1. Pile Foundation Reason for Piles Types of Piles
Capacity Prediction Methods
Load Tests

2. Reasons for Piles Large Structural Loads
Settlement Intolerant Structures
Addition to Pile Supported Structure
Low Strength Soils at or near Ground Surface

3. Reasons for Piles Piles transfer loads:
To suitable bearing strata through toe resistance (end-bearing piles)
To strata in which pile is embedded through shaft resistance (friction pile)
Through a combination of both shaft and toe resistance (most common)

4. Types of Piles Timber OK for capacities less than approx 25 tons
OK for length less than 60 ft
Protect from rotting above groundwater table

5. Types of Piles Concrete
Appropriate where homogeneous soil conditions allow driving to a specific length
Not appropriate in the upper Midwest due to variable soil conditions (variable length)

6. Types of Piles
H-Piles
Good choice when driving to bedrock or deep penetrations
Limited uplift capacity

7. Types of Piles Pipe Piles Higher capacity per unit length that H-piles
May not drive as deep as H-piles

8. Capacity Prediction Methods
Static
Dynamic Formulae
Wave Equation Analysis of Piles (WEAP)
Dynamic Measurement and Analysis

9. Static Capacity Prediction
Estimates probable capacity range for a given length of pile, or probable length for a given capacity
Requires exploratory borings to about 10 – 15 feet below deepest anticipated penetration
Groundwater, qu, SPT

10. Static Capacity Methods
B – Nt Method
Inputs include qu, SPT N values, location of groundwater, unit weights
Overburden stress major controlling factor, soil strength next
Takes soil/pile set-up into account
Variations of + 25% should be expected

11. Static Capacity Methods
Qu = Qp + Qs
Qu = Ultimate capacity of pile
Qp = point capacity
Qs = frictional resistance along shaft

12. Point Capacity in Sands
Qp = Ap qp = Ap q’ N*q < Ap ql
Ap = area of pile tip
q’ = effective vertical stress at tip
N*q = bearing capacity factor (F13.9)
ql = limiting point resistance
ql (kN/m2) = 50 N*q tan f

13. Point Capacity in Sands
For homogeneous sands (L = Lb)
qp (kN/m2) = 40 Ncorr L/D < 400 Ncorr

14. Point Capacity in Clays
For undrained, saturated clays (f = 0)
qp = 9 cu Ap

15. Frictional Resistance
Qs = S p DL f
p = perimeter of pile section
DL = incremental length of constant p & f
f = unit frictional resistance at depth d

16. Frictional Resistance in Sands
f = K s’o tan d
K = earth pressure coefficient and varies with depth and pile type
K ~ 1.0 to 1.8 (1 – sin f)
s’o increases with depth to about L = 15D
d ~ 0.5 to 0.8 f
fav (kN/m2) = 1 to 2 Ncorr

17. Frictional Resistance in Clays
l Method
fav = l (s’o + 2 cu)
l varies with depth of penetration (F13.12)
Qs = p L fav

18. Frictional Resistance in Clays
a Method
f = a cu
a = empirical adhesion factor (F13.14)
Qs = S f p DL = S a cu p DL

19. Static Capacity Examples

20. Dynamic Formulae Theoretically unjustifiable, purge from practice
ENR Method is a common example

21. WEAP Analysis
Predicts dynamic behavior of pile driving by modeling driving assembly/pile/soil system
Estimates penetration resistance required for a given end-of-initial-drive (EOID) in the form of a graph

22. WEAP Analysis Provides estimate of probable capacity at EOID
When set-up is considered, provides embedment-dependent penetration resistance criteria
Provides design- and/or construction-phase flexibility w.r.t. selection of hammer/pile combinations
May allow use of lower FS

23. WEAP Input Parameters Pile Properties
Dimension (known)
Material properties (known)
Efficiency of driving assembly (assumed)
Resistance distribution (assumed)
Shaft and toe: quake and damping (assumed)

24. Dynamic Measurement and Analysis
Performed in Field with a Pile Driver Analyzer (PDA)
At pile head, strain measured to determine force, acceleration measured to determine velocity
Provides estimate of toe resistance, magnitude & distribution of shaft resistance, ultimate capacity

25. Dynamic measurement and Analysis
CAse Pile Wave Analysis Program (CAPWAP)
Performed on PDA measurements
Selects appropriate WEAP inputs to match predictions with measurements
Can be very cost-effective
May allow for lower FS

26. Load Tests Provides a proof test for design assumptions
Most valuable if pile fails (plunges)
Wait at least 30 days after EOID to allow for set-up
Load Pile to 250% of design load

27. Load Test Readings Applied axial compressive load Head deflection
Deflection of reaction piles
Deflection of pile at depth
Strain in pile section at depth

28. Load Test Instrumentation
Telltales
Measures pile head movement relative to specific location in pile
Backcalculate average load in pile above telltale based on known elastic modulus

29. Load Test Instrumentation
Strain Gauges
Vibrating-wire strain gauge most common
Measured strain used to calculate load at specific depths
Estimate load distribution in pile shaft