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Normal practice is not to desludge but to backfill and move the trench by a metre or more, depending on the soil.
Unsupported soil walls will usually not support de-sludging, but may collapse during the operation. Are the trenches lined?
If you really want to desludge, a honey-sucker will not shift solid material. You will have to add water to make the material soft enough for a desludging pump.
Digging out may be your best option. However, shifting the material will be unpleasant and may be a health problem.
Regards
John Cosgrave