Existing Masonry Wall and sliding foundation

[vincentpa]

The hillside along the south wall of a big box supermarket is sliding away. The geotechnical engineers are stabilizing the hillside. We are analyzing an existing 12" load-bearing reinforced concrete masonry unit wall that is 20 feet high and is 5.5" out of plumb to determine if the wall is safe. The construction tolerance for the wall is 0.5" for variation from plumb. When we analyze the wall, the stresses fall within the allowable range. However, it just doesn't feel right. A variation from plum of 5.5" in a 20 foot high wall is a lot. I am afraid that the wall will collapse below the allowable stress range. Am I being too cautious? What would you say to the owner? I am thinking of telling him to rebuild the wall even though my analysis shows the wall is understressed.

 

[concretemasonry]

The wall itseld may be designed properly, but it is a cantilever wall. Could the footing be failed or rotating because of the soil?

Dick

 

[vincentpa]

The building is a CMU load-bearing wall designed for vertical loads from the bar joists and wind load. The building walls were designed to be plumb. The building is less than 10 years old and is like any other big box retailer like WalMart or Home Depot; load bearing masonry walls and steel columns, steel joist girders and bar joists. The hillside is sliding away taking the footing and the bottom of the masonry wall with it. I imagine the walls were designed as a pin-pin condition at the roof and footing. Now we have increased moment on the wall and moment from P-Dleta.

 

[JAE]

vincentpa -

A couple of thoughts here:

1. Can the footings be underpinned and jacked back into place? Many times simply re-setting the footings back to their original location will solve the problem. Helical piling can be used, or resistance piling, to underpin the footings and then they can be hydraulically jacked upwards.

If there is lateral sliding of the footings - that is a whole different (and more challenging) problem. For this helical piling can also be used laterally to pin the footing back into the soil.

2. If the wall is out of plumb, is the whole building also out of plumb? If so, then you might have a lot more P[Δ] thrust than you think.

3.

 

[JedClampett]

I would have the owner fix it. They're already fixing the hillside. Maybe their insurance is covering it. But if there's an unexpected high wind or seismic loading, the wall might fail due to the combination of stresses. Or the joists or putting a tension load in the top of the wall that might fail the attachment. The results will be much worse if it happens at some time in the future than fixing it now.

 

[msquared48]

I agree with JAE about the pin pile, but am concerned about the plane of the rotational failure. How far into the building structure did the failure plane penetrate? With the pin pile solution, I am concerned about the lateral unsupported length of the pile above the failure plane. The soil would have to be completely stabilized to avoid loading the pin pile laterally.

HOWEVER, THAT DOES NOT SOLVE THE P DELTA PROBLEM. That's a totally different issue and gets into the repairability of the structure. I would be concerned with the connections at the roof of the structre, checking all of them for structural integrity, even with this small degree of rotation, to include the chord ties.

If this is in a seismiclly active area, I would ask the geotech if there is any danger of liquefaction during a major event, that could lead to more rotation. If so, then rebuild the wall and install pin pile too as a precautionary measure. If not, I would have to ask myself if I want to assume the liability should the wall not be repaired and fail later. I know what my answer would be.

Mike McCann
MMC Engineering

 

[kslee1000]

Agree with JedC. As JAE pointed out, the foundation maybe salvageable, but with the CMU wall on top, it will require design of temperary shoring to hold the wall during remediation. After all that efforts and weighing potential risks, maybe it is better off to rebuild the wall.

One question here, if only the south wall has slided with 5.5" out of plumb (assuming lateral movement occurred on top only), wouldn't be the joists/beams/girders have teared apart, fell off the support seatings, or the wall has cracked below the roof support level? If so, why save the wall?

 

[hokie66]

kslee,

As I read Vincent's problem, it is the bottom of the wall that has moved out, not the top.

Vincent,

I agree with others that the wall should be rebuilt following rectification of the embankment and footing.

 

[kslee1000]

hokie66: yes, you are correct - that makes sense.

 

[vincentpa]

The wall runs parallel to a tall (~60') MSE wall. The MSE wall is only about 15' from the CMU load bearing wall. There have been attempts to stabilize the wall. They have failed. The attempts did not cross the failure plane of the soil which is about 80' into the building. Helical piers and pin piles are not an option to jack the foundation. The geotechs will be using battered pin piles at 2' on center to stabilize the hillside. I don't want to put any more lateral load on the hillside. There are enough problems already.

The wall has moved from the bottom; i.e. the footing has slid with the hillside. I checked the joist bearing connections as well as I could. The joist seats are grouted into the wall. There were no signs of distress at the grout pockets or the surrounding CMU. This is not a seismically active area. But nevertheless, the stresses are within acceptable limits but I am still uncomfortable with this amount of distortion to the original design. Masonry is not steel. It seems most of you are thinking along the same lines as I am.

 

[idecharlotte]

I'm not sure why helical piers are not an option. I've seen this happen numerous times. I put in vertical and diagonal helical piers to restrain the foundation and it worked every time. You don't need to jack the foundation, just prevent it from moving any further. It's about the easiest repair you can do.

http://www.idecharlotte.com

 

[hokie66]

Good luck, Vincent. The problem is actually that retaining wall, and until any movement in the wall and the hillside is prevented, you are in an impossible situation in repairing the building.