What are Subgrade Soils?
Subgrade soils are native soils beneath organic rich topsoils and upon which foundations for sheds and other structures are built. Subgrade soils are not all alike nor are they all located at the same depth. Depending on your location your subgrade soils may vary from bedrock formations to loose unconsolidated sands and silts. The depth where these soils can vary from shallow (4- to 6-inches below ground surface) to deep (24-inches or more for fertile flood plains and agricultural lands).
Why should I construct my foundation on Subgrade Soils?
Organic rich topsoils are rich in decaying vegetative matter. These soils are often loose, unconsolidated and mixed with roots, leaves, branches and other plant debris. On its own, organic rich topsoil is highly desireable, for the garden. Under a foundation, it can lead to differential settling and foundation problems which can be expensive to resolve.
Subgrade Depth Determination:
Your depth to subgrade is either the deeper of two conditions:
- Your depth to subgrade is the depth of the remainder of the shed foundation not above ground. If only topsoils are reached when digging to the bottom of the foundation depth, proceed to #2 below. If the shed foundation is suitable per the manufacturer to bed on subgrade soils and they have been reached, then you are ready to go. If the shed foundation must be bedded on imported crushed rock, then depth to subgrade is the depth of imported crushed rock below that of the foundation.
- Remove the topsoil and find out. If the depth to subgrade is greater than the depth determined in #1 above, then proceed with depth determined in #2 and import crushed rock backfill to the desired depth to meet the bottom of the shed foundation.
Subgrade Soil Preparation:
All topsoils and organic matter should be cleared from the location of the proposed foundation. That can be a perimeter foundation or slab. Concrete or imported rock. In any case, the topsoil should be removed.
Undisturbed subgrade soils, if of a quality mix of sand, silt, and clay, with or without existing rock, are ideal to build up from as they have been consolidating and naturally compacting over years, decades or longer. However, some subgrade soils may be of a low bearing strength. That is, how much can an existing soil system be loaded with without the soils failing. These are often soils heavy with clay and/or silt.
Subgrade soils if not damp, can be lightly watered and compacted easily with a weighted tamper, hydraulic "Jumping Jack" or weighted roller. Avoid adding too much water if the soils are high in clay and silts. This will cause the soil to "pump" and liquefy under load.
Imported Crushed Rock:
Importing of crushed rock as a foundation system is beneficial for many reasons. It can be aesthetically pleasing as it is clean and often provides contrast. It provides a strong base foundation above subgrade soils for the shed foundation to be positioned. It provides a capillary break (an air gap between groundwater or saturated winter soils and the surface) below the foundation aiding in rot prevention.
Import crushed rock consisting of a well graded mixture from a #200 sieve sand to 3/4" maximum dimension rock. Place in loose 6-inch lifts and compact as previously described for subgrade soils. Crushed rock should be damp when compacting in order to achieve it's maximum density when placed and compacted.
The perimeter of your imported crushed rock can be neatly dressed with pressure treated 2x dimensional lumber prior to placement of rock. Care should be taken when compacting next to other structures and utilities.
Always identify where your underground utilities are located prior to performing any excavating either by calling your One-Call Before You Dig phone number or your local utility agencies.