Settlement due to adjacent excavations 8

[pelelo]

Engineers,

I am looking for references which can provide information about settlements and cracks in structures due to adjacent excavations.

My case is next to a house was made a 8 ft o 10 ft excavation on a loose to medium dense sand material. The wall of the house is 20 ft off the edge of the excavation (surprisely, it was a sloped excavation, looked around 0.8h to 1v, and did not fail).

House owners said they started to notice damages on the house once the excavation was made.

Thanks.

 

[EireChch]

As the house is 20ft from a 10ft deep excavation, I wouldn't think that the foundations of the house would even know the excavation as happened?! It is well outside the theoretical failure wedge of 45 + phi/2.

I would ackknowlede their concerns and tell them you will get a building surveys to make some measurements. I.e floor plane survey, crack monitoring.

What type of foundations is it on? Where is the water table, any dewatering? How long is it was the excavation open for?

 

[Ron]

This is not uncommon! As EireChch notes, the excavation most likely had no effect on the structure.

The occupants probably paid more attention to cracks and other anomalies that already existed in the structure but went unnoticed.

Cracks can often be evaluated for their cause and, to some degree, their age. I have seen this many times over the years. Some are legitimate....most are not.

 

[pelelo]

Thanks for your replies.

EireChch,

Using a phi of 26 deg (N between 6 and 12), 45 + phi/2 (measured from the horizontal), is 58 degrees, which results in about 4 feet of the wall. I don't think it is that far.

There is a separate team working on the measurements, such as floor survey and crack monitoring.

The foundation is a simple concrete wall footing (continuous footing) on top of a stem wall. No dewatering, WL was no detected in 10 feet borings. The excavation was open for about 1 month.

Ron,

I agree with you, many homeowners react to these situations once they notice something weird is going on next to their properties. But this is not the case. What happened was contractor did the excavation but halfway they noticed few cracks were showing up at the adjacent house, then contractor decided to back fill the excavation in order to avoid more damages.

I got few more questions:

1 - Other than the 45+phi/2 approach, in order to know if the footing would or would not notice there was any excavation nearby, there are few other approaches such as

a) Bousinnesq pressure isobars (I did a quick check and using this one the footing would not be affected by the excavation) and
b) the 45 deg method which is drawing a 45 deg line from the edge of the footing and check if it intersects the excavated wall (in this case, i did a quick calc, and it is about 10 feet below the bottom of the excavation).

2 - How would you take into account the time in which the excavation was open?. I mean, what is the difference is the excavation is open for 1 month and for 1 year?.

 

[dik]

My experience is similar to Ron's... once they notice an 'old' crack, then they start to look for other cracks... generally pre-existing.

Best to avoid by doing a pre-construction review of adjacent structures.

Dik

 

[oldestguy]

Check local laws. In some places the owner of a building has to protect himself from any damage done by work beyond the property line as long as the excavation is less than some fixed depth, such as 10 feet. Deeper than that the neighbor is thereafter responsible. I've seen this rule applied for filling of the neighbor lot beyond the property line, in addition to several excavation situations.. This also has been applied to vibrations from construction.

 

[dik]

OG: I was not aware that there are jurisdictions where the owner of a building had to protect his building from new construction; it's generally the new constructor has to protect adjacent existing buildings and reinforce them is there is a snow accumulation shadow.

Dik

 

[Okiryu]

Do you have groundwater and dewatering operations going on in that excavation?

 

[EireChch]

Pelelo.

58 degress from horizontal results in top of failure wedge being rougly 13ft from the adjacent structure. You have measured from vertical.

 

[pelelo]

Okiryu,

No WL level detected in 10 ft deep borings. I am sure Water table is not that far from the 10 feet depth. No dewatering operations were performed.

EireChch

You are correct, it is 13 ft off the structure, so should be fine.

Just to confirm, Would you also use Bousinessq pressure charts in order to verify if stresses could be felt by the adjacent structure?. or with the 45+phi/2, measured from the horizontal, you think it is more than enough?.

 

[PEinc]

Maybe the attached paper by Ralph Peck will help you.

www.PeirceEngineering.com

 

[pelelo]

Thanks PEinc!.

 

[Okiryu]

pelelo, see page 13 of this link:

https://www.cedengineering.com/userfiles/Intro to Deep Foundations.pdf

 

[BigH]

I doubt that there would have been any effect either BUT this goes to show that when one is carrying out any excavation with nearby structures - even if outside the theoretical lines etc., a thorough condition survey should be carried out; documentation such as photos, pre-existing cracks, deformations, locations of trees, etc. Then you have a base line on which to tell the owner of the adjacent property he is all wet . . . or you might have to own up to something.