Chapter 4: Water Management - Alternate Drain


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updated: 5/29/2018

Click on the topics below to view more information on the best practices for Allan Block segmental retaining wall design for residental and commercial applications.

Alternate Drain Water Management Drawing
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4.1    Alternate Drain Locations

water management heel drain

Figure 4-1: Alternate Drain

4.1   Location, type, and venting of drain pipes.

  1. When the drain pipe must be raised to accommodate outlets through the wall face.
    1. Low permeable granular soils should be used to create a shelf inside the mass, level to the height of the finished grade outside the wall to prevent water from ponding below grade. See Allan Block Design Detail – Alternate Drain, in the Allan Block Spec Book.
  2. Drain pipes existing to daylight through the wall should be installed with rodent screen to prevent nesting within the pipe. Cut adjoining block to allow for installation of a manufactured drain cover. Alternatively, Wall Drain Pro could be used without the requirement of cutting block. See more information here on Wall Drain Pro.

4.2    Heel Drain Recommendations

4.2   A heel drain should be specified for sites whenever grid is used or where migrating water from behind the mass is possible.

water management heel drain

Figure 4-2: Typical Drain with Heel Drain

  1. The purpose of the heel drain is to pick up any water that migrates from behind the retaining wall structure at the cut, and route the water away from the reinforced mass during construction and for incidental water for the life of the structure.
  2. The piping used at the back of the reinforced mass shall have a 1% minimum gradient over the length, but it is not critical for it to be positioned at the very bottom of the cut
  3. The heel drain should be vented at 100 ft (30 m) intervals along the entire length of the wall and should not be tied into the toe drain system.
  4. The pipe may be a rigid pipe with holes at the bottom or a corrugated perforated flexible pipe.
  5. For infill soils with a high percentage of sand and/or gravel the heel drain pipe does not need to be surrounded by wall rock. When working with soils containing fine grained cohesive soils having a PI of greater than 6 and LL of 30 or greater, 1 ft³ (.03 m³) of wall rock is required around the pipe for each 1 ft (30 cm) of pipe length.