Basement Wall Details

Referring to the attached sketch, typical basement retaining wall details include the following.

1. Footing - gross dimensions of footing with required reinforcement, for example ... 10 in. x 18 in. w/ 3 - # 5 evenly spaced and placed 3 in. clear from bottom. It is important to note here (or clearly elsewhere) the required condition of the bearing soil. For example, ... footing shall be cast upon undisturbed native soil or upon engineered fill certified by the Soil Engineer as meeting the Allowable Soil Bearing Pressure in the General Requirements of the Construction Documents.

2. Drain - drain diameter, minimum slope, and highest placement. For example, the drain should certainly be below the basement floor elevation, and preferably below the bottom of the footing. In areas where foundation zones may be infiltrated with fine (silt) materials it is recommended to require durable filter fabric around the drain or other effective means of protecting the drain system from plugging. I recommend that the drain be surrounded with at least 1 sq. ft of drain rock or gravel, and then that volume be protected with durable permeable filter fabric. Some details may show the whole backfill or drain chimney volume protected.

3. Toe Condition - for a retaining all important. In this detail the basement slab will used to provide lateral support at the bottom of the wall (keep the bottom of the wall from pushing in). In this regard the slab must be continuous concrete (no expansion/contraction joints ... well, unless you can accept the wall pushing in the amount that will `squish' them). If lateral support at the bottom cannot be provided by a basement slab (or other snug floor system), then lateral support must come about via the footing. Generally the passive pressure of the soil against the toe of the footing will be insufficient (and especially so if the soil there is not compacted, or if the form has been left there, to rot). In such case a deep footing may be required or a footing with a deep key.

4. Vertical Reinforcement - in this example the reinforcement is shown near the inside face betraying that the wall is acting like a vertical one-way slab (or beam). This will ONLY work if lateral support is also provided at the top of the wall (by a floor system and sufficient attachment to it). The reinforcement is shown embedded into the footing, and this Embedment should be clearly spelled out. In many designs a straight embedment of 6 in. will be satisfactory. Some jurisdictions require hooked embedment. Determining the spacing, size, etc. of the reinforcement is the subject of other lessons. Here or elsewhere the minimum wall thickness and the precise distance of the rebar from one or the other face must be specified. In this example the strength of the wall is a function of (among other things) the distance of the vertical rebar from the outside (soil) face. As such, it is not advised to detail a minimum distance from the inside face alone.

5. Horizontal Reinforcement - generally tied to the vertical reinforcement. Here the reinforcement is illustrated as tied to the vertical reinforcement toward the center of the wall. In two-way walls the majority of the resistance to the retained earth is provided by the horizontal reinforcement. In one-way walls the horizontal reinforcement provides additional strength and integrity and crack control.

6. Drain Chimney and Finish Grade. In this example a drain chimney is sketched with other backfill material beyond. Free drainage of the backfill behind any retaining wall is important. If the backfill materials are themselves not free draining, provide at least 12 in. of freely draining gravel. If top and backfill soils will potentially be the source of fine materials that could eventually plug the drain then the immediate backfill zone and drain should be protected with durable filter fabric or other proven means.

7. P.T. Mud Sill and Anchor Bolts - Should be Shown on the Foundation Plan / Detail. For a one-way wall where the floor at the top of the wall will be helping the wall retain the earth the anchor bolt schedule will be very robust. Likewise it will be necessary to specify the fastening of the floor system to the mud sill. Specifics are provided by prescription in the building codes or must be determined by calculation (and are covered in other lessons).

8. Waterproofing Seal or Membrane. The wall must be at least sealed against intrusion of moisture. In some cases a membrane may be used that both seals the wall and allows a vertical channel for backfill water to the drain.

This detail and list are not meant to be exhaustive. Shown are main structural items. Not shown are any number of energy or architectural or other items.

Related Links:

Prescriptive Basement Wall Design ... www.associatedcontent.com/article/1237026/prescriptive_basement_wall_design_.html

One-Way Basement Wall Design (ICF Example) ... www.associatedcontent.com/article/1242694/design_of_a_oneway_reinforced_concrete.html