Pile Capacity Flashcards
Motivation for piles
- Upper soils have a low bearing capacity
- Loads too large for suitably small spread footing
- Large uplift or lateral force
- Excavation planned near the site
- Foundation to be installed under water
can carry a single column or the entire load when usd as a group under a pile cap
Definition
L/D > 5
Types
Driven
- hammered into the ground or driven hydraulically so generally too noisy for use on land
- Steel or conc
- Classified as displacement piles
Bored and in cast piles
- Hole bored then filled with conc
- large diameter
- can be undereemed
Continuous flight augered
- type of bored
- less vibrations therefore less noise
- quick installations
- no support needed for walls
Piling considerations
Soil
- Driven better for loose and water bearing sands, not boulders
- Stiff clays require driven or undereemed
Durability
- Precast conc is better for saline marine environments
Disposal of spoil
- None produced for driven so its good for contaminated areas
Terminology
- Rs = Shaft resistance
- Qb = Base resistance
Sum = Pf which balances self weight and applied force P
- Pf dominated by Qb is an end bearing pile, dominated by Rs is a friction pile and no dominance is combined
- Friction piles have a failure mode along th chaft
- End bearing have a failure more resembling that of a shallow foundation
Activation of shaft and base resistance
Rs reached its max at settlements between 0.2% and 2% of the pile D
- Stiffer response therefore friction piles generally have less settlement
- At Rsf resistance can increase lowly or decrease slowly
Qb reaches its max at settlements between 10% and 20% of the pile D
Relative contributions of Rs and Qb
-Straight sided
-Bored undereemed pile
- Straight sided has base resistance much lower than shaft
- Bored undereemed has shaft much lower than the base
- The general shape for each is unchanged, just the magnitude
Determining pile capacity
- Definition of pile failure
- alpha/beta method are semi-empirical
Pile failure
- Plunging is when the piles settlements increase indefinitely (Pu) but hard to reach
* Pf0.1 is used. This is when the settlements are 10% of the piles diameter which is approximately Pf for a friction pile but they differ a lot for an end bearing one
Pile Testing
Used to verify the accuracy if designs or building methods
- Done before construction or on the piles that will acc be used in a Working Pile Test
* done under maintaned load or constant penetration
* loaded at 150% of the design load
* load applied using a jack against kentledge and the settlement is recorded
- Reduces Fos
- No testing –> 3
- 1% working pile test –> 2.5
- 1% working pile test + preliminary testing –> 2
Undrained base resistance
Qbf = Ab(Su,baseNc + p0)
- Su,base from lower bound of scatter
- Nc = 9 for depth >5D
- p0 cancels assuming Ysoil = Yconc
Undrained shaft resistance
-Next to the pile the soil is sheared and when this exceeds the strength there is failure
Tau = sigma_r’.Tan(delta’), with delta’ being the friction angle
-Driven piles increase the stress around the pile while bored piles generally have little effect but sometimes negative
* Affects the selection of alpha/beta
Rsf = As.su_bar.alpha
- su_bar taken at the middle of the shaft using a mean line of the scatter
- alpha is the Adhesion factor which is derived from a database pile test result
Adhesion Factor
- depends on soil type, pile type and method of getting Su
- typically 0.3 to 0.6 for a bored pile with straight sides but higher for ribbed/irregular piles or in stiff, over consolidated clay
*typically 0.7 for f=driven piles
*In london clay its 0.45 for 38mm triaxial tests and 0.5 to 0.6 for 100 =mm TT
uncertainty surrounding alpha leads to high FoS and site testing
* correlates with Su/sigma_v’ for driven
* correlates with dy for bored
Adhesion factor limitations
-Empirical therefore limited for applications outside of the test data
- Scatter in getting su
still widely use
Beta Method
-Also for fine grained soils but done in effective stresses with accuracy improved with correlations with tests (CPT or SPT)
- Shear at failure depends on
*overburden
*over consolidation ratio
*effects of pile installation
*change in vertical effective stress during loading - sigma_r = K.sigma_v with K not equal but close to K0 therefore:
Rsf = As(sigma_v’.Ks.Tan(delta’))
with beta = Ks.Tan(delta’)
delta’ = phi’ - 5
Beta method advantages
- sigma_v’ is accurately determined therefore beta is quite well defined
- Can be obtained empeically of though pile tests
