You’re enthralled by the majestic vistas only hillside homes can offer. You want to build a house in L.A. and a sloped lot is your only option. Whatever your reason for planning a hillside project, you’ve probably wondered why it’s a challenging, expensive undertaking.
Building on a slope requires heavy equipment, intense labor, and a bit of thinking outside-the-box when it comes to logistics. The selection of contractors able to perform such work is quite limited.
The design of a hillside home must overcome 2 major obstacles – an amplified seismic hazard and constant, yet unpredictable soil movement.
Hillside homes and earthquakes
Earthquakes pose a considerable threat to hillside homes, particularly to those built on a downslope – extending away from the hill with the entry floor at the top.
Older downslope homes were often built on stilts – tall, slender columns, that extend down to the footings. When an earthquake rattles the ground, its multi-directional lateral forces produce circular motion. The supporting stilts are often too frail to resist this rotational force, and many homes succumbed to this weakness as earthquakes shook California over the past century.
While this style of building foundation is better suited to withstand earthquakes than stilts, stepped foundations and braced walls have their shortcomings. Unlike the rectangular shear panels that reinforce most homes on flat ground, the shear panels at a hillside cripple wall are also stepped, with the shortest panel at the top, and the tallest at the bottom.
This discontinuity limits the wall’s shear strength. The shortest panel attempts to resist the entire load first and fails. The next shortest panel follows, and the failures progress. Even if the braced wall does not collapse, losing an individual panel may cause the house to displace enough to damage its foundation anchorage. In the 1994 Northridge Earthquake, many hillside homes were damaged or destroyed after detaching from their foundations in this way.
To reduce the risk of a home disconnecting from its foundation, most local jurisdictions have introduced appropriate regulations into their respective building codes. While these may vary, typically these regulations require the use of primary and secondary sets of anchors and struts that attach each floor assembly of a house to the closest foundation segment.
The primary anchors are meant to resist the lateral forces of earthquakes and mitigate the extent to which a home can separate from the foundation. The secondary set of anchors and struts are redundant, required where downhill foundations have a direct connection to less than 70% of the floor assembly above them, or are spaced more than 30 feet.
While following these code regulations may improve your hillside home’s seismic soundness, they are a bare minimum, designed only to keep you and your family alive. Every hillside project comes with variables. Slopes, soil conditions, the seismic design category (SDC) of the site, as well as the home’s architectural features are all vital factors in determining an appropriate seismic design. If you are wondering whether your hillside home is earthquake-ready, or are planning to build one, our team of engineers can lend a hand. Call us at (877) 704-5687 to discuss your needs.
Hillside homes and soil movement
Uphill homes fare much better in earthquakes than their downhill counterparts. They’re as susceptible to seismic damage as homes built on a flat grade. However, they do face their own, unique danger – the towering hill behind them. And, believe it or not, hills are always moving.
Known as soil creep, hills’ constant state of motion is caused by gravity and water. Although unperceivable to the untrained eye, this downward progression of soil and rocks leaves tell-tale signs in its wake, such as leaning fences and trees.
Sometimes, the soil’s movement can speed up and become a landslide with little warning. Erosion due to heavy rainfall, earthquakes, or volcanic eruptions may all lead to this sudden acceleration. Right in the soil’s path, hillside homes present an easy obstacle to these gargantuan forces.
Water and erosion are the leading causes of landslides, making Northern California a notably dangerous place to build a hillside home. That said, earthquakes are not discerning in the damage they inflict and may cause devastating landslides even in SoCal’s arid climate.
The only sure-fire way to prevent landslide damage to your home is to build it on a flat site. If this option is not for you, you must make every effort to stabilize the soil around your home. Several methods can help you do this:
You can’t stop the rain, but you can channel rainwater in a way that prevents erosion in your home’s vicinity. Besides reducing the risk of landslides, proper drainage will ensure that rainwater doesn’t flood your property and destabilize your home’s foundation.
Vegetation is a natural means of erosion control. Certain species of grass, shrubs, and trees anchor the soil beneath them with their roots. Not all plants are equally effective, and their performance will vary depending on the climate, soil type, and slope. As you plan your project, consult an arborist to find the most suitable species for your location.
Erosion blankets can help you stabilize a slope too steep to support vegetation. They can also reinforce your soil while the vegetation you planted takes hold; coir mats, made of coconut husks or other biodegradable materials are perfect for this application.
While retaining walls may not be strong enough to hold off a landslide, they can help reduce erosion. Along with other stabilization methods, strategically placed retaining walls can reinforce the soil on your site, particularly that above your home. They come in many shapes and sizes, and an engineer will know best how to incorporate them into the grading plan.
As with seismic design, every hillside home has different topography and geological conditions. A licensed engineer can assess your project’s needs and help you protect it from soil movement. Give our team a call at (877) 704-5687 to find out more.
Notable cost factors
Earthquakes, landslides, and the logistics of building on a hill all add up to make hillside homes some of the most expensive single-family residential developments in the state.
Seismic bracing and erosion control measures come at a premium, but it’s the foundation that really breaks the bank. It’s difficult to pinpoint a cost per square foot figure for a hillside foundation, simply because each site is unique. To provide a stable base that withstands soil movement and hydrostatic pressure, a hillside foundation must rest on bedrock, and bedrock can be deep. You will need a geotechnical report before your engineers can design a suitable foundation, the cost of which will depend on your site’s conditions.
Grading can also be a budget-buster. Your contractor will need to excavate, export, and import fill – activities that demand extra crew hours, heavy equipment, and a level storage area for the excavated soil. You will also need a geotechnical report to determine whether the existing soil can be reused on the site.
Site logistics are another challenge. You contractor will need to find a staging area somewhere where it’s flat, and build temporary roads down to, or up to the construction site.
In fact, all construction tasks may end up happening slower than normal and requiring additional resources. That’s why building on a slope should never be a DIY endeavor. If you want to build a home, that’s finished on time, on budget, and is safe to live in, hire seasoned, licensed professionals to design and construct it. Our engineers at Design Everest can help. Call us at (877) 704-5687 for a free consultation and quote.
