Deep Foundations

Question 1

Open vs Closed End Piles

Answer 1

Open ended piles - ONLY skin friction. No or little (from plugging during static loading) end bearing

Question 2

Skin Friction vs End Bearing Load Transfer

Answer 2

Skin Friction: load mobilizes/transfers fast vs end bearing

End Bearing - difficult to mobilize/transfer load tip resistance (regardless of soil type)

Question 3

Soil Setup vs Relaxation

Answer 3

Soil Setup occurs in clays bc pwp slowly dissipates due to consolidation

Relaxation occurs in dense, saturated sands. Driving process causes suction (negative pwp) which increases effective strength and dissipates with time

Soil Setup generates positive pwp while relaxation generates negative pwp

Question 4

Static Load Testing vs Dynamic Load Testing

Answer 4

Static is the most accurate, so recommended
-Expensive
-apply load SLOWLY with beam on top
-Install strain guages along pile and telltale on pile end
-evaluate creep

Dynamic - hammer used to load pile
-Measure resulting compression waves from hammer - down pile- and back up
-fast

Question 5

Driven vs Drilled Piles

Answer 5

Drilled
-Typically little/no construction induced vibration (good for urban areas)
-Large excavations minimized
-typically higher capacities
-Helical piles are easiest to install and less ground disturbance

Driven
-work is clean and neat
Driven favored for offshore piles

Question 6

Critical depth of pile

Answer 6

= Diameter of pile * depth to water