Underpinning Existing Adjacent Party walls 2

[MNBarch]

We are about to build a new four story infill house between 2 three story philadelphia row houses. The new basement is below the existing basements of each of the adjacent properties. Their exterior walls are 8" brick with 16" stone rubble foundations. These walls are load bearing and have floor joists pocketed into them at each level. The construction will require underpinning of about 3' below the existing adjacent foundations. Our engineer shows the underpinning to extend 1' below the footing of the new structure and max underpin size to be 4'. Should the underpins be smaller at the corners? Will the taller new structure impose uneven loading at the party wall causing settlement of the undrepins? Should the underpins be tied together with rebar? what would be the advantage or disadvantage of rebar since there would be alot of tied joints? Does anyone have any tips or precautions to share? this will be our company's first attempt at underpinning?

 

[JAE]

The larger the new footings are, beneath either the new structure or a common wall, the better as you "soften" the loading on the underlying soils, reducing the pressure as much as possible. This will tend to minimize the settlements that will occur.

Underpinnning also is best done sequentially, completing small segments along the existing wall so as not to undermine the entire existing footing.

Both the new and the existing structures will be supported on common footings so anything you can do to minimize settlements, and get the two structures to work together is best. If this means doweling the two together to create shear friction connections, then do it....but all this should be considered by your engineer. It is the engineer's job to ensure that the final structure will work, and that a proper sequencing of the work is outlined.

Engineers are not responsible for the means and methods of how the work is done, but the sequence sometimes is critical and this should be developed by the engineer.

Finally, a new four story between two three stories - in Philadelphia. Has your engineer considered the new snow drift loads that will occur on the existing roofs?

 

[PEinc]

I've designed and constructed many underpinning jobs in Philadelphia, For Philly buildings with rubble stone foundations, 4' is pretty typical for pier widths. Going 1' below the bottom of the new footing is not unreasonable. You should not need smaller piers at the corners (I assume you mean each end of the existing wall).

Since the existing walls are multi-story party walls, they are tall and heavy. Therefore, you will probably need continuous underpinning piers for the length of each party wall. Another reason for continuous piers is that you are not going very far below the existing rubble stone wall with your new footings. Therefore, your piers will probably be bearing in the same type of soil as the existing rubble stone walls.

I expect that your new building will have its own footings and columns and that the new floor slabs and roof will be supported on their own framing system, independent of the existing structures. If so, you will actually be reducing the loads on the existing walls - except possibly for the addition of drifted snow as JAE mentioned above.

Usually, adjacent underpinning piers do not have or need connecting rebar dowels or formed shear keyways. However, in Philadelphia, one particular structural engineering firm always insists that dowels and or shear keys be installed between piers. This engineer also insists that pressure treated lumber be used for all pit shoring and lagging boards. Reinforcing steel is almost never used in underpinning. The stresses in the underpinning concrete are usually very low and rebars make it hard to dig the pits and allow a worker to enter a deeper pit. Underpinning pits are also usually hand excavated, especially when underpinning rubble stone walls.

Philadelphia has at least three experienced underpinning contractors working there - Moretrench Geotec, D'Angelo Bros., and Schnabel Foundation Company.

Make sure that you do a pre-construction survey of the existing adjacent structures before starting any excavation or other work. Also, monitor on a regular basis these adjacent structures so that you know how much movement they may be experiencing. This will also be very important if the buildings see any damage during the work.

Finally, don't attempt the underpinning unless you have an experienced underpinning crew. Many contractors say they do underpinning, but there are very few who really know what they are doing. The rest just dabble in shallow underpinning and are accidents waiting to happen , especially with deeper underpinning projects. The company doing the underpinning should be doing the pit excavation and shoring, pouring the pier concrete, and doing the drypacking - in other words, the whole works. Underpinning is not work to be parcelled out to several different contractors. When this happens, no one has full responsibility - or authority.

 

[JAE]

PEinc - nice post.

 

[PEinc]

Thanks, JAE. I'd like to know who MNBarch works for and who the project engineers and CM are.