The construction of the tunnels between the City Hall and Raffles Place stations was difficult as it has to go underneath the Singapore River, which was still polluted at the time and made the soil acidic. An integral waterproofing system was then implemented for the tunnel, as the system is not to be affected by the acidic soil condition and should be an effective and fast system to avoid delay due to tidal condition. Parts of the Singapore River had to be "drained" in stages to facilitate the construction of the MRT tunnels. A cofferdam made of sheet-piles was used to drain the river to allow the construction using the cut-and-cover method in stages. The construction was done in two stages: from the riverbank at the Immigration Department site and the remaining at the other side.