Nothing like getting all of our rain for the year in a 3 week span....Right?
Well that is exactly what mother nature has done for us. We were on the verge of being very short for rain accumulation again for the year in Southern California and then the past 3 weeks happened, and BAM!!! we are ahead of schedule for our yearly average. Coming into the month of April we were about 2-3 inches off of our yearly average. Since then we are about 2-3 inches above our yearly average depending on which part of SoCal you are located in. That amount of rain, in that short amount of time can lead to some problems....Some MAJOR problems.
Once the last rainfall happened on the 12th of April, we received numerous phone calls about slopes and hillsides collapsing. Many of them had retaining walls that failed or worse yet, some didn't even have retaining walls in place. Definitely not a fun thing to wake up to as one customer did.
If you think of the soil that is under your home and surrounding your property and your neighbors, as a sponge it will give you a clearer picture of what we are talking about. When a sponge is introduced to water, it will absorb as much as it can and typically with expand in size. All the excess water that it can't absorb has to go somewhere, and that is when you get run off of the water.
Same can be said with the clay soils that we have in Southern California. The clay soils are able to absorb X amount of rain water, after that, the excess water has to go somewhere and that is where it runs off or puddles in the lowest elevation that it can. Yes, gravity plays a role in this as well. On top of the clay soils absorbing the water, just like the sponge, they start to expand and become slippery. Typically on a slope, there is a slip plane associated with the hillside. What this is, is where solid native soils or materials are in contact with the clay/fill soils. The native soils don't absorb much water and with the slippery clay and now heavy water absorbed, slippery clay sitting on causes gravity to kick in and creates a mother nature slip in slide.
The way that engineers design these retaining walls for slopes is to prevent or hold back these soils from being able to slide. When the wall is under designed or starts to become compromised, it can lead to massive problems. And with massive problems comes massive costs. It is always better and cheaper to take on these issues before they hit this tipping point.
Bottom line is, when we have this much rainfall in this short amount of time, it typically will lead to some soil problems to some degree. Either immediately or in the future. Don't wait for the problem, attack it and find a solution before you are left with no other choice. It will save you a ton of money, time, and headaches if you do.