Geosynthetics June/July 2019 - 38

Streambank restoration using engineered bank stabilization

The foundation for
the EBS units was
placed approximately
18-24 inches (46-61
cm) below the
streambed elevation
for scour protection. A
nonwoven geotextile
and base stone were
used to provide a
stable foundation.

cell wall. For small compaction equipment, such as vibratory plate compactors, the thickness of soil compaction
lifts often is limited at 6-8 inches (15.2-
20.3 cm) in order to achieve the specified
density (per ASTM D1557). Effective
compaction monitoring and verification
help reduce the potential for differential
settlement, which may adversely affect
long-term performance.

Geotechnical design concepts
The weight of the bastion units must
resist the lateral earth pressure due to
the retained soil tending to deform and
slide downward and outward due to the
pull of gravity. Additionally, it must resist
any lateral contribution from surcharge
loading in the backslope area (such as
permanent loads due to a building or
bridge foundation or temporary loads
due to traffic or construction equipment).
The lateral earth-pressure force, Fa (measured in force per unit length of wall face;
i.e., pounds/foot or kN/m) is estimated
using the Coulomb active earth pressure
coefficient Ka given by:
(1)

TABLE 1 Factors of safety for typical
geotechnical design criteria

38

Mode

Minimum FOS

Base Sliding

1.5

Overturning

2

Bearing Capacity

2

Global Stability

1.3-1.5

Geosynthetics | June July 2019

where: β = batter angle of wall
(inclination of wall face from vertical)
ϕw = friction angle at wall-soil
interface (typically 0.67 of the soil
friction angle)
ϕ = friction angle of retained soil
θ = backslope angle
The lateral earth-pressure force
(Figure 7) is:
Fa = 0.5γH2Ka
where: γ = unit weight (density)
of retained soil
H = height of wall system

(2)

Lateral force due to any surcharge
loading applied in the backslope area is:
Fq = qHKa

(3)

where: H = height of wall system
q = surcharge loading, expressed as a
pressure in units of pounds per square
foot (psf) or kiloNewtons per square
meter (kN/m2)
These forces tend to overturn (rotate
by tipping) the gravity-wall system or
cause it to slide horizontally outward
along its base; thus, they must be sufficiently resisted by the weight of the
wall units to provide long-term stability.
In addition, a bearing capacity analysis (foundation) and a global stability
analysis (overall stability of wall and
back slope area using a method-of-slices
approach) should be conducted to evaluate these potential modes of failure
external to the wall system itself (Miller
2017). Optional ground anchors can be
installed to form a "tieback" system if
local site conditions warrant.
Oftentimes, a coarse granular (crushed
gravel) backfill zone is included directly
behind the EBS units to allow free drainage of groundwater and to increase the
friction angle along the potential failure
path through the retained soil. Using a
weighted value of the friction angle along
this path (a path intersecting some gravel
and some retained soil) results in a lower
computed value of Ka and, thus, increased
stability of the wall system.
Geotechnical design criteria (i.e., factor of safety against failure) for the analyzed potential failure modes typically are
prescribed as depicted in Table 1.

Mountain Creek layout
Due to the streambank height, floodplain and scour, a two-tiered EBS system
with EEAs was used for the project and
were set back 18 inches (46 cm). HPTRM
Geosynthetics June/July 2019

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