Where the underlying ground consists of permeable materials, such as sand or sandy clay, the cost of the technique will be significantly increased due to the requirement of cut-off walls into non-permeable layers to seal off the vacuum. It has been suggested by Carter et al.
When the base consists of permeable substances like sand or sandy clay, the cost of this technique increases significantly due to the necessity of constructing cut-off walls in non-permeable layers to prevent the vacuum from leaking. This idea was suggested by Carter et al.