Summary of Case 6.2: "TunnelingProblems in Metamorphic Rocks
Metamorphic rocks are not the best to build tunnels according toengineering facts. These facts include the fragile nature of therocks. The fragile nature means that construction of a tunnel willrequire many steel rods to support the exterior rock wall. Theprocess of building tunnels in metamorphic rocks is hazardous to thehuman labor force involved in the construction. The rocks havedifferent characteristics as a number of folds, faults, thrusts ofvarious magnitudes and are heavily charged with sub-terrain water.The objective of the study is to uncover in detail, these and otherhardships that face tunnel construction in metamorphic rocks.
Due to the continued development in research, the need to harnesswater resources for human utility power generation and irrigationthe demand for a water diversion system. Such a diversion systemproves difficult especially in areas that are tectonically active.Over the years, researchers and engineers have investigated on thetopic, tried it out and researched on ways that could reduce theoverbreak. Overbreak refers to the excessive volume blown up in a bidto create the tunnel volume. Overbreak makes the operation costlybecause it has to be sealed with concrete and steel rods (Wahlstrom2012). Contrarily, overbreak gives engineers a chance to readjustmeasurements according to the newly available volume and have amplespace to put up a strong foundation for the tunnel.
FIGURE 1. Cross section of a tunnel showing the overbreak, or areaexcavated beyond the payline to accommodate steel supports. Thepayline is the outer boundary of the area estimated for excavationprior to construction.
Benefits of tunnels overweigh the problems and costs involved. Theessence of the overbreak volume is to create space to mount rods forstability in the tunnel. Since metamorphic rocks are susceptible topressure, construction should have a layer of igneous rocks,fortified by concrete, to form a strong cylindrical or cuboid shapethat gives the tunnel a firm support. The underlying metamorphic rockfor example in the Himalayas give the percentage support necessary tohold a certain tunnel volume. According to the graph on roughnessfactor, mean overbreak and eccentricity against percentage support,the slope is positive. This shows that the elements in comparison aredirectly proportional.
FIGURE 1. Plot of various outbreak parameters versus percentagesupport, showing that overbreak and therefore costs of tunnelingincreases in areas of foliated metamorphic rocks.
Overbreak is an inevitable factor in tunnel construction.Additionally, the volume of overbreak varies with the extractiveapproach used. When constructors use explosives to excavate thematerial in the designated area, it gets hard to control the volumeof overbreak surrounding the tunnel. However, the volume is useful informing a stable foundation for the tunnel, which is a preventivemeasure for occasions when geographical forces of the earth wouldcause movement in the tectonic plates. The main idea in the overallconstruction is to minimize the cost, maintain stability anddurability in the same. Tunnels are meant to serve the society foryears, which is its prime benefit. Therefore, construction shouldtake an approach that doesn’t necessarily work to reduce the costsinvolved, but on the quality and stability of the tunnel and itsdurability.
Wahlstrom,E. (2012). TunnelingIn Rocks (Vol.3). Elsevier.