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Precast
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Frequently Asked Questions
about Precast Concrete Retaining Walls
1. What are the different types
of precast concrete retaining walls and how does each work?
2. What are the advantages of precast concrete
retaining walls over other materials and construction methods?
3. What should a civil engineer, architect, general
contractor or developer know about retaining walls and precast
concrete wall systems in particular?
4. Do precast concrete manufacturers have their
own internal quality assurance program?
5. What is the NPCA Plant Certification Program?
6. What methodologies are used to design retaining
walls?
7. What safety factors should the retaining wall
engineer calculate?
1. What are the different
types of precast concrete retaining walls and how does each
work?
There are several different types of precast
concrete retaining walls, including:
- Mechanically Stabilized Earth Systems
- These walls are built from individual
panel sections.
- The panels interlock and are supported
by steel straps that are mechanically attached to the
back of the panel and extend back from the panel into
the retained soil, resulting in
a reinforced soil zone behind the wall.
- Large Precast Modular Block –
Unreinforced / Gravity Wall
- These walls are built from individual
large concrete blocks and generally stacked in a running
bond fashion.
- The wall systems usually have an
interlocking feature or shear key mechanism that locks
one course of block to the next.
- They retain the soil by virtue of there size and weight.
No additional reinforcement of the soil
is required.
- Large Precast Modular Block –
Reinforced Wall
- These walls are also built from
individual large concrete blocks and generally stacked
in a running bond fashion.
- Given the height of the wall, soil
reinforcement is now incorporated with these large blocks.
- The soil reinforcement used is generally a form of
geo-grid attached to the large blocks either frictionally
or mechanically.
- Cantilever Wall
- These are typically large “L”
type units with an extended footing on the front side
of the wall. The footing helps resist the overturning
moment.
- These wall systems retain the soil
by virtue of there size and weight. No additional reinforcement
of the soil is required.
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2. What are the advantages
of precast concrete retaining walls over other materials and
construction methods?
The advantages of precast concrete retaining
walls are significant and can include:
- Engineering.
Precast concrete walls are generally supported by
engineering specific to both the particular wall system
and to the project site conditions. This is not always the
case with walls built from natural stone materials (i.e.,
boulders).
- Durability.
Precast concrete walls are usually made from wet-cast,
air-entrained concrete that is very durable. Resistance
to the adverse effects of repeated freeze-thaw cycles and
road salts can be significant. Check with the producer to
verify the mix design used (including the strength of the
concrete) and to verify that the producer has a Quality
Assurance program in place.
- Aesthetics.
Many of the precast concrete wall systems are made with
an architectural finish that replicates natural stone. In
addition, precast concrete walls can be stained with a number
of commercially available stain products to further enhance
and customize the look.
- A range of Value
Added Solutions:
- Some systems are excellent for gravity wall applications,
minimizing excavation costs and eliminating the need
for a reinforced zone behind the wall which might otherwise
push the wall out from a property line or require easements.
- Some systems are excellent when site conditions and
wall heights require soil reinforcement.
- Some systems are designed to accommodate slight movement
or settlement and thus do not need to be placed below
the frost line, saving both material and construction
costs.
- The range of different precast wall systems provides
a variety of reinforcement options, from geo-grids to
steel straps and more. The choices in precast wall systems
also will impact the choice and/or required specification
of soil types in the reinforced zone. Some systems require
a “select granular” while others may work
well with the on-site soils.
- Construction
Ease. Most precast wall systems are installed with
equipment as opposed to manual labor. Combined with the
large square footage per precast wall unit, installation
is relatively quick and efficient.
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3. What should a
civil engineer, architect, general contractor or developer
know about retaining walls and precast concrete wall systems
in particular?
The quality of a retaining wall is generally
equal to the strength of the weakest link in the retaining
wall chain. That chain includes upfront planning and design,
quality of the retaining wall product chosen, quality of the
site-specific engineering for the wall, quality of the information
regarding the soils at the site, and the quality of the construction
of the wall.
- Retaining walls are frequently structural
in nature and often critical to the proper development of
a site. A first step in the proper planning for a retaining
wall is to collect information about the soils at the location.
Soil borings and follow-up shear testing of the soils are
important and perhaps critical. In addition, the expense
incurred to gather this information will generally pay dividends
in the long run. Without the proper soils information, wall
construction will be delayed until the information is collected,
or “worst case” assumptions may be made. This
may result in the wall being over-engineered, which may
in turn cost more than it should. Suggestion: When the
rig is on site taking borings for the building pad, have
the rig take borings every 50 to 75 feet along the foundation
wall.
- Develop a grading plan that shows existing
grades. The plan should include: those grades immediately
outside the property lines of the parcel being developed;
proposed grades; location of the wall(s); top of wall and
bottom of wall elevations along the proposed wall; location
of existing and proposed buildings, roads, etc.; location
of utilities; and location of construction/excavation limits.
- Proper engineering of a wall must
be done not only with knowledge of the soils, but also
with knowledge of slopes and surcharges that may exist
at the top of the wall and any slopes that may exist
at the toe of the wall.
- Select an engineer who is experienced
in retaining wall design and who is familiar with the product
chosen for the project.
- Make sure the producer of the product
has a Quality Assurance program in place.
- Make sure the product brand selected
has an engineering manual available that includes testing
for items such as shear, sliding, and reinforcement connection.
- Make sure the contractor selected for
installation has experience with the installation of the
product chosen. Is the company a certified installer? Does
it have a Quality Assurance program for wall installation?
For example, will the company perform compaction testing
and provide results to the customer?
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4. Do precast concrete
manufacturers have their own internal quality assurance program?
Most do, and all should. Ask your precast
concrete company about its Quality Assurance program. This
is very important in determining the quality of the precast
concrete product that you will be selecting for your project.
A number of the precast concrete manufacturers are enrolled
in third-party quality control programs such as the NPCA
Plant Certification Program. Others may be certified by
the state department of transportation.
5. What is the NPCA
Plant Certification Program?
The NPCA
Plant Certification Program is an effective way for precast
concrete manufacturers to demonstrate their commitment to
the production of high-quality products. The basis of this
voluntary certification program is the Quality
Control Manual for Precast Plants published by NPCA. The
program covers all aspects of a precast concrete operation
necessary for production of high-quality precast concrete
products and quality assurance. Compliance with the Quality
Control Manual is verified with inspections of the plant on
an approximately annual basis. The program certifies that
plants meeting the requirements set forth in the NPCA Quality
Control Manual are capable of producing high-quality products.
The program does not certify products.
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6. What methodologies are
used to design retaining walls?
The American Association of State
Highway Transportation Officials (AASHTO) uses the FHWA Publication
Number FHWA-SA 96-071 Mechanically Stabilized Earth Walls
and Reinforced Soil Slopes Design and Construction Guidelines
(known as Demo 82) dated October 1996.
AASHTO is generally used for public and
government projects. It calls for reinforcement depths equal
to the greater of 70 percent of the wall height or 8 feet,
requires uniform reinforcement depths, requires imported sand
or gravel in the reinforced zone, and calls for a minimum
embedment depth of 2 feet.
Other design methodologies may be available
depending on the system used.
7. What safety factors should
the retaining wall engineer calculate?
The retaining wall engineer should be calculating:
- External Factors of Safety, including
Overturning, Sliding and Bearing.
- Internal Factors of Safety, including
Pull Out.
- Local Stability, including Shear.
- Global Stability
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