Relief Trenches

“What is a relief trench?” you ask.  A relief trench is a ditch installed from the forming preparations to a drainage area as part of site preparation for construction of a slab on grade foundation to prevent accumulation of water in the forms in the event of rain. The next logical question is, “Why is it important to drain the water from the forms?”  In our area, forms for a slab on grade foundation are constructed by setting wood forming above the soil and digging trenches into the soil that will eventually be filled with reinforcement and concrete to form the stiffening beams and slab of a foundation.  If you could view the foundation from its underside this results in foundation having a final shape that resembles a waffle.  Without a relief trench the dug beams are susceptible to filling with water and holding it for a long time. Water standing in these trenches can liquefy the soil and damage the form preparations in several ways.

  • Standing water can result in caving of the sides of the trenches, causing the soil at the sides to slough to the bottom covering the reinforcement and reducing the depth of the beams.  This reduces strength and stiffness of the cast in place beams.  This condition must be corrected before placement of concrete.  Although cleaning of the beams without removal of reinforcement is sometimes possible when sloughing is not significant, in severe cases the reinforcing will have to be removed to allow the beams to be redug and to ensure proper placement of the reinforcement.  This can also increase the amount of concrete used and thus the cost of fabrication.
  • A sand layer is often added to the surface of the soil at the slab as recommended by the geotechnical engineer as a friction break and capillary break.  If water stands too near the top of the beams, it can liquefy the sand layer at the slab, damaging this smooth layer and possibly affecting the performance of the foundation.  This can require removal of the reinforcement and moisture barrier to allow the layer to be replaced.
  • Saturation of the soil supporting the slab between the beams will make this soil unstable until it dries.  Walking these areas when wet and soft during placement of concrete will result in undesirable roughness in the surface of the supporting soil, allow displacement of the sand layer, or allow reinforcement to be pushed out of place undetected, possibly affecting the long-term performance of the foundation.
  • Saturated soil can result in a failure of the interior drop forms used for forming the drop surfaces of porches or garages, or perimeter beam and slab forms, causing them to warp and drop from their intended positions.  This would require reforming but may not be detected if it occurs just prior to or during placement of concrete.

Preventing these concerns is as simple as digging one or more relief trenches when the beam trenches are dug but is commonly overlooked.  The relief trenches should be dug to at least the same depth as the beam trenches.  They should extend to a location where water can be safely discharged with a continuous downward slope.  This will limit the amount of water that will collect in the forms and the subsequent damage that can occur.  A simple relief trench could reduce a potential 2-week delay to a few days.  In our climate where rain is common and can occur suddenly and for several days, this can improve overall quality and save the builder money.