Stormwater management on retaining walls

Most concrete retaining block walls are semi-impervious, which means water cannot easily pass through. For this reason, efficient drainage is crucial to prevent a hydrostatic pressure build-up behind the wall that could cause damage such as bulging, cracking or even collapse.

The wall itself and the foundation are important, but the drainage layer is at least as important,” stresses Holger Rust, founder of Terraforce, a pioneering designer and manufacturer of precast concrete retaining blocks and allied systems.

This drainage layer should consist of a coarse, well-draining material of not less than 300 mm behind the blocks (as shown in Figure 1) and be well compacted. In certain situations, an engineer may also specify a permeable drainage pipe above foundation level. “It’s also crucial to channel surface water away from the top of the wall in an open swale,” Rust explains.

Gravity versus reinforced retaining walls

Terraforce retaining blocks can be used either as part of a gravity system or as the fascia of a geosynthetic reinforced segmental retaining wall structure, or as a fascia for a cement stabilised backfill.

Gravity retaining walls (either single or double skin) rely on their self-weight to resist lateral earth pressure, and such walls have been in use for centuries. In a gravity system, the Terraforce masonry units can be stacked several courses deep to provide support to the forces imposed by the retained soil. The weight and geometry of the stacked units prevent the constructed wall from sliding on its base or, at an intermediate height, toppling over or rotating out of position.

The key characteristic of geosynthetic reinforced soil segmental retaining walls is that they utilise reinforcing sheets of geogrid or suitable woven geotextile. These are attached to the fascia and are embedded in a body of engineered fill. The integrated nature of the fascia and the abutting large body of reinforced soil thereby support the applied earth forces. In this case, the ‘gravity’ component of the retaining wall is provided by the reinforced soil mass, which acts as a monolithic unit.

By virtue of their inherent flexibility, reinforced walls can accommodate movement and some settlement without suffering distress. This makes the system particularly suitable for construction on (engineered) filled ground or in areas which may be prone to settlement effects.

Construction in progress on a doubleskin segmental retaining wall

Drainage system options

The following are examples of drainage systems that could be required when designing and building Terraforce retaining walls, depending on the soil conditions and specific application. Fin drains: Fin drains, also known as wick drains, are typically installed at an angle of 45 to 60 degrees at predetermined centres against the cut face. Subsurface drains: Subsurface drains, also known as subsoil drains, are specified behind the bottom row of blocks. “Slotted or drilled drainage pipes should be installed within a gravel bed and wrapped in filter fabric to prevent clogging of the gravel with fine particles from the backfill,” Rust explains. “However, this may be waived in some conditions, depending on the soil analysis.” Drainage outlets: Another common design application is the installation of drainage outlets through the wall and at the end of the wall.

“From experience, we recommend that installers wrap the open end of the outlet pipe with plastic mesh to prevent spillage of gravel,” Rust continues. Hard lawn paving blocks: Drainage and aesthetics can also be combined successfully to striking effect. An example is the use of Terracrete eco-surface hard lawn paving blocks in conjunction with concrete retaining block wall system designs.

The Terracrete units can be laid in different patterns. They can also be used with or without ground anchors for the lining of riverbanks and other areas subject to soil erosion. Other popular applications include the paving of grassed roads and parking areas, as well as the stabilising of steep embankments. In- and outlets over the wall: “From the examples given, it’s clear that drainage is application and design specific,” says Rust, adding that – for in- and outlet drainage points – precast elements work well both over and through the wall.

Terraforce blocks Typical swale or concrete channel to engineer’s specification Sod laid on 200 mm topsoil Subsurface/subsoil drains are installed behind the bottom row of blocks

Impervious layer

Suitable nonwoven geotextile

Drainage fill

Mastering the science of retaining wall systems

When designed and built by industry experts, concrete retaining block walls are highly robust and will last a lifetime. Bearing testimony to this are the walls standing today that were built with Terraforce’s first product, known as the S block, which was first introduced in the mid-1980s.

As an added service to industry, in 1992, Terraforce introduced a design manual for local and international practitioners. The modern-day version has now been updated for release in 2022 with the participation of professionals in three countries.

Split into two sections, the Terraforce Design Guide deals with the theory and practice of gravity retaining wall design – with worked examples – as well as the design and installation of Terraforce reinforced soil retaining walls in accordance with BS 8006 and SANS 207:2011. All the procedural and sequential steps that need to be followed are covered to construct safe and sound structures.

Precast elements work well both over and through the wall for in- and outlet drainage points

Understanding segmentation

Alongside Terraforce’s products, there are a wide range of block designs on the local and international market. “Some systems are interlinked while others interlock on the vertical and/or horizontal plane,” Rust explains. “However, all these systems rely largely on interblock friction to derive sliding resistance.

“It’s important to emphasise that no amount of interlocking or interlinking will prevent a structure from collapsing when substantial active lateral earth pressure occurs. Under such circumstances, designers must utilise layers of geosynthetic reinforcement in the backfill to achieve stability and ensure that the drainage systems are adequate for seasonal and storm events,” Rust concludes.

Terracrete eco-surface hard lawn paving blocks form part of the drainage system on this concrete retaining block wall design

Installation of drainage outlets through the wall and at the end of the wall are common design features

Open drainage channel above the wall