Hillsides can be stabilized by installing “horizontal drains” in the slope to draw down the water table and reduce pore water pressures. Though they are called “horizontal drains,” the drains are usually installed at a slight upward angle, to facilitate drainage. They are usually 200 feet to 300 feet in length, and cost about $10 per foot installed.
This photo shows a drill rig installing a horizontal drain at La Esperanza Dam in Ecuador. A hole is drilled about 300 feet into the slope using a hollow-stem auger. Then a perforated PVC pipe is inserted into the hollow stem of the auger, and is held in place as the auger is removed. With this powerful drill rig, a 300 foot long drain hole could be drilled into the shale that formed the hillside in about 15 minutes.