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. 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. 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.

SRW blocks are less expensive to produce as they are usually smaller and their manufacturing process results in a much shorter production cycle. The wet cast concrete blocks that comprise PMB walls require longer production times as the blocks are larger and the more fluid wet cast mix is poured into molds to cure.

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 geotextile soil reinforcement, even at shorter wall heights. Geotextile 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 geotextile 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, geotextile soil reinforcement may be required when building with PMBs.

When geotextiles or other methods are used behind walls to secure the solution, the wall may also be referred to as a mechanically stabilized earth (MSE) or "reinforced" wall.

How are they installed?

SRWs employ a simple, but labor-intensive installation process. SRW blocks are hand-placed by a crew of approximately 5-8. A larger project footprint may be required for SRWs as any wall exceeding 4 ft (1.2 m) will likely require the installation of some method of reinforcement behind the wall, like geotextile material. The area behind the planned wall is excavated so the geotextile material can be stretched to specific lengths behind the wall. Geotextile 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. Gravity PMB walls may be installed in smaller footprints and don’t usually demand significant excavation as the inclusion of geotextile 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 and their geotextile reinforcement components are best used for tall wall and tiered wall scenarios that offer a generous project footprint to allow for necessary excavation associated with geotextile reinforcement installation.

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. Note that site conditions such as freeze-thaw or prolonged exposure to water may result in much shorter lifespans for SRWs due to their dry cast composition.

Both SRW and 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 geotextile or other reinforcement, depending upon the site. Employing additional reinforcement like geotextiles 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 geotextile materials, their heights may exceed 4 ft (1.2 m).

Are there cost differences?

It’s difficult to provide a truly accurate price comparison between SRWs and PMB walls. Cost is influenced significantly by specific project demands, site conditions, and other factors that may be unique to an installation location like available labor, freight costs, and machinery rental (if applicable), for example. We can, however, offer some general guidance on this topic.

SRW blocks are less expensive to produce as they are usually smaller 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 which keeps pricing lower. SRWs typically require the addition of geotextile reinforcement materials which adds cost, along with requiring larger installation crews, bigger machinery, and more time to install.

The wet cast concrete blocks that comprise PMB walls require longer production times as the blocks are larger and the more fluid wet cast mix is poured into molds to cure. This extended manufacturing time results in the production of fewer blocks during a particular period when compared to SRW blocks. Installation costs can sometimes be lower than SRWs since excavation may be limited and additional costs like geotextile reinforcement material may not be necessary.