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Tuen Mun Highway Rock Slopes Construction Phase, 1996
The Tuen Mun Highway is one of the major arterial roads from Hong Kong to China and a busy commuter link from the settlements to the north down to HK Island. As part of the new Chek Lap Kok airport development new roads join the Tuen Mun Highway at Ting Kau. Access ramps, lead onto the north-south Route 3, Ting Kau Bridge and Tsing Yi then onto Tsing Ma Bridge, Lantau Island and then Chek Lap Koi Airport.
The widening of the TMH had to be completed with the minimum of disruption to traffic. The old road section at Ting Kau presented some problems because the rock had to be removed on both sides only metres from the carriageway. Even during initial inspections, considerable effort was given to covering the face as shown below.
On behalf of the Ting Kau Contractors Joint Venture, rock mechanics advice, including a risk assessment and a design for protection measures was provided.
A risk assessment scheme was devised at an early stage of the excavation works. There was no as-built information available for the slopes. A rock structure assessment provided key data on the likelihood of large blocks and a strategy was developed for the excavation sequence. The report showed that the risk to public safety was too high during blasting and that no public access should be allowed. However, the risk during excavation/removal could be mitigated by the use of a specifically designed catch netting system. Due to the proximity of the highway, the slope profile, the slope condition and the nature of the excavation process, it was necessary to ensure that the system would not itself be damaged during these operations. The risk assessment included determining the block size distribution, the likelihood of disturbing the face above the road and the likelihood of blocks reaching the road using computer models. This scheme was accepted by the Client and a second report provided a novel scheme of rock nets which were anchored remote from the face and lifted at the crest to catch any blocks rolling over the edge. A scheme was finally agreed and verified in last spring 1996.
As part of the verification process, a series of block trials were carried out on site, away from the road. Opposite shows a trial net being prepared on a small trial slope. Two tonne blocks were rolled into the net and the resultant behaviour and damage to the net was assessed.
The bottom photograph shows the series of frames designed to lift the front face. The Tsing Ma Bridge is in the background. Anchors were placed remote from blasting areas to minimise possible damage. Each cable to the top of the net had at least two and often three grouted anchors in place.
The excavation works have now been completed successfully. Three rock falls did occur and the nets handled all of them. A minimum delay of twenty minutes per blast affected the TMH.
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