Concrete Retaining Walls on Piles 2

[SteynvW]

Hi All

We have this detail of retaining wall on piles floating around in our office.
It is a cantilever retaining wall that is built on a concrete base with beams on the edges.
The moment from the wall is transferred to the beams trough a coupling moment. These beams then
span to a pile cap with the forces (upwards and downwards).

Resistance against sliding is through a network of ground beams not shown.

I can not find fault with this, it is just perhaps a bit more difficult to build.
Has anyone seen something similar?

 

[SlideRuleEra]

Has anyone seen something similar?

No, but that's not surprising. I would never use, nor do I know any other engineer who would use that (complex / expensive) type design. If a retaining wall is to be constructed using piling, just drive steel or concrete sheet piling and be done with it.

http://www.SlideRuleEra.net

 

[milkshakelake]

Why not batter some piles instead of a network of grade beams?

 

[KootK]

1) I've run into and designed pile supported retaining walls in both Alberta and California. It's done.

2) The software package Enercalc actually has a module for designing these things which I've always found interesting. I does not explicitly address what I consider to be the most critical design element with this system however: the pile to retaining wall connection.

3) In my opinion, the pile to retaining wall connection is the trickiest and most difficult aspect of designing these things. The retaining wall "footing" has always been a flat slab element for me rather than discrete beams which makes the transfer of localized pile forces into a distributed wall element a tricky thing, particularly over the limited amount of space that is usually available for this. As much as I hate to defer to it for the design of something so routine, the connection may warrant some strut and tie checking. Or, at the least, explicit consideration of punching shear.

4) While I've never done this using discrete beams as shown in your detail because of construct ability issues related to the complex form work:

a) I've always wanted to as it's obviously awesome in a mechanical sense.

b) I've seen several instances where designers have designed "in-slab" beams in a similar vein to simplify the design.

 

[STrctPono]

Your detail is certainly more difficult to build. I wonder if it wouldn't just be easier to pour the pile cap solid. Do you need 2 rows of piles? Can you get away with 1? Retaining walls are bending elements and do not contain a significant axial load. I don't really see the need for 2 rows of piles. By doing so, you are actually make it worse for yourself since you now have the pile closest to your heel carrying all the vertical weight of the backfill.

The concept of supporting retaining walls on piles is definitely doable. I design pile supported retaining walls quite often but the details I employ are quite different from yours. What you are showing is quite complicated with the use of the torsion elements and grade beams.... When I design cantilever retaining walls on piles I use one line of piles/shafts and a pile cap about 6"-12" larger than the pile diameter. No force couple mechanism, just pure bending from top of wall to point of fixity in shaft. For tall walls, I will incorporate a GRS backfill behind the wall to reduce the static and dynamic earth pressures exerted on the wall. Most times, I will utilize a layer of geofoam to separate the GRS from the concrete wall and to avoid vertically surcharging the pile cap.

I regularly see other Engineer's designs that utilize this 2 shaft force-couple system for bridge abutments and 9 times out of 10 we successfully value engineer the design down to 1 row of shafts.

 

[r13]

I've never seen a design like this, and wonder if there was a specific reason/condition that require it. Note that the contractor would charge more for the work, and for some of the forms are not retrievable.

 

[Brad805]

We have done this many times. We use a buttress at each pile with the bending capacity you need and then size the pile for the bending forces using soil spring analogy. Your detail is not cost effective due to forming time.

 

[dik]

Can you use a larger diameter pile with a single row?

Dik

 

[milkshakelake]

@STrctPono "No force couple mechanism, just pure bending from top of wall to point of fixity in shaft." How do you attain that fixity with one line of piles? Piles are generally pinned.

 

[KootK]

@milk: you moment connect the wall to a torsion member at the bottom and them moment connect the torsion member to the piles which, then, are not pinned at the top. While the economy is undeniable, I prefer the two row system for large scale stuff because:

1) You don't have to rely 100% on soil lateral resistance for overturning resistance and;

2) While the detailing can be done properly (I do it myself occasionally), four times out of five I see the detailing done poorly.

In a sense, the one pile row system in very nearly a sheet pile retaining system and probably ought to be though of in similar terms in many respect. Not that that's a problem if the detailing is done properly.

 

[r13]

For one row of piles, I would consider battered pile. Then, soil-pile interaction could be another headache.

 

[dik]

Generally, but they don't have to be... I've often designed shafts for fairly substantial moments using active and passive soil pressure on the shaft... recently some sports signage enclosures... about 12'x10'x8' up about 12' from grade... on a single shaft... only a possibility... two lines make it a little easier... but not essential.

Dik

 

[milkshakelake]

@KootK: You don't have to rely 100% on soil lateral resistance for overturning resistance

What do you mean by that? How is that different from a 1 row system? I thought that the lateral resistance is done by the network of grade beams suggested by OP.

Edit: @dik "Fairly substantial moment" is different between a sign enclosure and a retaining wall, by about 10 times. I'm wondering about the connection between the pile and footing/pile cap to transfer the moment.

 

[dik]

Dowels... have done it lots of times, but, not for really high moments. If a high moment, then a couple of rows of piles might be in order, with a solid 'blob' of concrete for the transfer footing... sheet piling would be a real consideration depending on the soil.

Dik

 

[STrctPono]

@milk,

I think it's important to differentiate whether we are talking about prestressed piles, auger cast in place piles, or drilled shafts. All of these will have different connection details to the cap. I can't speak for ACIP piles since I have no experience with them. Drilled shaft connection details are pretty straightforward. Extend the vertical reinforcing out the top of the shaft into the cap with enough development on the bars. I'm a big fan of headed bars and spirals at 3" OC in this region to reduce cap height. Prestressed piles are a bit trickier since sometimes the lengths are determined by blow count rejection so are harder to estimate. I typically run additional #7 reinforcing in the top 40 ft and extend it out the top of the pile to achieve the same goal of moment fixity.

I've never designed a top pinned deep foundation but then again I don't design buildings and we don't utilize grade beams.

In a medium to high seismic zone, designing with 2 rows of shafts can be a very uneconomical choice. All the weight of the soil on the heel of the pile cap is now carrying the inertial weight of the soil above. You don't have that situation with 1 row of piles.

I'm surprised with some of the hesitance to utilize soil resistance for lateral loads.

 

[r13]

Pinned piles are mostly used in large pile group, and utilize soil resistance for lateral load. The reinforcing steel are extended into the cap with development length only.

 

[SteynvW]

Thanks for all the replies, but like most things in engineering context plays a big roll.

This project is in a rural town in South Africa quite far from one of our cities.
So the economics works a bit different than the US as our labor is relatively cheap.

In terms of the pilling itself, how the setup works here is the pilling contractor is
appointed at the last second and it is a design and supply contract with the forces that
the pile should resist supplied by the structural engineer. This implies we don't always now
even what pile type will be used up to just before construction.

So it is possible to install piles transferring moment but for simplicity and risk generally
we only rely on vertical resistance from the piles and design our structure around this.

The alternative to work around the above mentioned is to have a torsion element at the footing of the wall
and at the singe pile transfer the torsion into a ground beam as a moment running in the transverse direction, but the
problem with that is that we don't know the pile stiffness and don't know how much of moment will be transferred to
the pile and how much to the ground beam.

Or can one reason that only vertical forces will be taken up by the pile and size your ground beam to take
the full moment, this will thus increase the stiffness of the beam and the rotation of the pile will be minimal?

 

[r13]

Do you have soil report, or you just think you need the piles, I don't quite understand your ground beam approaches. Can you present something on paper and upload it for discussion?

 

[SteynvW]

They are doing the investigation later this week, but preliminary information indicated the requirement for piles.

Attached is a sketch that should explain the approach.