To build a driveway culvert, set the outside and inner rails in place, cut out cross rails, and put them in place temporarily to mark their locations. Install the rails in place by making a hole in each cross rail and the support rail it intersects. Make sure to put 1 1/2-inch washers onto the shafts of the bolts before placing bolts into each hole that you drilled. Put another washer onto the bolt, and apply the nut after that.
To create the driveway layout, measure the length of the driveway between the high points on both sides of the dip. Obtain 6-by-4-inch beams, and make four support rails out of them. To determine the support rail locations, first set the outside rails in place. Thee outside rails should be 6 inches inside the width of the driveway, and the two inner rails should be 2 feet inside the outside rails.
Use a trenching shovel to remove some ground at the ends of the rails; the ends should be 4 inches lower than the high points. Each cross rail should be 10 feet long, and you need twice as many rails as the length of the driveway in feet. To mark the cross rail locations, put one block of 4-by-4-inch rails on each outside rail, put a cross rail next to them, and mark the rails.