Drains are holes installed in a seawall, normally just above the footing. After drilling a hole through a seawall, a piece of PVC pipe with a patent pending vent is installed. In theory, drains are installed to relieve the water pressure from behind the seawall.
We are once again reminded that waterfront property owners must also prepare for the onslaught of the summer rainy season. What many waterfront residents don’t realize is that with this seemingly abrupt weather change comes shifting hydrostatic ground pressure that isn’t always immediately obvious at a glance. However, changes in ground pressure often result in marine structures (such as seawalls) experiencing drainage issues, causing them to ultimately fail. And we all know what that means for our once perfectly landscaped and aesthetically-appealing waterfront property: shoreline erosion due to improper drainage systems.
Improper drainage and insufficient tiebacks are the two most common reasons for repairs (and the combination of these two is just a recipe for disaster!). With either of these issues present in a seawall, the result is often a shifting seawall that looks as though it’s been pushed outward. This is very often due to built up hydrostatic ground pressure and/or water retention of the wall itself (in other words, poor drainage).
Because water seeks the path of least resistance, it will eventually find a weak area (panel joint, attachment anchor) or eventually crack a panel. In a “worst-case scenario,” the water will exit underneath the seawall panels taking soil with it. This can cause extremely weak soils and evacuate the upper soils at a very rapid pace. And, because the weak soil zones can and do happen at any vertical level in the surrounding, adjacent soils, collapse can occur. More commonly, the soils located directly behind the seawall begin to weaken and create voids. The same is not necessarily true when the pressures are reversed in a properly sealed and vented seawall.
After the drainage problem has been addressed, we are able to straighten out the seawall and prevent it from moving any further by installing shotcrete!