(AP Photo/Indianapolis Metro Police Dept, file)

PHOENIX - A U.S. man faces trial Wednesday on accusations that he forced his young grandsons to hike for several kilometres in the Grand Canyon without food or water in brutal August heat.

Christopher Alan Carlson's grandsons, who were 12, 9 and 8 at the time, told investigators that Carlson hit, pushed, choked, and squeezed them and forced their fingers down their throats to make them vomit during trips into the Grand Canyon.

Investigators say Carlson told them that the boys were overweight and that he thought hiking the Grand Canyon would help get them into shape.

Carlson has pleaded not guilty to six counts of child abuse.

A ranger with binoculars spotted the group on what would be the last of the hikes on Aug. 28, when the temperature soared to 42 Celsius and a man died on another trail from heat exposure. The ranger reported seeing Carlson shoving the oldest boy and whipping him with a rolled-up T-shirt.

Rangers fed the boys and gave them water after one showed symptoms of heat stroke and the others had signs of heat exhaustion and dehydration, authorities said. The boys were placed in the care of state Child Protective Services.

"He told me that he loved his grandchildren very much, but at the same time there were tough people in the world and his grandchildren needed to be tough as well," National Park Service Special Agent Chris Smith said at the time.

Defence attorneys have questioned the boys' statements, saying it seemed improbable that they could have gone on such a hike without food and water.

The boys' mother, Tara Danaher, sobbed at a court hearing Sept. 1 and said she talked with her children throughout the summer and they never expressed any concerns.