Hillside Multi-family projects need hand excavated french drains
Let’s talk about hand excavated french drains on view properties perched on a hill with a fantastic view. Lots of views, lot of slopes, lots of rock, lots of problems. I am working on my 4th building in such a development right now. It is a wonderful condominium project high in the west hills of Portland. Every building gets water in the crawlspace due to the rainwater running to the high side of the property sloping to the foundation wall, and saturating the wall creating hydrostatic pressure and leaking into the crawlspaces.
The management knows the severe nature of their problem, and they are doing their best to correct this with the construction of hand excavated french drain groundwater removal systems around the high sides of the buildings. That is to say, the sides of the building where the slope is negative toward the foundation, not away from it.
The ground consists of basalt volcanic rock with a little dirt. The slope allows for the construction of daylighted vents in most cases connected to the french drains. This is the best alternative for them. However, the buildings where there are longer distances of flat topography are the toughest buildings to deal with. In those cases we run out of depth for the french drains because the lineal distance is too long. Too many lineal flat feet, not enough drop. Therefore the excavation of 4 foot in diameter by 4 foot deep drywells becomes the only way to get the french drains 18″ deep where they enter the drywells. This is accomplished pick by pick, rock by rock and slowly excavating through the rock to a depth of 4 ft. Slow, tough and absolutely necessary. The best engineering to use is to create a slope traveling approximately 50ft. lineal distance up the affected area to the top of the french drain at a depth of approximately 8″.
The big problem here is that when the water does get sloped away from the building into the drywell, it will not perk. So, the installation of concrete cylinders, sump pump, conduit for electricity, and river rock surrounding the cylinders is a necessity. This is called a sump well.
The sump well is the only alternative to constantly remove large amounts of water on flat areas or below grade areas. The water is collected and pumped out to a low point drain installed already as part of the parking lot drainage system in this development. This system is called an exterior sump well. Not the first choice, but better than having water under the building. In this development the water is gravity flowed to the edge of the woods and drains into the creek area in all other cases. The long flat, or below grade areas are the ones that are really a challenge in time, materials, and cost, and usually require sump wells.