December 4, 2015

What is Wall Rock and Why Do I Need It?

Wall Rock is a compactible aggregate ranging in size from 0.25 in to 1.5 in. (6 mm to 38 mm) with no more than 10% fines and is used behind every Allan Block retaining wall whether it is just a small gravity wall or a larger reinforced wall. 

In the early years of the segmental retaining wall industry, Wall Rock was placed behind the wall to allow for the use of less energy directly behind the wall during the compaction process.  This eliminated the chance of rotating the carefully leveled block during compaction.  Also, a properly compacted Wall Rock zone directly behind the wall had less settlement behind the wall than the properly compacted native soil.  Less settlement over time prevented down drag forces on the geogrid from occurring.

Of course for many years this twelve inch deep zone was referred to as drain rock because the industry talked about this as a path to get water away from the structure.  Unfortunately by referring to this area as the drainage zone some people thought that it could handle large amounts of water without adversely affecting the structure.  Allan Block has always called this zone of rock “Wall Rock” in order to prevent the thought of Wall Rock being perceived of as a primary drainage path for surface water.  Wall Rock is only intended to drain away incidental water that infiltrates the soil mass and makes its way to the front of the wall structure.  A designer should always divert surface water flow away from the back of the retaining wall.

So in essence, the use of Wall Rock is to aid in compaction behind the wall, mitigate settlement and downdrag forces on geogrid, and to provide a means for incidental groundwater to find a path of least resistance out from the wall rather than to allow it to build up water pressure behind the wall over time.   Wall Rock is an integral part of a segmental retaining wall and it should always be used behind any Allan Block wall.

For more information on Wall Rock and retaining wall construction, visit our website at

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