CHP spokesman David Slate said the driver had reportedly fallen asleep behind the wheel while going eastbound on Interstate 205 near the northbound junction of I-5.
He said the driver allegedly awoke and overcorrected his steering wheel, hitting the back area of the big rig before crashing through a guardrail on the right shoulder of I-5.
The truck rolled down the embankment and came to rest on its roof, Slate said.
The driver had minor injuries, but the passenger reportedly became trapped inside the truck and sustained a broken arm and leg.
Both men were taken by ambulance to San Joaquin General Hospital.
The roadway was reopened to traffic at 2:47 p.m.
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