Underground projects such as tunnels, shafts, and caves almost always incorporate concrete ele-
ments. The most significant use of concrete underground is as a lining that provides initial and/

or final ground support and, if needed, protection from corrosive environments. Initial and final

linings may be cast-in-place (CIP) concrete, precast concrete segments, shotcrete, or combina-
tions thereof. CIP concrete uses forms into which the concrete is placed and allowed to set until

it attains a specified strength and the forms can be removed. Precast concrete segments are man-
ufactured at a segment manufacturing plant and installed in the tunnel behind tunnel boring

machines (TBMs). Shotcrete is transported, similar to CIP concrete, to the point of application
before being sprayed directly onto the tunnel surface using a spray nozzle without the need for

All three applications of concrete raise construction issues underground that differ from con-
siderations aboveground. The biggest differences arise from the confined nature of underground

construction, distance from point of delivery to point of placement, and the atmosphere or envi-
ronment underground. These issues will recur again and again as we discuss in later chapters the

construction and specification considerations related to each of the concrete applications.

Different methods of construction require different applications of concrete, whether exca-
vating in rock or soft ground and whether using drill-and-blast or mechanical methods of excavation.

Combinations of CIP concrete, shotcrete, and precast concrete segments are applied in almost all
underground excavations in either primary or secondary linings or in one-pass lining systems.


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