1.1 PURPOSE OF THE DOCUMENT
The purpose of this procedure is to outline the sequence of operations to be carried out for geometry control of segments cast using the long line casting method, and to ensure that all works will be conducted safely and in accordance with the drawings and the Project specifications.
1.2 PROJECT DESCRIPTION
Ho Chi Minh City Urban Railway Construction Project – Ben Thanh – Suoi Tien section (Line 1), Contract Package 2 consists of approximately 12 km long of U-girder segments to be erected using span-by-span method. The precasting yard (PCY) general layout is as follows:
General layout - Precasting yard
1.3 SCOPE OF WORK
This procedure is established for the geometry control of segments produced using long line casting method. Only straight span typical segments with no horizontal curve, no precamber and/ or no vertical curve are cast using long line casting method. This procedure shall include the following items:- - Establishment of Survey Reference Points and Survey Towers in the PCY - All measuring equipment and related procedures - Survey plates and its position at each segment. - A geometry control procedure for the vertical and horizontal alignment control for the precasting of segment including:-i) survey controls and procedures ii) checks iii) computational and/or graphical methods iv) correction measures. This method statement does not incorporate the pier segment. Pier segment will be treated in a separated method statement including slope management of top pedestal.
Reference is made to Outline Construction Specification Package 2: Civil (Elevated & Depot):- i) Clause 12.4.1 (4) - “Casting control geometry” ii) Clause 12.4.1 (12) – “Tolerance” related to fabrication of segment iii) Clause 12.4.3 (3) – “Erection Tolerance”
All reference documents are intended to refer to the last issued revision.
In this terms & conditions, the meanings assigned to words and expressions shall be the following: - “Geometry Control” means the reference to the segmentation, adjustment computations and production of as-cast coordinates and levels to enable the precast spans to conform to the design alignment and profile once erected. - “Segmentation” means the process of breaking the viaduct spans into individual segments, and pre-calculating the theoretical positions of every segment joint, in relation to the alignment and profile of the project. - “Survey plates” means the insert installed at predetermined position at mould prior to segment casting. The insert as-built position (level and coordinates) shall be taken and recorded before the mould is stripped and the insert as-built position used in setting up the next segment to cast. - “Local coordinates” means the information required to cast the segments in their correct spatial relationship, so that when erected the span will conform to the theoretical alignment of the bridge. It is computed from the supplied alignment geometry for each span. - “Global coordinates” means the information required to erect the segments in their correct spatial position, relevant to the theoretical alignment of the bridge. It is computed from the readings taken after casting of all of the segments in a span and is in the form of horizontal coordinates and levels of the survey inserts placed in the segment during casting. - “Formwork adjustment” means the preparation of formwork for casting a segment, based on the as-built survey of the segment previously cast. - “Survey tower” means the reinforced concrete or steel columns located along mould centerline and at extremity of mould such that it can be used as survey station or survey target. As survey station, it supports survey equipment to allow formwork adjustment and cast segments survey works. - “Survey reference point” means the reference point placed in a Precasting Yard used as reference target that will never move in time to calibrate the Survey tower position. - “Match Cast Segment (M/C)” means the previously cast segment and forms the stop end at the opposite end of the mould to the stop end. - “Wet Cast Segment” means the segment being cast and is between the stop end and the match cast segment.
Further definitions have been given in Section 1.5 “Definitions” of the method Statement “General precasting works at Precasting Yard” HCMC-261-CSYD-CWS-MST-00012.
2. SAFETY & ENVIRONMENTAL RISKS
2.1 HAZARD ANALYSIS & RISK ASSESSMENT
Hazards Analysis and Risks assessment are carried out and attached in the present method Statement. This document will identify all the hazards to ensure adequate control measures and strategies are in place to mitigate as much as possible the risks (Refer to appendix A).
All personnel carrying out the work will be properly trained by experienced supervision. Besides, all personnel shall wear the appropriate PPE (Personal Protective Equipment), such as safety shoes, safety helmet, safety gloves and safety harness if required.
######2.2 TOOLS AND EQUIPMENT All the electronic precision apparatus for survey readings and segment settings shall be calibrated and fit for its purpose in precasting operations, and covered to protect from the sun or the rain. Surveyors must ensure that they are in good condition to use. Where applicable, official documents and calibration certificates are to be produced.
2.3 ACCIDENT / INCIDENT
In the event of an accident/incident, response will be as per the Health and Safety Plan. Prestart and toolbox talk will reinforce safety requirements prior to commencement of the works.
3. CONSTRUCTION RESOURCES
The geometry control work involves the following resources:
- Survey Manager
- Assistant surveyor
Survey Manager shall have previous direct experience in geometry control of segments. The surveyor and assistant surveyor shall be appointed from qualified personnel with accurate survey experience.
3.2 PLANT, EQUIPMENT & TOOLS
- Surveying instruments (Total stations and auto-levels)
- Prism, prism pole, and steadying struts for prism pole
- Survey staff
- Measuring tapes, marking pens, pencils
- Chalk line and ink line
- Long Line moulds
- Survey towers
- Survey plates
This list only provides a preliminary overview of resources which can be possibly used during segment precasting operations, so that it does not necessarily prevent personnel to use other equipment plants and tools.
3.2.1 ACCURACY OF EQUIPMENT The measurement tape shall be equipped with scale reading allowing measurement possible to 0.25mm. The total station and auto-level including survey staff and prism pole (including steadying strut) shall be capable of reading to accuracy of 0.05mm. Brand and Model of instrument shall be chosen to meet the accuracy requirement.
3.2.2 INSTRUMENTS CALIBRATION & CHECKING Survey equipment and accessories shall be inspected, and adjusted if necessary, before starting work, to ensure that they are operating within the tolerances specified by the manufacturer. Instruments shall have calibration label affixed, giving the due date of the next calibration. During work, if it is suspected that instruments or ancillary equipment have been:- - damaged; - is out of adjustment; - is malfunctioning;
the instrument or ancillary equipment shall be withdrawn from service until it has been repaired and/or adjusted.
After the withdrawn equipment has been repaired and found to be in calibration, the equipment shall have a new calibration label affixed. Any unserviceable equipment shall be clearly marked “Not to be used” (or equivalent marking), and stored away from the equipment in use.
Form related to checking the equipment (total station and level) is attached (refer Appendix D).
4. ESTABLISHMENT OF SURVEY REFERENCE POINT AND SURVEY TOWER
4.1 PERMANENT SURVEY REFERENCE POINTS
For survey control at casting yard, 3 permanent survey reference points shall be established with relation to the government control benchmark located nearby. These survey reference points are located in areas where there is no movement of vehicle or workers. The survey reference points shall protected and shall not be disturbed.
These survey reference points shall be used as reference target to establish and calibrate the survey towers positions. These survey reference points shall be checked periodically based on Survey Manager recommendation.
4.2 SURVEY TOWERS
At both long line bed extremities, survey towers have been setup. At one end, the survey tower nearer to the long line bed shall be utilized as survey station where the survey equipment (total station) is installed. The other survey tower, on the other end, shall be used as “target” to initialize the survey equipment every day. Prior to use as survey station, the survey towers shall be calibrated with the 3 survey references points. The survey tower calibration shall be done on an agreed periodical basis based on the Survey Manager recommendation. Survey towers include a concrete column/steel L-beam for survey station (for both total station and level fixation), stair access and shelter to shade the survey equipment from weather.
The survey equipment (total station) installed at the survey tower is used for:- - Setup the centerline of the longline bed (which is also the centerline of the span cast) - Read the levels and coordinates of survey plates (refer 5.2) prior to segment casting - Read the as-built levels and coordinates of survey plates after segment casting prior to mould striking(removal)
5. GEOMETRY CONTROL PROCESS AT LONG LINE MOULD
The purpose of having a geometry control system is to ensure that information given by the Designer thru the General Alignment Drawings are transferred to the precasting factory such as the geometry of the precast segment manufacture will allow a perfect match with the theoretical span alignment after erection on site.
The manufacturing of segment is using the concept of “match casting” which is the technique of casting a new segment between a stop end on one end and its adjacent segment on the other end. The pattern of formwork and adjacent segment is imprinted on the new cast segment creating a unique relation between the segments.
The geometry control will also be used to control segments cast one after another within the same span such that the geometry of the span remains within the design tolerance in relation to level, alignment, dimension, position and profile.
The long line bed is designed and installed straight and flat. Therefore, the long line moulds are only for casting straight span with no horizontal curve, no precamber and/ or no vertical curve.
The straight centerline of the bed coincide and acts as centerline of the segment span.
5.2 SURVEY PLATES EMBEDDED ON SEGMENT
To implement a systematic geometry control scheme, survey plates are embedded in the segment at predetermined position.(Refer Appendix B).
Each segment require four survey plates. For the first cast typical segment (which is cast between 2 stop ends), 2 survey plates are bolted to each stop end. Subsequent typical segments requires 2 survey plates bolted to plates already embedded in match-cast segment and other 2 survey plates bolted to the stop end.
5.3 PREPARATION OF GEOMETRY CASTING DATA
The geometry casting data are prepared from the general alignment span drawings. Based on the general alignment drawing at rail level reference, the coordinate of each span is defined for points A, B, C and D.
Coordinate of point A, B, C, D are got by transferring from center of pier at rail level
The coordinates will then be transferred via AutoCAD into the reference system of the precast yard, considering segmentation and position of survey plates within the segment. AutoCAD will model the span with theoretical position of the segments (and therefore respective survey plates). Cast segment after cast segment the coordinates of the segment survey plates will be monitored and corrected to remain as close as possible from the theoretical coordinates of the span and avoid accumulating error due to match cast process.
Position of survey plates in transverse direction
The theoretical coordinates shall be used as the theoretical data to set up position of formwork stop-end every time a new segment is to be cast.
5.4 SURVEY WORK ON LONG LINE MOULD
5.4.1 SET-UP SURVEY FOR 1ST SEGMENT OF A SPAN
The 1st stop-end is installed at one extremity of the long line bed; the location of the 1st stop-end is fixed and given by the formwork concept. Using measurement tape, a line will be drawn on the long line bed at 2.85m (length of 1st segment to be cast as per shop drawing) away from the 1st stop-end to mark the position of the 2nd stop-end. Marking is made either using a chalk or ink line. The 2nd stop-end will be installed at the mark vertically and perpendicular to the long line bed.
After formwork closure and survey plates fixed at both 1st stop-end and 2nd stop-end, survey team shall take measurement, using measurement tape, between survey plates to confirm the stop-ends set-up is within the tolerance of positioning. Concreting is then allowed.
5.4.2 AS-BUILT SURVEY (AFTER CASTING) OF 1ST SEGMENT
As-built survey is typically done the next day at early morning prior to the arrival of formwork team. The surveyor, using total station and prism, shall take readings of the cast coordinates of all of 4 survey plates.
The survey manager or dedicated office surveyor shall superimposed the cast coordinates on the AutoCAD file comparing against the theoretical coordinates.
The survey manager or office surveyor shall then confirm the cast coordinates are within tolerance. Taking into consideration error of the 1st cast segment, the survey section shall also inform the position of the stop end for the next segment casting. This information is the dimension from match-cast segment to stop end that will be used by the formwork team, measurement using measurement tape, to set-up the stop-end for the next segment.
5.4.3 SET-UP SURVEY OF 2ND AND SUBSEQUENT SEGMENT (BEFORE CONCRETING)
The segments are cast between match-cast segment and stop end. The match-cast segment is left on the long line bed as it is. Any casting error of earlier cast segment length is taken into consideration by the survey team in issuing the dimension for setting up of the stop end position of the next segment.
Based on dimension issued by survey team, the mould team shall mark the stop end position on the long bed either using a chalk or ink line. The stop end is then installed at the mark; installed vertical perpendicular to the bed.
Once the mould is almost ready, the mould team shall check by using measurement tape the dimension between the points on survey plates located between stop ends. The mould team shall then inform the survey team.
The surveyor, by using total station and prism, shall take the reading and confirm the stop end is installed within tolerance. The surveyor shall sign the inspection checklist which allows the concreting of the next segment.
5.4.4 AS-BUILT SURVEY (AFTER CASTING) OF 2ND AND SUBSEQUENT SEGMENTS
As-built survey is typically done at early morning prior to the arrival of mould team. The surveyor -using total station and prism- shall take readings of the cast coordinates of all of 4 survey plates; 2 survey plates at match-cast segment and 2 survey plates at stop end.
The survey manager or dedicated office surveyor shall superimposed the cast coordinates on the AutoCAD file comparing against the theoretical coordinates. He shall then confirm the cast coordinates are within tolerance. Taking into consideration error of the wet cast segment, he shall inform the position of the stop end for the next segment casting. This information is the dimension from match-cast segment to stop end.
5.5 GRAPHICAL PRESENTATION OF AS-BUILT SURVEY
For each span, the vertical and horizontal as-built alignment check is presented in a graphical form. Each segment as-built value will be input in excel file which will generate the as-built line compared to theoretical line for each segment and the full span (Refer Appendix C).
6. ACTIVITY HOLD POINTS
Hold points will be identified in the Inspection & test Plan. Refer to the ITP “Precast segment production”.
7. INSPECTION AND RECORDS
Survey records shall be actual readings, measurement and notes taken during the segment as-built survey and shall be recorded as references in the ITP.
Field notes shall be on preprinted sheets and shall be in ball point pen, and any alterations shall be struck through and initialed by the surveyor or assistant surveyor actually taking the readings. Example of survey form is as attached (refer Appendix D)
The date of the survey, as well as segment marking of the wet cast segment and match cast segment, shall always be recorded along with the actual readings. Any major disagreement between two sets of readings, or any difficulties experienced in the course of the survey of a particular segment shall be resolved, or if necessary referred to the Survey Manager prior to any proceeding with the next segment casting #####8. ATTACHMENTS
APPENDIX A: HAZARD ANALYSIS AND RISK ASSESSMENT APPENDIX B: SURVEY PLATE PLACED IN SEGMENTS APPENDIX C: TYPICAL AS-BUILT GRAPH OF VERTICAL AND HORIZONTAL ALIGNMENT APPENDIX D: SURVEY FORMS – SURVEY AND CHECKLIST OF SURVEY INSTRUMENTS