AY1516 Cuthbert's Reflections on LTA C923

8/18/2019 AY1516 Cuthbert's Reflections on LTA C923

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Name: Cuthbert Yeo, Year 4

Organisation: Land Transport Authority

Area of Specialisation: Geotechnical/Project Management

VIP period: 18 May 15 – 6 Aug 15

Designation: Site Intern @ C923 Upper Changi Station

Personally, I really felt the need to see for myself what I have actually learnt so far from school that is

applied in reality, so I have requested from HR to place me on site nearest to my house. I was posted to

C923 Upper Changi Station of Downtown Line 3 Package C which would complete by 2017. Only

Entrance A was Top-Down (with secant bored piling) and the rest of the station box was constructed

Bottom-Up (with Dwall). This is a Design-Bid-Build project.

C923 site surrounded by various stakeholders; sheltered walkway to be provided within 400m radius ofstation to provide better connectivity for commuters to ride.


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The beginning weeks were rather daunting. I was inundated with meetings (project

safety/risk/design/instrumentation audit/concrete casting plans/CD Door inspections@Yongnam

factory/PDM/QPs audit/weekly and bi-weekly progress meetings from main contractor Samsung C&T) as

well as policies that you have to pick up and be aware of in 1 to 2 weeks’ time (mainly your TR26, CP65

Concrete bible, CP4 Foundation, LTA Construction Safety Handbook, LTA Materials and Workmanship

(M&W) Specifications bible and Civil Design Criteria RevA1 as well as some BCA Advisory) so that you

understand what is exchanged on the meeting tables. They can be found here:

https://www.lta.gov.sg/content/dam/ltaweb/corp/Industry/files/Construction_Safety_Handbook.pdf

and this VIP: https://www.corenet.gov.sg/general/e-info/codes-and-regulations.aspx

After you’re done, quickly move on to method statements (so that deviations can be spotted during PGI),

design change reports and GIBR for borelogs and preliminary damage assessment reports by consultants.

In my case, these were done by MEINHARDT, Amberg and instrumentation readings by TRITECH.

It is important to get the hang of how the drawings are to be read based on sections or gridlines (in my

case 1xGL ~ 11.6m) as well as understand their shorthand acronyms. You also have to make the personal

effort to break the ice and establish good relationships with not only your colleagues from your site

office, but also those from the main contractor and QPs teams. What I did was to went round the

Samsung C&T office and Parsons Brinckerhoff office and introduced myself personally and for staffs who

aren’t around at their tables, you could leave a POST-IT note leaving your name and school email (or HP

number if you really need to) so that you can get the required information you need later in your report.

It is good to do all of this early so it wouldn’t be so awkward later.

I will now briefly elaborate as much as I can remember in these 13 weeks based on topics:

Diaphragm Wall (Dwall) similar to CE3115

I was guided by HENO Hafiz on construction of Dwall.

The general sequence is to survey, mark, then hoard the areaperform trial trench whenever utilities

or SPPG or Singtel cables are thereconstruct guide wall(control alignment of box cutter)setup

bentonite plant (foundation has to be PE calc; pipes embedded must be properly connected; concreting

slurry to be fresh and not reused; working slurry can be reused)mobilise excavator then grab cutter

then box cutter(increasing depth for different reach of machine; bite sequence shown below

pic)KODEN machine to test vertical profile of trenchStop ends with water stops placed at corners of

panels to be castedRebar cage inConcreting through surging( where concreting bentonite slurry

floats up as concrete is tremie in; with pipe at least 3m embedded to prevent re-entry of slurry; RFI first

then cast!)Stop ends are removed when concrete still green leaving behind the water stop

(hydrophilic water strips)

https://www.lta.gov.sg/content/dam/ltaweb/corp/Industry/files/Construction_Safety_Handbook.pdfhttps://www.lta.gov.sg/content/dam/ltaweb/corp/Industry/files/Construction_Safety_Handbook.pdfhttps://www.corenet.gov.sg/general/e-info/codes-and-regulations.aspxhttps://www.corenet.gov.sg/general/e-info/codes-and-regulations.aspxhttps://www.corenet.gov.sg/general/e-info/codes-and-regulations.aspxhttps://www.corenet.gov.sg/general/e-info/codes-and-regulations.aspxhttps://www.lta.gov.sg/content/dam/ltaweb/corp/Industry/files/Construction_Safety_Handbook.pdf


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Any bulge would then be hacked. Major cracks or water seepages would then be Polyurethane grouted

depending on the severity of the finishes. If honeycombs were found in panels, they would be hacked,

BRC and casted.


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Key problems faced and solutions implemented are summarized in mindmap below:


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Utilities Gap and Diversion

I was guided by

ENO Vasu on

utility gaps. (See

Instrumentation

for settlement

limits of utilities

as an example)

The utilities were

either supported

by I-beams or

concrete

haunching.

Most of the utilities were not diverted despite being in the way of the designated construction area due to

the high cost of fines imposed for damages. The presence of the utilities would impede the use of grab

cutters or BAUER box cutters in the construction of diaphragm wall. Hence, the construction of diaphragm

wall beneath these utilities was constructed in stages as excavation progress. Based on the soil condition,

the utility gap excavation may vary with different depth and with or without the use of BRC. Exposed soil

surface at utility gap must be monitored for water seepage or soil collapse. Lean concrete was used as soilprotection wall to protect the soil face from collapse. At every section of the utility gap, hydrophilic strips

and re-groutable tubes would be installed along the perimeter of the opening to manage the water

seepage through the construction joint. After the main reinforcement is installed, formwork installation

follows and gaps will be casted with C40 concrete between existing Dwalls.

PUB, SP telecom and SPPG utilities

clearly surveyed and marked.


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Strutting with Bottom-Up Approach similar to CE5108/CE5112

I was guided by SPE Karthigesh on strutting systems used. It was unusual to use 3xUB as struts for deep

excavations (usually 2xUB) until after 2005. This also meant greater emphasis of settlement or

deflection readings by TRITECH and contractors. The lack of openings requires the Sub contractor

YONGNAM to lower all the struts in pieces. These pieces of struts would then be preloaded with

hydraulic jacks, before assembling them on site and installing them at required levels. For diagonal

struttings, shear ankles has to be placed between the waler and concrete packing; this would also mean

longer time for hacking and removal as compared to 20.2m long horizontal struts. LTA requires

reinforced concrete strutting system for certain area in this project.

Hydraulic Jacks used to preload

the horizontal struts


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Minimum of 2 Strain gauges attached on web of struts

Omission of S7 @Section F-F and G-GThis deep excavation requires 7 struts initially, which contractors themselves later found the last

unnecessary and the final strut (S7) was omitted at Section F-F and Section G-G. I made the attempt to

do a similar soil modelling with the help of FEM analysis tool PLAXIS. The estimated excavation depth

during construction is approximately 28m below existing ground level. The following results were found:


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BCA Advisory Note on ERSS: https://www.corenet.gov.sg/einfo/Uploads/Circulars/CBCA090402.pdf

Comparisons of design results from FEM programme, actual strain gauge readings from TRITECH and

ultimate deflection limits imposed by BCA would then be compared to see if they meet serviceability

state requirements. BCA also requires the analysis in the event of a one strut failure. When all

requirements are satisfied would then validate the omission of S7 strut from initial design.

Instrumentation similar to CE4216C923 tunnel and station box is mainly situated in Old Alluvium (OA). Hence, most report took only

drained analysis. EC7 requires both drained and undrained analysis. Also, with the presence of

important services and live utilities, settlement is closely monitored with settlement markers and optical

prisms to ensure they do not reach alert level or work suspension level. For example, for C923 project,

Singtel Submarine cable can only settle no more than 5mm; 15mm or more settlement would cause a

SWO. Monitoring frequency is arranged prior to and during excavation as well as during and after

backfilling. It is important for instruments to be placed in sets (in the event one spoils; the next closest

reading would provide an equally realistic result) as well as to monitor the drawdown of groundwater

table for excavations.

https://www.corenet.gov.sg/einfo/Uploads/Circulars/CBCA090402.pdfhttps://www.corenet.gov.sg/einfo/Uploads/Circulars/CBCA090402.pdfhttps://www.corenet.gov.sg/einfo/Uploads/Circulars/CBCA090402.pdfhttps://www.corenet.gov.sg/einfo/Uploads/Circulars/CBCA090402.pdf


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TR26 – Technical Reference for Deep Excavations

Site Language and Work Culture

Relationships between supervisors and workers must be good to ensure good exchange of information.

Whatsapp messenger is pretty much abused when reporting on site conditions haha, so it’s important

that you are in the group chat. It is also important to learn the Chinese, Blangadashi and Korean

language (in my case of Korean engineers) when communicating with the worker on progress of works.

The following are some important phrase you should learn to effectively communicate with workers:

Blangadashi Meaning in English

Shobu Shakal Good Morning

Shobu Dopul Good Afternoon

Shobu Aso Good EveningTara Teri Faster

Astei Astei Slowly

Aseh Yes

Nei No

Ak 1

Dul 2

Tin 3


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Cha 4

Pass 5

Dono Butt Thank You

I must say the work culture and perks given by LTA can be pretty good! There is travel credit provided to

full time personals and welfare events like yoga, dragonboat and even floorball for you to interact withthe people from other departments like the project, contracts or design teams even!

Floorball welfare organized monthly around months end by Racheal

AY1516 Cuthbert's Reflections on LTA C923

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