Okura tunnel is located in Route 4 of Kitakyushu Urban Expressway. The road in the up-line that has two lanes is restructured wider to have three lanes within 170m range from pithead. The construction is carried out while the two lanes are open by installing protectors as guardrail inside the tunnel.
Road widening without traffic control by using protectors is conducted in other cases before but this construction is recognized to be the first case for widening of two lanes to three under urban expressway's heavy traffic.
This report presents the construction's plan and design summary, and discusses the differences in the design and the actual construction of tunnel excavation.
