XN000022-1983-01-19 — Page 6

WEDNESDAY, JANUARY 19, 1983.

ROAD SUBSIDENCE STILL UNDER STUDY

****

THE GOVERNMENT IS STILL INVESTIGATING THE SUBSIDENCE OF A SECTION OF HENNESSY ROAD ON NEW YEAR'S DAY AND WILL, IF NECESSARY, MAKE RECOMMENDATIONS TO THE MASS TRANSIT RAILWAY CORPORATION TO MODIFY ITS TUNNELLING PROCEDURES TO INCREASE SAFETY AND PREVENT SIMILAR ACCIDENTS IN THE FUTURE.

THIS WAS STATED BY THE SECRETARY FOR LANDS AND WORKS, MR DAVID MCDONALD, IN REPLY TO A QUESTION BY THE HON ANDREW SO IN THE LEGISLATIVE COUNCIL TODAY.

HOWEVER, MR MCDONALD ADDED, IT SHOULD BE NOTED THAT THE MTRC ALREADY KEPT ITS WORKING PROCEDŪRES AND PRECAUTIONS UNDER CONSTANT REVIEW AND ALTERED THEM AS CONSIDERED NECESSARY TO SUIT PARTICULAR CIRCUMSTANCES.

MR MCDONALD RECALLED THAT PRIOR TO THE SUBSIDENCE A MTRC CONTRACTOR WAS CARRYING OUT TUNNELLING WORKS UNDER HENNESSY ROAD

IN HARD ROCK CONDITIONS.

+ALTHOUGH IT WAS KNOWN THAT THE WORKS WERE APPROACHING AN AREA OF SOFT GROUND THE RESULT OF PROBES DRIVEN FOUR METRES INTO THE ROCK FACE TO CONFIRM INFORMATION ON GROUND CONDITIONS OBTAINED FROM EARLIER PROBES AND BOREHOLES INDICATED TO THE ENGINEERS AND CONTRACTORS THAT FURTHER BLASTING COULD BE CARRIED OUT AND STILL MAINTAIN AN ADEQUATE ROCK COVER,+ HE SAID.

+ON THE AFTERNOON OF DECEMBER 31 CHARGES WERE SET TO REMOVE ABOUT 1.3 METRES OF THE ROCK, ABOUT FIVE HOURS AFTER THE BLASTING A SEEPAGE OF WATER OCCURRED THROUGH A SECTION OF MODERATELY DECOMPOSED GRANITE IN THE TUNNEL FACE.+

MR MCDONALD SAID THAT THE FLOW OF WATER AND MATERIAL INCREASED AS TIME PASSED AND BY MIDNIGHT THE FAULT HAD ENLARGED IN AREA AND A CHIMNEY OR VOID HAD DEVELOPED BEHIND THE ROCK FACE.

ATTEMPTS BY THE CONTRACTOR TO STEM THE FLOW OF MATERIAL FROM THE CHIMNEY BY ERECTING BULKHEADS IN THE TUNNEL WERE UNSUCCESSFUL AND THE FLOW CONTINUED.

AT ABOUT 4.15 AM THE INITIAL ROAD COLLAPSE OCCURRED FOLLOWED AT ABOUT 5 AM BY A FURTHER LARGE COLLAPSE AND ONLY GRADUAL ENLARGEMENT OF THE HOLE FROM THEN ON, MR MCDONALD SAID.

15