Full Issue (17 MB) - Pile Driving Contractors Association

The Bearing Capacity
of Vibrated
Cast-In-Place-Piles
By Hermann Zeilinger, Dipl.-Ing. TU, Partner, Harland Associates and Klaus Hudelmaier, Dipl.-Ing. FH, President BVV Ldt.
Investigation and Comparison of the bearing capacity of
cast in place full displacement piles
1. driven with impact hammer, standard extraction
2. driven and extracted with vibratory hammer
3. driven with the vibro-jetting method, extracted with
vibratory hammer
Objectives:
In the Kimmeridge Clay area, with its very specific soil
conditions, one of the standard foundation methods uses full
displacement piles. At the end, with a lost plate closed steel
piles were driven with an impact hammer down through the
soft soil layers into the hard clay, creating the required bearing
capacity. The reinforcement cage will be inserted within
the casing and concrete will be pumped from the bottom up
to the surface. The extraction of the steel pipes with hydraulic
powered jacking system is the more difficult part of the installation
and usually determines the time interval for one cycle.
Bigger diameter of the lost plate at the tip is one way to reduce
the skin friction making the extraction easier. But with a larger
diameter more concrete is needed and the soil looses stress
reducing the final bearing capacity of the skin friction. Because
of the long installation cycles, caused by the slow and difficult
extraction of the casing, two other methods based on vibrated
piles were investigated, searching a way making the installation
faster. But before this procedure could be accepted: the
key question concerning the bearing capacity of those vibrated
cast in place pipes, has to be answered.
Goal of the assignment:
The goal of a one to one field test was: to install displacement
piles of same size and length using the three above mentioned
installation methods and investigating and comparing
their bearing capacity.
For the static load testing reaction piles and a standard
loading platform were installed for a maximal load of 3.000
KN. The load tests were executed with a manual served press
system completed with a total of five measuring instruments.
The dynamic testing is based on the CAPWAP – method,
(Case Pile Wave Analysis Program). This program considers
the speed as function of the acceleration, the elasticity and
cross section area of the material as well as the dynamic and
speed of the wave. Comparing the different loads the results
are presenting: the contribution of the skin friction along the
shaft, the tip bearing capacity as well as a load/settlement diagram
under static load.
The Project:
The site of Palm Paper Inc. in Kings Lynn, England was
already under construction. The overlaying soft soil did require
a pile foundation system to transmit the heavy load of the construction
down into the firm clay. The planning did demand
a total number of 7.011 cast in place full displacement piles
of 406 and 610 mm diameter carrying the designed load considering
a safety factor of 2. The advantage of the site was the
fact that already a series of different test results were available
for further investigation and comparison with this newly organized
field tests.
Soil conditions:
The soil layers consisting of very young sediments created
after the last ice period. These soft muddy layers are subject
to severe settlement s under any load. Organic peat and clay
of high plasticity were followed by very soft clay with some
silty sand, in change with layers of fine sand, sandy silt and
sandy silty clays, soft and of very low density. About 13 meters
below surface the so called Kimmeridge Clay marks the stiff
to firm bearing layer. For further investigations SPT test were
executed exactly in the positions of the test piles to receive
additional data for the determination of the bearing capacity.
44 • Q4 • 2009