July/August 2023 - 16
(8.5x8.5x2 ft) precast concrete bearing blocks installed in five
rows above the highway, a tied-back concrete tangent pile
wall below the highway, and reconstruction of the highway to
original alignment and grade. The pile wall comprises 148
bored piles of 1,524 mm (60 in) diameter, ranging in length
from 20-26 m (65-85 ft); 125 tie-back soil anchors; a waler
assembly and a cap beam with a cantilever wall stem that
extends to 5 m (16 ft) in height. The piles are embedded 10 m
(33 ft) into the dense, free-draining, coarse-grained
undifferentiated glacial sediments.
Working top-down, each row of anchored blocks was
sized to slow, or even stop, the movement of the slide mass
above it. After installation and testing, the anchors of a given
row could be locked-off within a fairly short timeframe.
However, the pile wall required a large number of piles to be
installed, and during the time required, the soil mass below
the lowermost row of anchored blocks continued to move.
The design therefore required a pile solution that would
exhibit considerable early shear strength and be tolerant of
displacement until the tie-back anchors could be installed,
tested and locked-off.
The solution selected was to use steel shell piles with
concrete infill (composite piles), with the steel shell providing
initial shear stiffness across the slide plane while the
concrete infill cured. Installing tie-back anchors through
steel shafts would have been prohibitively expensive, so only
every third pile was a steel shell composite pile with the
remainder being reinforced concrete bored piles. Drilling
anchors through the concrete pile shafts is a common
construction task. The composite piles were installed ahead
of the concrete piles and successfully slowed slide
movements to allow the concrete piles to reach adequate
concrete strengths without being damaged.
To share the soil loading among both the concrete and the
composite piles after the tie-back anchors were installed, a
steel waler system was installed to link each composite pile
with its adjacent two concrete ones. The waler system
involved first attaching a waler stool to each concrete pile, the
stool allowing position adjustments to ensure a flat vertical
plane for the waler beam was achieved. The stools also
Pile wall details
incorporated a steel pipe shear stub that was inserted into the
hole that was cored through the concrete pile for the tie-back
anchor. The shear stub directly transferred the vertical
component of the anchor testing and stressing force to the
concrete pile shaft. The steel walers, comprising C380x50
(C15x33.9) channels back-to-back, were then attached to the
stools and the tie-back anchors stressed. Subsequently, a
concrete pile cap was constructed that encapsulated the pile
tops and extended down to cover the walers, with pockets
provided for permanent access to the tie-back anchors. By the
time the pile wall could be constructed, the design road
alignment was between 1-6 m (3-20 ft) above the pile tops, so
the pile cap was topped with a cantilever wall stem that acts
integrally with the pile wall system.
Design Methodology
Detailed 2D and 3D numerical time-step modelling of the
slope movements and soil-structure interaction was used to
predict stresses on the structural elements throughout
construction, and to define time restrictions for the
construction contracts. A combination of limit-equilibrium,
finite difference and finite element methods were used for
both the geotechnical analyses of the slope movements,
upslope anchor stresses and the soil loads on the pile wall,
and for the structural design of the pile wall components.
Geotechnical Analyses - Two-dimensional limit
equilibrium stability analyses were used to evaluate and
compare various landslide stabilization solutions. For the
selected solution, the limit equilibrium stability analyses
were then used for defining anchor spacing and loading and
for establishing the pile configuration and capacity. The limit
equilibrium stability analyses were also used for local
stability assessments of the construction excavation and
benches required for the installation of the stabilization
measures.
More detailed analyses of the slide movement and impact
of the stabilizing measures was then carried out on a crosssection
through the center of the landslide, which
corresponded to one of the limit equilibrium models thus
allowing for verification of results. These analyses were
carried out on 2D finite difference numerical models using
the computer program FLAC.
The staging of the stabilization work, and the coincident
continual movement of the slide was analyzed in discrete
steps. In general terms, each FLAC Step represented a stage of
construction, with sub-steps used to calibrate the model to
yield results that matched the known ground movements, see
Table 1. The strength reduction method (SRM) was used at
each step wherein the friction angle of the basal shear zone
was reduced slightly below the calibrated value for existing
conditions to initiate movement in the model, i.e., achieve a
serviceability limit state (SLS) with a FoS against sliding of just
under 1.0. The SRM was then used again to increase the FoS to
16 * DEEP FOUNDATIONS * JULY/AUG 2023
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Table of Contents for the Digital Edition of July/August 2023
TOC
July/August 2023 - Intro
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July/August 2023 - TOC
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