Repair recommendations for building with shallow footings experiencing subsidence

[pbc825]

I've been contacted to provide some repair recommendations for a building which has experienced shallow footing subsidence caused by an adjacent sub-grade parking structure excavation. The adjacent lot was excavated and the excavation walls were temporarily supported using anchors and reinforced shotcrete in several stages (I think 5). The tenants noticed several symptoms of subsidence and engaged some engineering service providers who engaged a geomatics firm to quantify the lateral and vertical displacement of several reference data. Surveys were performed every few days, and now the adjacent building is completed and lateral and vertical displacement is no longer increasing. So, it's time to fix the tenant's building.

The building is founded on shallow concrete footings with a series of structural-steel portal frames. There's not much to this building. Four gridlines one way and two the other. The displacement does not strain the strucutral steel or connections excessively.

The photographs of the building following subsidence show lots of cracks in the drywall, doors that are unable to function properly, and several large cracks in a concrete SOG floor.

I have two school's of thought on this repair.

1) Really Mike-Holmes it, and, "Make it right!" Recommend some screw piles to return the footings back to their original vertical location, repair/replace sections of the SOG floor (maybe some mud-jacking) and fix the drywall/doors. Some challenges with this. The piles would need to be installed from the interior (I have some experience with this in tight headroom locations), and there's no outside wall access (the new building is inches away).
2) Let sleeping dogs lie, and largely leave it alone. Repair/replace the slab, and fix the doors/drywall.

What am I seeking? Some advice from an engineer who's done this a few times. What's typically done? Any advice would be greatly appreciated.

 

[phamENG]

I haven't had quite your situation - I haven't done much dense commercial and everything outside of the CBD where I am has to be at least several feet apart. That, and nobody ever has that much data.

Even so, I usually recommend #2. What is there to gain by doing #1? You said yourself that the frame isn't being stressed excessively (I'm guessing you modeled it - computer or by hand - with a forced displacement to verify?) and the issues are almost entirely serviceability related. To do #1, the cost is likely going to be hundreds of thousands of dollars more than #2 and you'll have the same functionality when you're done.

The new building owner next door may be footing the bill, but you can bet they'll be picking through the plans and the costs with their own engineer to make sure you're not doing anything excessive.

 

[msquared48]

Kinda depends on the level of quality of the structure. Removing and reporting the slab will not level any floors or walls above.

Mike McCann, PE, SE (WA, HI)

 

[Ron]

Use helical or hydraulic pushed pin piles and bring the foundation back to level. Fix the slab either with epoxy injection or replacement.