Encountered loose soil

[inhinyera_sibil]

Hi,

I am new to this forum. I can see this forum is full of people with great knowledge and experiences, so trying my luck here to find answer.
I am handling a school project now. We had encountered loose soil. First reason is the architect decided to relocate the building. The farthest area is a nearby creek. On that point we have no geotech investigations. The structural specified a depth of 3m only. Upon excavation at that area the soil was very loose compared to the other side of the excavation. Note we are on a mat foundation if this is something that could help out.

Any solution besides going deeper? If were going deeper we will end up just putting a basement to be utilized so as not to waste the amount/cost of excavation.

attached is a photo on both sides.

 

[inhinyera_sibil]

oh below is the photo.

 

[MTNClimber]

First step would be getting a local geotechnical engineer involved. You need to find out what the subsoils are like under the new building location.

Depending on the building footprint size and available contractors you’re probably looking at ground improvement, stiffening the mat, or switching from a mat to a deep foundation.

 

[oldestguy]

Agree about a geotech. In looking at the photos, to have the ground cut stand vertically, is a positive sign.

 

[inhinyera_sibil]

We already opted for a ground improvement. Unfortunately they refused to do the work. Probably is because most areas are already excavated to the depth of the 3meters and they would just improve the remaining depth about 1.5-2m.

My bad forgot to tell about the ground improvement.

If deep foundation? is this still a mat footing or isolated one? Incase we are actually leaning towards a basement should we need to go deeper.

We are looking also for the possibility of mass concrete to replace the loose soil? Any thoughts about this? Since we won’t be needing to use other equipments.

 

[EireChch]

Your only answer is to get an experienced local geotechnical engineer. You dont even know the ground conditions at your building location other than its loose.

There are likely much better options that mass filling. Again, a geotech is best placed to advise.

 

[Ron]

I agree you need a geotech involved. A few observations though...

It appears you have sheet piles installed in your photo. That means you likely have access to a vibratory hammer. One quick and easy ground improvement method you can use in loose sands (another thing we don't know from your description...you said "loose" so I'll assume sands) is to create densified sand columns using a 24 to 30 inch diameter open end steel pipe inserted and extracted with the vibratory hammer. Create a grid that is 2x the pipe diameter. Insert at cross points in the grid. Extend to the depth of improvement needed. You'll see the soil densify within the pipe and after extraction you will see the soil column lower than the adjacent soil. You might need to add water within the pipe as you densify.

Will work well with mat foundation. Verify this with a local geotech. I've used it successfully in the past; however, you need to know the soil conditions first.

 

[inhinyera_sibil]

For our soil investigations results.

below are the depth when spt reached n values >50 hence they started coring.
BH-1 4.5-6.0
BH-2 1.5-3.0/3.0-4.5
BH-3 4.5-6.0
BH-4 3.25-4.5

above this depths the soil description are clayey sand, sandy lean clay, silty sand with gravel.

this is the two additional boreholes at the farthest location near the creek. Depths when spt reached n values >50. Note we just opted to test this area now due to relocation of the bldg.
Bh-5 6.0 (nearest on the creek)
BH-6 4.5

Does the Strl seems to missed this details when he specified the 3meter depth he required? even with the old test assuming we haven’t done our two additional test.

we are still trying to look for a geotech right now.