562 URBAN GEOLOGY and in costs of construction The process by which geological input is discovered and provided to permitting officials, planners, owners, designers, and construction contractors is termed ‘site characterization’, and this task must include the discovery and description of man-made wastes (perhaps of archaeological/ historical importance) left from previous site occupation Site characterization as a concept generally arose in about 1900, but has taken on the particular meaning described here only since about 1980 It is a well-accepted practice in the field of engineered construction and environmental permitting Engineering geologists are generally tasked with planning and conducting site and waste characterizations, and the most important aspects related to the effort are experience, training, and professional competence of the geologist and the provision of adequate exploration funding by the builder/owner In this respect, there has been a truly unfortunate trend towards unwarranted price-driven selection (‘bidshopping’, or commoditization) of site-characterization consultants, by owners and some engineers, worldwide, since about 1980 Suffice it to say that the geologist who will perform the site characterization should be selected on the basis of qualifications and experience, rather than on the cost (budget) accorded to the effort Engineering Geological Site Characterization Each construction site needs to be characterized for its geological, hydrological, and waste conditions, so that architects and design engineers have the necessary parameters when considering feasibility and design In some cases, when geological constraints (the preferred term for ‘geological hazards’) are identified, the proposed construction may be shifted to a geologically ‘less expensive’ site in terms of building costs, operation, and maintenance of the proposed works More often, however, for a variety of reasons, the owner of the planned construction, public or private, is committed to the site and the geological knowledge becomes essential to the success of the project Engineering geologists generally begin site work with a walkover of the site, attempting to ‘peer’ below the ground surface to generate a working hypothesis and conceptual geological model of what is likely present and what may be hidden The ultimate goal is to assess how geotechnical factors could impact the construction or performance of the engineered works Tables and provide a summary of geological conditions that might be expected to impact project feasibility, design, and construction processes Table Problems of urban construction: societal pressures causing geological impacts Condition Impact Geological considerations Cities require off street vehicle parking for a significant percentage of building occupants and visitors Above ground space is too valuable for parking vehicles, hence deep basements are required Water must be supplied to the city Need for conveyance projects and tunnels; importation of the water for the distribution system Reliance on tunnelling to achieve minimal grades between stations and minimal depth below ground for people movement Hardrock excavation costs are extreme; bedded sedimentary and jointed or foliated crystalline rock present slope stability problems for the basement excavation during construction; stability of adjacent buildings is a constant concern Linear projects such as pipelines, aqueducts, highways, and railroads are particularly susceptible to earthquake ground motion Places geological information at a premium for design and construction of stations; keeps transit tunnel alignment in sound rock, free of major ground support considerations Use archival records, old topographic maps, historic aerial photographs, and confirming geophysical traverses Transportation routes must not interfere with much of the existing ground surface infrastructure Abandoned waste dumps of all types Previous quarrying, mining, and other mineral extraction activity Excessive abstraction of groundwater Organic debris will naturally compress as much as 300% vertically (becoming as little as one third of its preconstruction thickness, hence causing considerable settlement) Many cities once relied on extraction of coal, building stone, gypsum, clay mineral, or mineral ore, leaving unstable ground Causes rotting of historic timber foundation piles; these may also rise on termination of water abstraction Present day hazards include rotting support timber, collapse prone workings, and movement of contaminated groundwater in workings Saline intrusion in coastal areas, activation of non seismic ‘growth’ faults in post Cretaceous coastal embayments