About two months ago, we injected a crack in a residential foundation. The crack was about 1/8 inch wide at the surface. We weren't sure if we could control resin drainage, so we used an epoxy gel rather than a lower modulus material. Since the wall was 8 inches thick, we installed the entry ports 8 inches apart, capped the crack, and injected the resin. We started injecting at the bottom of the crack and worked our way up, plugging the port when epoxy flowed out the next port. Everything proceeded normally. The owner called recently saying that another crack had developed next to the old crack. Conventional wisdom is that the crack we injected was a moving crack and shouldn't have been injected. But I'm sure the crack was not moving. Is there any other explanation?
We spoke to John Trout of Lily Corp. and he says he sees this problem often. The crack, 1/8 inch at the surface, probably narrowed at greater depths. Most likely, the epoxy gel you injected did not flow through the whole 8-inch-thick crack before appearing at the next port. If a crack is not completely filled, a plane of weakness is formed (a control joint, in effect) at that spot. Any stress on the wall is likely to induce a crack there. The new crack you see isn't really new at all. It's where the uninjected portion of the old crack resurfaced. To increase your chance of completely filling a crack, Trout recommends that you start injecting a crack at its widest point regardless of where that point is on the wall. Stay on the wider part until you get back pressure, not just until you get bleed at the adjacent port. If the port starts bleeding before back pressure builds up, plug it and continue dispensing until the machine stalls out.