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Road Skill

A set of alternative piling plans to improve a major road in Hong Kong has saved time and money for the government and main contractors on the job. Martyn Smith reports.
The Castle Peak Road improvement project is a $HK280M (!24.56M) highways project in Hong Kong. It involves reconstruction and improvements to some 8km of coastline road along Castle Peak Road through Ting Kau and Sham Tseng.

The first three phases of the project are almost complete with the fourth phase now under way. The two main contractors for phases one to three, China State Construction Engineering and Maeda Corporation, appointed Jacobs China (then Babtie Asia) as alternative designer for some structures.

The client is the Highways and Civil Engineering Development Departments of the Hong Kong government. Meinhardt Halcrow JV is the engineer with responsibility for the overall project. Construction work began in 2003.

The main contractors requested alternative designs in an attempt to be cost-efficient, ease construction in difficult site conditions, enhance final appearance and provide overall technical, economic and social benefits.

All three alternative phase designs went through the Advisory Committee on Appearance of Bridges and Associated Structures submission system and gained approval based on aesthetics and practicability of construction.

The alternatives include skewed concrete arch bridges, re-designed large diameter bored pile walls and excavation and reinforced fill retaining (RE) wall construction over poor ground conditions immediately beneath a curved flyover.

In one example, a large fill slope was discovered with tile drainage pipes at the base of stepped temporary excavations in a restricted space. Over time this had resulted in soft deposits at the base of the excavation. The soft materials had to be excavated and replaced by up to 6m of mass concrete, while the drainpipes were re-routed through the RE wall, using concrete surrounds for the pipes, to allow continuous future drainage from the ground next to the wall.

The modified designs also included three-arch bridges within RE walls, skewed paired concrete arches beneath the existing Ting Kau brides pillars and 10m to 25m high RE walls along the coastline – all to replace conforming concrete viaducts.

Bored pile walls and concrete walls have been substituted for soldier pile walls. Combined bridges using RE wall bank-seats, and abutments with central concrete pillar supports take the place of conforming bridges with deep pile foundations and pile caps. The bridges would have been difficult to construct along the coast at near sea-level.

Footbridge foundations changed from bored piles to pad footings, and in one area lifts replace conventional footbridges. Temporary works included temporary cuts with both normal and pre-stressed soil nails to maintain stability of the old road.

The team replaced a conventional L-shaped concrete retaining wall option, which would have involved excavating the road, with a small rock fill slope built while the road stayed operational. This made construction easier and caused minimal disruption to the public.

Jacobs' approach of using these alternative designs has made cost savings of !4.38M on the project for the Hong Kong government, with a similar amount saved by the main contractors.

The consultant's geotechnical team designed all alternatives, with assistance from external checker SMEC, before being issued for the construction work. The team had only a short timeframe to come up with the designs as the main contractors were already on site.

Martyn Smith is divisional director – geotechnical at Jacobs UK

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