How much reduction is to be expected in active earth pressure

[msh69]

Hi everybody,
Cut and cover construction. Excavation is supported by contiguous piles and anchors. I'd like to know whether the active earth pressure acting against the basement wall can be decreased or disregarded in structural design. Thanks in advance.

 

[MotorCity]

What is your justification for decreasing or eliminating the pressure? It doesn't sound like a good idea. The only scenario I can see for disregarding the pressure is if you have equal fill on both sides of the wall. But then its more of a footing instead of a wall.

To summarize, the answer is no.

 

[eric1037]

Do you mean that there is a retaining wall currently supporting the excavation? You say it consist of contiguous piles and anchors. Is this a secant or tangential wall with tiebacks? Was it designed to be permanent or temporary?

There are different design requirements for temporary walls.

At a minimum, if the walls are to remain in place, I think you be able to have a significant reduction in lateral earth pressure. It is quite common to construct external retaining walls to reduce lateral pressures on basement walls.

I guess we need more info at this point.

 

[PEinc]

If the excavation support wall is a temporary system, the basement walls must be designed for the full earth and surcharge pressures. For permanent tiedback walls, the basement is usually constructed up against the tiedback wall, with no backfill between the basement foundation wall and the sheeting. Permanent tiedback soldier beam walls usually have epoxy-coated soldier beams, corrosion protected tieback anchors, some type of wall drainage (such as Miradrain panels), and a premanent facing of cast-in-place or precast concrete. It is also preferable that the building not be rigidly connected to the tiedback wall due to differences in their stiffness. With permanent tiedback walls for a building, the tieback anchors are usually installed without wales that might project into the proposed basement area. Instead of using wales, the tieback anchors are installed between paired channels or wide flange beams so that wales are not needed and so that a permanent facing can be easily attached to the soldier beam wall.

It is a frequent mistake (incorrect assumption?) made by architects and structural engineers to think that because underpinning or temporary sheeting is needed to construct a building, the new building foundation wall doesn't need to be designed for the lateral pressures.

 

[msh69]

Thanks for the replies. Let me give you some more details.
Tiedback walls are temporary. Basement foundation walls were constructed to some extent. In the design, full lateral earth pressure was assumed. Because of some improper applications throughout the construction of basement foundation walls, it was needed to be searched the possibilities of decreasing the lateral earth pressures acting on the basement walls so that the design could still be safe under working loads and any justification for that decrease.

 

[DRC1]

If the tiebcks are temporary, then they are not designed for permenat use and will eventualy transfer load to the structure. Basement walls should be designed for at rest pressures (k=.5 +/-). You may want to consider backfilling with crushed stone. This will reduce lateral soil pressure and provide excelent draiage. You want to pay particular attention to te drainage so that the walls do not develop hydrostatic forces. Most plans show drainage, but many of them are not well thought out or are poorly installed. If lateral pressures are a concern, a working drainage system is vital.

 

[jxzohio]

I totally agree with DRC1's advice. Another thing you may consider is to use light-weight aggregates or slags to backfill the wall. They are good for drainage and also apply less lateral pressure due to their light density.

 

[msh69]

Thanks for all the replies.