I would run Schedule 80 PVC conduit large enough to carry the #2 or #4 wires for the service panel in the garage. You can use the bell ends and cement to make watertight connections for the conduit run. With very large gauge wire I like to run the wires through the conduit before cementing the sections as less force is required to pull the wires through each 10 ft section of conduit.
The conduit needs to be a complete system before the conductors are installed. Your method is against the electrical code.
Answers based on the National Electrical Code. Local amendments may apply. Check with your local building officials.