Foundation Systems

Question 1

What are the four types of deep foundations?

Answer 1

• drilled piers (caisson)
• socketed caisson
• end-bearing pile
• friction pile

Question 2

What are drilled piers (caissons)? (3)

Answer 2

• drilled into the earth with large auger
• belled (flared) at the bottom as necessary
• diameters range from 18” to 12’

Question 3

When are belled caissons practical?

Answer 3

practical only where the bell can be excavated in a cohesive soil that will retain its shape until concrete is placed

Question 4

How are belled caissons “belled?”

Answer 4

special “belling bucket” attachment used to bell bottom of caisson after shaft is drilled

Question 5

What are socketed caissons? (3)

Answer 5

• drilled into rock or hard stratum
• not belled at the bottom
• bearing capacity comes from end-bearing and friction between sides of caisson and rock

Question 6

What are piles? (4)

Answer 6

• used where drilled piers are not practical
• hammered or pressed into the earth
• classified as “end-bearing” or “friction”
• may be made of steel, wood, or precast concrete

Question 7

What are end-bearing piles? (3)

Answer 7

• forcibly driven until the tip encounters firm resistance
• bearing capacity comes from end of pile
• driven to refusal

Question 8

What are friction piles? (3)

Answer 8

• used where no firm bearing layer can be reached
• achieves load-bearing capacity through frictional resistance on surface of pile
• driven to a predetermined depth or until a certain level of resistance is encountered

Question 9

What are pile caps? (2)

Answer 9

• piles are typically driven close together in clusters

- a pile cap spreads load among clustered piles

Question 10

What is the purpose of grade beams?

Answer 10

spans between the pile caps or piers to provide continuous support for the wall above

Question 11

What are grade beams? (2)

Answer 11

• reinforcing bars project from the tops of completed drilled piers
• gravel is deposited between the piers, to form a base for concrete grade beams which will span between the piers

Question 12

What are mini-piles, or pin piles? (4)

Answer 12

• made of steel pipe or bar 2-12 inches in diameter
• pressed or rammed into holes and grouted in place
• can be installed in short sections
• good choice when vertical clearance is limited

Question 13

What are helical piles, or screw piles? (3)

Answer 13

• installed without hammering
• coupled end-to-end as drilling progresses
• no vibration/low noise: good choice for work close to existing buildings or when making improvements to existing buildings

Question 14

What is seismic base isolation, or elastometric bearings? (2)

Answer 14

• used in areas of the country where earthquakes are common

- base isolators flex or yield to absorb a significant portion of movement during a seismic event

Question 15

What are the major components of elastometric bearings? (3)

Answer 15

• rubber layers: provide lateral flexibility
• steel shims: provide vertical stiffness to support building weight while limiting lateral bulging of rubber
• lead plug: provides source of energy dissipation

Question 16

What is underpinning? (2)

Answer 16

• strengthening or stabilizing an existing foundation
• may be required when original foundation was improperly designed, where a change in use or addition increases foundation loads, or when new construction could cause damage to nearby foundations