The purpose of Survey work will include, but not be limited, to the following;
- Initial Surveys to confirm the site boundaries (plot boundaries); establish permanent marks and record site levels and details.
- Set out for control of formwork and all other structures.
- Lift core verticality
- As-built surveys.
The scope of this method statement is for the land survey works of the company projects.
Project Technical Specification .
PERMANENT MARKS At least 4 permanent survey marks will be established around the project in secure locations. The horizontal and vertical relationship of these marks will be determined to a relative accuracy of:
Horizontal ± 3mm
Vertical ± 2mm
In the event that any mark is disturbed it can be re-established or its position re- measured from the adjoining marks.
This surveying method statement should be read in conjunction with the below referenced project documentations.
- Contract Specification and drawings
- Approved Shop drawings
- Project Quality Plan
- Project HSE Plan
Roles & Responsibilities
The Construction Manager is over all responsible for the implementation of method statement and ensures that his team of Engineers, surveyor and foreman are aware of this method statement and the all resources – human, material and equipment’s are available to carry out the work as planned and without any delay.
The Project Engineer is responsible to carry out the work as per the approved shop drawings, method statements, contract spec. and documents. He is also responsible to ensure adequate equipment’s for concreting; finishing and curing materials are in place before concreting.
Construction Manager is responsible to plan the schedule of as-built surveys, identify non-conforming i.e. out of tolerance structures and issue the report to QA/QC department.
Surveyors are responsible for all the setting out done on site, As– built surveys and the details given for submission.
The Safety Engineer is responsible to ensure that all Safety precautions are in place, and that all personal on site are using the required Personal Protective Equipment’s like – Helmet, Gloves, Shoes, and coveralls.
All personnel will be trained, prior to start the site surveying work related HSE / Quality Control / Method statement.
Site working hours shall be as per company policy. However, if work demands survey work can be commenced in evening or night based upon site conditions (lighting arrangement, etc. shall be necessary).
Safety Engineer shall ensure to maintain safety for all aspects.
Safety assistant are allocated at all the site locations to maintain housekeeping.
Manpower, Material and Equipment Requirements
- Chain man
- Chalk, Marker, Spray Paint.
- Rope, Warning Tape.
The following are the major items of survey equipment that will be used:
- Three total station, accuracy angular 5 seconds, distance 3mm +
- Six precision automatic levels, 2mm / Km.
- One optical plummet
- Three laser plummet, accuracy better than 5mm @ 50m.
- All equipment will be checked and calibrated on a regular basis.
SURVEYING PROCEDURE / METHOD TO EMPLOY
Raft Foundation to 00 Grade parking
For the excavation, raft foundation and initial levels of construction, position and height will be established by direct measurement from the permanent marks and temporary control points established at each level. As soon as the slab is completed at each level then the permanent grid points and grid lines will be set out.
Grade parking to Level 10
- At 00 Grade parking level at least 4 permanent survey marks will be established within the ‘footprint’ of each tower. Block outs (150mm by 150mm) will be set in each slab vertically above the survey marks so that the control can be transferred directly to each level above using optical plumbing with the precision optical plumb.
- As soon as each section of the floor slab is cast, permanent grid points and grid lines will be set out. At level 30(Approx-100mtr) the 4 survey marks will be established, checked and adjusted and these marks will then be used to establish control at L30 and above.
- Traditional levelling techniques will be used to establish marks on the foundation slab.
- From the foundation slab to the top of the building the datum point for each floor will be set by measurement from the floor below; traditional levelling can be used but where more than one change point is required an alternative method will be used.
- A steel tape will be suspended vertically, secured at the upper end and tensioned at the lower end to about 3Kg. Two precision levels are set up; one nears the lower datum mark and one on the level above. Readings are made simultaneously to the suspended tape and then to the datum marks.
- The instruments are moved and a second set of readings taken. The mean value is used to determine the datum at the upper level. This procedure will give an accuracy of ± 1mm and as the errors are not cumulative over 10 or more floors.
- Because of the effects of column shortening it is important that each new datum point is established from the datum on the floor below as soon as practicable after the formwork has been stripped. This will result in a uniform floor-to-floor height, which will facilitate certain wall installation.
Transfer of datum
Line and level datum will be progressively transference up the building at every 10–12 floors and survey points closed off behind.
As built surveys
As built surveys will be carried out as follows:
- Lift shaft surveys
- Core Wall verticality surveys
- Column and shear wall position & verticality
- Precast setting out / Edge of slab surveys
- Slab level surveys
On site survey procedures
Primary survey control
The 4 permanent marks established surrounding the site are considered to be the Primary points and all precise work must be connected to these marks by closed traverse and/or level run.
Marks established by closed traverse and/or level run within or adjoining the structure are considered secondary control and are used to establish control at all other levels of the building. The traverse or level run miss-close must always be adjusted before the secondary marks are used for the set out of further points.
- Horizontal control at another level is obtained by setting the precise optical plummet on each secondary control mark and sighting vertically through the survey block-out, using the angle eyepiece to mark a point / line over the block-out at the target floor.
- Two observations (180 deg. apart) are made to the target and the mid position adopted.
- The instrument is then transferred to the target level and observations made to the other 3 or 4 secondary control points.
- An initial accuracy of better than 1:10,000 should be achieved and a ‘best-fit’ solution is the adopted for the marks and major grid points and grid lines are then set out.
- Vertical datum for a new level is, in all cases, established directly from the level below.
- A suspended, tensioned tape should be used whenever the level transfer involves more than one change point by traditional means.
- The level run must be closed and the miss-close must not exceed 2mm. If greater than 2mm – re-survey!
- When using the suspended, tensioned, tape the same tape and the same tension (about 3-4 Kg) must be used at all times.
- Two sets of readings must be taken and again the difference in the two vertical distances must not exceed 2mm.
Core wall control
- Two different systems shall be used on site with some formwork being 3 or 4 levels above the current slab and some formwork being on the slab and only rising to the next level.
- Control will be set out on the current slab and plumbed to the formwork system using the laser plummet or the optical plummet.
- The shutter positions are then checked / set out from these marks. The verticality of lift core walls shall be checked every floor, while form work moving up. But frequency of results shall be generated and recorded every 5 floors including two diagonally opposite corners of lift shafts.
- All shafts verticality will be monitored from the external wall co-ordinate results with the thickness of wall.
Three offset points or two lines are to be marked on the current slab defining the base of the column.
Survey points set out on the slab below each span are plumbed to the next level to control the edge of slab form. Grid lines shall be established top of the slab prior to casting and edges shall be marked and confirmed by representative of precast company. Once slab been cast all grid lines to be marked by surveyor.
At all times the aim is to achieve an accuracy of 5mm in relation to the secondary survey control in setting the formwork at any one level. This applies to both horizontal and vertical position.
- It will not be possible to achieve the required accuracy unless regular checks are carried out on all instruments and equipment.
- Horizontal and Vertical collimation on each total station should be checked at least every week and in particular if the instrument has been knocked.
- A standard “Three peg test” is to be completed each three months on the automatic levels.
- Laser plummets – The laser plummet should be checked against the total station about every two weeks and in particular if the instrument may have been knocked or propped.
SURVEY PROGRAM – KEY TASKS
- Set out core wall and column positions on the top of the re-bar of the foundation slab so that starter bars can be accurately positioned.
- Set out all core wall and column positions on the foundation slab as soon as possible after the slab is cast and completes an as-built survey of the starter bar locations.
- At each level, commencing at the foundation slab, complete an initial as-built survey as soon as the formwork on the core walls and columns is stripped / jumped. At each level complete a final as-built survey including all openings and slab levels when all propping and formwork removed.
- At each level, as soon as the slab is cast, set out permanent grid points and mark grid lines as required.
- As soon as the 00 grade parking slab is cast, establish precise control at 4 points within the ‘footprint’ of the tower and then place survey block-outs vertically above these marks on all subsequent floors to provide access for plumbing.
- Establish a primary TBM at each level and accurately level from the floor below. Place additional TBM’s at the new level and measure in relation to the primary TBM.
- A summary of the results should be submitted to the Technical or QA/QC dept. within 10 days of the survey being completed.
- Independent Survey company will carried out survey works and submit the reports for every 10 floors.
HEALTH, SAFETY AND ENVIRONMENT
Survey staff will comply with all on-site safety procedures as set out and updated from time to time.
In addition the following items have been identified as job specific potential hazards for the survey team:
- Use of Aluminum staff and measuring poles near overhead electrical cables – all personnel to check each location prior to commencing the survey.
- Potential hazard in moving materials or siding to clear a line of sight – staff to liaise with site foreman prior to moving any material.
- Hazard from material falling through survey block-outs – wooden covers to be placed on all block-outs and removed only during observations.
- Potential hazards from material falling down lift shafts – person to be stationed at the upper end of all shafts when internal surveys are in progress.
- Hazard of vertical collision while positioning of stations & associated protection, barriers etc., to be installed to protect personnel.