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Lateral Earth Pressure on Retaining Wall (Normally or Overconsolidated)

Lateral Earth Pressure on Retaining Wall (Normally or Overconsolidated)

Lateral Earth Pressure on Retaining Wall (Normally or Overconsolidated)

I am currently working on a project that has sparked a lot of discussion over the lateral earth pressure to be applied to below grade foundation walls. A structural engineer has approached us with his foundation design for a building with an unbalanced fill from front to back of approximately 13 feet and will include a walkout basement in its rear. Thus, he has a 13 foot tall retaining wall in the front of the structure that he is designing as a cantilever retaining wall with an at rest earth pressure. He is currently using an estimated at rest pressure of 70 pcf and would like for us to dial in value for his design. Should be simple enough, right?

Well this sparked a huge disagreement in our office about calculating the at rest coeffcient and what is acceptable as industry standards. The particular wall has parking and sidewalls within about 5 feet of the wall, which is easy enough to include as a surcharge load. However, how should the pressure applied by the soils be calculated? The 2 following formulas were discussed:

1) Ko = 1-sin(phi)

2) Ko = 1-sin(phi) + (compacted unit weight/loose unit weight)*5.5

The plan is to use a granular backfill. The discussion is that sidewalks and parking is close to the wall, which we obviously don't want those to settle, the backfill must be compacted to 95% of the standard proctor using a plate tamper. However, this now places the backfill in an overconsolidated state and warrants a larger at rest pressure coefficient. Projects of this nature are very common and I just wanted to get some input from the community on how they handle such design situations. Thanks.

RE: Lateral Earth Pressure on Retaining Wall (Normally or Overconsolidated)

Are you sure the formula listed for #2 above is correct? It implies the at rest earth pressure coefficient will be increased by no less than 5.5.. seeing compacted unit weight is higher than loose.

RE: Lateral Earth Pressure on Retaining Wall (Normally or Overconsolidated)

No, you're right. I typed it in incorrectly.

2) Ko=1-sin(phi) (compacted unit weight/loose unit weight-1)*5.5

RE: Lateral Earth Pressure on Retaining Wall (Normally or Overconsolidated)

I'm not familiar with the approach you have mentioned. However, the below is a standard approach:

K0,OC = (1 - sinφ).OCRsinφ (based on Mayne and Kulhawy, 1982)

When overconsolidation ratio (OCR) equals 1 (i.e. the soil is normally consolidated) the above equation returns to:

K0,NC = 1 - sinφ (based on Jaky, 1948)

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