Soil varies greatly by region in it's structural abilities. Top soil has greatly less strength ability and depth varies by region. Most likely 4 foot deep straight sides will need some type of support to hold up long term. Since it's straight vertical either a deep concrete collar around the top circumference or some type of wall.
Hand dig a test hole and see the depth and type structure you're getting into and then make a decision if a full wall is needed.
Sloping the walls a little greatly reduces the risk of cave ins. Option 2 in the diagram is less risk.
Last edited by batman; 12-02-2021 at 09:24 AM.
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