1. Introduction

The design covered two areas, namely geotechnical and C&S works. Geotechnical design was covered by M/s Kiso-Jiban Singapore Pte Ltd, and it involved mainly the temporary supporting system to facilitate excavation of the underpass. Briefly, sheet pile wall was used for excavation away from Merdeka Bridge. Excavation below the bridge was facilitated by jet grouting and micro-tunnelling. The overall concept of the underpass design was by T.Y. LIN South East Asia Pte Ltd (TYL), which also took into account geotechnical constraints.

Micro-tunnelling was adopted below the bridge as the retaining system due to insufficient height for the installation of diaphragm wall or similar system. Jet grouting was provided to stiffen the soft Marine Clay and that would reduce deflection caused by excavation.

There were various concerns TYL had regarding the project:

(a) The soft Marine Clay stratum encountered at the site.

(b) Sensitivity of Merdeka Bridge foundation to the excavation system adopted. TYL was aware that Merdeka Bridge was upgraded from a simply-support deck system to that of a continuous frame structure. This upgrading work performed some time ago in 1993 also enabled the bridge foundation to absorb more lateral forces.

(c) Buildability to improve speed of construction and ease of application of waterproofing system.

With the above concerns, the following design strategies were formulated:

(a) Reduction of excavation depth was a prime objective and will be most beneficial in minimising disturbance to the bridge foundation. This was achieved by designing the wall as pure cantilever without structural props at the top. The design was actually more expensive but it would be compensated by simpler construction requirements.

(b) Changing the open box drainage system to that of a conduit drain, thereby making the soffit of the underpass flat and without haunch.

(c) Adoption of three-dimensional analysis techniques to assess actual stress patterns and structural behaviour. In the process, achieving the necessary structural efficiency which a two-dimensional analysis could not be attained.

3. Final Structural System Adopted

The bored pile foundation set during tender was retained, as the layout was efficient. A check was made on the adequacy of piles installed by the previous contractor and they were found to be compatible. The pump sump sizes were check against ENV requirement and were found to be adequate and hence maintained.

Underpass slab was changed to a flat plate system giving rise to speedier design, construction and placing of waterproofing system. It is to be noted that movement of retaining system for strutted excavation is also time dependent, particularly in sensitive Marine Clay environment. Disturbance to the bridge foundation would be reduced if less time was spend in construction. 3-D shell analysis was performed with the cantilever wall and skip loading was carried out to derived the worse possible moment and force for the structural design.

The stretch of underpass below the bridge was analysed as 3-D shell elements suspended from the sump walls outside the bridge. It was designed as an RC structure with post-tensioning provided to control deflection and formation of early thermal and shrinkage cracks.

The prop beams above the underpass was omitted and the lowest point of the underpass was then raised by the depth of the beam. However, the length of the underpass was maintained by reducing the gradient of the vertical alignment. This was to enhance the safety aspect of the road design.

Micro-tunnelling steel pipe and some of the sheet pile wall would not be extracted at the bridge location, as extraction would cause further unnecessary ground movement. Since they would be permanently installed, they were combined with the concrete structural walls to enhance the final retaining wall system.

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