Monday, November 12, 2012

Basement lateral pressure

Basement lateral pressure, backfill pressure, is a tricky subject. Most books say it is Ka times the vertical, and I believe that true, right after construction. Over time I am not so sure. With high plastic clays, dessication - wetting cycles, freeze thaw cycles, for shallow fills, I think the residual stress dissipate, and the wall lateral pressure increasing over time to Ko. This is not what some of the books say. I think this condition takes 30 to 40 years to develop. Are there any long term lateral pressure studies on house basements, high plastic soils, deep frost penetration? none yet found.

After the wall has failed, the lateral repair must be stronger, because prior to failure, there was a diaphragm moment due to the end walls. During failure, the end cracks relieve this moment, hence all the lateral moment must now be taken by the vertical stiffness of the wall.

When we add potential swelling pressure and potential freeze expansion pressures, the lateral pressure becomes a large transient value. The upper bound of the pressure would be Kp; however, Kp exceeds the typical probable pressure. To use Kp for design would be excessive.

There are areas of the wall that have no lateral support, either through wall openings or through no floor diaphragm as dropped floors and stairs parallel to the wall. These may need abutments or counterbalancing exterior stairs or steps. 

Epoxy injection can reach the strength of the previous wall; however, to do that, all the cracks would need to have 100 percent penetration. The other issue is that after the wall cracked, the stresses were relieved. After epoxy injection, the critical failure start point will have changed. This results in loss of effective strength. The corners have loss the deflection necessary to develop the support it had before.

I think the only way is to do a structural design accounting for all the loss of strength, using a Ko = 1.0, plus a live load for all construction equipment, and reconstruct the wall or equivalent repair. Outside stiffeners can be used, but they must account for the actual openings in the wall and diaphragm. Typical spacing may not be adequate. There will now be no (very little) support from the end walls.

Over the years these are the methods I have seen utilized are as follow:

  1. Shotcrete reinforced wall inside
  2. Concrete abutment inside
  3. Reinforced shotcrete wall outside
  4. Wall replacement with proper reinforced design accounting for openings. One required an inside abutment.
  5. PWF inside wall, without plywood
  6. PWF inside wall with an plywood abutment under the beam. All the PWF were inset into the floor to existing foundations. New foundations may also be required. Bottom and top lateral support is required.
  7. Exterior vertical stiffeners, back to back channel bolted through the wall to bring the wall back into line.
All these methods require the wall to be excavated full length and depth, exterior water proofing, good condition weeping tile to provide drainage, and some kind of vertical water drainage is recommended on the exterior wall face.

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