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# Passive friction for soldier piles

## Passive friction for soldier piles

(OP)
Hi all

I have been engaged to design 4km of soldier pile wall ranging from 0.2m to 2.4m high. Due to the length of the wall I would like to make sure I am not being unnecessarily conservative in the design. The walls will be founded in class 1 clay fill in a new residential estate. The typical design is a steel UB post encased in 450 dia concrete at 2.4m centres. Concrete panels span between the posts. I am calculating 1.6m embedment for a 1.2m wall, and 2.6m for a 1.8m high wall, but believe this can be reduced. The geotech has provided long term design values of density = 20kN/m3, c'=0, phi =30 deg.

• I have taken the friction angle of 0.66*phi into account to reduce the active pressure on the concrete lagging. Is it possible to take the friction into account for the passive pressure and if so over what width would it act? Assuming friction between pile and soil of 15 deg, kp = 4.3. Assuming an effective width of 3*pile dia this is an effective kp of 12.9. Is this friction already taken into account in Broms kp factor of 3?
• Does anyone have any opinion on the suitability of Broms for small retaining walls? I have used Broms assuming rotation about the base for the walls up to 1.4m high and then used the traditional limiting equilibrium method, calculating the actual rotation point for the higher walls (however still using an effective width of 3x dia as opposed to 0.08*phi).
• Some publications suggest ignoring the active pressure behind the pile, some say it acts over the pile width only and others suggest it acts over the full effective width of up to 3 x dia. I have assumed it acts over 1 x dia for walls over 2m and ignored for lower walls.
• Does anyone have any thoughts on phi of 30 deg and c'= 0 for stiff clay? I assume that phi is a bit high but accounts for any long term cohesion that may be present.

### RE: Passive friction for soldier piles

There are more qualified members than I to answer this, but here are my thoughts:

#### Quote (TrisM)

• I have taken the friction angle of 0.66*phi into account to reduce the active pressure on the concrete lagging. Is it possible to take the friction into account for the passive pressure and if so over what width would it act? Assuming friction between pile and soil of 15 deg, kp = 4.3. Assuming an effective width of 3*pile dia this is an effective kp of 12.9. Is this friction already taken into account in Broms kp factor of 3?
If I were considering this friction I would probably assume that it only acts over the actual pile width. I have never read that the effective width already considers friction. However, I can't remember seeing an example where they use the effective width and friction either.

#### Quote (TrisM)

• Does anyone have any opinion on the suitability of Broms for small retaining walls? I have used Broms assuming rotation about the base for the walls up to 1.4m high and then used the traditional limiting equilibrium method, calculating the actual rotation point for the higher walls (however still using an effective width of 3x dia as opposed to 0.08*phi).
To me it they are basically the same approach, no? You are summing forces on either side of the pile to check stability. I have seen both methods used for short walls. Do you get different results?

#### Quote (TrisM)

• Some publications suggest ignoring the active pressure behind the pile, some say it acts over the pile width only and others suggest it acts over the full effective width of up to 3 x dia. I have assumed it acts over 1 x dia for walls over 2m and ignored for lower walls.
I usually see that the active pressure is considered to act on 1 x dia. That is probably what I would do.

#### Quote (TrisM)

• Does anyone have any thoughts on phi of 30 deg and c'= 0 for stiff clay? I assume that phi is a bit high but accounts for any long term cohesion that may be present.
Your assumption seems correct based on what I have seen from geotech reports, but I am no geotech.

### RE: Passive friction for soldier piles

(OP)
Thanks RFreund

I think I will ignore the friction between the pile and the soil then.

My understanding of Broms original theory for short piles in cohesionless soils is that the point of rotation is about the base.

For the equilibrium method the point of rotation is calculated to balance horizontal forces, resulting it being located at a depth of around 80-85% of the total pile depth.

Would you agree?

### RE: Passive friction for soldier piles

I have some thought on the phi of 30deg and C = 0 for stiff clay.

The friction angle does seem high for a clay, though I know not its origin, and increasing the friction to account for long-term cohesion may not be the best idea in a residential clay fill.

If a phi of 30deg is the basis for your design, I would avoid over-refining other elements to reduce your embedment.

### RE: Passive friction for soldier piles

It would appear that the backfill is fully drained and there is a level surface front and behind. If saturation occurs and the backfill is sloped, soil in front is not level, your numbers may be way off.

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