Landslide Prevention Is Safer and Cheaper : There are many geologic variables that trigger a slide. Proper planning and analysis are essential.

Aren’t geologists supposed to examine and certify safe building sites before construction begins? Aren’t the builders placing the foundations in good, solid rock that will not fail? Aren’t public officials supposed to check building sites before they approve construction?

I have been asked these and similar questions after a rash of recent slope failures that brought down houses in Laguna Beach, San Clemente and Anaheim Hills. There is no single answer because, although the end result looks the same, the cause is not always the same.

There are two factors of primary importance that trigger slope failures such as landslides. The first is gravity. This force never changes. Water is the substance that works with gravity to increase slope failures such as landslides and mudflows.

Rainwater soaks into the hillside, adding mass, which increases the force of gravity. It also reduces the frictional resistance of rocks and soil. The buildup of water creates hydraulic pressure on the underlying impermeable bedrock and literally lifts the surface material up off the bedrock, thereby destabilizing it. There are also clay-rich beds in some of the bedrock and in the soil that contribute greatly to the instability of hillside slopes.

Unlike gravity, water is seasonal. In Southern California, many years may pass between heavy rains. When the heavy rains do arrive, all of the water-related phenomena that have been lying dormant for years combine with gravity to cause earth materials to move down the slopes of our hillsides. Water is the variable, and gravity is the constant.

There are other factors that play pivotal roles. Earthquakes help trigger landslides. If the quake occurs during a rainy season, like this year, slope failures will be more numerous and individually more extensive.

And there are certain formations in Southern California that are more susceptible to slope failure than others. In Orange County there are the Puente Formation (Anaheim Hills and Puente Hills), the Monterey formation (Dana Point), and the Capistrano Formation (Capistrano Beach cliffs, San Clemente and San Juan Capistrano). These formations have many clay-rich beds, and they are extensively exposed in the hills and shore cliffs of Orange County, so it is not unexpected that cities in this area are experiencing extensive slope failures.

Also, bedrock layers sloping downward parallel to the hillside increase the potential for failure because when these rock layers become water saturated they slide down hill like cards in a deck. This condition exists at the slide sites in Anaheim Hills and Dana Point.

In addition, it should be recognized that prehistoric climatic conditions in Southern California were very different than today, even though the record of the last 100 years shows considerable variation.

Rainfall in Southern California was much higher during this prehistoric period and, consequently, landslides and mudflows were much more widespread then than we see today. As the climate changed gradually, many of these slides became dormant. The tell-tale geologic clues have gradually become concealed by vegetation and modified by later geologic processes making hazards very difficult to recognize even by an experienced geologist.

Decades ago, when building activity was beginning to move into higher ground--and before geologic building site evaluation was required--many landslides were overlooked. These hazards are now coming back to haunt us.

With almost no flat land left in Orange and Los Angeles counties, developers are having to move their operations higher into the hills to find open space on which to build houses. This increases the chances of placing houses on unstable ground because the greatest potential for future slope failures is on the higher hillsides.

Even though a geological site evaluation may result in a building permit, development activity can create conditions that render a site unstable. Working in hillside terrain involves an enormous amount of earthmoving that can destabilize dormant landslides or create conditions that will trigger new landslides.

Landscaping is another problem. Native plants are rarely used, which means that much more water is spread on the terrain than was natural before development. In many slope failures reported in recent years, too much garden watering on hillside yards has been a contributing factor.

Add to this condition the weight on the slope of things such as houses and cars and the unusually heavy rainfall this season and the geological terrain becomes overloaded. The combination can speed slope failure that under more natural circumstances might not occur for centuries.

Serious geological residential building site analysis has been going on in Los Angeles and Orange counties only in the last 20 years. Geologists are continuing to improve their ability to evaluate hillsides for potential hazards such as old and inactive landslides, and fault zones. Governmental agencies are getting better at requiring more intense analysis.

Even after the best analysis and review, there are still situations where the geological consequences of grading and building cannot be foretold. This is partly because geologists cannot physically probe all the underground terrain and partly because there is a practical financial limit to what can be spent to develop guarantees.

So, there is always a trade-off when anyone decides to build a home on a hillside in Southern California. The geologists and grading contractors have to work within certain cost-benefit limits set by the governmental agencies that are responsible for issuing the building permits.

In the final analysis, there is no substitute for good geological terrain analysis.

Some of the recent slope failures and subsequent loss of houses are probably because the houses were built before geological analysis was required or before such analysis improved to its present high level. There is, therefore, a large inventory of houses that may be located where slope failure will occur.

I recently toured the Anaheim Hills landslide where many fine homes are involved. Several aspects of this disaster impressed me greatly. It is obvious that the initial geological survey conducted many years ago before development failed to identify a very large, ancient landslide even though tell-tale geological signs were evident. The geologists called in to stabilize the slide and save the homes are doing a magnificent job. I believe they will be successful, but it will take many years and the cost will be very high.

Anaheim should be a model for disaster response for all cities. Anaheim employed all its personnel and available technology and has maintained a staff of support personnel on the job to mitigate the problems.

As I look at the hillsides and shore cliffs of Southern California, it is clear that we will see many more homes sliding down the hills in the years to come.

Some property may have to be abandoned. Some homeowners will be able to take preventive measures at a modest cost. In other cases, however, the cost will equal or exceed the value of the house and property itself. It is understandably difficult for those owners to invest the large amounts of money needed without convincing evidence, such as seeing their home, or a neighbor’s home, starting to slide down the hill.

In this business, however, it is a truism that it is cheaper to take preventive action than to repair the damage to a house that is breaking in half as it slides down the hill on the back of a landslide.