Study On Entrance Technology Of Xiaokang Freeway Tunnel Portal

The constrution of tunnel portal is always difficult. Relying on the tunnel portal constrction of xiaokang freeway, tunnel portal "zero excavation" constrction methods was drawn throuth summary of portal constrction technology, environment investigation and statistics and analysis for advanced support measures, excavation methods and support parameter basing on different topography and geology condition. The contents are as follows.(1) For bad geological conditions, "early detection, early exclusion" should undertaken outside tunnel and the measures taken mainly include clearing the slope stones,anti-slide pile and so on. Inside tunnel, the measures including strengthening advanced support, reducing excavation progress and disturbance to rock and timely and strengthening support should taken to make sure the tunnel security.(2) According to topographical conditions, the tunnel constrution measures can be divided into the following four types:orthogonal hole method, oblique hole method, the lateral hole and one-way hole. The main method of orthogonal hole in xiaokang freeway tunnel is "umbrella arch method" and "half umbrella arch method". To avoid bad construction condition, the lateral hole and one-way hole choose better conditon beneficial to constrtion and reverse the constrution sequence. The lateral hole is suitbale for long tunnel with short transverse hole and one-way hole is suitbale for short tunnel with short construction period.(3) The whole process of construction of "umbrella arch method" was simulated by finite element software MIDAS/GTS in the situation of shallow, V grade surrounding rock support and ring excavation leaving core soil. The simulation was proved that "umbrella arch method" was effective in the shallow condition.The safety of tunnel portal constrtion was closely raited to advanced support measures, inside excavation methods and support scheme. As long as the rational use of these three factors, we can create a series of security construction methods. "umbrella arch method" and "half umbrella arch method" was succeed by strengthening advanced support, reducing excavation progress and disturbance to rock and timely and strengthening support, so the two metheds has extensive applicability in mountain tunnel portal constrution.