Podcast
Questions and Answers
What is the primary purpose of site reconnaissance?
What is the primary purpose of site reconnaissance?
Which of the following observations is NOT typically made during site reconnaissance?
Which of the following observations is NOT typically made during site reconnaissance?
What is the aim of the data and map study phase following reconnaissance?
What is the aim of the data and map study phase following reconnaissance?
During in-depth investigation, what is a critical technique used for soil sampling?
During in-depth investigation, what is a critical technique used for soil sampling?
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What depth should test borings ideally reach during site investigation?
What depth should test borings ideally reach during site investigation?
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What type of testing is performed on soil samples during the laboratory testing phase?
What type of testing is performed on soil samples during the laboratory testing phase?
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Which of the following factors is evaluated during site reconnaissance?
Which of the following factors is evaluated during site reconnaissance?
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What is one approach for determining the number of borings needed based on investigation results?
What is one approach for determining the number of borings needed based on investigation results?
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What tests are performed for cohesive soils during the classification test?
What tests are performed for cohesive soils during the classification test?
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Which of the following is NOT a purpose of subsurface exploration?
Which of the following is NOT a purpose of subsurface exploration?
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What is the depth of exploration primarily dependent on?
What is the depth of exploration primarily dependent on?
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What type of test is used to analyze short-term stability in cohesive soils?
What type of test is used to analyze short-term stability in cohesive soils?
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Which of the following factors is NOT typically considered when planning subsurface exploration?
Which of the following factors is NOT typically considered when planning subsurface exploration?
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In granular soils, what tests are performed during the classification test?
In granular soils, what tests are performed during the classification test?
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What does subsurface exploration help determine regarding lateral earth pressure?
What does subsurface exploration help determine regarding lateral earth pressure?
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Which of the following aspects is a key consideration in determining potential foundation issues?
Which of the following aspects is a key consideration in determining potential foundation issues?
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What is the primary factor determining the size of a test pit?
What is the primary factor determining the size of a test pit?
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At what depth do excavations typically not require lateral support to side walls?
At what depth do excavations typically not require lateral support to side walls?
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When should a test pit be backfilled?
When should a test pit be backfilled?
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Which location is NOT commonly used for the placement of boreholes?
Which location is NOT commonly used for the placement of boreholes?
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Which boring method is particularly suitable for cohesive soils above the water table?
Which boring method is particularly suitable for cohesive soils above the water table?
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What is a benefit of using auger boring for investigations?
What is a benefit of using auger boring for investigations?
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What is one function of exploratory bore holes?
What is one function of exploratory bore holes?
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In what situation are vertical boring methods typically used?
In what situation are vertical boring methods typically used?
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What is a primary advantage of percussion drilling?
What is a primary advantage of percussion drilling?
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What is a significant disadvantage of rotary drilling?
What is a significant disadvantage of rotary drilling?
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Which statement accurately describes the rotary drilling process?
Which statement accurately describes the rotary drilling process?
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Which condition is rotary drilling particularly useful for?
Which condition is rotary drilling particularly useful for?
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What is a drawback of using percussion drilling when temporary casing is required?
What is a drawback of using percussion drilling when temporary casing is required?
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What is a key feature of geophysical methods in subsurface investigation?
What is a key feature of geophysical methods in subsurface investigation?
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Which drill method is known for potentially drilling past multiple water tables?
Which drill method is known for potentially drilling past multiple water tables?
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Which disadvantage is common to both percussion and rotary drilling methods?
Which disadvantage is common to both percussion and rotary drilling methods?
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What principle does the seismic refraction method rely on?
What principle does the seismic refraction method rely on?
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What are the two types of electrodes used in the electrical resistivity method called?
What are the two types of electrodes used in the electrical resistivity method called?
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What is the primary purpose of the magnetic survey method?
What is the primary purpose of the magnetic survey method?
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How are seismic waves generated in the seismic refraction method?
How are seismic waves generated in the seismic refraction method?
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How is apparent resistivity calculated in the electrical resistivity method?
How is apparent resistivity calculated in the electrical resistivity method?
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What is a common application of the magnetic survey method?
What is a common application of the magnetic survey method?
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What is recorded by geophones in the seismic refraction method?
What is recorded by geophones in the seismic refraction method?
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Which of the following statements about the application of electrodes in the electrical resistivity method is true?
Which of the following statements about the application of electrodes in the electrical resistivity method is true?
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Study Notes
Site Reconnaissance
- Visual inspection of the site is the first stage of site investigation.
- Topographical and geological features are collected during reconnaissance.
- Groundwater table location is determined by observing wells in the area.
- High flood level marks are observed on bridges and high-rise buildings.
- Presence of vegetation and soil type are noted.
- Past records of landslides, floods, shrinkage cracks, etc. are reviewed for the region.
- Aerial photographs, blueprints, geological maps are studied.
- Deep cuts are observed to understand soil stratification.
- Settlement cracks on existing structures are noted.
Data and Map Study
- Information from reconnaissance is analyzed to determine necessary tests and samples.
- Licensed water abstraction and discharge consents, landfills, and waste disposal sites are identified.
- History of hazardous incidents is reviewed.
- Surface water and groundwater quality and vulnerability are assessed.
- Past local borehole records are examined.
- Presence of coal and other mines is investigated.
- Historical mapping is studied.
- Buried and overhead supply and utility lines are identified.
In-Depth Investigation
- Soil samples are taken at this stage.
- Test borings are used to sample disturbed and undisturbed soil.
- Samples are collected from various depths for testing and observation.
- One boring per corner and center of the intended structure is a common starting point.
- More borings and tests may be added based on results uniformity.
- Borings should extend past topsoil into firm layers, accommodating structural loads and design intent.
Laboratory Testing
- Soil samples are classified and characterized in the laboratory.
- Geotechnical parameters are provided for the design phase based on project needs.
- Basic tests include classification and shear strength tests.
- Classification tests for cohesive soils include moisture content, plasticity index, particle size distribution, and bulk density.
- Classification tests for granular soils include particle size distribution and bulk density.
- Shear strength tests for cohesive soils include short-term and long-term stability, and residual shear strength properties using the shear box test.
- Shear strength tests for granular soils include short-term and long-term stability analysis using the shear box test.
Subsurface Exploration
- Identifying underlying soil deposits and their characteristics is the purpose of subsurface exploration.
- Subsurface exploration program provides engineers with essential site characteristics and properties for design and construction.
- Foundation type and depth selection is informed by subsurface exploration.
- Foundation bearing capacity is evaluated.
- Potential foundation issues (e.g., collapsible soils, landslides) are identified.
- Groundwater table location is determined.
- Lateral earth pressure for retaining walls and other structures is determined.
- Pile bulkheads and braced cuts are planned.
- Methods for modifying ground conditions are considered.
Planning of Subsurface Exploration
- Areal extent of the investigation, methods of exploration, boring locations, spacings, depths, and sample types are defined in the exploration program.
- The program aims to maximize information on subsurface conditions at minimal cost and time.
Depth of Exploration
- Depth of exploration is determined by considering the pressure increase due to structural loads.
- This depth is where perceptible settlement or shear failure of the foundation is likely to occur.
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Depth of exploration depends on the following factors:
- Type of structure
- Load on structure
- Size and shape of the foundation
- Position of loaded areas
- Soil profile and properties
Thumb Rule for Deciding Depth of Exploration
- Depth of exploration should be at least 1.5 times the width of the foundation.
- For wider foundations, the depth may need to be increased.
- This is a general guideline and may vary depending on the specific project.
Size of Pits
- Pit size is determined by the space needed for excavation and sample collection.
- Economic limits also influence pit size.
- Test pits are typically square or circular with dimensions of 1.2–3 meters.
Stabilizing Side Walls
- Side walls up to 1.5 meters deep often do not need lateral support.
- Shallow test pits can be stabilized by sloping side walls.
- Deeper excavations are often more economical with sheeting.
Collection of Soil Samples
- Soil samples are collected from the bottom of the test pit.
- Samples are collected using sampling techniques.
- Laboratory tests determine strength and other engineering properties of the soil.
Back Filling
- Test pits are backfilled and compacted immediately after completion.
- Open test pits are not left unattended overnight.
Boring Methods
- Boring methods are used for subsurface investigations to collect samples.
- These methods are suitable for various soil types and are commonly used for deeper exploration.
- Borehole locations are determined based on the structure's type, size, and weight.
- Boreholes are typically located at building corners, the center of the site, and where heavily loaded columns are proposed.
- At least one boring should extend to a deeper stratum.
- Vertical boring methods are used for deep excavations.
- Samples are extracted from boreholes and tested in the laboratory.
- Groundwater table is located and in-situ tests are conducted using boreholes.
Different Boring Methods
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Auger Boring:
- Used for cohesive soils and soft soils above the water table.
- Suitable for shallow foundation exploration.
- Economical and rapid method.
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Percussion Drilling:
- Can remove boulders and break harder formations.
- Can drill through water tables.
- Heavy and expensive equipment is required.
- Slow drilling process.
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Rotary Drilling:
- Forms deep boreholes for rock samples.
- Uses a powered rotary cutting head.
- Suitable for highly resistant strata.
- Can be used for sands and silt.
- Quick and efficient method.
- Capable of large diameter boreholes and deep depths.
- Costly in terms of operation and initial cost.
- High fuel and maintenance costs.
- High water requirement.
Geophysical Methods
- Geophysical methods indirectly investigate subsurface conditions by measuring and interpreting physical fields at the surface.
- These methods are used when the depth of exploration is large and speed is important.
Seismic Refraction Method
- Based on the principle that elastic shock waves travel at different velocities in different materials.
- Shock waves are generated at the ground surface using a sledge hammer.
- Waves travel deep into the ground and are refracted at interfaces between different materials.
- Time of arrival of waves is recorded by geophones at various locations.
- Geophones convert ground vibrations into electrical impulses for recording.
Electrical Resistivity Technique
- Measures soil strata resistivity, correlating it to soil properties.
- Involves introducing a direct current into the ground using electrodes.
- Outer electrodes are current electrodes, inner electrodes are potential electrodes.
- Voltage drop or potential difference between inner electrodes is measured.
- Apparent resistivity is calculated using the potential difference for interpretation.
Magnetic Survey
- Measures magnetic field distortions caused by ferrous materials (e.g., steel, iron, iron-rich rocks and soil).
- Often uses two sensors to increase resolution and detection ability.
- The magnetic method can be used for large-scale geological mapping or detecting buried metallic objects.
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Description
This quiz covers essential techniques used in site reconnaissance for geological investigations. It examines the collection of topographical and geological features, groundwater table observations, and the assessment of historical environmental records. Dive into the methods of data analysis and map studies crucial for understanding site conditions.
