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1.Introduction
The most important part of any Surveying project is the Control Network. Control network or the control traverse is the collection of control points in a common reference system.Whether it is a construction or cadstaral surveying reliability of the project highly depend on the the control network . Accuracy of the control network varies with the requirement of the project. Generally the accuracy of the control network has been defined by the relevant body for the particular project.
Before the beginning of Surveying work with the total station some important preliminary factors should be considered,
Be mindful of calibration condition of the instrument.
Spectial attention should be given to the targets and tripods conditions.
Check whether legs of the tripod have been properly clamped and adjust the centering eyepiece and optical piece of the targets without any parallax error. After centering the target over the control point, Rotate the target from its tribrach by 90 and 180 degrees and check the centering again if there is any deviation from initial centering the centering lens mechanism has to be adjusted.
Legal Boundary Survey
In a typical boundary survey or cadastral survey ,following department survey regulations are followed by the Survey Department of Sri Lanka.
1. In a detail traverse control points must not exceed the maximum limit of 20.
2. The accuracy or the closeness factor is defined by
,k is the total length of traverse in km
Construction Survey
In a construction survey accuracy of the control network is depends on the requirement of the project. When considering building survey for high-rise buildings, high accuracy control network is vital for the success of the project.
Accuracy of the control network should be around 1:100,000 to 1: 120,000. Different zero traverse should be run to establish a control network.
Different zero traverse
To achieve a high accuracy in a traverse, only distance and angles are measured with the instrument in a repetitive manner.
Following is the process which has to be followed for the no of times required(zero).
Face left observation
Observe the back sight. Set instrument reading for zero and get the distance and record then observe the foresight recording the horizontal angle and the distance change the face.
Face right observation
Observe the back sight recording the horizontal angle and the distance similarly observe the fore sight the record the details.
One set of recordings are completed
Similarly, with number of zeros needed to set the back site reading to (0+180/n degrees) and repeat the process.
For four zero observation in degrees
1st zero = 0
2nd zero = 45
3rd zero = 90
4th zero = 135
I would like to share the real world approach for better understanding of establishing high accuracy control network in a construction project. We will look in to the project Waterfront Integrated Resort (Cinnamon Life).
2.Waterfront Integrated Resort ("Cinnamon Life") Colombo, Sri Lanka.
Client - John Keells Holdings; Waterfront Properties - Cinnamon Life.
Under construction since March 2014.
The development features an 800 room, 5 star luxury hotel with entertainment and associated facilities, conferencing & exhibition centre, spa, gym, banqueting, all-day dining, specialty restaurants, 2 residential apartment towers, 1 office tower, waterfront esplanade, landscaped public areas, retail mall, 2500+ carparking and civil works (Including roadways and a 6 lane vehicular/pedestrian access bridge).
Project Value / Size $1billion+, 422000m2 GFA
In a summary the project required construction of six different towers with various height levels connecting each other in different floor levels in varying angles. Due to this complexity and the structural accuracy desired, a control network with an accuracy around 1: 100,000 is required.
This number was finalized by various survey teams and many other resource persons after a period of trial and error.
Achieving this was highly challenging and to maintain it throughout the completion of the structural work was even harder.
Following special process was followed apart from common procedure to achieve the accuracy
Two zero traverse method used to establish the control network, only 10 seconds error was allowed within a single face angle observation.
Highly visible and places with minimum disturbed points within and external to the project premises used.
Special care was taken to protect the control points and creating new points mainly exterior to the site.
After all these conscious efforts control traverse was run for every month.
All the survey teams have been coordinated to check their internal networks with the main traverse and report any issues immediately. Occasionally their control points were cross checked with the main control network.
Sometimes achieving this accuracy was nearly impossible until we found the cause of the problem. Main trouble faced and the solution
With the restriction in the instruments and it's accessories , we found out that standards of instrument accessories is not satisfactory to meet the requirements.
After many analysis by checking all the instruments and the targets at the site we found out that the issues are with the targets. All the instruments we used were in the brands of Leica, Sokkia and Topcon but the accessories specially the targets were not in good conditions.
Problems faced with targets
a) The Prism constant of the target prisms was not the commonly used ones.
b) The centering of the target sets was erroneous which is explained no 2 in introduction.
Solution: for all main traversing full set of Leica total station and targets were used.
When running a traverse in mid-day even between two control points there was a considerable error in the distance measurement. This was due to the refraction error cause in the rays emitted from the total station.
Usually refraction error is not considered when running a traverse in Sri Lankan weather conditions. Here in this situation we had to consider especially in couple of control points in the network.
In between those control points the earth surface was widely varying. Respectively from tar road to gravel then to the water (Bere wewa) and finally to the concrete slab. Which probably the reason for the error.
Solution: the traverse should be run before the environment get heated. The best possible time to start it in between 8 am to 9 am
After considering these we were able to successfully complete our task.
Following are some data sheets collected from the field and the processed output for your reference.
Results of the New Control Traverse done on 13th June 2017 are submitted herewith
for your information.
The Angular Misclosure obtained was 02"
Relative Accuracy was 1:179,966
Drawing file showing the locations of the Control Points is also attached along with a jpg file of same.
Following file shows the initial designing of control traverse for the Waterfront Integrated Resort .
M.F.M. Mafaz
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