Abstract：Construction of crossing tunnels underground in urban area inevitably disturbs the surrounding ground，posing a series of problems on the environment，especially for the old Beijing city. In the construction of Subway Line 6 in Beijing，two tunnels cross each other one over the other in the section between Beihai north to Luoguxiang south，which is a first case of underground tunnel crossing in Beijing. Numerical simulation and field measurement were adopted to study the surface deformation characteristics of five typical sections under different excavation sequences，layouts and grouting methods. The maximum ground surface settlement and the width of settlement trough were smaller and the tunnels were more stable in the case of the lower tunnel excavated first than the ones the upper tunnel excavated first. The surface deformation was found to increase notably with the increase of the crossing angle of two tunnels. When the crossing angle was greater than 60 degrees，the maximum value of ground surface settlement exceeded 30 mm which was above the control standard. The stratum needed to be strengthened through grouting in this case. The maximum settlements at ground surface and tunnel crown decreased with the increase of the thickness of grouted layers. Having considered the economic costs and the unfavorable effects of grouting pressure，the thickness of grouted layers was suggested to be 1.5–2.0 m. The monitored field data showed that the ground surface settlements after grouting met the requirement of control standards and the grouting was an effective method to reduce the ground surface settlement and the width of settlement trough.
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2014, Vol. 33 
