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Holistic Assessment Strategy for Hung Hom Station Extension Stage 2 Investigation - Results of Workmanship Quality for Coupler Connections

According to the holistic assessment strategy of the MTR Corporation Limited (MTRCL) regarding the platform slabs and diaphragm walls of the Hung Hom Station Extension under the Shatin to Central Link (SCL) Project accepted by the Government on 5 December 2018, the MTRCL commenced the concrete opening up works of the holistic assessment strategy stage 2 on 10 December 2018.  The purposes of opening up the concrete are:

(1) Physical investigations will be carried out at locations with gaps in the construction documentation, so as to verify the as-constructed conditions of the connections between the platform slabs and diaphragm walls.
(2) in view of the allegations on the cutting-short of steel bars, the MTRCL needs to open up certain connections between the platform slabs and diaphragm walls for detailed inspection, and to conduct non-destructive tests (Phased Array Ultrasonic Test (“PAUT”)) to determine the embedded length of the threaded steel bar inside the coupler.

According to the information from the coupler’s supplier, proper installation requirements of coupler are:
(1) there shall be a maximum of two full threads exposed; and
(2) the embedded length of the threaded steel bar inside the coupler shall be at least 40 millimetres.

The MTRCL uses the PAUT to verify the embedded length of the threaded steel bar inside the coupler. As the allowable measurement tolerance of the test is 3 millimetres, equipment readings below 37 millimetres are regarded as not meeting the requirement. The test is carried out by the professional technicians of A.E.S. Destructive and Non-Destructive Testing Limited.

Large deviation was observed between the embedded length measured using PAUT up to 29 January 2019 and direct measurement after removing the steel bar with the coupler by cutting. The MTRCL and its technical team have conducted a detailed investigation on the incident, and carried out in-depth study and enhanced the PAUT in order to improve its accuracy and reliability. The MTRCL also conducted validation on the enhanced PAUT.  Measurement signals or readings could still be unclear due to certain limitation in the enhanced PAUT, for example, uneven end face or damage of the threaded steel bar, insufficient area for the probe to operate due to site constraints.  These could result in failing to successfully complete the test.

The Government accepted the proposal from the MTRCL for the enhanced PAUT on 14 March 2019, and the MTRCL fully resumed the investigation test on 15 March 2019.

  • Past test results (from 14 December 2018 to 28 January 2019) using original PAUT (Details are here)
  • Hung Hom Station Extension Layout Plan of Platform Slabs of East West Line and North South Line (Layout Plan is here)

    The results using enhanced PAUT conducted by the MTRCL are given below:

Table 1: Embedded length not less than 37mm
(Verified test results as at 18 March 2019, with the latest information highlighted in blue)

Item

Coupler No.

Location of Coupler Tested

No. of Exposed Thread

Embedded Length
(mm)

Remarks

1

EWL-W68-BB-B1-02-C1

At the bottom of Area C1 of EWL slab near western diaphragm wall

1-2

37.8

 

2

EWL-E107-BB-B1-01-C1

At the bottom of Area C3 of EWL slab near eastern diaphragm wall

0

38.3

 

3

EWL-E107-BB-B1-03-C1

At the bottom of Area C3 of EWL slab near eastern diaphragm wall

0

39.9

 

4

EWL-E32-TT-T1-01-C1

At the top of Area HKC of EWL slab near eastern diaphragm wall

0-1

44.5

 

5

EWL-E50-BB-B1-01-C1

At the bottom of Area B of EWL slab near eastern diaphragm wall

0

42.3

 

6

EWL-E70-BB-B1-02-C1

At the bottom of Area C1 of EWL slab near eastern diaphragm wall

1-2

41.3

 

7

EWL-W58-BB-B1-01-C1

At the bottom of Area C1 of EWL slab near western diaphragm wall

0-1

39.2

 

8

EWL-W58-BB-B1-02-C1

At the bottom of Area C1 of EWL slab near western diaphragm wall

0

45.5

 

9

EWL-W113-BB-B1-01-C1

At the bottom of Area C3 of EWL slab near western diaphragm wall

1-2

40.2

 

10

NSL-W52-TT-T1-01-C1

At the top of Area B of NSL slab near western diaphragm wall

0-1

38.4

 


Table 2: Embedded length less than 37mm
 (Verified test results as at 18 March 2019 with the latest information highlighted in blue)

Item

Coupler No.

Location of Coupler Tested

No. of Exposed Thread

Embedded Length
(mm)

Remarks

1

EWL-E44-TT-T1-02-C1

At the top of Area B of EWL slab near eastern diaphragm wall

9-10

9.5

 

2

EWL-W113-BB-B1-02-C1

At the bottom of Area C3 of EWL slab near western diaphragm wall

7-8

16.4

 

3

EWL-W115-BB-B1-03-C1

At the bottom of Area C3 of EWL slab near western diaphragm wall

2-3

35.5

 

4

EWL-W112-BB-B1-01-C1

At the bottom of Area C3 of EWL slab near western diaphragm wall

3-4

29.0

 

5

EWL-W112-BB-B1-02-C1

At the bottom of Area C3 of EWL slab near western diaphragm wall

2-3

35.1

 

6

EWL-W112-BB-B1-03-C1

At the bottom of Area C3 of EWL slab near western diaphragm wall

1-2

35.4

 

7

NSL-W52-TT-T1-02-C1

At the top of Area B of NSL slab near western diaphragm wall

1-2

33.7

 

8

NSL-W52-TT-T1-03-C1

At the top of Area B of NSL slab near western diaphragm wall

0-1

36.4

 

 

Table 3: Unsuccessful in completing the test
 (as at 18 March 2019 with the latest information highlighted in blue)

Item

Coupler No.

Location of Coupler /steel bar

No. of Exposed Thread

Remarks

1

EWL-E107-BB-B1-02-C1

At the bottom of Area C3 of EWL slab near eastern diaphragm wall

6-7

Remark 1

2

EWL-E40-TT-T1-01-C1

At the top of Area B of EWL slab near eastern diaphragm wall

9-10

Remark 1

3

EWL-E44-TT-T1-01-C1

At the top of Area B of EWL slab near eastern diaphragm wall

1-2

Remark 1

4

No coupler no.
(Date of discovery: 20 December 2018 to 12 January 2019)

At Area C1 of EWL slab:

Not applicable

3 no. of couplers, which are not part of the structure
(Remark 2A)
(Remark 4)

5

No coupler no.
(Date of discovery: 27 February 2019)

At Area C3 of EWL slab

Not applicable

1 no. of coupler
(Remark 2A)
(Remark 5)

6

No coupler no.
(Date of discovery: 24 December 2018 to 24 January 2019)

At Hong Kong Coliseum and Area B of EWL slab

Not applicable

3 no. of couplers (Remark 2B) (Remark 4);
1 no. of coupler (Remark 2B) (Remark 5)

7

No coupler no.
(Date of discovery: 4 January to 16 January 2019)

At Areas C1 and C3 of EWL slab:

11-12;

2-3;

to be confirmed;

3-4

(Remark 2C) (Remark 4);

(Remark 2C) (Remark 4);

(Remark 2C) (Remark 5) ;

(Remark 2C) (Remark 4)

8

No coupler no.
(Date of discovery: 13 February to 28 February 2019)

At Area C3 of EWL slab:

0;

0-1;

3-4

(Remark 2C) (Remark 4);

(Remark 2C) (Remark 4);

(Remark 2C) (Remark 4)

9

No coupler no.
(Date of discovery: 31 January 2019)

At Area C3 of NSL slab:

10-11

(Remark 3)
(Remark 5)

Remark 1: PAUT cannot provide reliable result
Remark 2A: Coupler with no steel bar connected at both sides
Remark 2B: Coupler connected with steel bar at one side only and no reinforcement at the other side
Remark 2C: Steel bar not connected with adjoining coupler
Remark 3: No coupler at the location
Remark 4: The status is confirmed
Remark 5: Details are being investigated

Other findings:

(1) During 17 January to 26 January 2019, a total of 5 no. of hydrophilic strips were found at Areas C1 to C3 of EWL slab.  After verification, it was confirmed that all hydrophilic strips were part of the design details.