Ah!!!! my Archilees heel. Hammer the wedges in to hard and the tenon shears along the grain of the wood. Solution, Insert a dowel through the tenon at a point midway between the wedge hole and the end of the tenon, This greatly increases the strength of the joint probably by a factor of ? (over to you Steve) The builders of Stonehenge would have been expert woodworkers, as carpentery must surely be the oldest profession of all.