Energy release process of surrounding rocks of deep tunnels with two excavation methods

Numerical analysis of the total energy release of surrounding rocks excavated by drill-and-blast （D＆B） method and tunnel boring machine （TBM） method is presented in the paper. The stability of deep tunnels during excavation in terms of energy release is also discussed. The simulation results reveal that energy release during blasting excavation is a dynamic process. An intense dynamic effect is captured at large excavation footage. The magnitude of energy release during full-face excavation with D＆B method is higher than that with TBM method under the same conditions. The energy release rate （ERR） and speed （ERS） also have similar trends. Therefore, the rockbursts in tunnels excavated by D＆B method are frequently encountered and more intensive than those by TBM method. Since the space after tunnel face is occupied by the backup system of TBM, prevention and control of rockbursts are more difficult. Thus, rockbursts in tunnels excavated by TBM method with the same intensity are more harmful than those in tunnels by D＆B method. Reducing tunneling rate of TBM seems to be a good means to decrease ERR and risk of rockburst. The rockbursts observed during excavation of headrace tunnels at Jinping II hydropower station in West China confirm the analytical results obtained in this paper.

Control of settlement and volume loss induced by tunneling under recently reclaimed land

The proposed Tuen Mun-Chek Lap Kok Link(TM-CLKL)project comprises a 9-km-long dual two-lane carriageway between Tuen Mun and North Lantau,Hong Kong.Construction of the two tunnels at the Northern Landfall Fall was carried out by two TBMs:the northbound is referred to as ML03 and has an excavated diameter of 17.63 m,and the southbound is referred to as ML02 and has an excavated diameter of 14.00 m.The use of TBMs minimized the amount of dredging and removal of soft marine deposits,resulting in less impact to the environment within and near the work site of the project.Ground treatment in the form of deep vibro-compaction was carried out at the sand fill and band drains installed in the marine deposit.A surcharge was also added above the final formation level to limit the residual settlement and differential settlement of the reclaimed land to less than 500 mm and 1 in 300,respectively,for a design life of 50 years.Finite element analyses were performed to predict the settlement trough under different confinement pressures along the tunnel alignment.Extensive monitoring of the ground movement was carried out during the driving of the two TBMs to ensure that the design confinement pressure of the slurry would not cause excessive settlement,heave,or blow out at the shallow tunnel sections or different facilities along the tunnel alignment.The monitoring data was then used to back-calculate the percentage of volume loss,which is a key index describing the behavior of the ground during excavation.The monitoring data showed that the measured settlement and back-calculated volume loss were much lower than the predicted values.