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Design for Liquifaction (or mitigation thereof)

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Dear Colleagues;

 

I’m looking at a new project where liquefaction is an issue. According to the soils report, total induced settlements, should liquefaction occur, are estimated to be approximately 3 to 8 inches.

 

The geotechnical engineer has suggested that I consider the following five options:

 

  1. Densified potentially liquefiable sand/silt layers at 10 to 40 feet depth by use of vibro-compaction, vibro-replacement, compaction grouting, or deep dynamic compaction.
  2. Deep dynamic compaction of the upper 40 feet of soil by use of falling weights.
  3. Foundations that use grade-beam footings to tie floor slabs and isolated columns to continuous footings (conventional or post tensioned). Flexible connections for utility tie-in required.
  4. Structural flat-plate mats, either conventionally reinforced or tied with post-tensioned tendons.
  5. Deep foundations (drilled piers, geopiers, stone columns or piles) founded at a depth of 40 feet.

 

The geotechnical engineer has no experience with option 1. He thinks option 2 will be unacceptable because the project site is in a populated area and dropping 2.5T weight from 40 feet might be objectionable. We both believe option 5 would be cost prohibitive for this project.

 

For some background, the structure is a one story medical office building in UBC/CBC country. The site is 15.6 km from a Type A fault. Groundwater was encountered at a depth of 9 to 15 feet. The soil is classified as 5 feet of sandy silt over 10 feet of silty sand over 5 feet of sand.

 

A foundation I recently designed for a similar structure can be found here:

 

http://www.allendesigns.com/projects/20325/20325S11.pdf

 

The geotechnical conditions were different in that this foundation was moderately expansive and was designed using UBC section 1815. However, you can see the types of loads from the foundation design. The exterior walls are bearing walls and there is a center girder line with columns and pad footings. The proposed structure will be framed similarly.

 

If it helps, the roof framing plan can be found here:

 

http://www.allendesigns.com/projects/20325/20325S21.pdf

 

In talking with the geotechnical engineer, I told him that I would have to charge the client extra $$ if I designed using either option 4 or 5. I only included conventionally reinforced UBC 1815 type footings in my Basic Scope of Work. He said he thought that a grade beam foundation would be acceptable and he referenced a subgrade modulus in his report of 200 pci and an allowable soil bearing pressure of 2,000 PSF.

 

One of the problems I’m having is that, for a uniform pressure of 2000 PSF, based on k = 200 pci, I’m only getting a deflection of 0.07 inches. I guess I was expecting something like 50 pci or an allowable soil bearing pressure of 500 PSF during a seismic event. Fortunately, I only have to consider dead load since this is a one story building. FYI, the proposed building is approximately 141 feet x 225 feet.

 

Now my question(s):

 

  1. If I choose a grade beam system similar to the job I recently completed, what should I use for methodology/criteria in designing the grade beams?
  2. Any other observations/recommendations?

 

TIA,

 

T. William (Bill) Allen, S.E. (CA #2607)

ALLEN DESIGNS

Consulting Structural Engineers

http://www.AllenDesigns.com

V (949) 248-8588

F (949) 209-2509