What is Engineering Geology of tunnels??
Engineering Geology of Tunnels
A tunnel is much more than just a tunnel. It serves ant of myriad functions highway, railroad, or rapid artery; pedestrian passageway; fresh water conveyance, cooling water supply, waste water collector or transport; hydropower generator; or utility corridor. In cross-section it takes one of the several shapes – circular, mullticurve, horseshoe, cathedral arch, arched, or flat-roofed, and withy clear spans of from a few feet to more than 50 ft and, in cavern form, much wider, its length can vary from less than 100 ft to more than 30 miles. A tunnel can be located in any of variety of places – under mountains, cities, rivers, lakes, sea estuaries, straits, or bays. Finally, a tunnel is constructed in one of innumerable media – soft ground, mixed face, rock uniform, jumbled, layered, dry, wet, stable, flowing, and squeezing.
Influence of Geological Conditions on Design and Construction
There are several particular geological features which are commonly encountered in tunneling operations. These can give rise to difficulties especially in impeding progress and/or increasing the hazardous nature of the operation. Awareness of geological conditions on tunneling operations is a natural extension of a thorough site investigation. Changes in geological conditions which result in decreased competence of the rocks surrounding the excavation often result in increasing tunneling costs in addition to affecting operational and safety aspects.Consequently, in-depth appreciation of the geological conditions plays an important role from design and planning through to construction and eventual commissioning and operation of the tunnel.
The Geological Environment
Tunnels are driven in virtually all the main rock types. Wahistrom gives a detailed account of geological aspects related to tunneling operations and draws attention to the importance of petrology and alteration aspects.
The crystalline nature of igneous rocks signifies high compressive strength with potential difficulties in rock excavation processes, but can also indicate marked competence with the advantages of decreased support needs to achieve an acceptable degree of stability. Igneous rocks can be classified under two broad categories, namely extrusive and intrusive bodies. The size often crystals of such rocks is generally dependent on the size and geological history of the igneous body. Localized and relatively thin intrusives are usually fine-grained and often possess high strength and significant resistance to weathering by comparison to coarser grained igneous types with similar mineral constituents. Intrusive igneous bodies can range from granitic batholiths exhibiting surface exposure areas in excess of 100 km2, to dyke swarms, sills etc.