Seismic foundation behaviour

The issue is far too important to trust comments unknown people give you over the Internet. An expert should always be contacted.

I suspect that "liquefaction" would make sand a very bad choice. Wikipedia it for more info.

When considering the interactions between the soil and a building's foundation during an earthquake, we have to not only consider the soil types, but the types of waves that earthquakes generate and how the soil transfers the energy to the foundation.

If you are using this question as a test to determine if a particular candidate belongs on a design team, first look at their transcripts to see if they have taken a course in geology. If they have, you would expect them to have a valid answer, and you can see they have exposed themselves to a variety of subjects and not just the engineering, math and physics courses required to get their degree. If they haven't and they give a good answer anyway, that means they studied this on their own, or they were paying attention when the question came up. In either case, if the candidate doesn't give a valid answer, you want to know if they are willing and capable of doing the research to produce a valid answer. In short, what you are looking for in the candidate is not just knowledge, but flexibility and resourcefulness.

I have no experience with earthquakes, but I have taken a geology course. I do remember back in 1968 when the shock wave from a dynamite plant explosion near Joplin MO was felt in the Tulsa OK area.

Check http://en.wikipedia.org/wiki/Soil_liquefaction for a little information on this. There have also been a couple of shows on this over the years. There was one that was very good and probably was on the Discovery Channel. It is pretty scary actually.

Wet sand or sand with the water table near the surface is very bad. Most areas built up by river deposits or human land fills are very bad too. Shake with pressure and the solid ground your city is built upon turns into chocolate pudding. Some of the demonstrations are unbelievable. Some of the bad characteristics seem to be small size of particles, somewhat round in shape particles and being wet. Some "soils" turn into "lubricated ball bearings" when shaken with pressure.

Japan has some cities built on "bad soil". I think we have some too.

Backing up a bit, there are two or three major types of failure mechanisms for those of us who don't know what we are doing to consider. First would be the building breaking apart due to the "true shaking". A building on bed rock would be good to think about here. Some large buildings in San Francisco are being lifted and placed on concave bearing plates to help reduce this risk. There is a little bit of "a building at rest tends to remain at rest" going on here. The bearing plate allows the earth to slip under the building and the concave shape keeps the building from slipping off the plate.

Buildings also need to be made/retrofitted with materials that won't completely fail with shaking. Wood frame construction with full sheets of plywood on the walls under the dry wall will take a lot of shaking with only cosmetic damage. Rebar in poured concrete is pretty good. Brick on 100 year old mortar is just a pile of loose bricks once you shake it.

Resonances in the building is a problem. Some tall buildings are being fitted/retrofitted with huge weights on a pivot point. Somehow when the building starts to move they dampen the motion and keep the building together. The demonstrations look very impressive, but this is still magic to me.

Back to the soil/sand/rock you started with. There are at least two things going on here. One is the liquefaction that can cause both building tilt and cracking of the foundation (actually cracking of everything) and therefore a total failure. Another would be the constructive and destructive interference of energy waves. I think that a large plate of bedrock will act as a pretty good transmission medium for the sliding/grinding/snapping energy from the epicenter/fault-line. Mixing areas of sand, bedrock, gravel, farm dirt, etc. will (if I recall correctly) allow the energy waves to bounce around and create nulls and peaks. Thus, on a somewhat "local scale" some areas will have much stronger shaking than other areas. If I recall correctly, this stronger shaking can have a more random collection of frequency components and have more of a "snap action" on the buildings.

If you are going to build something the most important thing for you to remember is that I don't actually know what I am talking about.

Hi!

The question is not which is better as a foundation - rock or sand. Foundation conditions usually dictate the type of foundation a building requires.The situation assumes special significance in regions subjected to seismic activity. What is desirable is for both the sub-structure - foundation - and the superstructure, to be able to safely withstand the imposed earthquake loads and the displacements thereof - both vertical and lateral. It is the soil-structure interaction and the structural response to dynamically imposed loads - such as due to the earthquakes - that has to be studied carefully in each type of foundation so as to arrive at the right stability of both sub- and super-structures under such imposed loads. While sound rock is an accepted medium of foundation under normal static conditions, loose sandy soils, particularly fine-to-medium sands - pose the problem of "liquefaction" under dynamic loads, if they are not densified or treated properly by suitable foundation treatment methods that are available to the Foundation Engineer.

Regards.

PLAN