Segmental
retaining walls (SRWs) are gravity retaining walls that rely primarily
on their mass (weight) for stability. The system consists of concrete
masonry units which are placed without the use of mortar (dry stacked),
and which rely on a combination of mechanical interlock and mass to
prevent overturning and sliding. The units may also be used in combination
with horizontal layers of soil reinforcement which extend into the
backfill to increase the effective width and weight of the gravity
mass.
Design
Flexibility - The SRW system is composed of units whose size and
weight makes it possible to construct walls in the most difficult
of locations. Curves and other unique layouts can be easily accommodated.
Segmental retaining walls have the ability to function equally well
in large-scale applications (highway walls, bridge abutments, erosion
control, parking area supports, etc.) as well as smaller residential
landscape projects.
Aesthetics - Since SRW units are available in a variety of sizes,
shapes, textures and colors, segmental retaining walls provide designers
and owners with both an attractive and a structurally sound wall system.
Ease
of Installation - Most SRW units can be placed by a single construction
worker. The dry stack method of laying units without mortar allows
erection of the wall to proceed rapidly.
Economics
- SRWs provide an attractive, cost effective alternative to conventional
cast-in-place concrete retaining walls. Savings are gained because
on-site soil can usually be used eliminating costs associated with
importing fill and/or removing excavated materials, and because there
is no need for extensive formwork or heavy construction equipment.
Durability
- Segmental units are manufactured of high compressive strength,
low absorption concrete which helps make them resistant to spalling,
scour, abrasion, the effects of freeze-thaw cycles, rot, and insect
damage.
Performance
- Unlike rigid retaining wall structures, which may display cracks
when subjected to movement, the flexible nature of segmental retaining
walls allows the units to move and adjust relative to one another
without visible signs of distress.
Site
Design Consideration - Typical designs and specifications for
segmental retaining walls should be prepared by a designer who has
technical knowledge of soil and structural mechanics. Each SRW unit
manufacturer can provide design information tailored to that product,
which will indicate the wall heights and design conditions when an
SRW should be designed by a qualified engineer. In addition, SRW systems
should be designed by a qualified engineer when:
- structures
will be surcharged
- walls
will be subjected to live loads
- walls
will be founded on poor foundations
- the
nature of the design conditions requires special consideration.
Geosynthetic
Length and Spacing - For soil-reinforced segmental retaining walls,
geosynthetic reinforcement increases the mass of the composite SRW
structure, and therefore increases the resistance to destabilizing
forces. Length of the geosynthetic is typically controlled by external
stability calculations. Increasing the length of the geosynthetic
layers increases the SRW's resistance to overturning, base sliding,
and bearing failures. In some instances, the length of the uppermost
layer(s) is locally extended in order to provide adequate anchorage
(pullout capacity) for the geosynthetic layers. The strength of the
geosynthetic and the frictional interaction with the surrounding soil
may also affect geosynthetic length.
A sufficient number and strength of geosynthetic layers must be used
to satisfy horizontal equilibrium with soil forces behind the wall
and to maintain internal stability. In addition, the tension forces
in the geosynthetic layers must be less than the design strength of
the geosynthetic and within the allowable connection strength between
the geosynthetic and the SRW unit.
Drainage
System - Drainage is an essential part of a properly designed
SRW. Drainage materials are generally well-graded aggregates. A properly
designed drainage system relieves hydrostatic pressure in the soil,
prevents retained soils from washing through the face of the wall,
provides a stiff leveling pad to support a column of stacked facing
units, and provides a working surface during construction. Surface
water drainage should be designed to minimize erosion of the topsoil
in front of the wall toe and to direct surface water away from the
structure.
Batter
- Segmental retaining walls are generally installed with a small
horizontal setback between units, creating a wall batter into the
retained soil. The wall batter compensates for any slight lateral
movement of the SRW face due to earth pressure, ensuring that the
finished wall does not appear to rotate.
Unit
Size and Shear - In conventional (gravity) SRWs, where the stability
of the system depends primarily on the mass and shear capacity of
the SRW units, increasing the SRW unit width or weight provides greater
stability, larger frictional resistance, and larger resisting moments.
In soil-reinforced SRWs, heavier and wider units may permit a greater
vertical spacing between layers of geosynthetic.
All SRW units provide a means of transferring lateral forces from
one course to the next. Shear capacity provides lateral stability
for this mortarless wall system. This is accomplished by shear keys,
leading lips, trailing lips, clips, pins, or compacted columns of
aggregate in open cores.
Embedment
- The primary benefit of wall embedment is to ensure the SRW is
not undermined by erosion of the soil in front of the wall. Increasing
the depth of embedment also provides greater stability when site conditions
include weak bearing capacity of underlying soils, steep slopes near
the toe of the wall, potential scour at the toe (particularly in waterfront
or submerged applications), seasonal soil volume changes, or seismic
loads.
Elements
- The basic elements of each segmental retaining wall system are
the foundation soil, leveling pad, segmental retaining wall units,
retained soil, drainage fill, and, for soil-reinforced SRWs, the soil
reinforcement.
Foundation soil: The foundation soil is the soil which supports the
leveling pad and the reinforced soil zone of a soil-reinforced SRW
system.
Leveling pad: The leveling pad is a level surface, consisting of crushed
stone or unreinforced concrete, which distributes the weight of the
SRW units over a wider area and provides a working surface during
construction. The leveling pad typically extends at least 6 in. (152
mm) from the toe and heel of the lowermost SRW unit and is at least
6 in. (152 mm) thick.
Segmental retaining wall units: Segmental retaining wall units are
concrete masonry units that are used to create the mass necessary
for structural stability, and to provide stability, durability, and
visual enhancement at the face of the wall.
Retained soil: Retained soil is the undisturbed soil for cut walls
or the common backfill soil compacted behind infill soils.
Drainage fill: Drainage fill is free-draining granular material placed
behind the wall to facilitate the removal of groundwater and minimize
buildup of hydrostatic pressure on the wall. It is sometimes also
used to fill the cores of the units to increase the weight and shear
capacity. The dry stacked method of construction used for segmental
retaining walls permits water to drain through the face of the wall,
aiding in the removal of groundwater. In some cases, a geotextile
filter is installed between the drainage fill and the infill to protect
the drainage fill from clogging.
Reinforced soil: Reinforced soil is compacted structural fill used
behind soil-reinforced SRW units which contains horizontal soil reinforcement.
A variety of geosynthetic and steel soil reinforcement systems is
available.
Construction
The success of any segmental retaining wall installation depends on
complete and accurate field information, careful planning and scheduling,
the use of specified materials, proper construction procedures, and
inspection.
It is good practice to have the retaining wall location verified by
the owner's representative. Existing and proposed finish grades shown
on the drawings should be verified to ensure the planned design heights
are in agreement with the topographic information from the project
grading plan.
The contractor should coordinate the delivery and storage of materials
at the site to ensure unobstructed access to the work area and availability
of materials. Materials delivered to the site should be accompanied
by the manufacturer's certification that the materials meet or exceed
the specified minimum requirements.
Construction occurs in the following sequence:
1. excavation and leveling pad construction
2. setting, leveling, and backfilling base course
3. placement and backfilling of units in succeeding courses
4. placement, tensioning, and backfilling of soil reinforcement (when
required)
5. compaction of backfill to the specified density
6. capping and finish grading.
As with any structure used to retain soil, careful attention should
be paid to the compaction equipment and procedures used during construction.
When compacting soil within 3 ft (0.9 m) of the front face of a wall,
compaction tools should be limited to hand operated equipment, preferably
a vibrating plate compactor. Reinforced soil can be compacted with
walk-behind or self-propelled riding compaction equipment.
