Effect of Inclusion Piles on Mitigation of Seismic Surface Ground Motion

The need to construct structures on soft and unstable soils due to the appropriate technical and economic
conditions has led to the development of various soil remediation methods. Moreover, the experience obtained from
recent earthquakes has indicated the influence of sites’ stiffness on the surface seismic ground response. One of the
ways to increase the stiffness to improve the soil, especially in soft soils, is to employ inclusion piles. These types of
piles can be used at the bridge's piers to reduce the seismic response of the aboveground structures. In this regard,
the role of the geometry characteristics of the inclusion piles can be significant. This paper investigates the effect of
changes in the geometric parameters of inclusion piles such as diameter, length, the distance between them, and
surcharge on the ground seismic response based on the offshore Turkish Izmit Bridge as a case study and base
model. The effective depth was obtained by comparing the ground response spectrum of the two-dimensional model
with inclusion piles using FLAC2D software based on the nonlinear hysteresis model, with the depth equivalent to
the acceleration response spectrum of the free-field model. The geotechnical subsurface conditions at the North
Tower Izmir bay bridge consist of 10 meters of loose to medium dense sand layers with silt, underlain by 127 meters
of dense sand and hard sand clay. Bedrock lies approximately 144 meters below the mudline datum. The 1D
responses obtained from the FLAC 2D and DEEPSOIL 1D software have been compared using the nonlinear soil
behavior to verify the numerical modeling results. Then, with the calibration of soil parameters and lateral and
bottom boundaries, inclusion piles have been added to the validated free-field model in FLAC2D software.
In this study, the 2D modeling process includes introducing soil layers’ characteristics and determining the
lateral free-field boundaries and the quiet boundary as the bottom boundary subjected to the seven earthquake
excitations is performed. The inclusion pile was modeled using the beam and cable combine elements in the
FLAC2D. Besides, inclusion piles are two-dimensional elements with 3 degrees of freedom (two displacements and
one rotation) at each end node. Piles interact with the FLAC grid via shear and normal coupling springs.
The obtained results indicated that by increasing the ratio of distance to the diameter of inclusion piles (S/D), the
effective depth decreases due to reducing the stiffness of the inclusion pile system, and after reaching a ratio of 5, it
has reached a constant value. In other words, with increasing stiffness of the soil-pile system, the effect of kinematic
interaction on the soil-pile system increases. Moreover, by increasing the length to diameter ratio of inclusion piles
(L/D), the effective depth will first increase and then reach a constant value, in which the optimal range for the
length to diameter ratio of piles is 15 to 30. Also, the effective depth increases linearly with an increasing surcharge
ratio above the inclusion piles ( q ).
Finally, it should be noted that the soil improvement using inclusion piles due to the kinematic interaction can
apply a new foundation input motion altered from the free-field ground response. This interaction increases the
effective depth of the equivalent free-field model, which can reduce responses of the aboveground structures (e.g.,
buildings or bridges, etc.). Therefore, the use of this type of piles due to having more stiffness than traditional soil
improvement approaches such as stone columns or deep soil mixing, etc., can be effective in order to optimally
design structures located on loose or soft saturated soils.