Seismic risk assessment for oil and gas pipelines - Seismic Risk Analysis and Management of Civil Infrastructure Systems - page 698

Ground improvement is emerging as one of the widely adopted mitiga-
tion measures to reduce the impact of earthquake-related ground displace-
ment hazards. In mitigation works, the design philosophy often revolves
around implementing ground improvement measures to limit deformations
in a given pipeline to acceptable levels (e.g., design to minimize the loss of
pressure integrity in pipelines). Observations following major earthquake
events have indicated that sites with improved ground had performed well
during earthquakes (Mitchell et al. , 1995).
When ground improvement is considered to be the desired option, the
selection of the most suitable remedial option is governed by many factors
including, but not limited to: soil conditions, space restrictions, issues related
to the protection of existing structures during ground improvement, opera-
tional constraints, environmental regulatory requirements, and land avail-
ability. Historically, ground improvement has been used as a means of
improving the post-construction bearing capacity and settlement perfor-
mance of soils under static loading conditions, and a variety of ground
improvement techniques have evolved in the past few decades. In addition
to resisting static loads, some of the ground improvement measures have
been effectively used to retrofi t facilities that are located within, or that
have foundations supported on liquefi able soils. These measures include
dynamic deep compaction, vibro-replacement using stone columns, com-
paction piling, explosive compaction, and compaction grouting.
The method of vibro-replacement using stone columns is the most pre-
ferred technique of ground improvement in sandy soils. The method can be
effectively used to densify soils within about 25 m below existing ground
level (see Fig. 25.4). The method is attractive because of the potential avail-
ability of drainage through stone columns for the dissipation of excess pore
water pressures in addition to the densifi cation effect. Compaction grouting
is a useful tool not only in fi ne-grained soils, but also in improving sites that
have physical constraints such as low headroom. Deep dynamic compaction
is a viable means of improving the settlement characteristics and liquefac-
tion resistance of random fi lls and alluvial soils that are in a state of loose
relative density and diffi cult for a probe to penetrate through. In-situ veri-
fi cation using penetration resistance measurements confi rm that this method
can be used to a maximum depth of about 10-12 m below existing ground
level. Below this depth, the achieved improvement in penetration resistance
diminishes considerably.
Detailed site-specifi c studies are required to quantify potential for pipe-
line damage, and to determine whether or not practical alternatives exist
to reduce the risk. Development of site-specifi c recommendations requires
careful consideration of many factors including site geology, environmental
conditions, pipeline response characteristics, and system performance
requirements. The ground improvement confi gurations used in practice are