What are the differences between segmental retaining wall (SRW) and precast modular block (PMB) retaining wall solutions?
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When managing the contour of landscapes and controlling soil, retaining walls play an essential role. They prevent erosion, manage grade changes, and create usable spaces. Segmental retaining walls (SRW) and precast modular block (PMB) retaining walls are two widely used systems that work to ensure property owners get the most from their land.
The Base Course
Short on time? Here are this article’s key takeaways…
Segmental retaining wall (SRW) blocks are machine-produced with dry cast concrete and their length, width, and height are similar in dimension. The same SRW block size will be used throughout an entire wall. SRW blocks typically weigh less than 150 lb (68 kg).
Precast modular blocks (PMBs) are produced from wet cast concrete and feature base length and width dimensions greater than their vertical height, with varying block widths. PMB walls include blocks of varying sizes and weights, with the larger blocks installed at the bottom of the wall for gravity applications. PMBs range between a few hundred pounds to several thousand pounds.
The installation of SRW blocks is usually more labor-intensive because they are hand-set and more blocks are required to cover the same wall area. PMBs are machine set and their size means fewer blocks are required for the same job. PMB gravity walls can exceed heights of 20 ft (6.1 m) depending upon the individual site. The additional reinforcement used to install MSE wall structures allows them to achieve heights of 60 ft (18.3 m) or more, in certain site applications.
Take a holistic approach when evaluating project cost. SRW blocks are typically cheaper to manufacture, but can incur substantial additional costs during the construction phase of a project.
If you’re wondering about the differences between the systems and which solution might work best for your land retention needs, keep reading.
What are the primary differences between SRW blocks and PMBs?
SRW blocks are machine-produced with dry cast concrete. Learn more about the differences between wet cast concrete and dry cast concrete HERE. Their length, width, and height are typically similar in dimension. This means that the same SRW block size will be used throughout an entire wall. SRW blocks typically weigh less than 150 lb (68 kg) and sometimes require alignment mechanisms to be installed during construction. To achieve structural stability, an SRW wall is comprised of blocks and geosynthetic soil reinforcement, even at shorter wall heights. Geosynthetic materials, including geogrid or metal straps, are attached either directly to the SRW blocks or between the tops and bottoms of stacked blocks, employing friction to secure the geosynthetic material.
PMBs are produced from wet cast concrete. They feature base length and width dimensions greater than their vertical height, with varying block widths. PMB walls include blocks of varying sizes and weights, with the larger blocks installed at the bottom of the wall.. The blocks range between a few hundred pounds to several thousand pounds, and their heavier weight allows the wall to rely on the blocks’ weight to achieve structural stability. PMBs typically feature shear connections – knobs, lugs, or similar features – that establish setbacks between successive courses (rows) of blocks and allow the transfer of forces between units in the wall, adding to its stability. Depending on wall height and site scenarios, geosynthetic soil reinforcement may be required when building with PMBs.
How are they installed?
SRWs employ a somewhat simple, but labor-intensive installation process. SRW blocks are hand-placed by a crew of approximately 5-8. A larger project footprint is required for SRWs due to the need to install geosynthetic reinforcement behind the wall. The area behind the planned wall is excavated so the geosyntheticmaterial can be stretched to specific lengths behind the wall. Geosynthetic reinforcement is installed course by course with soil being piled on the material before moving to the next course.
PMBs are machine-placed by a small crew of approximately 3-6. PMB walls require smaller footprints and don’t usually demand significant excavation as the inclusion of geosynthetic materials is often not needed. The project site must allow access to the appropriately rated machine (backhoe or loader) which will be used to lift the blocks.
What types of projects are best for SRWs and PMB walls?
Both SRWs and PMB walls can be employed for residential and commercial projects. Their unique characteristics make them better suited for several specific application types.
SRWs are best for fill sites that are not exposed to deicing products, water, and freeze thaw cycles as the dry cast production process creates product durability concerns in these types of environments.
PMB walls rarely require excavation so they work well for tight site conditions. Their minimal footprint demands work particularly well for transportation applications as those job sites are often constrained. Their wet cast composition also makes PMB walls good choices for water applications like detention/retention basins, flood walls, storm channels, and river and lake walls.
How long do SRWs and PMB walls last?
If installed properly and maintained well, the design lives of both SRW and PMB walls can last 75-100 years or more. Site conditions such as freeze-thaw cycles or prolonged exposure to water or road can lead to premature failure for SRW block walls due to dry cast’s typically more porous mix.
Image: If installed properly and maintained well, PMB walls can last 75-100 years or more.
How tall can they be built?
Gravity PMB walls can exceed heights of 20 ft (6.1 m) without geosynthetics or other reinforcement, depending upon the site. Employing additional reinforcement like geosynthetics to create MSE wall structures allows PMB walls to achieve heights of 60 ft (18.3 m) or more, in certain site applications.
Unreinforced SRWs typically can only be built up to 4 ft (1.2 m) if the site's soil conditions are less than ideal or surcharge loads are present. When SRWs are augmented with soil reinforcement like geosynthetic materials, their heights may exceed 4 ft (1.2 m).
Are there cost differences?
SRW blocks are usually smaller in comparison and the manufacturing process (compressing dry cast concrete mix using massive pressure, requiring minimal curing time) results in a much shorter production cycle. This means more product is available in a shooter time. PMB wet cast concrete blocks require longer production times as the blocks are larger and the more fluid wet cast mix is poured into molds to cure. As a result, fewer blocks are produced during a particular period when compared to SRW blocks.
This means that SRW blocks are usually less expensive to purchase than PMBs, but that’s typically where SRWs’ cost savings end.
Site conditions, freight, installation demands, and the duration of the project can have substantial impacts on the total project cost. Focusing only on the pricing of the material and failing to consider additional costs associated with its use will likely leave you disappointed with your project’s final bill.
Consider a project scenario where a taller wall is required, but the site offers minimal additional space. A taller PMB gravity wall can be installed (depending on specific site conditions) without back-of-wall reinforcement, which means less excavation (time and machinery savings), no geotextile costs (material savings), and a faster installation (labor savings). The latter consideration can act as a savings multiplier when the wall’s completion is necessary to allow another phase of a project to proceed. Each of these factors has the potential to substantially reduce project costs.
To gain a better understanding of the costs associated with any retaining wall project, a holistic approach is recommended. Want to learn more? We break down the pricing process of a Redi-Rock wall and why considering the lifetime value of your wall is the recommend approach to choosing your wall solution.