Lateral Load Tests on Piles in Liquefied Sand to Develop P-Y Curves
Kyle M. Rollins1 , Travis M. Gerber 2 , J. Dusty Lane 3
ABSTRACT
To improve our understanding of the lateral load behavior of deep foundations in
liquefied soil, a series of full-scale lateral load tests have been performed at the National
Geotechnical Experimentation Site (NGES) at Treasure Island in San Francisco,
California. The ground around the test piles was liquefied using explosives prior to
lateral load testing.
The goal of the project was to develop load-displacement
relationships for bored and driven piles and pile groups in liquefied sand under full- scale
conditions. Pilot liquefaction studies showed that excess pore pressure ratios great than
0.8 could be maintained for 4 to 6 minutes after blasting in a relatively large test area.
The first set of foundation tests involved the lateral load testing of a single steel
pipe pile (324 mm OD) reacting against a single H-pile (12 x 53). Prior to blasting,
lateral load tests were performed with a maximum pile head deflection of 38 mm in one
direction. Following the blasting, lateral tests were performed with a maximum pile head
deflection of 228 mm in the opposite direction. Load was applied using a 2000 kN
hydraulic actuator which could move at a rate of approximately 20 mm/sec. Following
blasting and the generation of excess pore pressure ratios near 1.0, the lateral resistance
of the soil- foundation system dropped significantly, but eventually reached a relatively
steady state condition after multiple load cycles. At steady state conditions, about 7 to 9
times more movement was required to reach a given load than for the pre- liquefaction
condition. The load-deflection curves exhibited a concave upward shape with the slope
increasing with displacement rather than the typical case where the slope of the curve
decreases with increased displacement. This phenomenon appears to be connected to
dilation of the sand at high displacements. As the pile moves laterally, the soil dilates
leading to a decrease in the excess pore pressure ratio. As the pore pressure ratio
decreases, the effective stress in the sand increases which leads to an increase in the
resistance of the sand surrounding the pile.
After the single pile testing, a similar procedure was used in the testing of a ninepile group driven open-ended in a 3x3 pattern at a spacing of 3.3 pile diameters. The
piles were attached to a relatively rigid load frame so that the deflection of each pile was
essentially constant. The load carried by each pile was measured using tie-rod load cells
that were attached to each pile with a pinned connection. A similar loading procedure
was employed as for the single pile tests. During the pre-blast loading, the piles in the
leading row carried significantly greater load than the piles in the trailing rows for a
given displacement. This reduction in resistance is attributed to overlapping shear zones
and is often referred to as a group reduction effect. After the blasting produced a
liquefied zone around the pile group, the load carried by each pile in the group was
essentially constant indicating that group reduction effects are minimal in the liquefied
state, presumably due to the fact that well-defined failure zones do not fully develop.
1
Prof., Brigham Young Univ., 368 CB Provo, UT 84602, USA, e-mail:rollinsk@byu.edu
Project Engr., URS Greiner, Inc., 6975 Union Park Ave Ste 400, Midvale, UT, e-mail:travis_gerber@urscorp.com
3
Prof., Construction Management., Brigham Young Univ., Provo, UT 84602, USA, e-mail:jdlane@et.byu.edu
2
Five of the test piles in the group were instrumented with strain gauges along the
length of the pile. The strain was measured at 0.1 second intervals throughout the testing
along with the pile head load, deflection, and rotation. The stain gauge data was then
used to determine the bending moment distribution along the length of the pile as a
function of time. This curve was then double integrated and double differentiated to
produce the lateral load per pile length, p, as a function of the horizontal deflection, y, at
a given depth. This procedure was employed at each time interval to produce p-y curves
for the entire test sequence. The p-y curves developed give reasonable estimates of the
measured load-deflection and bending moment curves when used in a program such as
LPILE to model the lateral pile behavior. When the steady-state load-deflection
condition is reached, the p-y curve shape is flat at low deflections but increases as
deflection increases (concave up). This shape departs significantly from the traditional
curve shape for soft clay which is often used in modeling liquefied sand along with the
residual strength from the SPT (N 1 )60 value. Use of the soft clay curve shape would
significantly overestimate lateral resistance in liquefied sand for small displacements but
may be approximately correct at relatively large displacements.