This paper reports the ground improvement and excavation of a cross passage between two shield tunnels for the construction of Tu-chen Line of Taipei Rapid Transit Systems. Jet grouting was conducted for soils around the cross passage before the tunneling with the shield machine. Water-leak tests were conducted to detect any possible crack in the JSG soilcrete, and chemical grouting was conducted from the shield tunnel to fill any possible crack. It is concluded that the subsurface conditions encountered at great depths might be quite complicated. The geotechnical engineer should never overestimate the watertight characteristics of the soilcrete formed around the cross passage by jet grouting and chemical grouting. Under the threat of high water pressure, the contractor is suggested to bear the multiple-defense-lines concept to keep the excavation work on the safe side.