WorldWide Drilling Resource

85 sales@aardva Manufacturing the Be Mechanical Inflatable Environmental Geotechical Packer Parts 5-546-6488 rkpackers com r r st Packers, Including: Wireline Straddle Mining G out Packe s Custom Design . 16 NOVEMBER 2021 WorldWide Drilling Resource® NATM - Tunnel Vision of a Different Sort Compiled by the Editorial Staff of WorldWide Drilling Resource® Tunnel vision, defined by Merriam-Webster, is “single-minded concentration on one objective”. While focus and concentration can be generally positive, viewing situations too narrowly may lead to an inability to consider all factors necessary to get a job done successfully. When constructing an actual tunnel, a team must have a different sort of “tunnel vision,” still focused, but with knowledge obtained through monitoring of many ongoing factors. The New Austrian Tunneling Method (NATM), also known as the sequential excavation method in the U.S., first garnered recognition during tunnelling operations in the Austrian Alps. NATM is both a construction method and a design philosophy. The philosophy uses the geological stress of the surrounding rock mass to stabilize the tunnel structure itself. In other words, ground conditions drive the tunnelling operation. The same philosophy also emphasizes constant monitoring. It is an effective method where there are diversified geological conditions, making forecasting of the rock mass difficult due to rapidly changing geology because it works on the observation approach to control stability. Often referred to as a design-as-you-go approach, NATM could be better characterized as a design-as-you-monitor approach. Generally, the process involves letting the weak ground around the tunnel deform in a controlled way by applying a flexible rock support. Later, when the rate of displacement reaches a specified limit, the permanent rock support is installed, designed to withstand future loads. Actual construction operations usually include the following: j Profile marking by the survey team to maintain the minimum excavation line. j Working face drilling a pattern of holes. j Charging and blasting - inserting the explosives into the drilled holes and charging them. j Defuming to expel harmful gases after the blast - ventilation ducts and fans. j Mucking to remove material produced as a result of blasting. j Scaling and chipping to check for undercuts or loose material or cracks - remove with excavators or breakers. j Geological mapping to determine the types, number of joints, and type of rock conditions of the face. j Face sealing shotcrete as a protective layer applied on the face and periphery to avoid falling of loose materials. j 3D monitoring targets installation to check deformations inside the tunnel after the excavation. j Lattice girder erection to act as initial support and provide a defined shape to the tunnel. j Forepoling for additional support in the case of weak rock in the crown portion. j Shotcreting, either sealing shotcrete or main shotcrete, to prevent loose material falling and provide load support. j Rock bolting to stitch the shotcrete with the rock as one unit to support the whole burden. Versatility is an advantage of NATM technology as it can be used for hard rock, soft rock, and blended ground conditions. It offers flexibility to build tunnels of almost any size or shape; however, curvilinear (oval) is preferred to allow smooth stress redistribution. Other advantages are its ability to deal with locally unexpected ground conditions and the possibility of optimizing equipment and crews by allowing multiple concurrent heading operations. More economical support can be provided only as required by the encountered ground conditions rather than a one-size-fits-all support system designed for worst load case. NATM is not perfect as it requires much coordination, cooperation, and communication, as well as a higher technology requirement for monitoring, to compete with the other construction methods. Even at peak performance, NATM typically performs at a slower rate than other methods as the team adapts to changing conditions. For NATM to compete successfully, the operation must reduce interruptions in excavation and support work by closely working with all elements of the operation to assess and respond to the changing conditions. Thus, NATM’s broader vision of progress monitoring to inform construction is the key to successful completion for professionals choosing this tunnelling method. C&G