What Wolfie has built is essentially a gravity wall. A gravity wall relies on its own mass to counteract the force of the fill against it. The hollow block wall has virtually no mass and would have failed without any additional water pressure load. A gravity wall must have a width at the bottom of between ½ and 1/3 the height of the wall. So if that wall is 10’ high it would have to be about 4’ + wide at the bottom with the width of the wall reducing as it gets closer to the top, ie at 5’ from the top it would need to be 2’+ wide. And if constructed with concrete blocks would have to be core filled with concrete to have enough mass.
The wall at 10’ high is supporting fill that has an angle of repose of 2 to 1. (When you add hydrostatic loads there is no angle of repose because water flows level so the force exerted goes up dramatically) So at that, the volume of fill is a triangular prism 20’ long x 10’ high (x 1/2 because it is a triangle) x 30’ which is the width of his shed giving a total volume of 3000 feet cubed. That volume of fill would have a mass of about 120 tons (plus any hydrostatic load) which can never be counteracted by a wall having a mass of bugger all.
Concrete blocks when used for retaining are designed as cantilever walls. A cantilever wall has a wide footing with the blockwork built at the outer edge of the footing to form an L shaped solid unit. Reinforcing steel runs across the footing width and bends at 90 degrees up into the wall which after core filling the blocks is carried out. What stops the cantilever wall from overturning or sliding is that the wall is designed so that the downward pressure exerted by the fill onto the footing, is more than the sideways pressure exerted by the fill.
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