We have a 5-foot high by 60-foot long retaining wall in our back yard made of creosoted timber, like old railroad ties. Some of the timbers have rotted and were sticking out, while some have voids that have accumulated soil and have weeds growing in them. I've been able to repair some of the timbers with lag screws, but what can I use to fill the voids, both horizontal and vertical--mortar mix, cement, what? (Replacing the wall at $24,000 isn't in the cards.)