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By Doug Blackburn, EI
Sep 29, 2008 - 12:16:02 PM

Construction Review is the most important part of segmental retaining wall construction.  It protects the owner, design engineer, contractor and insurance companies from improper wall construction.  Poor wall construction is the main reason retaining walls fail.  The following is a checklist of proper techniques to follow during construction review:

Foundation Materials

Exposed bearing surfaces should consist of stiff undisturbed soils.  The bearing surfaces should be cleaned prior to placement of leveling pad stone.  The bearing surfaces should be suitable for net allowable bearing capacities specified on the design drawing (full dead and full live load).

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Segmental Retaining Wall Blocks

The design drawings should specify wall facing units and those units should be verified in the field. The installation procedure should be in general accordance with the design and with the manufacturer's recommendations.  Blocks should be installed approximately level when viewed front-to-rear and side-to-side.

Stone Backfill

The Contractor should install a DGA (Dense Grade Aggregate) crushed limestone (at a minimum) as the leveling pad stone and No. 57 stone as infill between the wall blocks and to 1 foot behind the blocks as a drainage column.  Stone backfill minimizes or prevents the build-up of pore water pressure, has a high internal strength (i.e., friction angle), provides compaction, prevents freeze/thaw, and provides filtration for the segmental retaining wall.

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The stone should be installed in loose lifts and compacted with appropriate equipment (i.e., flat plate vibratory compactor).

Sand Backfill

The Contractor should install sand within the reinforced zone of the wall.  The sand backfill should be installed in loose lifts (specified by design) and compacted with appropriate equipment (i.e., flat plate vibratory compactor).

Geogrid Reinforcement

The Contractor should install geogrid as specified on the plans.  This material should be verified on site so that the proper material is installed.  Geogrid reinforces the wall structure and provides strength so that the wall can withstand soil pressures and bearing loads (dead and live) applied to the wall. The geogrid should be installed properly in lengths (from the face of the block to the back of the grid area) as provided by the plans.

Geotextile and Drainage Pipe

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Drainage pipe (i.e. 4 inch diameter) such as slotted, perforated HDPE or slotted schedule 40 PVC should be installed per the design drawings at the appropriate locations.  A geotextile sock should be considered during design and installation to prevent clogging of the drain pipe.

The angular No. 57 stone for backfill purposes act as drainage stone and should be installed immediately behind the wall blocks to a minimum depth of 12 inches.  Geotextile should be installed between the reinforced backfill and the lean clay cap to a distance specified on the design drawings from the face of the wall.