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View Full Version : My observations of the Laguna slide



Mark Singer
06-04-2005, 9:37 AM
Bluebird canyon is known to have the worst Geology in Laguna. I am on bedrock with slight bedding angling to the south. My lot is very gently sloping and is among the very most stable lots in the town. The home I am building at the top of Portifino is on solid Brescia including a great deal of the "Blue Brescia" which is extemely hard. This is why it took months to excavate the site. Then we poured many deep caissons ....it ain't going nowhere!

Yesterday I drove up to a home under construction on Summit Way, that I designed.. This put me eye level with the slide and extremely close. I was able to analyze the movement of earth and have a pretty good idea of what happened.
The winter rains weakened the bond between the harder and softer layers of soil. The moisture adds cohesion , but as it starts to dry the cohesion is reduced and failure occured. It is similar to making sandcastles on the beach. When the sand is wet it sticks very well. As the sun dries the sandcastles they fail in shear along a weak plane.

Many of the residents and home owners blame the newer home for the slide. They say because it was not built well , it somehow created the slide. This is just not true! In fact , I am sure that the slide started above the home , at the visible shear plane in the photos. The newer home was in the path of tremendous earth movement , and failed and distorted under the force of land mass that was moving. The caisons helped the newer home to remain fixed to its bedding and it forced the moving soil mass around and to the right (as you look up the hill) of the home. This is evident, because there is a new slope where the land mass over lies the original grade ....maybe 10' high. I think the caissons of the newer home helped deflect the moving earth mass away from the home I remodled (Duka home) and added to many years ago. It lies behind the newer home and was protected.

When I remodled the Duka home , the geoogist and I added 12 caisons to stabilize the home. At the time Lonnie Duka really didn't want to spend the $60,000 for the caissons. This was about 1990. The other day he thanked me , and said tose caissons saved his home. It appears to be 100% in tact with not even a crack.

The most disturbing part for the local residents is the media. I have had so many phone calls I can't even start to tell you . They have invaded every hillside wit cameras and trucks....helicopters in te sky. Really probing questions ....trying to fabricate stories from threads of information...
Much of what is publishd is incorrect.

The home owners that lost their homes would naturally like to think it was someones fault.... a newer home, the City, anyone. I understand, and feel very sorry for them. The facts are the facts and the history shows a slide in 1978 , long before any new home was built...and many warnings about poor geology....it was well known among residents. Hopefully there are funds to rebuild their homesites and their homes.....In 1994 we had a huge fire here and the poorly designed original homes which had cedar shingle roofs and wood eaves were rebuilt with new and much better built, safer homes. In time, I think the same will occur here...

Jamie Buxton
06-04-2005, 9:50 AM
..[snip]...rebuilt with new and much better built, safer homes. In time, I think the same will occur here...

YIKES!! You think that people will build in this slide area? Amazing!

John Miliunas
06-04-2005, 9:54 AM
Mark, I for one, truly appreciate your insight into this natural disaster. I have no doubt in my mind that you design and build home, NOT just for today, but for the generations hereafter. :) Coincidently, my buddy and I were talking about the exact same thing yesterday evening. Seems that we figured some type of caissons would've been in order for construction in any of those home not solidly sitting on bedrock. Mind you, we're thousands of miles away, pretty much on Terra Firma, but have that understanding. Strange how folks right in the middle of such an environment don't wish to follow "best practices" when building in that environment. After the fact, they are quick to point the finger elsewhere. Only human nature, I guess.:rolleyes: In the aftermath, what I find most troubling is, as you say, the media being totally ignorant of the true science to it, yet just as willing to place fault on other persons and/or unproven "facts". As much as I like to be informed, seems that often media has a way of causing more harm than good and, in this case, adding fuel to a fire which shouldn't be in the first place. Just MHO, of course.... Thanks again for your insightful views!:) :cool:

Roy Wall
06-04-2005, 11:11 AM
The most disturbing part for the local residents is the media. I have had so many phone calls I can't even start to tell you . They have invaded every hillside wit cameras and trucks....helicopters in te sky. Really probing questions ....trying to fabricate stories from threads of information...
Much of what is publishd is incorrect.

.

I wish I were surprised......but this is typical media coverage IMHO. Several years ago, a local station pinned our subdivision as have a "major dam break". We have two little ponds in the neighborhood...what a crock! It cost us residents thousands $$$ to have testing, monitoring...and - of course we became a "bad name" because of this....

Irritation for sure:confused:

Norman Hitt
06-04-2005, 4:20 PM
Unfortunately, the problem with the Media just gets worse as time goes by and has been Prompted considerably, I feel, by the advent of all the Media Awards Banquets, Pulitzer Dreams, etc. Everyone Wants to be a Star. Over the years I have on several occassions had first hand knowledge, and in some cases, witnessed an accident or "Event" and have been interviewed by the media, but later when I saw it on the news, (TV being worst and newspaper not much better), it seemed that ANY Similarity between what Actually happened, and what was reported in the News was Purely Coincidental, (after their in Studio Editing).

Unfortunately Headlines with a Humonguous Tragedy, Sex Scandal etc., etc., sells news papers And Advertising, and when the Media goes out to report, if there's not a story there, they just make one. As one famous news person once said, "NEVER let the FACTS get in the way of a Good Story".

Pete Lamberty
06-05-2005, 5:33 PM
Thanks Mark for giving us some of the facts. It is very interesting to me. You pointed out that you have built in the area on bedrock. The Duka home that you were involved with had caisons. Do the caisons go down to bedrock? How deep is the bedrock in the area of the slide? I can't see how one could have built safely in the area of the slide. Is this correct? Would it be better if people just didn't build in some of these areas? Thanks for your thoughts.

Mark Singer
06-05-2005, 11:15 PM
Pete ,
It should have not ben built on....you are correct....many of the homes are 40 to 50 years old and were built on the standard of care at the time.....this area is very week geotechnically and extrene measures are needed to stabilize the condition
Thanks Mark for giving us some of the facts. It is very interesting to me. You pointed out that you have built in the area on bedrock. The Duka home that you were involved with had caisons. Do the caisons go down to bedrock? How deep is the bedrock in the area of the slide? I can't see how one could have built safely in the area of the slide. Is this correct? Would it be better if people just didn't build in some of these areas? Thanks for your thoughts.

Kelly C. Hanna
06-06-2005, 12:02 AM
Mark...you insight is great to read. Can you link to a drawing of a caisson? I don't know this term and would love to in case I ever build a home for myself. I have noticed many failed foundations here in the black gumbo mud clay soil.

The other day I watched a very large foundation project for a few minutes. The norm here is to go down a few feet (3'-5') and call it good for pylons or footings. This house had columns 2' in diameter and at least 20' down. It's going to be a very modern house with glass and stone walls. Can you tell me why they are going so deep? Is it to get past the shifty clay? White rock is below the clay in some cases...limestone in others.

Vaughn McMillan
06-06-2005, 4:18 AM
Kelly, those 2' diameter x 20' columns you saw going into the ground the other day were, in effect, caissons. There are a number of kinds, and Mark can describe the specific ones he's used. I'm not really familiar with the ones used on hillside structures around here. Some are driven (a.k.a. pilings), some are drilled and filled with concrete and steel reinforcing, and there are other types. Here's one definition: http://www.encyclopedia.com/html/c1/caisson.asp.

My dad and brother-in-law have both been involved with inspecting large caissons after they were drilled but before the rebar was placed prior to the concrete pour. (I never had a chance to inspect any in the years I was doing civil inspection for a living, but I heard quite a few descriptions of the experience. It apparently wasn't real fun.) The holes they were being lowered into and inspecting were about 3' diameter by 30' or 40' deep, with a belled bottom -- 6' to 8' in diameter. These particular caissons were used in deep sandy soils (not to the bedrock), and the wide bottoms were to prevent downward movement. The reason for the inspection was to confirm the dimensions of the bell, and to ensure there wasn't too much loose material.

To keep this from being a complete threadjacking, I want to say thanks to Mark for the insight on the Laguna slides. Living in the LA area (on a hill, but not perchecd on the edge), I hear only the "details" discussed in the local press, and I truly appreciate getting the real scoop from someone who has intimate knowledge of the situation. Having grown up in a civil engineering environment, this type of stuff fascinates me.

- Vaughn

Kelly C. Hanna
06-06-2005, 8:09 AM
Thanks Vaughn...don't worry about the thread jacking...this is completely related to what Mark was posting about.

The place where I saw them being poured is in fact only a few hundred yards from White Rock Lake in Dallas. I wonder if that's why they are being used (besides the fact the home will cost well over a million bucks).

Mark Singer
06-06-2005, 8:53 AM
Kelly,

Vaughn is correct, they are holes drilled into the ground, inspected, a steel reinforced cage of rebar is lowered in and the concrete is placed in each hole. Over the caissons are grade beams that connect the caissons and tie them together. Then the normal slab or framing can begin.

Take a look at this post , it is a new home I am building.....


http://www.sawmillcreek.org/showthread.php?t=14842&page=1&pp=15&highlight=caissons

Kelly C. Hanna
06-06-2005, 9:15 AM
Wow...stunning views! That site is a challenge I see...now I understand the caissons. Thanks for the link to the thread...I'll be watching that one!