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**Unformatted text preview: **Pile Load Testing
Foundation Design
CE 482 Introduction
sophisticated structural analyses using
“rough” geotechnical design parameters
heterogeneity of soil (unlike construction
materials)
geotechnical design often empirical
conservative factors of safety
site-specific performance testing
more aggressive design, verified safety
factors Types of Load Tests
proof-test
pile is loaded to the ultimate design load
(allowable design load times the factor of
safety) and the deflection is measured at the
pile head
generally performed during installation load-deformation test
pile typically tested to failure, deformation (and
often stress) measured at several points along
the pile shaft and at the pile tip as well as at the
pile head
detailed load-deformation data obtained allows
more efficient design
usually performed during the design phase Types of Load Tests
static
load pile & measure deflection
dynamic
static pile capacities generally using
stress wave analyses of pile
deflection caused by dynamic loads Static Pile Load Tests
Conventional
construction of reaction frame and incremental
loading of the pile, usually with a hydraulic jack
[Figure 1]
test load measured with load cells and pile
head deformation measured with strain gages
and surveying equipment . Figure 1: typical setup for compressive testing Static Pile Load Tests
Conventional
uplift and lateral load tests performed by
modifying reaction frame and loading
(jacking) direction
[Figures 2 and 3]
costly and time-consuming but generally
provide the most reliable performance data
because the loading method is similar to
service loading Figure 2: typical setup for uplift test Figure 3: typical setup for lateral test Static Load Tests
Osterberg Cells
often more cost-effective than conventional
static load tests
Osterberg cells are in essence largediameter hydraulic jacks with built-in load
cells cast within the pile with twin reaction
plates similar in diameter to the drilled pile
at the top and bottom of the cell
movement is measured using strain gages
and reference rods isolated from strain
(sleeved) extending from the top of
Osterberg cells to the ground surface Static Load Tests
Osterberg Cells
typically not used for uplift testing because
conventional uplift tests are generally less
expensive
Osterberg cells cost-effective for
compression and lateral load tests because
reaction piles or anchor are not required Static Pile Load Tests
Osterberg Cells
single cells typically used for compression
proof tests
load cell cast near the bottom of the pile and
expanded to obtain load and deflection data
some interpretation of the data is required
because the test loading is differently from
service loading Static Pile Load Tests
Osterberg Cells
during the test, the cell is expanded near the
bottom of the shaft, causing uplift above the cell
and settlement below the cell
because cell loads are resisted by shaft resistance
above the cell and pile end bearing below the cell,
load-deformation data for the pile tip and pile shaft
can be obtained independently
multiple cells can be cast within a test pile to
isolate end-bearing and shaft friction effects or to
evaluate directional effects of shaft friction
[Figure 4] Figure 4: installation of Osterberg cell at pile tip Dynamic Load Tests
developed from research funded by the
Ohio Department of Transportation and
the Federal Highway Administration at the
Case Institute of Technology in Cleveland,
Ohio
using measurements of strain and
acceleration and the principles of wave
mechanics, dynamic test methods are
used to estimate static pile capacity,
inspect pile integrity, and evaluate pile
driving systems Dynamic Load Tests
two types of dynamic pile testing:
large-strain and low-strain methods
low-strain methods are typically
performed using hand-held hammers that
measure pile top velocities and are used
mainly to inspect integrity and length of
concrete piles. Anomalies in the velocity
record are used to evaluate pile integrity.
low-strain methods to inspect pile
integrity are limited to depths of about 30
times the pile diameter Dynamic Load Tests
Large-Strain Methods
Large-strain methods are used almost
exclusively for driven piles to evaluate the
driving system as well as for estimating
static axial pile capacity
Strain gauges and accelerometers are
installed near the top of the piles and
measurements are taken during pile
driving Dynamic Load Tests
Large-Strain Methods
typically performed during the indicator
pile program
(The indicator pile program is a field test
of the selected driving hammer and
system to evaluate the driving criteria,
driveability, and production rate) Dynamic Load Tests
Large-Strain Methods
because the cost of installing the strain
gauges and accelerometers and
monitoring the measurements is relatively
inexpensive compared to the total cost of
the indicator pile program, dynamic pile
testing is a cost-effective way of
optimizing the driving system and
estimating static pile capacity
for driven piles, optimization of the driving
system may be as important as estimating
pile capacities
A typical setup is presented in Figure 5 Figure 5: typical
PDA setup Dynamic Load Tests
Large-Strain Methods
measurements of strain are converted to
force and the measurements of
acceleration are converted to velocity for
input into dynamic resistance equations to
estimate static pile capacities.
the most popularly used dynamic
resistance equation is the Case Method
(Goble et al., 1975) Dynamic Load Tests
When a hammer or drop weight strikes the top
of a foundation, a compressive stress wave
travels down its shaft at a speed c, which is a
function of the elastic modulus E and mass
density
The impact induces a force F and a particle
velocity v at the top of the foundation
The force is computed by multiplying the
measured signals from a pair of strain
transducers attached near the top of the pile by
the pile area and modules Dynamic Load Tests
The velocity measurement is obtained by
integrating signals from a pair of accelerometers
also attached near the top of the pile
Strain transducers and accelerometers are
connected to a Pile Driving Analyzer® (PDA),
for signal processing and results Dynamic Load Tests
As long as the wave travels in one direction,
force and velocity are proportional:
F = Zv,
where:
Z = EA/c is the pile impedance
E is the pile material modulus of elasticity
A is the cross sectional area of the pile
c is the material wave speed at which the
wave front travels Dynamic Load Tests
Soil resistance forces along the shaft and at the
toe cause wave reflections that travel and are
felt at the top of the foundation
The times at which these reflections arrive at
the pile top are related to their location along
the shaft
The measured force and velocity near the pile
top thus provide necessary and sufficient
information to estimate soil resistance and its
distribution Dynamic Load Tests
Total soil resistance computed by the PDA
includes both static and viscous components
The static resistance can be obtained by
subtracting the dynamic component from the
total soil resistance
The dynamic component is computed as the
product of the pile velocity times a soil
parameter called the Damping Factor
The damping factor is an input to the PDA and
is related to soil grain size Dynamic Load Tests
Total soil resistance computed by the PDA
includes both static and viscous components
The static resistance can be obtained by
subtracting the dynamic component from the
total soil resistance
The dynamic component is computed as the
product of the pile velocity times a soil
parameter called the Damping Factor
The damping factor is an input to the PDA and
is related to soil grain size Dynamic Load Tests
The energy delivered to the pile is directly
computed as the work done on the pile from the
integral of force times incremental displacement
( ∫Fdu ) which is easily evaluated as force times
velocity integrated over time ( ∫Fvdt )
Maximum compression stresses at the pile top
come directly from the measurements
The measurements also allow direct
computation of the compression stress at the
pile toe and the tension stresses along the shaft Dynamic Load Tests
Pile integrity can be evaluated by inspecting the
measurements for early tension returns (caused
by pile damage) prior to the reflection from the
pile toe; lack of such reflections assures a pile
with no defects Dynamic Load Tests
Large-Strain Methods
a specific hammer blow can be analyzed using the
Case Method and a soil model to estimate the shaft
friction, end bearing, dynamic damping factors, and
soil stiffness
a computer program called CAPWAP® for Case Pile
Wave Analysis Program from Goble, Rausche, Likins
and Associates, Inc. can be used to perform this
analysis
the compression and uplift static pile capacities can
then be estimated Dynamic Load Tests
Large-Strain Methods
although dynamic testing can used to estimate
static pile capacity for drilled piles, the cost of
mobilizing a pile-driving hammer and rig may
make it inappropriate Weap
In the 1950’s, E.A. Smith of the Raymond Pile
Driving Company developed a numerical analysis
method to predict the capacity versus blow count
relationship and investigate pile driving stresses
The model mathematically represents the hammer
and all its accessories (ram, cap, cap block), as well
as the pile, as a series of lumped masses and springs
in a one-dimensional analysis
The soil response for each pile segment is modeled as
viscoelastic-plastic Weap
All components of the system are thus realistically
modeled
The analysis begins with the hammer ram falling and
attaining an initial velocity at impact
This method is the best technique for predicting the
relationship of pile capacity and blow counts (or set
per blow), and the only method available to predict
driving stresses Weap
Improvements to Smith’s method include work by
GRL to incorporate a thermodynamic diesel hammer
model and residual stresses
The GRL Wave Equation Analysis of Piles
(GRLWEAP) program is based on Smith’s method
The wave equation approach it is an excellent
predictive tool for analysis of impact pile driving, but
it has some limitations
These are mainly due to uncertainties in quantifying
some of the required inputs, such as actual hammer
performance and soil parameters Pseudo-Static Tests
impact energy prolonged to better simulate static
loading Statnamic pile loaded by acceleration of reaction mass
with ignition of propellant fuel Pile Load Tester spring system used to dampen impact and
delay (prolong) energy transfer onto pile ...

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