The No. 20 contact of shielding tunnel in Beijing Metro Line 4 project is located in Yongding River piedmont alluvial-pluvial fan, where the outcrop of rock is discontinuous and uneven.

• 摘要北京地铁4号线20标盾构隧道位于永定河冲洪积扇，岩石不均匀出露，在掘进过程中遭遇水下、软硬不均岩石地层和全断面极硬岩等不良地质情况。