Excavation and Soil Bearing Capacity

Today’s the big dig. You have the excavator lined-up. Your house plans still look like they are fresh of the printer. You’ve staked out the excavation area. You’re ready to go.    

Before you sink that shovel in the ground there’s few things about which you familiarize yourself. Excavation and soil bearing capacity is the most crucial component of the construction process for several different reasons that I will discuss in this article.

When excavating the foundation of a building, all the organic material needs to be removed. Materials such as grass, vegetation, top soil and roots. In some instances you may have to excavate deeper than anticipated in order to remove all organic materials. In this case you may be faced with the costly predicament of having to import granular fill material.

The next component that plays an important part is water. There should be no standing water in any excavations. If water is present its source needs to be investigated in order to determine what precautions need to be taken to remove or eliminate the water. The underground water table also plays a crucial part in to structural integrity of the building. Most Building Codes outline the necessary steps that have to be taken when footings are located within a certain distance of the water table.        

Once organics have been removed a soil having a proper soil bearing capacity must be present. Different soils have different bearing capacities. For example, clay when dry has a higher bearing compared to other soil types, as oppose to sand which much lower. The minimum soil bearing capacity prescribed by most Building Codes is 75 Kpa. By way of example sand is at the minimum requirement with a soil bearing capacity of 75 Kpa.

The other thing to keep in mind is that these soils have to be original and undisturbed. In cases where material has to be imported it must be compacted properly so that it meets the minimum soil bearing capacity. Keep in mind that most Municipalies require and soils Engineer to certify the compaction.   

If you are excavating at a depth greater than 6 feet then there are some additional precautions that need to be taken. These are in respect to the occupation and safety act. There are some acceptable and unacceptable excavations.

There are four main types of soils namely clay, silt, sand and granular. Clay and Silt look similar when dry. When silt becomes saturated it turns soft and like quick sand which is different than clay. Sand is the soil type with the lowest soil bearing capacity. Granular is slightly higher. For most residential applications and for water drainage purposes, granular  material is my preferred soil type as it offers a fairly sound bearing capacity but does not retain water therefore eliminating water penetration issues.

Remember the house your building is only as good as the soil it’s being built on. Take all the necessary precautions to make sure you are building on suitable soils, even if that means retaining a soils engineer to determine the soil bearing capacity.