Designing a retaining wall system for an already excavated deep open cut prone to collapse 3

[ysna]

Hi,
A client of mine have asked me to suggest and design a retaining wall system for an 18 meter deep excavation next to a series of adjacent 5 to 6 story residential buildings alongside one edge of the excavation site. The problem is, almost 90 percent of excavation to the desired depth have already been completed, and now that there seems to be some excessive settlements near adjacent pavements on three other sides of excavation site, he is concerned about probable collapse of open cut, and asked for a professional advise on the matter. As you can see from attached photo, about 4 to 5 meter of soil width have been kept alongside residential buildings (right side of the photo), but excavation on other three sides of the site are almost completed. There seems to be no sign of excessive settlements on the adjacent buildings, but in other three sides of the excavation, there is signs of local collapses on the face of excavation. You can see some temporary steel shoring elements near those areas. My question is, regarding that other than alongside residential buildings, the excavation is almost completed on three other sides, what is most logical solution for retaining of an already excavated cut face? The soil of the excavation site is moderately to highly cemented medium to fine sand (SP) and construction site is about 70 meter wide and 100 meter long.

 

[SlideRuleEra]

A little more information requested:

1. What type foundation do the adjacent buildings have? Depth of these foundations below original grade?

2. "Construction site is 70 meters x 100 meters." What is the minimum footprint size of the full depth (18 meters) excavation?

3. What potential solutions have you considered, even if you have ruled them out as impractical?

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[ysna]

1. What type foundation do the adjacent buildings have? Depth of these foundations below original grade?

Adjacent buildings have grid foundations of probably 1.2 to 1.6 meter grid width. They are based on between 1 to 1.5 meter below original grade.

2. "Construction site is 70 meters x 100 meters." What is the minimum footprint size of the full depth (18 meters) excavation?

Footprint size of the excavation is equel to construction site dimention, i.e. 70 meters x 100 meters.

3. What potential solutions have you considered, even if you have ruled them out as impractical?

Well, The first thing that comes to my mind is retaining excavation using soil nail walls. But working on 18 meter cut face seems to need a lot of falsework constructed in front of it. Also they may remain some extra excavation on those sides too. So I think contractor needs to excavat cut slop working on falsework too.

Since the safety factor of cut in it's current condition is very low, excavating boreholes for drilled piles are very dangerous. So we can't use any type of drilled pile walls. Driving piles near those cut faces could be dangerous, so driven pile walls are out of question too.

 

[SlideRuleEra]

Humm, quite a number of constraints...steep,unstable slopes - limited horizontal clearances - deep excavation - adjacent buildings on shallow foundations - no pile driving, etc.

One big advantage... the situation has been allowed to get so far out of hand that budget concerns should not be a primary issue.

First, completely document all existing conditions. This will include a thorough field survey by a qualified, independent third party. This information will be needed no matter what solution is selected.

Three potential solutions come to mind. None are "cheap" or cost effective and would be laughed at under normal circumstances:

1. Stop work. Backfill the entire excavation with compacted fill. Start over using proper excavation techniques. On some projects, I have used this approach... with no second guessing later. It was the best way.

2. Stabilize the existing conditions; look into Ground Freezing. I have not used this method, but did seriously evaluate it on a deep excavation (25 meters) project, in poor soil.

3. Call in a specialist. In the USA, perhaps Hayward Baker. In the long run, often the way to go - have done this, too.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Retrograde]

Shotcrete with ground anchors could work.

 

[EireChch]

Soil nails/ground anchors would require approval from the adjacent land owners as they would be extending benesth their buildings. Also - if any adjacent buildings are piled then this could be an issue.

Could shipping containers be used? I've seen them used as rockfall barriers for response to EQs. What if the face of the container was strengthened by fabricating some steel plates to the face.

Then they could be easily moved around the site to act as temporary support while shotcreating and propping or the likes are constructed?

I think any solution here is going to be drastic!

Where in the world is the job located?

Good luck and keep us updated.

 

[ysna]

Using shipping containers as falsework, interesting idea. Since there seems to be no problem of getting approval from adjacent land owners, and there remains 4 to 5 meters of soil width to excavate near adjacent buildings, I think we will finally go for soil nailing. I guess, maybe, we can move excavated soil on that side of cut to the opposite side, which is in dangerous condition, and compact it as temporary shoring. I'm now talking with some of local contractors, to see what they think about construction difficulties.

By the way, the excavation site is located in Iran, and it is more than a year now, that it is in this condition. As you can see, we have some very brave people here

 

[EireChch]

Very brave is right. I would not stand within 20m of one of those sides. Amazing that its standing up for a year now. Those adjacent buildings gotta be experiencing some settlement/cracking.

I would like to see the contractors methodology for soil nailing in terms of health and safety considerations. I was thinking if you start at top down might be better but you could have the entire block of soil fail of the un-nailed face beneath starts to slip/move.

 

[PEinc]

Brave? Or just stupid? I agree with SlideRuleEra. Has a slope stability analysis been run with correct soil properties and appropriate surcharge loads applied? Backfill the hole and start over. I'm assuming the excavation was made in soil, not rock. In the US, it would be illegal to work in front of an unsafe slope - even if trying to stabilize the slope.

http://www.PeirceEngineering.com

 

[HENRYZAU]

Agree with PEinc

 

[dik]

Whatever you do... SRE's pretty well sums it up... you should do it quickly; you have a real impending mess on your hands...

Dik