GEOLOGY

• Site Investigation (SI)


  The exploration or discovery of the ground conditions to enable engineers to make informed design decisions
• Purpose/Importance of Site Investigation

  • Suitability - to assess the general suitability of a site and its environs for the proposed work
  • Design - to enable and adequate and economic design, including for temporary works
  • Construction - to plan the best method of construction and, for some projects, identify sources of suitable materials such as concrete aggregate and fill and to locate sites for disposal of waste
  • Effect of changes - to consider ground and environmental changes on the works (e.g. intense rainfall and earthquakes) to assess the impact of the works on adjacent properties and on the environment
  • Choice of site - where appropriate, to identify alternative sites or to allow optimal planning of the works
• Parameters to be determined in a Site Investigation

  • Geological structure of the area
  • Lithology of the area
  • Ground water conditions in the region
  • Seismicity of the region
• Stages of a Site Investigation

  1. Desk Study

  2. Site Reconnaissance

  3. Preliminary Site Exploration

  4. Detailed Exploration

  5. Preparation of soil investigation report
• Desk Study


  The collation and review of information already available about a site, carried out at an early stage of site appraisal to inform and guide the remainder of the site investigation
• Site Reconnaissance


  The initial document search should be followed by a walk-over survey of the site and its surroundings
• Preliminary Site Exploration


  Carried out for small projects, light structures, highways, airfields, etc. The main objective is to obtain an approximate picture of sub-soil conditions at low cost
• Detailed Exploration


  Preferred for complex projects, major engineering works, heavy structures like dams, bridges, high rise buildings, etc. A huge amount of capital is required
• Sections of a soil investigation report

  • Introduction
  • Scope of site investigation
  • Description of the proposed structure, purpose of site investigation
  • Site reconnaissance details
  • Site exploration details such as number, location and depth of boreholes, sampling details etc.
  • Methods performed in site exploration and their results
  • Laboratory tests performed and their results
  • Details of Groundwater table level and position
  • Recommended improvement methods if needed
  • Recommended types of foundations, structural details, etc.
  • Conclusion
• Guidelines for depth of Site Investigation

  • At least one boring carried to bedrock, or to well below the anticipated level of influence of the building
  • For light structures, boring should be to a depth equal to four times the probable footing width or to a depth of 6m below the lowest part of the foundation, whichever is deeper
  • For more heavily loaded structures, at least 50% of the borings should be extended to a depth equal to 1.5 times the width of the building below the lowest part of the foundation
  • Bedrock should be proved by coring into it to a minimum depth of 3m
• Surface Exploration Methods

  • Topographic mapping
  • Aerial Surveys
  • Photogeology
  • Hydrogeological Surveys
• Subsurface Exploration Methods


  • Direct Methods - Examination of rocks or materials of the underground by digging of drill holes, trial pits, adits, shafts, galleries and exploratory tunnels
  • Indirect Methods - Application of geophysical techniques for obtaining fairly accurate idea of subsurface geology
• Open Excavation Techniques


  • Trial pits
  • Cost of open excavation increases rapidly with depth
• Boring Techniques


  • Auger boring
  • Auger and shell boring
  • Wash boring
  • Percussion boring
  • Rotary boring
• Subsurface Sounding


  Measuring the resistance of the soil with depth by means of penetrometer under static or dynamic loading
• Geophysical Methods


  • Seismic refraction method
  • Electrical resistivity methods
• Geophysical methods are generally non-invasive or non destructive methods long used in the construction industry for investigation of the subsurface
• Geophysical methods provide advantages such as speeding up the process of investigation, providing continuous streams of information not otherwise available in discrete sampling or invasive procedures, and giving advance information on what to expect for a given locality before a more detailed and costly soil exploration is even planned
• Geophysical methods


  • Generally speed up the process of investigation
  • Provide continuous streams of information not otherwise available in discrete sampling or invasive procedures
  • Give advance information on what to expect for a given locality before a more detailed and costly soil exploration is even planned
• Geophysical methods


  • Force multiplier for the engineer
  • Allow the user to identify potential problem areas or target areas even before the start of a detailed Soil Exploration program
• Ground Penetrating Radar (GPR)


  Uses high-frequency pulsed electromagnetic waves to map subsurface information
• GPR


  Transmitting antenna radiates short pulses of high-frequency radio waves into the ground
• Parts of a GPR

  • Control Unit
  • Antenna
  • Roller
• Applications of GPR


  • Used for detection of Cavities and Geologic Anomalies
  • Used for detection of Buried objects such as pipes, Improvised Explosive Devices (IED's) mines, subsurface disturbances and Archeological artifacts
  • Used for environmental scanning to determine waste landfills
  • Used For determination of structural thickness of Roadways and pavements
  • Used for detection of Rebars and other embedded Objects in Concrete
• Limitations of GPR


  • Moist, clayey soils, electrical conductivity
  • Plastic
  • Sand
  • Noise
  • Water
• Seismic Refraction Method


  • Consists of sending shock waves into the soil either by use of hammer striking a steel plate or with the use of explosives
  • The vibrations induced are picked up by a Seismograph through an array of geophones which pick up the refracted and reflected signals
• Seismic Refraction


  • Geophysical method used for investigating subsurface ground conditions by utilising surface-sourced seismic waves
  • Data acquired on site is computer processed and interpreted to produce models of the seismic velocity and layer thickness of the subsurface ground structure
• Applications of Seismic Refraction

  • Measures Bedrock Depth & Overburden Thickness
  • Determines Seismic Rippability Parameters
  • Investigates Pipeline Routes
  • Locates Geological Structures
  • Evaluates Sand & Gravel Deposits
• Seismic Refraction


  1. Sending shock waves into the soil

  2. Picking up vibrations with a Seismograph and geophones
• Georesistivity Method


  • Sends electrical current into the subsurface
  • Measures the resulting electrical resistivities and correlates them with various soil types and water bearing aquifers to yield layering or stratification information as well as identify other layer properties
• Schlumberger Electrode array


  More popular for use in well or aquifer surveys
• Gravitational Method


  • Works because different earth materials have different densities (mass) and hence produce different gravitational fields
  • Gravitational field variations can be interpreted to determine a source's depth, geometry and density
• Magnetic Method


  • Maps variations in the magnetic field of the Earth that are attributable to changes of structure or magnetic susceptibility in certain near-surface rocks
  • Sedimentary rocks generally have a very small susceptibility compared with igneous or metamorphic rocks
• Standard Penetration test (SPT)

  • Common in site testing method used to determine the geotechnical engineering properties of subsurface soils
  • Simple and inexpensive test to estimate the relative density of soils and approximate shear strength parameters
• SPT is the most common and widely used in geotechnical investigations for various projects
• SPT


  • Used to determine the in situ density and angle of shearing resistance of cohesionless soils and also the strength of cohesive soils
  • Useful in cases where it is difficult to obtain undisturbed samples for testing, for example, in gravelly, sandy, silty, sandy clay, or weak rock formations
• Core Drill


  • Hollow, cylindrical drill used to make holes through a surface
  • Made of metal, with drill tips usually coated with either diamond or carbide
• Core Drilling


  • Best method for testing concrete, but very expensive
  • Used when the quality of concrete in a structure is suspected to be weakened with general inspections
• Scaling, leaching, or pattern cracking can be signs of the need for core drilling
• Drilling


  Cutting process that uses a drill bit to cut or enlarge a hole of circular cross-section in solid material