Geomaterial Excavation in Surficial Environment

Some materials and energy needed to operate modern societies are conveyed and transmitted through lifelines that are buried in excavated trenches in the subsurface. Over a span of millennia, excavation technology for geomaterials has gone from the use of flint stones and sticks to dislodge soil to sophisticated machinery and techniques for intelligent removal of soil and rock in complex geohydrological environments. Developments in geomaterials excavation technology have also been matched by advances in analytical techniques to support mineral resource recovery, waste burial, transport of pipeborne water, oil and gas operations, geothermal energy development, large-scale civil and industrial construction, tunneling operations for mass transit, and even extraterrestrial excavation.

Large-scale projects are being implemented in many countries as part of economic development programs. Energy generation and supply require the design, construction, and maintenance of buried pipelines, cables, and control stations. In early 2005, more than $\mathrm{83,528}\mathrm{km}$ of oil and gas pipelines were under construction through various terrains globally, of which $\mathrm{22,873}\mathrm{km}$ were in North America. For projects completed between July 1, 2004, and June 30, 2005, data obtained from the United States Federal Energy Regulatory Commission indicate that the average implementation cost per kilometer of pipeline was US$0.755 million, within an approximate range of US$0.332 million to 3.734 million.

Despite the high pace of pipeline construction and the high volume of materials excavated, the exceedance of current and potential energy delivery capacity of available infrastructure by current and projected demand is a concern that has driven the initiation of many additional construction projects worldwide. For example, the European Union is expected to become dependent on external energy supplies by as much as 70% by 2030. A larger fraction of oil and gas supplies to the EU will need to be conveyed by buried pipelines that will traverse rugged and rocky terrains, as at present.

Energy supply is not the sole driver of geomaterials excavation projects. Despite improvements in material recycling and the extraction efficiency of valuable minerals from excavated materials, mining and processing operations still leave behind huge quantities of tailings and other residuals. Geomaterial excavation for developing civil and industrial infrastructure is a feature of most urban areas. Construction of physical facilities, and hence excavation of geomaterials, is increasing to keep pace with high urbanization rates (averaging 3%) of an increasing total population in most countries.

With congestion constraining the lateral expansion of new housing in central parts of large cities, it is reasonable to expect that extending housing to much deeper levels beneath large cities will become the norm within the next two decades. One city that exemplifies this expectation is Tokyo. This development will call for much more intelligent excavation machines that can avoid critical lifelines and structural foundations on which the sustenance and stability of aboveground systems depend. For almost all projects, minimization of costs, including excavation costs, while maintaining an acceptable level of service or facility quality, is an objective. Satisfaction of this objective requires the development and application of models and analytical techniques.

Invariably, excavating of geomaterials requires the exertion of mechanical, electrical, or chemical energy on the excavated geomaterials through a device or machine. The required energy and the operational efficiency of excavation processes depend on the mechanics of the excavating machine or instrument, its interaction with the geomaterials, and the resistance of the latter, as determined by its strength and intactness. The manner in which the excavating machine imposes stresses on the rock is one of the determinants of its excavation efficiency and ultimately the project cost. Although force and stress balance approaches usually satisfy theoretical requirements, the use of energy expended per unit volume of excavated space (specific energy) is usually found to be convenient in practice. The conversion from imposed stress to imposed energy is constrained by uncertainties regarding the surface area of rock contact for sharp cutting bits (required to compute force from stress) and by the distance of translation of force to compute energy (considering that energy is the product of force and distance).

Many analysts have attempted to circumvent these analytical constraints by assuming various fracture geometries of rock fragments, with or without material crushing, to estimate energy expenditure in rock drilling processes. In cases such as tunnels and trenches for civil, industrial, and military operations, excavations are developed to specified geometries; whereas in cases such as mining and dredging, the geometrical configuration of the excavated space is less important. The introduction of intelligent systems such as robots has advanced autonomous excavation in risky environments, typified by radiologically contaminated sites and rarefied atmospheres of planetary bodies such as the moon. Indeed, the integration of these systems with basic terramechanics of excavation processes has advanced the state of the art. These systems are needed to improve the cost-effectiveness of physical infrastructure development projects in economic sectors such as energy and transportation.