VTL
FEATURE
WHEN YOU NEED MORE THAN JUST LEADS
Precision Placement
Long Piles or Large Batters
Piling or Drilling
Rental or Purchase
600 Ferguson Avenue North
Hamilton, Ontario, Canada L8L 4Z9
Tel: 1.905.528.7924 Fax: 1.905.528.6187
Toll Free: 1.800.668.9432 (in Canada and USA)
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Email: bfssales@berminghammer.com
Since the effects of pile
installation are so different
between driven and drilled
foundations, it is important
to understand, at least
qualitatively, what happens in
the ground and at the pile/soil
interface during pile installation.
elevated pore pressures around the pile that could temporarily
reduce the effective stress in the ground. The occurrence of significant
pore pressures around piles in sand commonly occurs
only in sands with sufficient fines (silts and clays) so that the
sand is not freely drained. The writer has observed “sand volcanoes”
in the area around pile driving operations in loose silty
fine sands with shallow groundwater, but this phenomenon is
relatively rare. Elevated pore pressures typically dissipate within
minutes in well drained sandy soils.
Remolding and consolidation of clayey soils
In fine grained soils such as silts and clays, pore water pressures
are typically generated with the soil displacement and
increases in stress, and these soils have such low hydraulic conductivity
that the water does not drain away quickly. The effect
of pile installation on the soil and state of stress is complex and
changes with time.
As the pile is driven, the soil around the pile is sheared and
distorted by the displacement from the pile, with remolding of
the soil fabric. After the initial displacement and radial stress
increase as the toe of the pile passes a given depth, the soil adjacent
to the pile is subject to cyclic shear stress at the pile/soil
interface with each successive blow, developing a zone of very
remolded soil immediately adjacent to the pile wall.
As excess pore pressures around the pile gradually dissipate,
pore water will flow radially away from the pile and the soil
adjacent to the pile consolidates and the effective stress in the
soil increases; however, this change in volume near the pile wall
redistributes the relative ratio of radial stresses to tangential
stress around the pile. An early result of time-dependent consolidation
of soil around the pile is illustrated in measurements
reported by Flaate (1972) and also discussed by Karlsrud (2012).
Flaate identified the effects of consolidation in clays around
piles by his measurements of shear strength and water content
between piles in a group of timber piles during a test program
conducted over five years after installation. A shaft was
excavated to a depth of about 25 feet to obtain soil samples
between the piles. Water contents of the soil at the site were
typically in the range of 28 to 32%, but in the zone between
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