Guidelines for selection and development of the site for building in hill areas, Part 1: Microzonation of urban centres
1995 Edition

According to IS 14243 Part 1, key geological considerations for microzonation include: regional geology encompassing physiographic features like landforms and drainage patterns; the geological evolution and formations including surficial deposits; structural elements such as folds, faults, and fractures; active fault zones susceptible to seismic activity; and hydrological factors including groundwater table configurations. Locally, the uniformity of lithology (rock types, terraces, fans, flood plains) and slope stability are assessed, with slopes under 25° in rocky terrains and below 20° in surficial deposits generally regarded as stable. Also, zones with elevated groundwater or saturation are examined for liquefaction and settlement risks during seismic events. Mapping scales are recommended at 1:10,000 with 2 m contours for high relief, and up to 1:25,000 with 3-5 m contours for moderate relief. These elements collectively guide risk evaluation and safe urban planning in hilly environments.

IS 14243 Part 1 defines seismic hazard classes by evaluating the Peak Ground Acceleration (PGA) at the urban location as follows: Class I (Low risk) for PGA below 0.10g; Class II (Moderate risk) for PGA between 0.10g and less than 0.25g with no active faults within 10 km; Class III (High risk) for PGA exceeding 0.25g without nearby active faults; and Class IV (Extreme risk) when PGA is above 0.25g with active faults located within or up to 10 km from the urban center. The presence of active faults within this proximity elevates the classification to the highest hazard category. This classification framework is essential for seismic design parameters and stability assessments in microzonation.

The standard covers a spectrum of ground failure types relevant for safe site selection, including inherent material weaknesses in soil and rock, instabilities triggered by external forces such as earthquakes, slope failures, and floods, as well as those induced by human interventions like excavation, loading, reclamation, and waste dumping. Specifically, it includes slope failures identified by terrain profile disruptions, liquefaction and settlement concerns in saturated soils, and inundation or flooding hazards. Site selection is guided by demarcating homogeneous lithological zones and integrating geological, topographical, and hydrological mapping to distinguish stable from unstable areas, with particular caution advised around previously failed slopes and reclaimed or waste disposal regions.

Hydrological evaluation as per the standard involves analyzing surface runoff characteristics, including stream permanence, inlet and outlet elevations, and terrain features; assessment of water spread and marshy or waterlogged conditions; and determining groundwater tables, springs, and seepage occurrences that affect pore water pressures and shear strength. Flood risk classification includes inland river floods, deltaic floods, cyclone-induced sea waves, and heavy precipitation events like cloud bursts. Flood levels are established using historical data, with construction recommended above maximum flood heights. The approach also factors in extreme hydro-meteorological occurrences, sea tides, backflows, and drainage conditions to ensure appropriate siting and design.

Human-induced slope instability arises from mining and excavation without adequate planning, unregulated urban expansion, disruption of natural drainage systems, and construction on weak or flood-prone substrates, including flood plains and shorelines. IS 14243 Part 1 integrates these factors by mapping zones vulnerable to ground failures and inundation, incorporating regional and local geological and hydrological data alongside anthropogenic hazard influences. Areas affected by unplanned development or mining activities are flagged as high-risk, facilitating informed urban planning and risk mitigation strategies for safer construction and land use.