The wall design was “adequate,” based on a review of the original design, he said.

Ayub termed the use of horizontal dowels between the north wall and the east and west walls as “adequate” but said they “reduced the efficiency of the joints.”

The exterior walls are all 18 in. thick and the interior baffle walls 12 in. thick, according to the specifications in the design documents, he said.

The basin was built to hold 30 ft of liquid, or about 1.5 million gallons, and has an average operating depth of 4 ft to 8 ft, the report said. The collapse followed two days of heavy rain, and the basin held 25.5 ft, or about 1.3 million gal, according to plant data. More than two million gallongs of sewage poured into the Little Pigeon River after the incident.

Wastewater levels had been greater than 20 ft “on multiple occasions” and had reached 26 ft twice during the past year, the report said.

The flow control building where the men were working is still covered with parts of the wall and effluent piping.

Ayub and Misciagna, however, said, “There is no probable reason that access to this area would reveal any additional information that would result in citation being issued to Veolia.”

The only previous incident involving structural integrity entailed cracks and bowing in the north wall soon after construction, which was remedied by the installation of a reinforcing concrete buttress, they said.

Soon after the collapse, the city of Gatlinburg hired Construction Engineering Consultants, Knoxville, to investigate.

Albert J. Harb, a Knoxville attorney hired by the city, said that no report on the cause of the collapse will be issued until it can be "complete, accurate and based upon full knowledge of all available facts."