Boring and Geophysical Methods in Geology

Boring Methods

• Auger Boring: A method used for drilling holes in the ground.
• Wash Boring: A method that uses a water jet to remove soil and rock.
• Percussion Boring: A method that uses a hammer to drive a drill bit into the ground.
• Core Boring or Rotary Drilling: A method that uses a rotating drill bit to extract core samples of the ground.

Geophysical Methods

• Used for deep exploration and rapid investigation.
• Common methods include:
  • Gravitational methods: Measure variations in gravity to identify underground structures.
  • Magnetic methods: Detect variations in the Earth's magnetic field to map geological formations.
  • Seismic refraction method: Analyze the travel time of seismic waves to determine the layering and properties of the ground.
  • Electrical resistivity method: Measures the resistance of the ground to an electric current to identify different soil types and water content.
  • Seismic refraction method and electrical resistivity methods are commonly used in civil engineering.
• These methods are non-intrusive ways of "seeing" into the ground.
• Can be used to identify subsurface hydrological, geological, and contaminant conditions.

Factors Affecting Sample Disturbance

• Area Ratio: The ratio of the borehole cross-sectional area to the sample cross-sectional area.
• Inside Clearance: The space between the borehole wall and the sample.
• Outside Clearance: The space between the sampling tool and the borehole wall.
• Inside Wall Friction: The friction between the sample and the borehole wall.
• Design of Non-Return Valve: The design of the valve that prevents the sample from being lost during retrieval.
• Methods of Applying Forces: The way in which forces are applied during sampling.
• Recovery Ratio: The ratio of the length of the recovered sample to the total length of the sample.

Types of Foundations

• Shallow Foundations:
  • Individual Footing or Isolated Footing: Used for a single column and supports loads from structures.
  • Combined Footing: Used when two or more columns are close together and their isolated footings overlap.
  • Strip Foundation: Runs along the length of a wall, used for supporting walls and bridge piers.
  • Raft or Mat Foundation: Spreads across the entire area of a structure to support heavy loads.
• Deep Foundations:
  • Pile Foundation: Used to transfer heavy loads deep into the ground to rock strata.
  • Drilled Shafts or Caissons: Created by drilling a hole and filling it with concrete to provide support.

Shallow Foundations

• Individual Footing or Isolated Footing:
  • Most common type of foundation used for building construction.
  • Also known as a pad foundation.
  • Shape is typically square or rectangular.
  • Size is calculated based on loads and soil bearing capacity.
  • Rectangular footings are used when they experience moments due to eccentric loads or horizontal forces.
• Combined Footing:
  • Used when two or more columns are close together and their footings overlap.
  • Has a rectangular shape.
  • Supports loads from structures.
• Spread Footings or Strip Footings and Wall Footings:
  • Have a wider base than standard load-bearing wall foundations.
  • This wider base spreads the weight from the structure over a larger area, providing better stability.
  • Used for individual columns, walls, and bridge piers where the bearing soil layer is within 3 meters (10 feet) of the ground surface.
  • Soil must be able to support the weight of the structure.
  • Should not be used on soils where there is a possibility of groundwater flow above the bearing layer, which can cause scour or liquefaction (loss of strength).
• Raft or Mat Foundations:
  • Spread across the entire area of a building to support heavy structural loads from columns and walls.
  • Used when loads from columns and walls are extremely high.
  • Prevents differential settlement (uneven sinking) of individual footings.
  • Suitable for expansive soils with low bearing capacity that cannot support spread footings or wall footings.
  • Economical when half of the structure's area is covered with individual footings and wall footings.
  • Should not be used where the groundwater table is above the bearing surface of the soil, as this could lead to scour or liquefaction.

Deep Foundations

• Pile Foundations:
  • Used to transfer heavy loads from the structure to hard rock strata deep below the ground level.
  • Used when shallow foundations are not suitable due to weak soil conditions near the surface.
  • Can also help prevent uplift of the structure due to lateral loads like earthquakes and wind.
  • Used when the ground is very weak, and shallow foundations would not be effective.
  • Used when there is a possibility of uplift of the structure, as pile foundations can resist these forces.
• Drilled Shafts or Caissons:
  • Created by drilling a hole in the ground and filling it with concrete.
  • The concrete shaft provides support for the structure.
  • Used when the groundwater table is high, and the soil is unstable.
  • Used when there is a possibility of uplift of the structure.

Other Important Structural Elements

• Beams: Horizontal members that carry loads from slabs and masonry walls.
• Columns: Vertical members carrying loads from beams and upper columns.
• Foundation: The load transmitting member, transferring loads from columns and walls to the ground.
• Shear Walls: Large columns that resist horizontal loads like wind and earthquakes (work in one direction only).
• Elevator Shaft: Vertical concrete boxes for elevators.