week 4

• Foundations


  Provide support to the structure, transfers the loads from the structure to the soil
• Types of foundations


  • Shallow (high level) footings
  • Deep foundations (piles)
• Shallow (high level) footings

  • Used for light loads on almost any strength soil
  • Used for heavy loads on high strength soil or rock
  • Easy to construct
  • Relatively inexpensive
• Types of shallow (high level) footings

  • Strip footings
  • Pad footings
  • Raft footings
• Strip footings


  • Continuous strips of concrete with a length that far exceeds the width
  • May be unreinforced but more common to be reinforced
  • Used for domestic constructions, and for larger buildings where load-bearing walls are employed
• Pad footings


  • For framed buildings, where the super-structure loads reach the foundations through separate columns
  • Support concentrated loads (columns)
  • Square or rectangular concrete
  • May need to consider starter bars, hold down bolts, or pockets
• Raft footings


  • Footing used to "float" over soil and spread load where soil is of very low strength or expansive
  • May be relatively thin for residential and industrial buildings to several metres thick for high rise buildings
  • Where a single basement is required and the raft can provide the slab, structure footings and retaining wall footings
  • Where isolated pads would overlap
• Deep foundations (piles)


  • Used for heavy loads
  • Used when the soil at shallow depth is compressible
  • Used when the groundwater table is high
  • More complex to construct
  • More expensive
• Types of piles


  • Displacement piles
  • Replacement piles
• Displacement piles


  • Piles which are driven, displacing soil laterally
  • Driven by drop hammer or rotary
  • Examples: steel, precast concrete, timber
• Replacement piles


  • Formed by creating a borehole into which the pile is then cast or placed, removing existing material
  • Examples: bored piles, continuous flight auger (CFA) piles
• Pile foundation classification by function

  • End bearing piles
  • Friction piles
  • Tension piles
• End bearing piles


  Transfer load on to a firm stratum
• Friction piles


  Carry the major part of loads only by means of friction developed between pile shaft and soil
• Tension piles


  Used to resist uplift forces as a result of hydrostatic pressure, seismic activity or overturning moments
• Pile Proof Loading


  1. Load testing of a representative sample of installed piles

  2. Ensures piles and founding material can resist design loads

  3. Two methods: static testing, dynamic testing
• Static testing


  • Uses reaction methods like kentledge, anchor stressing, and tension method
  • Determines the relationship between load and settlement or verifies pile bearing capacity
• Dynamic testing


  • Measures strain and acceleration near the pile top to determine pile capacity, stresses, energy transfer and resistance distribution, and assess any damage
• Pile cutting down


  Soil strata not accurately predictable so some pile length may be left extending from the soil which has to be removed without damaging the pile or soil surrounding the pile
• Pile caps


  • The mat that rests on groups of piles and forms part of the foundation
  • Shape and plan dimensions depend on the number of piles and their spacing
• Working platforms for piling operations

  • The foundation for a piling rig and ancillary equipment
  • Necessary if there is a lack of understanding of the ground conditions
• Pile


  • Pile length may be left extending from the soil
  • Pile has to be removed without damaging the pile or soil surrounding the pile
• Pile cap


  The mat that rests on groups of piles and forms part of the foundation
• Pile cap


  • Shape and plan dimensions depend on the number of piles in the group and the spacing between each pile
  • Common shapes include triangular for 3 piles, hexagonal for 6-7 piles, rectangular for all other number of piles
• Working platform


  The foundation for a piling rig and ancillary equipment
• If there is a lack of understanding regarding the importance of preparing and maintaining a safe working platform capable of supporting the equipment to be used, this can happen
• Slab on ground


  A type of foundation construction
• Construction sequence for slab on ground

  1. Building foundations

  2. Structure framing

  3. Roof cladding

  4. Building Slab

  OR

  1. Building foundations

  5. Building Slab

  6. Structure framing

  7. Roof cladding

  
• Pouring methods for slab on ground

  • Long strip pour method - recommended
  • Continuous pour method - most efficient over large areas, difficult to control levels
  • Chequerboard pour method - not recommended, difficult to control levels, access problems
• Design considerations for slab on ground

  • Base and subbase materials
  • Slab thickness
  • Compressive and flexural strength values
  • Concrete mix
  • Vapour barrier requirements
  • Insulation requirements
  • Joints details, locations and filling
  • Reinforcement - type, size, detailing and locations
  • Surface treatment and finish requirements
  • Tolerances for base, sub-base, slab thickness and surface
  • Curing of the concrete
  • Embedment details
• Slab components


  • Subgrade
  • Sub-base
  • Vapour barrier
  • Slab
  • Topping slab or wearing surface
• Common reinforcing types


  • Steel mesh
  • Post tensioning
  • Steel reinforcement
• Vibrating screed


  Machine used in finishing large concrete slabs, expels air from concrete to increase density and improve durability
• Slab joints


  • Control cracking from drying shrinkage, expansion and contraction
  • Control warping stresses from differential temperature and moisture
  • Dowel bars are often included to spread applied load between neighbouring bays
• Types of slab joints

  • Contraction joints
  • Construction joints
  • Saw cut crack control joints
  • Isolation joints
• Contraction joints


  Allow horizontal movement of slab, provided transverse and parallel to direction of concrete placement
• Construction joints

  Permit horizontal slab movement, placed transversely at end of pour or unplanned interruptions, and parallel to direction of concrete pour
• Saw cut crack control joints

  Transverse joints formed by sawing a 40mm deep groove in the surface of the concrete over dowel bars, to manage cracking
• Isolation joints


  Permit horizontal and vertical slab movement, placed between abutting elements like columns, service pits, machinery bases
• Curing of concrete


  1. Water curing - ponding or mist spraying

  2. Mechanical barriers - waterproof building papers or plastic film

  3. Water retaining materials - applied by hand or power sprays

  4. Absorptive covers - sand, hessian or canvas kept damp