关键词: 隧道, 上导坑法进洞, 仰坡稳定, 土压力, 挡土墙, 洞口段, 浅埋等压, 偏压

Abstract: The "guiding wall + large pipe shed" is basically the only method for tunnel entry at present. When the surrounding rock condition is good, it is a waste to use guiding wall and large pipe shed, whereas when the surrounding rock condition is bad, it is insufficient in specificity and safety. Therefore, the Coulomb soil pressure theory is used to calculate and analyze the soil pressure on the front slope of the tunnel entrance during the construction process, obtaining the conclusions that the end wall of the entrance is an important measure to stabilize the front slope of the entrance and controlling the primary support settlement is the key to the construction of the portal section. Then, the Rankine earth pressure principle is used to calculate the "shallow buried equal pressure" burial depth of surrounding rocks at all levels of the tunnel. The tunnel support structure is in a favorable stress state without bending moment under shallow buried equal pressure, and that is in the most unfavorable stress state under only lateral load. The maximum internal force of the support structure is calculated, and the reasonable thickness of the structure is obtained. This serves as a reference value for increasing the stiffness of the "exposed" and near exposed shallow buried eccentric pressure tunnel support structure by increasing the thickness of the structural section. The essence of the bias state is the vertical deflection of the direction of the main stress action under the positive pressure state, which simplifies the calculation and analysis of the primary support under bias pressure to the deflection of the main stress direction under the positive pressure. Finally, an engineering example is used to illustrate the advantages and disadvantages of the "guide wall + large pipe shed" method and the "shed hole" method for tunnel entry, proving that the current tunnel entry plan still needs to be optimized. As weill, an engineering example with fully-and stronglyweathered granite geology with significant shallow buried bias with support structure "exposed" is used to verify the advantages of the above technology. It is concluded that using retaining walls to stabilize the front slope of the tunnel entrance is a key measure for tunnel entry; the construction of the portal section should focus on controlling support settlement, the method of increasing the stiffness of the support structure in the exposed part of the shallow buried bias tunnel with exposed support structure should be adopted.

Key words: tunnel, top heading method, front slope stability, soil pressure, retaining wall, opening section, shallow isobaric, bias voltage