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1. Introduction

Given the wide network of the MRT system in Singapore and rate of building development, the potential of land development along MRT lines is very high. In order to carry out such development, it is very common to see building structures and basement structures adjoining to or directly above the existing MRT lines.

Such construction must be carried out in a manner according to the Code of Practice to avoid detrimental effects on the sensitive MRT structures. It is important to have proper and adequate precaution/ protection measures incorporated in the design and construction works to avoid such effects.

2. Safety and Protection

2.1 Code of Practice for Railway Protection

New developments that are taking place in close proximity to existing MRT lines are subjected to the approval of Land Transport Authority (LTA). Full compliance of the technical requirements must be achieved as specified in the Code of Practice for Railway Protection. The Code requires the developers to ensure that the movement of the MRT structures is limited to the following requirements:

a)       Movement in any direction limited to 15mm;

b)       Rotation of the tracks limited to 1:1666 in any plane.

However, there are cases where the allowable limits may be adjusted according to the site conditions (Doran, et al, 2000). Special care has to be taken when undertaking installation of piles and diaphragm walls with the 1st and 2nd Reserve Lines.

 Safety measures must be adequately in place especially during the installation of piles and diaphragm walls within the 1st and 2nd Reserve Line of the MRT structure. Usage of permanent casings can minimize the potential for any imposed stress on the MRT structure for installation of piles and diaphragm walls.

2.2 Planning/ Design Consideration

Necessary permits from LTA must be approved before any new development in close proximity to existing MRT structures can take place. The various documents to be submitted to LTA for approval includes the following:

a)       Method Statement of Work;

b)       Instrumentation Proposal;

c)       Review Levels for various instruments.

All documents are to be submitted by the Qualified Person (QP) to LTA before the undertaking of any construction of such nature. Site activities are strictly allowed to proceed after approval by LTA. For construction of underground tunnels, exploration, foundation works and development work which would involve the use of a crane, an application for the exemption of the provisions under the Rapid Transit Systems (Railway Protection, Restricted Activities) Regulations* shall be made by the qualified person in writing.

LTA will approve the developmental proposal in writing if the following requirements are compiled with:

a)       Qualified person has complied with the requirements of Clauses 4.2 to 4.5 of the “Code of Practice for Railway Protection” and that all items submitted are found to be in order;

b)       Qualified persons has fully demonstrated and confirmed that it is feasible for the development works to fully meet the technical requirements of Section 10 to 12 of the “Code of Practice for Railway Protection”.

*Application for such exemption can be made using FORM LTA DBC RAIL EXEMPT which is available in the “Guide To Carrying Restricted Activities within Railway Protection and Safety Zones”.

3. Protective Measures

Protective measures are essential when carrying out construction work in close proximity to the MRT structures. The protective measures can include the following:

a)       Rigid retaining structures (e.g. diaphragm walls);

b)       Proper construction methods (e.g. top down construction methods);

c)       Good water tight retaining structures;

d)       Water recharging wells;

e)       Soil improvement techniques (e.g. jet grout);

f)        Debonding of piles and diaphragm walls in the 1st and 2nd Reserves;

g)       Silent Piler.

Instrumentation are also necessary to monitor the changes that might affect the tunnels during the construction works.

3. Conclusions

1)       Good planning and control system consisting of Codes, Guidelines and proper procedures are essential in ensuring the successful execution of new developments near existing MRT structures and maintaining public safety. The establishment of the allowable limits by local authorities based on design conditions and field performance provide a useful guide to the engineers/contractors of these critical projects.

2)       Construction activities within the Railway Protection Zone shall be regularized and controlled by the Code of Practice for and executed under the supervision of competent engineers for minimizing detrimental effects on the existing MRT structures.

3)       The effects of construction on the existing MRT structures hall be properly evaluated by necessary analyses in the design and verified by suitable instrumentation system during construction.

4)       The application of suitable protective measures, e.g. rigid retaining system, soil improvement techniques, special equipment and advanced automatic monitoring systems are fond with significant contribution to the safety control of underground MRT structures in close proximity to construction sites in view of its sensitivity and restricted accessibility. However, proper interpretation of the instrument readings, application of good engineering judgment and supervision by competent geotechnical engineers are needed.

References

1)       Land Transport Authority 2000, Code of Practice for Railway Protection, Singapore

2)       S.K.Kong, Moh and Associate Pte Ltd, Safety Control for Development Adjacent to Existing MRT Structures , Singapore

3)       Doran S.R., Wood T, Tham S.K., Copsey J.P. Shirlaw J.N. & Wen D. 2000, The Assessment of Limits for The Movment of Subway Tunnels and Trackworks Due to Adjacent Construction. Proc International Conference On Tunneling and Underground Structures 26 – 29 November, Singapore

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Source by Zeng Han Jun