560 URBAN GEOLOGY Table Geotechnical influences on urban construction Geotechnical influence Emphasis Key considerations Site geological knowledge reduces the risk of unknown ground conditions Unknown, undetected, or undisclosed geological conditions can compromise scheduling, cost, and operational performance of engineered works Avoidance of constraints or slowdowns to construction process Capacity of foundation soils to support loads of the structure; stability of surrounding facilities; excessively ‘weak’ or ‘strong’ ground exacerbates the construction effort Detection in the site exploration process, notification of owner and design engineer; incorporation into the design and construction specifications Avoidance requires adequate site characterization, funding by the owner, and judicious selection of geological and geotechnical support consultants Some earth materials have undesirable properties or characteristics ‘Faces’ across which geologic character of construction ground changes Pockets or zones of ‘bad’ ground Near surface groundwater Perched water or groundwater is generally detrimental to the construction process Nature of site preparation or construction ‘spoil’ (soil) or ‘muck’ (tunnel spoil) When not anticipated by the contractor, can cause unwanted perturbations in schedule and in project cost; where spanned by one ‘bay’ (segment) of construction, may lead to unacceptable differential settlement of portions of the foundation Three dimensional bodies of degraded earth material not able to support design, construction, or operational efforts or roles of project Always a problem Control and removal without impairment of construction or with performance of completed facility Generally related to geomorphic or tectonic considerations of origin and may have characteristics detrimental to construction or of performance of the facility Generally interferes with construction, particularly in placing the foundation; may require dewatering, which may affect performance of surrounding existing foundations When truly ‘perched’, drains into the construction excavation within hours and does not replenish Must be removed from the construction site and reused in some worthwhile manner acceptable to the community Spoil and muck have geotechnical characteristics that must be heeded in considering their reuse Table Engineering geological contribution to urban geology Contribution Purpose Important considerations Stratigraphy Define the nature and bounds of soil types and of geological formational units Must be capable of bearing the combined live and dead loads of the engineered works to be constructed Controls the suitability and relative cost of siting and dimensional design of virtually all projects Acceptability measured in terms of nil compressibility and sufficiently high shear strength to support the loads of the project Locate and define fault displaced geological units and discontinuities that are of sufficient length of exposure to cross any one dimension of site excavation Protection from damage by human activities; avoidance of premium costs for foundations Using geological evidence to define the nature of flooding as an economic and human welfare concern Avoidance of adverse geological structures, premium foundation conditions, and expensive or ‘bad’ ground in terms of underground construction In no way can groundwater be of beneficial consequence to the construction process Mainly involves interpretation of present geomorphic features that control the path, depth, and velocity of low frequency/high impact flood events Mainly ground that is unstable under gravitational and slope water conditions, along with rock falls Engineering properties of the foundation soil or rock Geological structure of bedrock exposed in construction Occurrence of groundwater Surface water hydrology Avoidance of surrounding unstable ground Avoidance of existing subsurface voids Protect from what may slip onto the site or move down slope from the site Avoid presence of abandoned tunnels, mine shafts, mines in general, and natural voids such as karst caverns All subsurface voids suffer from decreasing ‘stand up’ time, which is the ability to span loads imposed at the ground surface, and from the surrounding overburden weight of the geological column of materials