Deep hole monitoring technology and engineering application of groundwater stratified hydraulic connection for deep tunnel

[Objective] The issue of external water pressure is a key problem in the construction of deep-buried tunnels. It is a great challenge to determine the external water pressure of deep tunnel due to the large depth of deep tunnel, high groundwater level and complex hydraulic relationship between strata. Clarifying the hydraulic connection law of groundwater in the whole life cycle of tunnels is the key to accurately evaluate the external water pressure of deep tunnels and the impact of tunnel construction on groundwater environment. [Methods] In this paper, two different initial groundwater distribution conditions are applied to two geological models of different stratum types respectively, and a total of three stratum models are generalized for seepage calculation by numerical simulation. By monitoring the groundwater pressure of the corresponding strata and the amount of water inflow in the tunnel during the tunnel excavation process, the influence of different stratum types on groundwater evolution and distribution after tunnel excavation is studied. Then, on this basis, a groundwater stratified hydraulic connection monitoring technique for deep tunnel is proposed. This monitoring technique uses deep holes on the surface to monitor the undisturbed groundwater pressure in different strata above the tunnel in advance in front of the tunnel face. The strata with poor water permeability are regarded as water-proof strata, and the strata with better water permeability are regarded as water-permeable strata. The monitoring section is arranged in the water-permeable stratum, and the blocking section is arranged in the water-proof stratum. And the core key issues of this monitoring technique are verified by indoor experiments in the laboratory. Finally, this monitoring technique is successfully applied to Caijiacun tunnel of the Central Yunnan Water Diversion Project. A deep hole named ZKSY301 is divided from bottom to top into "monitoring section-blocking section-monitoring section-blocking section-monitoring section". The blocking section is used to restore the water-proof characteristics of the water-proof stratum. The monitoring section is used to monitor the groundwater pressure of the water-permeable stratum. Several monitoring instruments are used to monitor groundwater pressure in different water-permeable strata during tunnel excavation. [Results] The numerical simulation showed that whether there was water-proof formation above the tunnel had different influence on the groundwater hydraulic connection after excavation. When there was a water-proof stratum, tunnel excavation only affected the groundwater in the stratum that the tunnel traversed but had no effect on the groundwater above the water-proof stratum. Groundwater stratification occurred between different strata, and led to that the amount of water inflow into the tunnel was significantly reduced. The feasibility of stratified monitoring technology and the existence of stratified groundwater hydraulic connection in deep tunnels were verified by engineering application. And it founded that as the Caijiacun tunnel continued to advance, the groundwater pressure in the stratum where the tunnel traversed dropped rapidly, while the groundwater pressure in the stratum above the water-proof stratum dropped slowly or was basically unchanged. In-sile monitoring results confirmed the existence of stratification in the hydraulic connection of groundwater in deep tunnels. Finally, based on engineering applications, several suggestions were put forward for the in-sile application of layered monitoring technique for technical personnel's reference. [Conclusions] The stratified monitoring technique pioneered in this paper provides the real initial external water pressure in deep tunnels. And the evolution law of groundwater distribution throughout the life cycle, before construction-construction-operation, can be obtained. The stratified monitoring technique provides an effective mean for the study of high external water pressure in deep tunnels and improves the deficiencies in the existing tunnel external water pressure design specifications. © 2024 Tsinghua University. All rights reserved.