The three most common ground
improvement methodologies
used by engineers to mitigate
the potential damage from liquefaction
include densification, drainage and reinforcement.
Conventional driven timber
piles can be used as ground improvement
by providing densification and reinforcement
to the subgrade. Timber piles modified
to provide drainage may be able to
achieve all three of the improvement methodologies
in one improvement method, or
improve the magnitude of densification by
virtue of relieving driving-induced excess
pore pressure.
Conventional and drained timber
piles were installed to investigate the effect
of pile spacing, time and drainage on densification.
Following installation of timber
piles spaced at two, three, four and
five diameters, cone penetration tests were
conducted at various durations following
installation to evaluate the degree of
densification, the effect of time and the
role of fines content on the degree of densification.
In general, the relative density
of the soils improved from approximately
40 to 50 percent pre-installation to 60 to
80 percent, depending on the pile spacing
and the presence of drainage elements.
The effect of improvement on liquefaction
resistances is described in terms of the factor
of safety against triggering for each of
the pile spacings.
Introduction
Among other recent earthquakes, the
Canterbury sequence, largely characterized
by four large (magnitude, Mw = 5.6
to 7.1) earthquakes between September
2010 and December 2011, highlights the
significant damage to civil infrastructure
caused by liquefaction. The Christchurch
events were characterized by significant
and widespread liquefaction-induced damage
to commercial structures – lifelines
such as pipelines, bridges and residential
buildings, the latter of which suffered
from ground subsidence, differential
settlement and lateral spreading with
remarkable frequency and intensity (Van
Ballegooy et al. 2014). These observations
indicate a continuing need for the evaluation
of cost-effective ground improvement
methods to mitigate the effects of liquefaction.
Common ground improvement
techniques used to counter the effects of
liquefaction fall into three general categories:
densification, drainage and reinforcement.
FEATURE
Methodologies that can perform
each of these functions – such as vibroreplacement
(i.e., stone columns) – represent
desirable alternatives owing to the
secondary effect of drainage in addition to
densification and reinforcement. However,
the performance of accepted techniques
such as stone columns can suffer from
inherent limitations, such as the reduction
of densification and drainage capacity
due to the presence of fines (Mitchell
1981), and the drainage effect is not typically
incorporated into the design of liquefaction
mitigation. The combination
of ground improvement methods such as
stone columns with pre-fabricated vertical
drains (PVDs) has been shown to successfully
improve densification in silty sands
(Rollins et al. 2006, 2009).
However, the use of more than one
technique and its attendant equipment necessarily
incurs greater mobilization, supply
and labor expenses, and may disqualify its
use based on cost considerations.
Oft-overlooked, driven timber piling
has been shown to densify loose, sandy
soils (Mitchell 1968). Despite the inexpensive
nature of this renewable material,
reservations against driven timber piles
may stem in part from perceived long-term
durability concerns or the usual questions
regarding time-dependent densification
(Mitchell and Solymar 1984, Slocombe et
al. 2000) and the effect of fines impeding
drainage, thereby limiting the magnitude
of densification. This paper presents a field
trial of drained and conventional, driven
timber piles conducted for the purposes of
evaluating the magnitude of densification
possible in soils considered to be susceptible
to liquefaction. First, the subsurface
conditions at the test site are described,
including the development of a site-specific
cone penetration test-based fines content
correlation. Then, the liquefaction hazard
for the test site is described in the context
of the regional seismicity. Thereafter,
the timber pile test program is presented,
including the evaluation of PVDs fitted on
timber piling for possible improvement in
the magnitude and time-rate of densification.
Comparison of improvement through
densification is made as a function of pile
spacing (ranging from two to five diameters,
D), elapsed time-since installation
and the presence of drainage elements.
Observations derived from the test program
show that significant magnitudes in
densification, and thereby mitigation of
Bartek Zyczynski/Shutterstock.com
This paper was originally published in
the 6ICEGE Proceedings, 1-4 November
2015, Christchurch, New Zealand. It is
reprinted with permission.
PILEDRIVER | 53
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