excavating utility trench beside settled footing

[USER10]

Hello, I thought that this could be interesting problem to share.
A utility trench (almost 50 m long) with depth of 2 m is proposed at horizontal offset of 2 - 3 m from a wall of 3 storeys house . Unfortunately, the wall has been reported to have cracks due to both structural issues and foundation settlement. Horizontal offset between the problematic wall and trench can't be increased. As understood there 2 to 2.5 m of compacted fill overlaying firm to silty clay at subject site. The excavation of trench may compromise the stability of the wall . Conceptual solution options:

First obtain more accurate soil profile at site and information on the depth of the foundation of the wall with respect to depth of the trench. Solution options would be :


1- Lower the burial depth of the utility to the minimum possible (1.5 m ) and if the depth of trench is shallower than the depth of the wall foundation ( wall foundation settlement is not likely to happen). Even with the shallow trench of 1.5 m , structurally support the trench side (soldier timber piles system) without causing any tangible vibration (more than 5 mm/sec) (Utilize experienced excavation contractor)

2-If you can't figure out the depth of the wall foundation or if it is shallower than minimum trench depth (1.5m) , use proper trenchless technique ( that results in minimum gaps between drilled hole and pipe) to install the utility pipe in the subject trench.

3-other option is just fix the problem of the settled wall before excavation of the trench

Your thoughts and discussion on this matter will be appreciated

 

[oldestguy]

Get legal advice as to reducing liabilities. Get detailed before and after info on the existing house, with photos, measurements, including measurement of cracks with a system capable of many repeat measurements. Finding foundation depth may take actual limited excavation test pits. I'd not trust hand augers or probes. If some noticeable work the house owner could do to help him and incidentally help this job, be sure to mention that in a written communication. Oh yes, keep detailed records, even showing time of day, weather. Establish a reference line with stationing for reference purposes. Tie all photos, etc. to that reference system. If possible do some elevation observations of the building and possibly any affected landmarks.

 

[oldestguy]

The crack width measuring method I use is a pencil line across the crack and a small cross on each side placed a given distance apart. Record this distance and sny off-set of the lines on eachside.

 

[Ron]

Option 3 is better!

 

[USER10]

Maybe option 3 cant be done as it may cause significant delay of the project

 

[Ron]

User10...are you concerned about doing it right or doing it fast? Doing it right works. Doing it fast doesn't always work.

 

[USER10]

oldestguy , thanks for your good thoughts