PROBLEM TO BE SOLVED: To provide an economical multi-layer mat structure dispensing with maintenance and facilitating the purification of bottom sediments in a river and a lake and to provide a purification maintenance method for the bottom sediment. SOLUTION: This multi-layer mat structure 1 for the bottom sediment purification is provided with an intermediate layer including silt 4a having ferric oxide as its main component in an interlayer of a base material composed of a biodegradable sheet or a vegetation mat. In this purification maintenance method for the sediment, the multi-layer mat structure 1 is used instead of a sand covering material and laid on a prescribed region of the bottom. After the sheet for the multi-layer mat structure 1 is fed from a sheet roll unit disposed on a laying pontoon or in water and its end is fixed to the ground, the sheet roll unit is remotely operated, and the multi-layer mat structure 1 is developed and laid on the sediment as covering the bottom sediment, while preventing the dispersion of floating mud. COPYRIGHT: (C)2003,JPO
