Segmental Block Wall (also known as Modular Block Wall)
Used to construct near vertical hard faced systems. These walls can be either designed at low height as a gravity wall or for greater heights as a reinforced soil wall with the incorporation of polymeric or steel reinforcement extending into the retained soil behind the block facing.
Low height modular gravity walls
This type of wall relies on gravity action with minimal or zero tensile forces allowed to develop. The walls can be installed quickly by placing prefabricated modules on top of each other. Gabions walls, crib walls, dry stack walls and masonry walls are all examples of this type of geocomponent.
Crib walls are formed by a grillage of header and stretcher reinforced concrete or treated timber elements, filled with free-draining material. The headers form the front face of the wall, and they are connected by the interlocking stretchers.
The open structure of crib walls allows vegetation to grow or climb/cascade on the exposed face; with or without vegetation, they visually integrate in rural applications. Depending on their height, crib walls are supported on either a granular or a reinforced concrete foundation. They are normally laid to a batter, which should not be steeper than 1 horizontal to 4 vertical.
Drystack walls are formed by interlocking precast concrete blocks, and occasionally bricks. The interlocking ensures the blocks are correctly placed and provide shear resistance to the lateral pressure of the retained soil.
A strong foundation is required for this type of wall, which is often thicker at its base to withstand the weight of the structure and provide additional stability against overturning failure. As the blocks are light enough to be manually handled, the most exposed element at the top may be bonded with mortar or resin to avoid it being displaced.
Drystack walls can be vegetated at the top and/or bottom and/or over the surface by leaving small openings filled with topsoil. Masonry walls can be also used as modular retaining walls using bricks, blocks, natural or manufactured cut stone elements which are bonded together on a concrete foundation.
Specific frost resistant materials should be chosen and the walls protected from water ingress with damp proofing and detailing at the top. Brick masonry walls are usually built in continuous panels of 10-15 m in length separated by movement joints to accommodate thermal expansion and differential movement effects.
Higher modular gravity walls
Modular walls which exceed about 3m in height normally cannot rely upon gravity alone for their stability but need to incorporate soil reinforcement. The soil reinforcement mobilises the strength of the retained soil and is connected to the facing units so retaining them in place.
For example, steel reinforcement strips can be anchored to the back face of concrete blockwork or panels, the free edge of geogrids can be connected between two hollow blocks and kept in place through the interlocking of the blocks and by their coarse aggregate in-fill. Retaining walls, incorporating reinforced soil, can be used for bridge abutments and wing walls.
A 14m high reinforced soil wall to support an access road to a distribution centre in Purfleet was built using concrete panels facing units to which 11m long reinforcement strips are anchored .
A dilapidated brick retaining wall required replacement as part of the A321 improvements scheme in Aldershot, Surrey. A crib wall solution incorporating the site arisings was used as an alternative to a concrete retaining wall .
- BS 5628-1:2005 Code of practice for the use of masonry. Structural use of unreinforced masonry
- BS 6031:2009 Code of practice for earthworks
- BS 8002:1994 Code of practice for earth retaining structures
- BS8006-1:2010 Code of practice for reinforced soils and other fills
- BS EN 1996-1-1:2005 Eurocode 6 - Design of masonry structures. General rules for reinforced and unreinforced masonry structures
- BS EN 1997-1: 2004 Eurocode 7: Geotechnical design
- BS EN 14475:2006 Execution of special geotechnical works – reinforced fill
- BS EN 15258:2008 Precast concrete products. Retaining wall elements
- DMRB Volume 2 Section 1 Part 1 (BD 41/97) Highway structures: Design (substructures and special structures) material. Substructures. Reinforced Clay Brickwork Retaining Walls of Pocket-Type and Grouted-Cavity Type Construction
- DMRB Volume 2 Section 1 Part 3 (BD 68/97) and Part 4 (BA 68/97), Highway structures: Design (substructures and special structures) material. Substructures. Crib retaining walls
- DMRB Volume 2 Section 1 Part 5(BD 70/03) Strengthened/Reinforced Soils and other Fills for Retaining Walls and Bridge Abutments. Use of BS 8006: 1995, incorporating Amendment No.1
- CIRIA Publication C516 Modular gravity retaining walls - design guidance, London 2000
- CIRIA Publication C676 Drystone retaining wall and their modification – conditions appraisal and remedial treatment, London 2009
- CIRIA Publication C516 Modular gravity retaining walls - design guidance, London 2000
