mod6 Flashcards
why do we use deep foundations
- soil at the surface is soft
- large horizontal loads present on uplift
- scour can occur
end-bearing pile
- pile derives much of its carrying capacity from the resistance of the stratum at the toe of the pile
- bearing stratum is hard and relatively impenetrable material, such as rock or very dense sand or gravel
friction pile
- pile does not reach an impenetrable stratum
- derives its carrying capacity partly from the end-bearing and partly from skin friction between the pile shaft and surrounding soil
displacement pile
- pushes soil out radially and down vertically when it is installed
- usually leads to high horizontal stresses in the soil acting on the pile shaft, improving the shaft resistance
examples of displacement piles
- driven piles
- driven and cast-in-place piles
- jacked or pressed-in piles
non-diplacement pile
- constructed by boring a hole in the ground and filling with concrete, reinforced with steel reinforcing if on-shore or by a steel tube or “insert pile” if offshore
examples of non-displacement piles
- bored and cast-in-place piles
- concrete or grout intruded piles
axial capacity
arises from base and shaft resistanc
lateral capacity
arises from horizontal soil pressure acting along the shaft
“friction fatigue”
for displacement piles
- lower shear stresses in the top of the pile where soil has undergone most shearing than towards the tip
Horizontal pile loads may be due to:
- wind
- wave action
- vehicle braking and acceleration
- soil loads
4 main factors on selection of pile types
- location
- ground conditions
- durabilty
- cost
piles for water based works
- driven piles
- driven and cast-in-place piles
piles for moderate land, unhampered site
- bored piles
- driven and cast-in-place piles
piles for land with proximate structures
- bored and cast-in-place piles
- jacket piles
piles for land with heavy load
- large diameter bored piles
piles for subsea environments
- corrosion of steel may cause durability problems
- favours adoption of precast concrete piles
piles for water-line areas
- timber piles may be attacked by molluscs and can decay where there is wetting-drying
piles for soils high in sulphates
- can cause corrosion to steel reinforcement
- better to use precast units using sulphate resistant cement then placing insitu where it is difficult to monitor defects
Types of Piles
- timber
- driven/jacket prefabricated and prestressed concrete piles
- steel
- driven and cast-in-place concrete
- bored and cast-in-place
advantages of timber piles
- easy to handle
- relatively inexpensive
disadvantages of timber piles
- low capacity
- can easily be damaged during driving
- difficult to splice
- at risk from biological action
advantages of driven/jacket prefabricated and prestressed concrete piles
- resistance to corrosion
- easy to splice
- relatively inexpensive
- good quality control
- can be re-driven
disadvantages of driven/jacket prefabricated and prestressed concrete piles
- relatively difficult to cut
- high displacement of soil
- can be damaged during driving
- can be noisy
advantages of steel
- easy to handle
- easy to cut and splice
- can be driven through dense layers
- low displacement
- high capacity
disadvantages of steel
- susceptible to corrosion
- may deviate during driving
- expensive
advantages of driven and cast-in-place concrete
- relatively inexpensive if case removed and reused
- easily cut or extended to length
- enlarged base can be formed for higher bearing capacity
disadvantages of driven and cast-in-place concrete
- relatively high displacement
- difficult to control quality
- shells may be damaged during hard driving
- cannot be used immediately
advantages of bored and cast-in-place
- length easily varied
- soil removed can be inspected/tested
- can be very large diameters (large capacity)
- low noise/low vibration
- can be installed in urban areas
disadvantages of bored and cast-in-place
- difficult to control quality
- cannot be readily extended above ground level
- not suitable in compressible soils (sinking and settlement of adjacent structures)
Global FOS
Qa = (Qs + Qb) / 2.5 = (Qs + Qb) / F
Partial FOS
Qa = Qb / 3 + Qs / 1,5
reason for using Partial FOS
because base resistance requires much greater settlement to fully mobilise, compared with shaft friction
