Hydraulic Porta Power 

Our timbers are #2 and better. They are 6" x 6" spruce, pine or fir. We notch them on site and fill the mat.
Once the mat is full we jack the wood with a hydraulic jack, then we measure the gap and saw a 6" x 6" timber to fit the gap and drive the timber in.
 
The wood in our mats are always tight then we weld the holes shut on top of the mat so as they will be often inspected.