为解决软岩隧道开挖过程中初期支护整体下沉普遍较大的工程难题，依托郑州至西安高速铁路大断面黄土隧道及成昆铁路第三系昔格达地层软弱围岩隧道工程，通过理论计算及现场实测，对软弱围岩隧道初期支护普遍沉降较大的原因以及采取的工程措施的可靠性进行分析，得到以下成果： 1）软弱围岩隧道下沉量往往超过20 cm，现场实测的拱脚承受最大荷载为897.4 kN，初期支护整体下沉大的主要原因是拱脚压力较承载力大一个数量级； 2）锁脚锚杆靠近钢架位置的轴力最大，为55 kN。大拱脚的承压特性显著，其压力极值达到0.9～1.7 MPa； 3）增设锁脚锚杆（管）、扩大拱脚和及时闭合仰拱是控制软岩隧道初期支护沉降的关键措施。

The problem of large deformation of primary support during soft rock tunnel excavation is studied by taking the large crosssection loess tunnels on ZhengzhouXi′an Highspeed Railway and tunnels on ChengduKunming Railway in Tertiary Xigeda Fm with soft and weak surrounding rocks for examples. The causes of large settlement of soft rock tunnel primary support and its countermeasures are analyzed by theoretical calculation and site monitoring methods. The study results are drawn as follows: (1) The settlement of soft rock tunnel mostly exceeds 20 cm; and the main cause of large settlement of primary support is the overlarge pressure (897.4 kN in maximum) on arch foot, which significantly exceeds the bearing capacity of arch foot(arch springing). (2) The largest pressure of feetlock bolt is 55 kN, which occurs close to the steel frame; the pressurebearing property of large arch foot is significant and the extreme value is 0.9-1.7 MPa. (3) The methods of adding feetlock bolt (pipe), expanding arch foot and closing inverted arch timely are the keys to control the settlement of primary support of soft rock tunnel.