Chapter: 06 Construction Survey must be appropriate to the type of construction. Bench marks should be checked regularly.

The
elevation of any station / points is the vertical control. It is established by leveling process.
6.1 Introduction
6.3 Positioning of a structure
Construction surveying or building surveying (otherwise known as "staking", "stake-out", "lay-
out", "setting-out" or "BS") is to stake out reference points and markers that will guide the The new structure must be correct in all three dimensions both relatively and absolutely.
construction of new structures such as roads or buildings as per plan or map. i) correct level, size, plan and position.
ii) Once setting out begins it must proceed swiftly with little delay so that costs can be
Elements of Construction Survey: minimised.

 Survey existing conditions of the future work site, including topography, existing buildings Techniques used to achieve these aims are based on three general principles.
and infrastructure, and underground infrastructure whenever possible (for example,
measuring invert elevations and diameters of sewers at manholes); a) Horizontal control points must be established within or near the design area.
 Stake out lot corners, stake limit of work and stake location of construction trailer (clear of b) Vertical control (bench marks) must be within or near the design area.
all excavation and construction) c) Accurate positioning techniques must be adopted to establish design points from horizontal
 Stake out reference points and markers that will guide the construction of new structures and vertical control.
 Verify the location of structures during construction;
Positioning of any structure can be done by
 Provide horizontal control on multiple floors.
 Conduct an As-Built survey: a survey conducted at the end of the construction project to Basic Tasks
verify that the work authorized was completed to the specifications set on plans.
i) Setting Out of a point
ii) Setting Out of a line
6.2 Horizontal and Vertical Control iii) Prolongation of a line
iv) Setting Out of Angles
Horizontal Control v) Setting out of lines disturbed by obstacles.
Setting out is about working from the whole to the part. Secondary points (Established from the
The most commonly used methods are
primary control points) are used to the site to enable fast location of design points. Primary a) Orthogonal co-ordinates (Offsets)
control points should be far enough away from building activity to avoid damage. Primary b) Polar co-ordinates.
control points should be set in concrete. c) Intersections (Angle of Intersection 30o <<150o)
d) Linear Intersections (ties)
Reference grids

 Survey grid- usually based on original traverse. For Vertical Control the basic tasks are:
- Known as Easting and Northing
- Control points are original points. i) Transfer of height
 Site grid - Specified by designer ii) Setting out a horizontal line.
- Usually coincides with survey grids. iii) Setting out a gradient line.
iv) Setting out a contour line.
 Structural grids- Use to ensure building accuracy ( particularly of the verticals)
The methods used are
Vertical Control
a) Ordinary levelling
Points of horizontal control are often used for levels (vertical control) as well. One of these is b) Projected heights (e.g. in Mining Surveying)
c) Trig Heighten
usually made the Master Bench Marks, while others becomes Temporary Bench Marks(TBMs).
TBMs can be established in existing structures, or could be steel bars set in concrete. Accuracy

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6.4 Setting out of Building, Culvert and Bridge 6. If the plan is much too complicated and follows a zigzag pattern, then the Centre pegs are kept
at suitable positions according to site conditions.
Setting out a Building:

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.

Fig.1: Example plan to be set out on the ground

1. From the plan (fig 1), the Centre line of the walls are calculated. Then the Centre lines of the
rooms are set out by setting perpendiculars in the ratio 3:4:5. Suppose the corner points are a, b,
c, d, e, f and g which are marked by pegs with nails on top.
2. The setting of the corner point is checked according to diagonals ac, bd, cf and eg.

3. During excavation, the Centre points a, b, c, d, e, f, g may be removed. Therefore the Centre
lines are extended and the Centre points are marked about 2m away from the outer edge of
excavation.
The co-ordinates of different points are found from the foundation plan and indicated in the
Thus the points A1, A2, B1, B2 and likewise, are marked outside the trench. Centre line are tabular form.
shown clearly by stretching thread or rope. The Centre points fixed 2m away from the excavation
are marked with sit out pegs. 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.
4. From the plan details, the width of excavation to be done is also marked by thread with pegs at Procedure:
appropriate positions. (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.
5. The excavation width is then marked by lime or by with furrow with spade.

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

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(iii) Stretch the cords along lines PQ and Rs. Set off the distances OA, OB, OC etc on PQ and Setting out a bridge:
OE, OF, OI, OJ etc. on RS and fix arrows at all these points. 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
(iv) Set out comer 1 by measuring co-ordinates x1 and y1 from A and F respectively with the help dams, the flow of water cannot be diverted and also the length may be very long. Therefore
of two tapes and marks it with a peg. setting out is not possible from the centre of the bridge.
(v) Similarly, fix other points by their co-ordinates and drive pegs at each point.
The setting out involves the following operations:
(vi) Pass a cord around the periphery of abutment and wing wails as 1, 2, 3, 4, 5, 6, 7, 8 and mark 1. Preparation of topographic map of the bridge site.
the outline of the foundation with lime and by nicking i.e. cutting a narrow trench along this line.
2. Determination of the length of the bridge.
(vii) Similarly fix the corners of other abutment and wing walls and mark the outline of the
foundation. 3. Location of piers.

(viii) Take levels at all pegs and determine depth and quantities of excavation. 1. Preparation of topographic map:
A topographic survey of the bridge site and approaches to the bridge is required for long and
(ix) If the wing walls are curved, the points on the curve may be set out by offsets to the chords important bridges.
1-8, 6-7 and 2-3, 4-5 as indicated in fig. 14.6.
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.

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(xi) The detail and results of trial pits and borings etc. (b) Second method (Fig. 14.8):
(i) Set out the base lines AC and BD along both banks of the river.
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, (ii) Join CD, BC and AD. Then ABDC is a quadrilateral having BC and AD as their diagonals.
the length may be measured directly with a steel tape but that of a large bridge is measured by
method of triangulation. (iii) Measure the base lines AC and BD and the eight angles as shown very accurately.

Let A and B be the two points on opposite banks on the Centre line of the road. Any one of the (iv) Adjust the quadrilateral by approximate method or by method of squares.
following methods may be adopted to find the length of the bridge
(v) Calculate the length AC from the corrected length of BD and the adjusted values of the
(a) First method (Fig. 14.7): 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.
(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.

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

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(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.

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