DFI Magazine November/December 2020 - 99

ROW limitations (outlined in red) and utility locations (magenta) guided foundation
design decisions

Global stability was also evaluated
using Slope/W software by Geoslope. The
reinforcing elements were modeled as
" nail " reinforcement and assigned a design
load determined during the SNAILZ
analyses. The results of both Slope/W and
SNAILZ models were compared to select
the appropriate nail lengths for the final
design section.
The relatively weaker overburden
material was modeled separately to
analyze nail lengths and for local stability,
as a s taged ex cavatio n . T h en th e
overburden was included with the
complete design section to determine
minimum nail lengths. Temporary nail
lengths ranged from 8 ft to 25 ft (2.4 m to
7.6 m), with loads of approximately 14 kips
to 32 kips (3.1 kN to 7.2 kN). The
permanent nail lengths ranged from 8 ft to
10 ft (2.4 m to 3.0 m), with loads of
approximately 10 kips to 32 kips (2.2 kN
to 7.2 kN). Shotcrete was designed with a
4 in (101.6 mm) initial facing and a 7 in
(177.8 mm) final facing

In areas of the site where utilities
(proposed storm basins and existing storm
sewers), or ROW limitations, precluded soil
nail use, micropile-type soldier piles were
designed and installed to the full depth of
excavation. Once excavation depths cleared
the utility zone (the upper ±10 ft [3 m]), soil
nails supplemented the soldier piles for the
remainder of the excavation. This hybrid
soldier pile system was also applied where
ROW precluded soil nail use in upper layers.
Once the excavation encountered rock, the
local municipality allowed permanent soil
nails to extend beyond the ROW, with the
understanding that future utilities would not
be installed into the bedrock.
Soil Nails - All soil nails were
designed and tested according to Federal
Highway Administration guidelines
(FHWA 0-IF-03-017, March 2003). A 250
psf (12 kPa) surcharge was included atop
the wall for a minimum distance of 25 ft
(7.6 m) behind the wall, except along East
Beaver Avenue (State Route 26), where
PennDOT required a minimum surcharge
of 350 psf (16.8 kPa). Overall system
stability was analyzed to a minimum safety
factor of 1.35 in the temporary condition
and of 1.5 for the permanent condition.

Soldier Piles - In areas with utility and
ROW restrictions, the temporary use of
micropile-type soldier piles was specified
to support the upper 10 ft (3 m). The
remainder of the element, while drilled to
below the bottom of the excavation, was
supported with permanent nails and

The bottom of excavation and hybrid wall, with variability in initial shotcrete due to
bedrock excavation challenges

Global and internal stability of the soil
nail system was checked using SNAILZ
software from the California Department of
Transportation. The estimated nail loads
and lengths were determined from analysis
iterations for each section to find the
critical failure surface.

DEEP FOUNDATIONS * NOV/DEC 2020 * 99
DFI Magazine November/December 2020

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