Shallow foundation on loose sand

[tclat]

Hi,

I have a small two storey structure to be built near the coast line.

The foundations proposed are strip footings at a depth of 3 feet below the surface. The sand is loose sand (based on visual assesment) and the water table is about 6 feet blow the ground level.

I propose to flood the trenches and compact with a wacker plate and design the foundations for 2000psf. Does this sound like a reasonable allowable bearing capacity?

I am also in a seismic region. How concerned should I be about liquefaction?

I am located on a small island in the Caribbean with no experienced Geotechnical Engineers so some advice would be much appreciated.

Thanks

 

[ishvaaag]

http://www.ce.washington.edu/~liquefaction/html/main.html

http://yoga10.org/Documents/Liquefaction_of_fine.pdf

http://www.zetas.com.tr/pdf/paperno59.pdf

Without proper investigation of the soil it is impossible to lay an informed opinion on the behaviour of the soil. However there are bearing wall structures laying on those sand islands that have stood quite a long time. I don't see your working pressure on the soil high, only tests showing very poor strengths would make me change this preliminar opinion.

Also, hopefully your site is not in one of these that are caving from below, if one of these ensure you make some tests.

 

[oldestguy]

Flooding is not a good idea. Then your compactor has to drive that water out. Some watering can help, but I'd not go for saturation. You get much better compaction with the machines than depending on flooding.

Adding longitudinal reinforcing top and bottom of your grade beam wall also is a good idea to even out possible uneven support effects. Be sure it has good laps and goes around corners.

Check with area structures as to their usual foundation sizes, but I'd have to guess your 2,000 is OK. Up here that works for most weak bearing conditions as you describe.

 

[FixedEarth]

Have you considered a Mat foundation. In the past, I have used them for similar jobs and soils.

 

[Mccoy]

I believe with such a shallow water table you should be very much concerned about liquefaction, a way to assess its potential would be to run in situ tests (CPT, SPT's) after you've done your compaction interventions.

IF field tests are not possible for lack of proper equipment, then I'd go for a thorough compaction strategy which should be able to densify the upper layers, which are the ones which govern the negative effects of liquefaction on structures. As I understand it, trouble is that you have a limited choice as far as compaction intervention is concerned.

Also, the random pattern of liquefible pockets might cause differntial settlements so a rigid slab or a rigid grid of beams on grade wouldn't be a bad idea

Then you should check the whole site stability to lateral spreading for example (is the ground level or has it a slope?), large lateral spreading phenomena as seen in the Haiti main harbour might jeopardize your whole project.