By Dennis Ellman
Over the past several months, San Diegans have witnessed the dramatic risk that comes with living in one of the most desirable locations on the planet.
San Diego is a hilly desert community lined with canyons, endangered by wildfires and mudslides, earthquake fault lines and a wonderful coast line, which brings its own challenges when building a risk-reduced home.
"A substantial amount of San Diego residential property is built at the top or bottom of a canyon and terraced to accommodate housing, said GDC Construction in La Jolla. "Some 'cut and fill' terracing leaves slopes vulnerable to slippage and can cause foundations to crack and slip as the earth shifts over time."
GDC noted many factors cause earth slippage, including seismic activity, improper drainage, erosion and man-made challenges caused by moving and properly compacting vast volumes of earth during residential tract development.
As new residential communities are built throughout the county and homes reconstructed after the wildfire, homeowners and building contractors must pay close attention to elements that can affect both short-term and long-term structural integrity.
It's important to know that landscapes occur where they have occurred before. Large, deep-seated slides tend to be a reactivation of existing landslide complexes. They usually occur on steep slopes, like those found throughout La Jolla and other parts of San Diego's coastal communities.
Landslides are also triggered by the failure of drainage systems. "A contractor should understand and take appropriate measures to assure proper drainage," GDC said. "Large amounts of water flowing from driveways, roof areas, roads and other impermeable surfaces can cause slides. Slope stability also is a key concern, particularly in areas ravaged by the fires and the potential of rain that can cause mudslides.""When you seek advice from a contractor, make sure they are qualified to address geological and slope stabilization issues," GDC added. Some contractors may only recommend those measures they are prepared to construct. Make sure you identify a contractor familiar with your area, the potential for landslides and local regulations that might impact decisions about developing the site or stabilizing a slope.
"Do check the contractor's references specific to these issues," GDC said.
A variety of methods exist to stabilize steep slopes. These range from simple drainage improvements to complex retaining structures.
"In general," GDC said, "engineering measures involve either reducing the driving forces that cause landslides, such as drainage and regarding the slope, or increasing the resistance of the slope to these forces," i.e., toe buttresses and retaining walls.
Often, according to GDC, the homeowner may be expected to submit a report to support an application to build on or near a potentially unstable slope. "It's always a good choice to obtain a geotechnical evaluation prior to constructing any kind of slope stabilization project," GDC said.
Water is commonly the primary factor triggering a landslide. Slides most often occur following intense rainfall, when storm water runoff saturates soils on steep slopes or infiltration causes a rapid rise in groundwater levels. Human actions can worsen the situation when drainage systems fail or development increases runoff near steep slopes.
"Drainage improvements may often be the most cost-effective means of reducing the likelihood of landslides," said GDC.
Simple solutions include inspection and repairing of existing drainage systems or directing runoff to a storm sewer. More complex solutions may require drilling wells, installing pumps or inserting compress-air caissons -- which are retaining, watertight structures -- into the soil.
Where possible, GDC noted, site design and landscaping should minimize the volume of runoff that flows toward the edge of the slope. Through grading and vegetation choices, the volume and rate of surface flow can be reduced. Runoff from driveways and gutters can be collected and directed to a storm sewer or closed pipe that carries water to a safe point below the slope.
Homeowners and contractors alike should be vigilant about potential landscape hazards. In coastal areas, these include the following:
- Head scarps or steep cliffs at the top of a slope
- Benches, scarps and large cracks
- Exposed clays uplifted on the beach
- Hummocky and uneven terrain
- Sagging or taut utility lines
- Separation of foundation from sill plate
- Growing cracks in walls and window corners
- Broken or leaking water or sewer lines
- Doors not closing properly
- Significant crack of concrete slabs and pavement
GDC also cautioned homeowners to be on alert for the following:
- Small ponds on otherwise sloping terrain
- Disrupted natural drainage
- Unusually heavy or muddy seepag
"The key," GDC said, "is to identify and correct potential problems as early as possible in the building or rebuilding phase, and recognizing the signs of potential problems and getting the right help involved as early as possible."