. on RS and fix arrows at all these points.

(iv) Set out comer 1 by measuring co-ordinates x₁ and y₁ from A and F respectively with the help of two tapes and mark it with a peg.

(v) Similarly, fix other points by their co-ordinates and drive pegs at each point.

(vi) Pass a cord around the periphery of abutment and wing wails as 1, 2, 3, 4, 5, 6, 7, 8 and mark the outline of the foundation with lime and by nicking i.e. cutting a narrow trench along this line.

(vii) Similarly fix the corners of other abutment and wing walls and mark the outline of the foundation.

(viii) Take levels at all pegs and determine depth and quantities of excavation.

(ix) If the wing walls are curved, the points on the curve may be set out by offsets to the chords 1-8, 6-7 and 2-3, 4-5 as indicated in fig. 14.6.

Setting out a bridge:

The setting out of a culvert is quite simple because there is only one span and the flow of water is less. Even if the flow of water is more, it can be easily diverted. But in the case of Bridges and dams, the flow of water cannot be diverted and also the length may be very long. Therefore setting out is not possible from the centre of the bridge.

The setting out involves the following operations:

1. Preparation of topographic map of the bridge site.

2. Determination of the length of the bridge.

3. Location of piers.

1. Preparation of topographic map:

A topographic survey of the bridge site and approaches to the bridge is required for long and important bridges.

Tacheometric methods are used for the survey work and contouring and the map must indicate the following:

(i) The north line

(ii) The name of river and the direction of flow of water,

(iii) The name of the nearest town or village on either side of the bridge.

(iv) The width of the proposed roadway.

(v) The width of existing roadway if any.

(vi) The radius of curvature of the curve at approach road.

(vii) The position and description of Bench Marks and the ground levels on both the banks for a distance of about 150 m both on upstream as well as downstream sides.

(viii) The normal, lowest and highest levels of water.

(ix) The catchment area.

(x) The maximum velocity and discharge at the bridge site.

(xi) The detail and results of trial pits and borings etc.

2. Determination of the length of the bridge:

The length of the bridge is required to be measured along the centre line. If the bridge is short, the length may be measured directly with a steel tape but that of a large bridge is measured by method of triangulation.

Let A and B be the two points on opposite banks on the centre line of the road. Any one of the following methods may be adopted to find the length of the bridge

(a) First method (Fig. 14.7):

(i) Draw base line AC perpendicular to the centre line AB.

(ii) Measure the base line AC very precisely as also the angle ACB (θ₁) with theodolite by method of repetition.

Then AB = AC tan θ₁

(iii) To check the length AB, set out a line BI) perpendicular to AB at B. Measure BC and the angle ADB (θ₂) as before. Then AB = BD tan θ₂.

(iv) If the two distances are almost equal, the mean of the two may be taken as the length of AB, otherwise the whole operation should be repeated and accurate length should be computed.

(b) Second method (Fig. 14.8):

(i) Set out the base lines AC and BD along both banks of the river.

(ii) Join CD, BC and AD. Then ABDC is a quadrilateral having BC and AD as their diagonals.

(iii) Measure the base lines AC and BD and the eight angles as shown very accurately.

(iv) Adjust the quadrilateral by approximate method or by method of squares.

(v) Calculate the length AC from the corrected length of BD and the adjusted values of the angles and compare it with its measured value. If the discrepancy is less than 1 in 5000, the length of the centre line AB is accepted.

3. Location of piers:

After measuring the length of the bridge, mark the position of central points of piers along the centre line on the plan.

The piers are located by intersection of sights from the ends of the base line by the following methods:

(a) First method (Fig. 14.9):

(i) Measure the base lines AC and BD and the angles BAC and ABD.

(ii) Compute the angles ACP₁, ACP₂, BDP₁, and BDP₂ from the known lengths of base lines and the angles BAC and ABD.

(iii) Set up the theodolites at AC.

(iv) Direct the theodolite at A to B and set the angle ACP₁ at theodolite at C. The intersection of these two lines of sight gives the position of the central point P₁.

(v) Similarly locate the second central point P₂.

(vi) Check the locations of Pi and Pi by setting theodolites at B and D.

(b) Second method (Fig .14.10):

(i) Set up the base lines at A and B perpendicular to AB extending on both upstream and downstream sides.

(ii) Set off on each base line and on each side of the center line the distances equal to the center points 1 and 2 as shown.

Thus the intersection lines 1-1 and 2-2 make angles of 45& with base line on opposite banks and also with the center line AB.

(iii) Locate the center point P₁by simultaneously sighting at the intersection of the two lines 1-1 .Similarly establish P₂ at the intersection of the lines 2-2

This method is the fairly accurate and easy but is suitable only when perpendicular base lines on both sides of the center line of bridge are possible.

Setting out Slope of Earth Work:

This consists in setting stakes at a given slope or grade. The indicate how much cutting or filling has to be done to bring the surface of the ground to a given grade. After marking the grade or formation line on the profile, the formation levels are determined for each station. Knowing the formation level and the height of instrument, the staff readings required to set the stakes at given grade may be obtained by subtracting the formation level from height of instrument.

Illustration (Fig. 14.11):

Suppose a point A is to be established on a given grade. Let the formation level of point A = 60.500, the height of instrument = 62.605 and ground level at A = 60.255, the reading of the staff placed on the top of the peg i.e. formation level should be H.I. Formation level.

= 62.605-60.500 = 2.105 m.

 

The stake can be raised or lowered to give the desired reading. In this case, the top of the stake will be 60.500-60.255 = 0.245 m above the ground.

Similarly other points are established on the given grade.

The above method is suitable only if a few points are to be established on a given grade but if the number of points is fairly large such as in the case of a road, railway, canal, sewer etc., the grades are to be set out by a theodolite, an Abney’s level, a tilting level, sight rails and boning rods etc., the method being explained below:

1. Setting out grades by a theodolite (Fig 14. 12):

The method is used to establish grade stakes when the grade is uniform for a considerable distance. A line of sight parallel to the grade is established with a theodolite So that one grade rod (levelling staff) is sufficient for all points.

First of all establish two fixed points (A and B) at the required grade. The theodolite is then set up and levelled at one of them say A. A target is fixed at the staff at the height of horizontal axis say ‘h’ and the staff is placed at the other stake say B.

The telescope is then directed towards the staff, raised or lowered until the horizontal hair bisects the target. It is the clamped. The intermediate stakes C,D, E etc are established by observation the same staff reading. This method is suitable for comparatively uniform slopes.

2. Setting out grade by the Abney’s level or tilting level:

The index of the vernier of the Abney’s level is set to the reading corresponding to given grade. The instrument is then held over the starting point at a known height, say, 1.45 m above the point by supporting it against a pole. Another pole, on which the vane is fixed at same height, is held over the next point at some distance apart say 30 to 40 m.

The rod is then raised or lowered up or down hill until the vane is bisected and the bubble centered. The line joining the instrument station to the point on which is the ranging rod is held is then parallel to the line of sight and is, therefore, on a given grade. The point so obtained is then pegged, and is used as the next instrument station, the successive points are located in the same manner.

This method is not so accurate. Fairly good results can be obtained by using a tilting level fitted with micrometer drum in place of the Abney’s level and a target staff in place of the vane-rod.

3. Setting out grades for sewers and pipe lines with sight rails and boning rods (Fig. 14. 13):

A sight rail is a horizontal strip of wood 15 cm wide and 5 cm thick which is set across the centre line and nailed to two vertical posts 1.5 m long firmly embedded into the ground on either side of the centre line. The sight rails should be white washed.

A boning rod is T-shaped, and consists of a long piece of wood 10 cm wide and 3 cm thick and of varying length, across which is nailed a cross-piece 40 cm long and of the same cross-section. The length of the boning rod is constant for any one section of the alignment hut may vary for different sections. The top of the honing rod should he blackened.

The sight rails and honing rods are used to set the proper gradient between any two points on the alignment of sewers, pipe-line, road or railway etc. The sight rails are placed at regular intervals of distances along the centre line and at each change of gradient and direction. The top of each sight rail should be exactly horizontal and is; accurately set to the desired elevation by the level.

The line joining the top edges of the two consecutive sight rails should have exactly the same gradient as that of the centre line of the work. A cord of string should be stretched between the nails driven at the top of each sight rail.

Knowing the elevations of the actual centre line of work to be laid out and that of the top of the light rail, length of the boning rod is determined which is the difference of the two elevations. The boning rods are placed with their tops touching the string and the line joining their bottom points is parallel to the string and thus lies on the required gradient.