Nordlund Method Procedure
STEP 10 Compute the ultimate capacity, Qu.
Qu = Rs + Rt
STEP 11 Compute the allowable geotechnical pile load, Qa.
Qa =
Qu
Factor of Safety
Example 9-2
Single Piles in Cohesive Soils
g Total
stress method
− α-method or Tomlinson method
g Effective
stress method
− β-method
Tomlinson or α-Method
Unit Shaft Resistance, fs:
fs = ca = αcu
Where:
ca = adhesion (Figure 9-14)
α = empirical adhesion factor (Figure 9-15)
Tomlinson or α-Method
Shaft Resistance, Rs:
Rs = fs As
Where:
As = pile surface area in layer
(pile perimeter x length)
Tomlinson or α-Method (US)
Figure 9-14
Concrete, Timber, Corrugated Steel Piles
Smooth Steel Piles
D = distance from ground surface to bottom of
clay layer or pile toe, whichever is less
b = Pile Diameter
Tomlinson or α-Method
Sand or
Sandy Gravels
D
b
Stiff Clay
Tomlinson or α-Method (US)
Figure 9-15a
D = distance into stiff clay layer
b = Pile Diameter
Tomlinson or α-Method
Soft Clay
D
b
Stiff Clay
Tomlinson or α-Method (US)
Figure 9-15b
D = distance into stiff clay layer
b = Pile Diameter
Tomlinson or α-Method
D
b
Stiff Clay
Tomlinson or α-Method (US)
Figure 9-15c
D = distance into clay layer
b = Pile Diameter
HIGHLY OVERCONSOLIDATED CLAYS
In highly overconsolidated clays, the undrained shear strength
may exceed the upper limits of Figures 9-14 and 9-15.
In these cases, the adhesion factor should be calculated
according to API procedures based on the ratio of the
undrained shear strength of the soil, cu, divided by the
effective overburden pressure, po’. The ratio of cu / po’ is Ψ.
For Ψ ≤ 1.0,
α = 0.5 Ψ-0.5
For Ψ > 1.0,
α = 0.5 Ψ-0.25
Tomlinson or α-Method
Unit Toe Resistance, qt:
qt = cu Nc
Where:
cu = undrained shear strength of the soil at pile toe
Nc = dimensionless bearing capacity factor
(9 for deep foundations)
Tomlinson or α-Method
Toe Resistance, Rt:
Rt = qt At
The toe resistance in cohesive soils is sometimes ignored
since the movement required to mobilize the toe resistance
is several times greater than the movement required to
mobilize the shaft resistance.
Tomlinson or α-Method
Qu = R S + R T
and
Qa = QU / FS
Example 9-3
Which pile has the highest toe
resistance ?
Plugging of Open Pile Sections
D
fso
fso
fsi
qt
qt
qt
b
Figure 9-18
(a) Open Toe Condition
(b) Plugged Toe Condition
Plugging of H-Pile Sections
Figure 9-19
The DRIVEN Computer Program
g Developed
by FHWA in 1998
g Use for calculation of static pile
capacity
g Demonstration
program
of the DRIVEN computer
Piles Driven to Rock
The capacity of piles driven to rock should be based on driving
observations, local experience, and load test results.
RQD values from NX size rock cores can provide a qualitative
assessment of rock mass quality.
What is RQD? See Chapter 3
RQD
Rock Mass Quality
90 – 100
Excellent
75 – 90
Good
50 – 75
Fair
25 – 50
Poor
0 - 25
Very Poor
Piles Driven to Rock
Except for piles driven to soft rock, the structural capacity of
the pile will be lower than the geotechnical capacity of the rock
to support a toe bearing pile. (Fair to excellent quality rock).
The structural capacity of the pile then governs the pile
capacity.