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Traffic congestion in existing tunnels has increased due to increased traffic volume and enlarged vehicles. Enlarging existing tunnels has advantages over constructing new tunnels by reducing land purchasing costs as well as minimizing natural environment destruction. In fact, many overseas projects for enlarging existing tunnels have been reported. Thus, it appears that the demand on enlarging existing tunnels continues to rise in Korea in near future. Nonetheless, the studies related to the enlarged tunnels have been relatively rare since there have been few tunnel enlargement projects in Korea. In the present study, the tunnel behavior and the stability of rock pillar when enlarging existing parallel tunnels were investigated by performing FE analysis and using existing theory and empirical relationships. Four different enlarging cases, depending on the enlargement types and directions, were examined in the study. According to the results, for the tunnels with the same pillar width after enlarged, the uni-laterally enlarged tunnel indicated 5 to 20% higher crown settlement compared to the bi-laterally enlarged tunnel, and for the tunnel with the narrowest pillar, the highest shotcrete stress was observed. Also, the strength/stress ratio for rock pillar was more than 1.0 for all four enlargement cases, and the Matsuda's method was found to give higher strength/stress ratio by about 50% compared to the Peck's method.

In this study, characteristics of the long-term landfill settlement are analysed and the starting date of residual settlement is deduced using the data of the landfill 1 of the Gimpo Metropolitan Landfill (GML) measured for 27 years. The landfill 1 is a multi-staged municipal solid waste landfill where dykes are constructed after landfilling for subsequent waste fills. The landfilling began in 1992 and finished in 2000, and the waste settlement measurement continued throughout this period. The older waste of the lower lift shows higher biodegradation and larger settlement than the waste of the upper lift, which acts as vertical load. Large settlement occurred in the lower lifts due to the collapse of waste voids caused by the overburden load of the upper lifts after biodegradation of waste in the lower lifts during long-term landfilling. In the conventional landfills, that is, mono-layer landfills, the time-dependent settlement generally occurs after the stress-dependent settlement. But, most of the time, this landfill showed mainly the time-dependent settlement. The duration, at which the biodegradation of organic matters is reduced and the residual settlement begins, was 8.8 years in average and the date range was from 7.6 to 11.7 years considering each disposed block in landfill 1. This date is obtained by calculating from the mid-point time of the landfilling duration.