TLDR;
This lecture discusses the phases and classifications of subsurface investigation, emphasizing the importance of preliminary investigations to determine site suitability before detailed analysis. It covers reconnaissance, map studies, and aerial photographs as key components of preliminary assessments. The lecture also classifies detailed investigations based on project type: new projects, defect analysis in existing structures, ensuring the safety of adjacent structures, and sourcing construction materials.
- Preliminary investigations determine site suitability.
- Detailed investigations vary based on project objectives.
- Safety and financial considerations guide site selection.
Introduction [0:29]
The lecture introduces the topic of phases and classification of subsurface investigation, building upon the previous lecture's discussion of the requirements and objectives of such investigations. It highlights the importance of understanding subsurface conditions for civil engineering structures, as they transfer loads to the subsoil. The lecture outlines how different phases of investigation are necessary before starting geotechnical or geophysical tests, emphasizing the need for background information.
Phases of Subsurface Investigation [1:36]
The discussion covers the phases of subsurface investigation, noting that multiple locations or alignments are often considered for large projects like dams, bridges, and nuclear power plants due to geological, environmental, political, and financial constraints. Preliminary investigations are conducted to assess the suitability of tentative sites before detailed investigation. The findings from preliminary investigations determine whether to proceed with detailed investigation or seek an alternate site. The depth and details of the investigation depend on the project, structural requirements, and external characteristics governing design and load transfer.
Need and Objectives of Subsurface Exploration [7:26]
The lecture revisits the needs for subsurface exploration, including ensuring the safety of proposed and adjoining structures, collecting samples, and forecasting potential risks. Objectives include understanding soil and rock types, strength variations, water table levels, seepage characteristics, erosion properties, and identifying problematic soil. Solutions for distressed structures are also considered.
Preliminary Investigation [9:42]
Preliminary investigation, the initial phase, assesses site suitability for a project without detailed investigation. It is crucial for site selection in major projects like dams, bridges, and waste containment facilities. This phase relies on existing literature from agencies dealing with geological conditions, active fault lines, mineral deposits, and soil types. Collecting existing information is key to deciding site suitability without extensive on-site investigation.
Steps of Preliminary Investigation [13:06]
The steps include reconnaissance, study of maps, and aerial photographs. Reconnaissance involves studying topography, geological deposits, drainage patterns, vegetation, and land use to understand soil, groundwater conditions, and site feasibility. If the site is unsuitable, detailed investigation is unnecessary.
Reconnaissance Survey Details [13:53]
Reconnaissance involves studying topographical variations, geological deposits (quaternary sediments, geological time scale), drainage patterns, and vegetation. Vegetation indicates soil nutrients and type. Land use provides insights into soil strength and overall site feasibility. This survey informs planning for further investigation, including foundation and equipment choices.
Study of Maps [16:14]
Topographical and geological maps from sources like the Geological Survey of India provide existing information on regional variations in properties. Groundwater table fluctuations can be observed through ponds, wells, and springs. Vegetation types indicate soil characteristics. Detailed exploration depends on the project and the details being sought.
Maps and Project Types [17:41]
The extent of exploration depends on the project type, with routine buildings requiring information on foundation depth and type from nearby buildings. For dams, bridges, and highways, reconnaissance helps identify alternate locations and possible alignments. Knowledge of fills and contaminated ground can be obtained from land use data and resource maps. Geologists, geophysicists, and field engineers use existing information for decision-making.
District Resource Maps and Geological Maps [21:22]
District resource maps from the Geological Survey of India provide geological information at the district level. Borehole information is mapped in some countries to aid future investigations. Geological maps, Topo sheets, and mineral deposit maps are valuable resources. Fault identification, aided by geophysicists and seismologists, helps understand seismic activity.
Seismic Considerations and Flood Susceptibility [24:07]
Seismic atlas maps identify active faults and lineaments, crucial for projects in earthquake-prone areas. Isoseismal maps indicate damage extent from historic earthquakes. Agencies like the Indian Meteorological Department provide data on historic and current earthquakes. Studies of flood plains, alluvial fans, and soil types are available from concerned agencies, aiding in flood susceptibility assessment.
Aerial Photographs [31:57]
Aerial photographs facilitate quick surveys of large areas and help divide study areas into zones based on observed characteristics, optimizing resource allocation for detailed investigation. They provide insights into surface and underground conditions by correlating landforms and vegetation with geology.
Geomorphology and Site Suitability [35:32]
Geomorphology, the study of landforms and the processes that create them, aids in understanding regional geology. Reconnaissance, map studies, and aerial photographs help determine site suitability based on soil and rock types, accessibility, and availability of construction materials.
Classification of Detailed Investigation [37:13]
Detailed investigations are classified into four categories based on the purpose: new projects, addressing defects in existing structures, ensuring the safety of adjoining structures, and sourcing construction materials. The type of project, complexity, location, and required details dictate the classification.
Investigation for New Projects [40:57]
These investigations are wider in scope, ensuring the safety of new structures without compromising existing ones. They aid in foundation design and handling potential construction challenges. Details depend on the project and foundation type, including dynamic characterization of subsoil, flow characteristics, chemical analysis for groundwater contamination, and dewatering requirements.
Investigation of Defects or Failure in Existing Structures [45:33]
Detailed investigations help understand the reasons for distress or failure, aiding in suggesting remedial measures. They identify defects in data interpretation, design, or execution. Simplified assumptions in design, changes in groundwater table, land use, or climatic conditions, and external sources of issues are also investigated.
Ensuring Safety of Adjoining Structures [52:18]
These investigations ensure that construction activities do not compromise the safety of nearby structures. The choice of machinery, dewatering processes, and excavation methods can cause disturbance. Vibration, deformation, uneven settlement, and removal of lateral support are potential issues. Identifying buried utilities is also crucial.
Investigation for Construction Material [56:07]
Required for mega-structures like dams, nuclear power plants, and roads, this involves searching for suitable construction materials. Queries for aggregates and borrow pits for earth are essential. Materials must meet quantity and quality standards, with strength characteristics and load settlement curves being critical. Environmental impact assessment is also a key consideration.