Hi Norah,
Load bearing walls must be designed using verifiable measurements. Both above ground and below ground load bearing walls are subject to a range of different forces; however, the ranges are not the same. For example, below ground walls and above ground walls both must have the compressive strength to equalize the force of gravity, but above ground walls must have adequate compressive and tensile strength to counteract wind forces where below ground walls are not subject to wind forces. Below ground walls can be subject to hydrostatic forces, where above ground walls usually are not in inland locations.
All the forces acting on a wall must each be measured so the wall can be designed to equalize them; we're in luck, because the forces gravity and wind exert on walls have been studied for ages and there is publicly accessible data available to help us design our walls. To begin your design, gather all the data you can on the type of soil you will be building your foundation upon so you can calculate the compressive strength and shrink/swell characteristics of it. Your foundation wall design must be able to equalize these forces as well as any hydrostatic and lateral pressures from the earth, the dead load of the building above ground, the uplift and torsion the wind forces acting on the above ground walls that are transferred to the foundation, and many others.
If you excavate a portion of the basement hole to a depth of 3 meters, you can take soil samples based on the color and physical composition and look up their respective compressive strengths
online to begin designing the footing you will construct to support your foundation walls. If you have trouble finding a good match for your soil types, photograph them and post the photos here and I will help you. Be sure to note any subsurface
water or bedrock encountered when you excavate.
