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Excavating close to a building

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I am surprised that the city didn't take any borings in this area
as this is HDPE pipe (and fairly large).  The contractor could run
into bad material and the city would have to pay an extra (expensive).
I would want to compact the backfill in lifts of around 1' max.  I
don't think sheet piling in this instance is reasonable that is unless
the soil is very poor.  You might want to get a copy of the con-
struction specifications for the job and check the compaction re-
quirements. 
 
You may want to hire a surveyor and get multiple floor shots along the
wall of the building in question.  Have the surveyor sign and
seal the drawing (shouldn't cost to much) - then you would have
a good record in case there are future problems.  During excavation
and placement of the pipe, make sure the city representatives
(inspector and engineer) are on site. 
 
-----------------------------------------------------------------------------------------------------
Mr. Rissell spoke as follows:
 

Subject: Excavating close to a building

My home town in west central Missouri is installing new sewers. My dad has
a 30 year old pre-engineered metal building, 45'x60', slab on grade with
footings 3' below grade, I think. The city wants to run a new 22" HDPE
sewer along the 60' side of the building, 6' from the edge of the building,
10' deep. The building is still in excellent shape and there has never been
any problems with settling, concrete cracking, etc. Their construction plan
is to use trench boxes at the joints and then backfill in 2' to 3' lifts and
compact to 95% Proctor. I indicated a big concern with undermining of the
foundation and thought sheet piling was the only sure way to keep the
integrity of the foundation and surrounding soil intact. Obviously the city
does not want to go to that expense and stands behind the contract with
their contractor that says he is bonded and insured. Their engineer also
pointed to a similar installation they performed 2 years ago adjacent to a
house, with no problems resulting. As most of the houses in this area have
basements, I think we have an apples and oranges comparison. No soils
reports exist for this area and the city also doesn't want that expense. My
concern also is that even if this gets installed with no problems at the
time, settling could occur 2, 3, or 5 years down the road and it would be
hard/impossible to get any relief at that time, with legal proceedings
almost surely required. Prevention is much better than a cure anytime!
Anyone have any thoughts on ways to convince them to do this the right way?
Or am I being overly conservative?

 

Gary Bunkers P.E.