Retaining walls are structures that resist soil pressure from external forces, which are formed outside the soil failure zone.


Retaining walls are structures that resist soil pressure from external forces, which are formed outside the soil failure zone. Retaining structures are subjected to lateral soil pressures as well as water pressures (if water is trapped behind the retaining structure). These pressures are restrained by;

  • Friction and adhesion between retaining wall base and soil beneath the retaining wall
  • Passive soil pressures acting in front of the retaining wall
  • Bearing capacity of the soil beneath the toe of the retaining wall

Overtime, conventional gravity retaining walls transitioned into reinforced concrete types followed by thecrib and bin-type walls. In 1966, H. Vidal initiated, developed, patented, and promoted the mechanically stabilised retaining walls. The reinforcement was initially steel-mesh and in 1980’s polymeric geotextiles and geogrids were introduced (Koerner and Koerner, 2013;Koerner and Soong, 2001). The basic function of these reinforcements (polymeric geotextiles and geogrids) is to strengthen the soil mass and compacted backfill supports to anchor the geogrids beyond possible failure plane.

Segmental retaining walls (SRW) or modular concrete block walls have become increasingly popular fora number of reasons such as; dry-cast fabrication of the blocks, flexibility in design, ease and rapid installation, durability, outstanding aesthetic appearance, cost effective, ecologic friendly, and versatility (Koerner and Soong, 2001;Ren et al., 2016). SRW was first introduced mid-1980’s and were presented in Australia in the early 1990’s (Ren et al., 2016). The application of SRW is ranging from highway earth retaining systems, commercial centres and car parks to landscaping applications (Figure 1). The height of SRW varies from 200 mm to greater heights depending on the application (Chan et al., 2008).

Figure 1 Segmental Retaining walls used in landscape application (Austral Masonry, 2017)

Behaviour of Soil-Reinforced Segmental Retaining Walls

In order to achieve greater height SRW, reinforced-soil was introduced. The reinforcement resists the lateral earth pressure by mobilising its tensile strength. The required strength of the reinforcement is calculated based on the fully developed Rankine or Coulomb lateral earth pressure (Leshchinsky et al., 2004).
With regard to the Keystone system, reinforced-soil SRW consists of geosynthetic material, which is mechanically connected to the Keystone Compac or Keystone standard units and placed as ahorizontal layer on the compacted backfill that enables design to greater heights and to accommodate significant loads. As a result, the combination of geosynthetic and Keystone units provides an integrated wall system that can be used in different applications ranging from residential landscaping walls to structural highway walls, some exceeding 15.2 m in height (Keystone Retaining Wall Systems LLC, 2016).


There are two main types of Keystone units used in constructing retaining walls, namely Keystone Standard Units and Key Stone Compac Units. Both of these Keystone products are vertically interconnected using high-strength pultruded fibreglass pins. In addition, Keystone units consist of cores that can be filled with clean crushed stone, which provides additional mechanical interlock and internal drainage (Keystone Retaining Wall Systems LLC, 2016).
The Keystone standard unit is still the industry leader for tall retaining walls. The height-to-depth ratio of Keystone standard unit provides an excellent wall system with sound strength, durability, and stability. Keystone Compac units are used to construct walls, which require lower embedment depth. In addition, Keystone compact units have lighter weight and lower depth than those of Keystone Standard units. Details of Keystone standard and Keystone compact units are shown in Figure 2.

Figure 2 Keystone Standard and Keystone Compac Units(Keystone Retaining Wall Systems LLC, 2016)

According to the Keystone Retaining Wall Systems LLC (2016), dimensions of Keystone Standard Units and Keystone Compac Units are shown in Figure 3.

Figure 3 Variation in dimensions between Keystone Standard and Keystone Compac Units(Keystone Retaining Wall Systems LLC, 2016)

Variation involume of voids in tail and volume of voids to depth between Keystone standard and Keystone compac units is presented in Figure 4.

Figure 4 Variation in volume of voids in tail and volume of voids to depth(Keystone Retaining Wall Systems LLC, 2016)

Connection strength (Positive Vs Friction)

The essential design components of SRW include Geogrid length and strength, adequate connections, and effective drainage system to release water pressure acting on the wall.Among those design considerations, connection strength between Keystone unit and Geogrid reinforcement is important. There are two types of connection between Keystone unit and Geogrid reinforcement, namely “frictional connection” and “positive connection”.

Frictional connection is the most common form of connection, in which geogrid is sandwiched between successive Keystone blocks by means of gravel-filled cores, pins, bars, or other forms of fasteners. There is usually frictional component between the Keystone unit and geogrid, and connection relies on the weight of the blocks above the connection. In addition, geogrid strength is not fully utilized at the front of the wall.

In Positive connections, geogrid is wrapped through the Keystone unit. The resulting connection is not weight dependent and prevents the pull out failure. In positive connections, there are no extra componentssuch as pins, bars or other forms of fasteners to assemble the connection.

Achievable heights

Segmental Keystone retaining walls can achieve heights far greater, exceeding the limits of simple gravity walls. The world’s tallest single-tier segmental keystone retaining wall constructed to date is “Wall 15 at Hacienda Real” in Carolina, Puerto Rico with 19.9 m in height (Figure 5). The use of product KeySteel® Reinforcement KeySystem™ I, allowed builders to save enormous time and labour for reinforcing of steel, temporary formworks, and concreting of footings.

Figure 5 Wall 15 at Hacienda Real built using KeyStone(Keystone Retaining Wall Systems LLC, 2015c)

Heavy Loadings

Keystone compac units are capable of withstanding dynamic loads. Keystone compact unit walls with total of 9940m2 were built in Manchester Boston Regional Airport in the UK. These walls include a bridge veneer at Main entrance overpass (4830m2) and sectioned wall along the expanded runway (3810m2). In addition to withstanding dynamic loads, Keystone compact units provided the best solution for construction and engineering challenges such as installation efficiency, structurally sound, and aesthetically pleasing finish.

Figure 6 Keystone wall at Manchester Airport (Keystone Retaining Wall Systems LLC, 2015a)

In addition, walls constructed in the Western Ring Route are another example of structurally sound Keystone retaining wall systems that carry dynamic loads. This wall,featuring a wave pattern from top to bottom, consists of three Keystone compact blocks namely, one with aconcave face, once with a convex face and half-arc matching the curves of other two blocks.

Figure 7The Wavy Walls of the Western Ring Route, New Zealand (Keystone Retaining Wall Systems LLC, 2015e)

Water Applications

Segmental Keystone can also be applied as water application walls. The project of developing Gulf Harbour Marine Village consisted of the world’s largest canal development using Keystone wall system (Figure 8). This Keystone system consists of high strength segmental concrete units, fibreglass pins, and Geogrid soil reinforcement that provides weather resistant and design flexibility, accommodating different geometric Keystone units such as curves, internal and external right angle corners. In addition, the Keystone Compac Units allow the passage of water through the wall, thus reducing the hydrostatic pressure due to the changes in tidal level at Gulf Harbour as high as 3 m. More importantly, this Keystone retaining wall system provides the cost-effective solution for constructing avertical wall that maximises the available land while providing sufficient water frontage.

Figure 8 Keystone canal wall at Gulf Harbour Marine Village (Keystone Retaining Wall Systems LLC, 2015d)

Harlan, Kentucky flood wall constructed along the Cumberland River in London was another water application of Keystone walls (Figure 9). The wall area was approximately 2575m2, 381m in length, and 11m tall at its highest point.

Figure 9 Harlan, Kentucky Flood Wall(Keystone Retaining Wall Systems LLC, 2015b).


Aesthetic appearance is one of the major considerations on all above case studies. Keystone segmental retaining walls present a pleasing appearance as they provide various colour and face textures. One of thegreatexamples that provide creative and aesthetically pleasing Keystone SRW is the Wavy wall in Waitakere, New Zealand as shown in Figure 7.


  • 1 AUSTRAL MASONRY. 2017. Retaining walls & Pavers – Style and function [Online]. 27 Lawson Crescent, Coffs Harbour, NSW 2450: Brickworks Building Products. Available: [Accessed September 01 2017].

    2 CHAN, C., HOVER, K. C. & FOLLIARD, K. J. 2008. Segmental retaining wall (SRW) split face delaminations and practical implications. Construction and Building Materials, 22, 1749-1757.

    3 KEYSTONE RETAINING WALL SYSTEMS LLC. 2015a. Manchester Boston Regional Airport – Case Study [Online]. 4444 West 78th Street Minneapolis, Minnesota 55435 Available: [Accessed Aug 30 2017 ].

    4 KEYSTONE RETAINING WALL SYSTEMS LLC. 2015b. Upper Cumberland River Flood Project – Harlan, Kentucky [Online]. 4444 West 78th Street Minneapolis, Minnesota 55435 Available: [Accessed Aug 30 2017 ].

    5 KEYSTONE RETAINING WALL SYSTEMS LLC. 2015c. Wall 15 at Hacienda Real,Carolina, Puerto Rico – Case Study [Online]. 4444 West 78th Street Minneapolis, Minnesota 55435 Available: [Accessed Sep 01 2017].

    6 KEYSTONE RETAINING WALL SYSTEMS LLC. 2015d. Water Application – Gulf Harbour Marine Village – Case Study [Online]. 4444 West 78th Street Minneapolis, Minnesota 55435 Available: [Accessed Sep 2 2017].

    7 KEYSTONE RETAINING WALL SYSTEMS LLC. 2015e. The Wavy Walls of the Western Ring Route – New Zealand – Case Study [Online]. 4444 West 78th Street Minneapolis, Minnesota 55435 Available: [Accessed Sep 01 2017].

    8 KEYSTONE RETAINING WALL SYSTEMS LLC. 2016. Keystone Construction Manual [Online]. 444, West 78th Street, Minneapolis, MN 55435. Available: [Accessed August 31 2017].

    9 KOERNER, R. M. & KOERNER, G. R. 2013. A data base, statistics and recommendations regarding 171 failed geosynthetic reinforced mechanically stabilized earth (MSE) walls. Geotextiles and Geomembranes, 40, 20-27.

    10 KOERNER, R. M. & SOONG, T.-Y. 2001. Geosynthetic reinforced segmental retaining walls. Geotextiles and Geomembranes, 19, 359-386.

    11 LESHCHINSKY, D., HU, Y. & HAN, J. 2004. Limited reinforced space in segmental retaining walls. Geotextiles and Geomembranes, 22, 543-553.

    12 REN, F., ZHANG, F., XU, C. & WANG, G. 2016. Seismic evaluation of reinforced-soil segmental retaining walls. Geotextiles and Geomembranes, 44, 604-614.

Article by John Pagsolingan

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