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p 36
Chapter 2
Helical Foundation Systems
CHAPTER 2
HELICAL FOUNDATION SYSTEMS
force than tiebacks for a similar size wall, which
results in potential cost savings by using soil nails
due to reduced wall thickness requirements.
The following should be considered when
designing soil nail walls.
• Not all soil conditions are suitable for
construction of helical soil nail walls.
Excavations are generally made in 3 to 5-foot
steps, depending upon soil type and strength.
The soil should be able to stand unsupported
for a period of at least one day after the
vertical cut is made. Soil conditions that
may not be favorable for helical soil nail wall
construction include:
oo Dry, poorly-graded cohesionless soils; e.g.,
clean sands or sands with SPT N-values
less than 5 blows/foot
oo Highly plastic clays, expansive soils, organic
soils, or soils with a liquidity index of 0.2
or greater
oo Clays with SPT N-values less than 4
blows/foot
oo Soil profiles with high groundwater levels
- dewatering may be required to facilitate
installation
oo Soil with cobbles, boulders or weathered
rock lenses
oo Highly corrosive soils
oo Collapsible soils
oo Very dense sands and hard clays - may be
difficult to penetrate without pre-drilling a
pilot hole
• A failure plane generally develops at the top
of the wall at a horizontal distance of about
0.7 to 0.8 times the height of the wall away
from the wall face (Lazarte, Elias et al. 2003).
This distance may be reduced by battering the
wall face. Any structure, utility, roadway, etc.
that would be impacted by the wall movement
and/or failure plane should be considered
during the design phase.
• Top of wall lateral movements on the order
of 0.2% to 0.3% of the wall height should be
expected with soil nail lengths to wall height
ratios between 0.7 to 1.0, negligible surcharge
loading and a design including a global factor
of safety of at least 1.5. As a general guide, the
soil mass located between the failure plane
and the wall facing may slump approximately
1/8
-inch laterally and
1/8
-inch vertically for each
5-foot depth of excavation.
• Soil nail walls may be designed with a slight
batter to account for anticipated lateral wall
movement.
• There may be restrictions to the design soil
nail lengths, including property lines, right-
of-way (ROW), underground utility corridors,
bridge abutments or existing structures.
• Consider temporary and/or permanent
surcharge loads from adjacent structures,
roadways, construction equipment, fill
placement, etc.
• Maximum wall heights for helical soil nail
walls are practically limited to 20 to 30 feet.
Increased heights may be considered with a
stepped wall design.
• Helical soil nails are typically installed in a grid
pattern, spaced 3 to 5 feet vertically and 4 to
7 feet horizontally.
• Helical soil nails are typically installed at an
angle of 10 to 15 degrees downward from
horizontal, although a batter in not required.
The downward installation angle is a carryover
from grouted nail design where an angle is
required to prevent wet grout from flowing out
the hole.
• Soil nails may be installed with consistent
lengths for all rows, or be longer at the top
of the wall, becoming shorter with successive
rows toward the bottom. Nail length
determination depends upon soil strength
parameters, location of the failure plane, and
design for critical limit states as discussed in
Section 2.9.2.2.