本文叙述了家竹箐隧道不用折扣取值而用数解法通过卡斯特纳公式求解塑性区半径的方法。这要求在公式中除了应恰当地采用因随开挖面积不同而形成的形常数即开挖圆半径之外，还应特别计及开挖时人为的支护抗力。此二者都是通过家竹篙隧道施工实践而总结出来的宝贵经验。

The paper describes the evaluation method for the radius of plastic region by numerical solving through Kastner formula instead of evaluation by discount. It is required that not only the radius of excavation circle, i. e. the shape coefficient formed according to different excavation area should be adopted properly, but also the man-made resistance of support during excavation should be taken into account specially. These two aspects are valuable experiences summarized from construction practice of Jiazhuqing Tunnel.