Surcharge load is 1.2m away from retaining wall

[tngv752]

I try to design a retaining wall with surcharge load of 80Kpa. The surcharge load is 1.2m away from the wall and the wall is 1.8m high. The base is 400 thick, the key is 400 high.

When I check the sliding, I end up the base of 3.5m long when I use horizontal load due to 80KPa surcharge applied to the full hieght of retaining wall.

My question is that I am right when I applied the full horizontal load due to 80KPa surcharge applied on the full height of the wall or I should project the line of 45 degree under surchagre load and apply the horizontal load which start from the intersection of the line and wall to the base. Therefore, I can end up the base to 2.3m (small sliding load now)

 

[msquared48]

Project the 45 degree line down.

If the width of the base is driven by the load to the heel to resist sliding, consider putting a vertical key below the footing to help take the lateral.

Mike McCann
MMC Engineering

 

[Lion06]

I think I would actually project a failure line that corresponds to the failure slope of the soil under consideration.

 

[tngv752]

The surcharge load is not small therefore I have to be careful to do the right calculation.

The hard thing is when using Boussinesq, you can project 45 degree line down and applied the lateral loading below that. This method is suggested from a senior structural engineer.

Another method from US Army Corps of Engineers "Retaining and Flood Walls", we use full lateral pressure applied to the wall. Thus is suggested from a senior geotechnical engineer.

 

[DRC1]

Bousinesq is appropriate, or Terzaghi has a simplified method using m & n exponents. The 45 degree line is not at all applicable. It is used to determine the influence of a higher footin on a lower footing, but has some how been extrapolated to surcharge loads in all kinds of wierd misapplications. Using the full lateral pressure is applicable to a continuous surcharge starting at the wall and continuing behind the wall some distance. For your situation, it will be very conservative. The solution forms that exist depend on the distribution of the surcharge. The complex shape of the Boussinesq curve can be simplified to two triangles or a trapiziod to aid computations.

 

[STEVE97]

If the surcharge load is applied within a distance of the failure influence line it must be applied to the wall. The failure line is based on the shear stress of the soil and is assumed to start at the bottom of the footing and extend up to grade at a slope of (45 - phi/2).

 

[tngv752]

Retaining wall of 1.8m high and 300 thick base.
Surcharge load of 80KPa is 1.2m away from the wall give lateral loading of 21KN using Bousinesq and 27KN using Tezaghi.

I think Bousinesq method is a common way using of the engineer. Tezaghi which is suggested by geotechincal engineer, is likely more accurate to use.