1. Introduction
Given the extensive network of the MRT system in Singapore and the rate of building development, the potential for land development along the MRT lines is very high. To carry out such development, it is very common to see building structures and basement structures adjacent to or directly above the existing MRT lines.
Such construction must be carried out in accordance with the Code of Practice to avoid detrimental effects on sensitive MRT structures. It is important to have proper and adequate precautionary/protective measures incorporated into design and construction work to avoid such effects.
2.Safety and Protection
2.1 Code of practice for railway protection
New developments taking place in the vicinity of existing MRT lines are subject to approval by the Land Transport Authority (LTA). Full compliance with the technical requirements as specified in the Code of Practice for Railway Security must be achieved. The Code requires developers to ensure that movement of MRT structures is limited to the following requirements:
a) Movement in any direction limited to 15 mm;
b) Track rotation limited to 1:1666 in any plane.
However, there are cases where allowable limits can be adjusted based on site conditions (Doran, et al, 2000). Special care must be taken when installing piles and diaphragm walls with the 1st and 2nd Reserve Lines.
Safety measures must be adequately implemented, especially during the installation of piles and diaphragm walls within the 1st and 2nd Reserve Line of the MRT structure. The use of permanent linings can minimize the potential for any stresses imposed on the MRT structure for the installation of piles and diaphragm walls.
2.2 Planning/design consideration
The necessary LTA permits must be approved before any new development can take place in the vicinity of existing MRT structures. The various documents that will be submitted to LTA for approval include the following:
a) Statement of Work Method;
b) Implementation Proposal;
c) Revision levels for various instruments.
All documents must be submitted by the Qualified Person (QP) to LTA prior to undertaking any such construction. Site activities are strictly allowed after LTA approval. For the construction of underground tunnels, exploration, foundation works and development works that involve the use of a crane, the request for exemption from the provisions of the Rapid Transit Systems Regulation (Railway Protection, Restricted Activities)* must be made by the person in writing.
LTA will approve the development proposal in writing if the following requirements are met:
a) The qualified person has complied with the requirements of Clauses 4.2 to 4.5 of the “Code of Good Practices for Railway Protection” and that all the elements presented are in order;
b) The Qualified Persons have fully demonstrated and confirmed that it is feasible for the development works to fully comply with the technical requirements of Section 10 to 12 of the “Code of Practice for Railway Protection”.
*The request for said exemption can be made using the EXEMPT RAILWAY LTA DBC FORM that is available in the “Guide to carry out restricted activities within the railway protection and security zones”.
3. Protective measures
Protective measures are essential when construction work is carried out in the vicinity of MRT structures. Protective measures may include the following:
a) Rigid retaining structures (for example, diaphragm walls);
b) suitable construction methods (eg top-down construction methods);
c) Good watertight retention structures;
d) Water recharge wells;
e) Soil improvement techniques (eg jet grout);
f) Detached piles and screens in the 1st and 2nd Reserves;
g) Silent stacker.
Instrumentation is also necessary to monitor changes that may affect the tunnels during construction work.
3 Conclusions
1) A good planning and control system consisting of proper Codes, Guidelines and procedures is essential to ensure the successful execution of new developments near existing MRT structures and to maintain public safety. The establishment of allowable limits by local authorities based on design conditions and field performance provides useful guidance for engineers/contractors on these critical projects.
2) Construction activities within the Railway Protection Zone will be regulated and controlled by the Code of Practice and executed under the supervision of competent engineers to minimize detrimental effects on existing MRT structures.
3) The effects of construction on existing MRT structures shall be properly assessed by necessary analysis at design and verified by a suitable instrumentation system during construction.
4) The application of adequate protection measures, e.g. e.g. rigid restraint system, soil improvement techniques, special equipment and advanced automatic control systems contribute significantly to the safety control of underground MRT structures in the vicinity of construction sites in view of their sensitivity and accessibility restricted. However, proper interpretation of instrument readings, good engineering judgment, and supervision by competent geotechnical engineers are required.
References
1) Land Transport Authority 2000, Code of Practice for Railway Protection, Singapore
2) SKKong, Moh and Associate Pte Ltd, Security Control for Development Adjacent to Existing MRT Structures, Singapore
3) Doran SR, Wood T, Tham SK, Copsey JP Shirlaw JN & Wen D. 2000, Assessing the limits to movement of underground tunnels and pathways due to adjacent construction. Proc International Conference on Tunneling and Underground Structures 26 – 29 November, Singapore