July/August 2023 - 17

FLAC model, overall model (left) and pile wall with moment diagram (right)
1.2 to determine the loads on the structures for the ultimate
limit state (ULS). The models were run under SLS conditions to
achieve the slide movements anticipated between each
phase of construction. By FLAC Step 27, the model assumed all
earthflow soil downhill of the pile wall and above the bottom
of the basal shear zone will slide away and cease to support
the pile wall.
For both the limit equilibrium and finite difference models,
a sensitivity analysis was completed to evaluate changes in
loading on structures to be expected from a transient rise in
pore pressure. Furthermore, the predicted seismic response of
the stabilization system was evaluated for the design
earthquake by applying eight earthquake time histories to the
long-term SLS case where it is assumed all soil downhill of the
pile wall and above the basal shear zone moves downhill and
ceases to support the pile wall. Each time, history was
synthesized to match the site-specific target spectrum as
defined by Canadian Highway Bridge Code.
Table 1. Description of FLAC Analysis Steps
FLAC Steps
1 to 4
5 to 7
8 to 15
16
17
18 to 20
21 to 25
26
27
28
Description
Model development (boundary conditions, soil properties,
initial conditions)
Installation of anchor row 1 and subsequent slide movement
Installation of anchor rows 2 to 5
Installation of composite pile steel shell
Increase composite pile stiffness to account for hardening
concrete fill
Installation of concrete piles
Excavation of tie-back and waler construction bench
Tensioning of tie-back anchors
Loss of soil downstream of wall and above basal shear zone,
add earthquake
Check case of resetting and re-stressing anchor rows 1 to 5
Structural Analyses - The FLAC numerical model, being
2D, incorporated both composite and concrete piles as tiedback
elements. Thus, the FLAC results do not provide a true
representation of the load sharing between the directly tiedback
concrete piles and the waler-restrained composite pile in
between. For the detailed structural design, 3D structural
analyses were conducted of the pile wall system incorporating
the three triplets of one composite pile and two concrete piles
each and the waler, tie-back anchors, pile cap cantilever wall
stem and roadbed. Three models were developed to assess variations
in pile lengths and location of the basal shear zone. This
wall system was modeled and analyzed using the finite
element program RFEM. The RFEM structural analyses results
in the same load demand on tie-back system as the FLAC
analyses but provides for a more accurate local distribution of
load among the different pile types and the anchors.
For the composite and concrete piles, the effect of the
landslide loading was transferred from the 2D FLAC model to
the 3D RFEM model as follows:
1. The profile of shear force along the length of each pile type,
as determined by FLAC, was used to calculate the profile of
soil reaction loads acting on each pile for each relevant
analytical FLAC Step.
2. The incremental loads on the piles were then derived for
each construction stage. FLAC Step 20 was taken as the
starting point. Step 26 minus Step 20 represents the effect
of anchor stressing, Step 27 minus Step 26 that of SLS
equilibrium being reached, etc.
3. For the pile embedment below the shear plane, the RFEM
model used soil-springs determined by P-Y methods.
4. The FLAC analysis had all piles anchored, and as noted
above, the anchor load is implicit in the pile soil loads
obtained from the FLAC modelling. Therefore, in order to
model the wall with only the concrete piles anchored, half
of the composite pile soil loads above the plane of slip, for
the incremental Step 26 minus Step 20, were added to that
of each of the concrete piles.
DEEP FOUNDATIONS * JULY/AUG 2023 * 17
July/August 2023

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