Drilled pier with compressible foam in pier tip?

[dirtwoman]

I am the geotech for a project with a retaining wall that will be supported by drilled piers. The fine-grained soil gets stiffer with depth. The maximum depth of the drilled pier is 6 feet and the maximum wall height is almost 8 feet. This is a redesign of a retaining wall with a keyway. Currently, the wall is vertical above the grade. Below grade, the wall has a "shallow" foundation that varies in width to a maximum of 7.5 feet. The 2 foot diameter drilled pier is connected to the uphill side of the wall, 2 inches from the footing edge. The strucutral engineer designed the wall using the coefficient of friction along the "shallow" portion of the foundation. Because the soil becomes stiffer with depth, I am concerned that the friction along the "shallow" portion would not always be available. The structural proposes to use about 6 inches of compressible foam material in the tip of the drilled pier so that the stiffer soil would not be engaged during loading. Is this a reasonable solution? Thanks for any help! This project is under time constraints. (as usual)

 

[born2drill]

Can you post a sketch? It's not very clear what the existing conditions are, what the original design is, and what you are proposing to redesign (and why).

 

[dirtwoman]

Here is a rough sketch of the existing and proposed conditions. The keyway is not relevant at this point.

We are considering requesting that the structural engineer redesign the wall without using the friction along the "shallow" foundation because that friction would not be available long term. The load would transfer would be to the stiffer soil below.

The structural engineer is proposing to place compressible foam at the base of the drilled pier so that the load would not transfer to the stiffer soil along the base of the pier.

Thanks.

 

[emmgjld]

The purpose of the foam is not clear!!!
Is this an attempt to deal with expansive clays?

 

[dirtwoman]

No, no expansive soils. The purpose of the foam is to act as a buffer between the stiffer native soil and the base of the drilled pier to keep from transfering lateral loads to the base of the pier. Axial loading is not an issue for this condition.

 

[born2drill]

I'm still unclear on what's existing. What is there right now? A wall? A slope? Something else?

If you are installing this drilled shaft and loading it laterally at subgrade you have a condition just like a soldier pile toe, which means the shaft will resist the load by developing passive pressure over its length. What is foam at the bottom going to do? And why wouldn't you want to resist the lateral loads with the stiffer soils? Otherwise why are you drilling a shaft in the first place?

 

[dirtwoman]

There is nothing there except a slope. The existing slope will be cut into and retained with the proposed wall attached to make room for a basement addition.

The project plans are currently in review. The City reviewer took exception to the use of frictional resistance along the "shallow" portion of the footing because of the stiffer material below. I agreed with the City reviewer and he notified the structural engineer.

This wall design is the 3rd design for this project. The drilled shafts are needed to resist the wall loads. Passive pressure on the pier is being used by the structural and I want the shaft to function normally in the passive region.

I spoke with the structural engineer this morning. He needs to completely redesign the wall if he can't use the frictional resistance on the "shallow" portion of the wall. He proposed placing 6 inches of compressible foam in the bottom of the drilled pier to keep the lateral loads from reaching the stiffer soil down there. Hw would still need some redesign if I allow his proposal, it would not be as dramatic.

I am trying to find a technical reason to allow or disallow his proposal.

 

[InDepth]

Case 1: Are you having the pile resist all the load? To me this would be the best retrofit to the existing wall.

Case 2: Are you having the pile only resist uplift loads (and having the footing resist downward bearing and lateral through passive pressure)? There may be stiffness compatibility in this case.

If you say that the pile is to resist all the loads, then I don't think the foam gives you anything, especially if the pile is suppose to resist lateral anyway (Not understanding why you want to prevent lateral resistance of the toe). From what I can see you have a "free head" condition with 1) an applied moment at the top of the pile, and 2) an applied lateral load at the top of the pile. Vertical axial load should be minimal (just the vertical weight of the soil above the footing).

I would say run a pile analysis with these loads and assume the upper mid-stiff silt doesn't contribute to the lateral resistance. Of course ask the structural for a preliminary reinforcing and assumed pile stiffness (cracked stiffness). You could probably use L-Pile.

I guess, the foam could be a solution. Usually 2-3" of soil is uncompacted due to the drilling process. I think part of your geotechnical calculations should include an expected settlement for load on the stiff soil....so the foam or whatever you stuff down can be sized (usually stick foam to bottom of rebar cage.

 

[dirtwoman]

Thanks for your input.

This wall is not constructed yet.

Case 1: The structural designed the wall as a unit. I did not design the drilled piers. It appears that the structural is using the frictional components from the bottom of the "shallow" foundation and the bottom of the drilled pier. He is using passive pressure to resist movement along the front of the "shallow" footing and the front of the drilled pier.

Case 2: The way he designed it, the "shallow" foundation and drilled pier are providing both passive resistance and frictional resistance.

 

[FixedEarth]

Often when we have pier supported retaining wall, the footing is designed as a pier cap with "butcher hooks" reinforcement connecting stem reinforcement and pier rebar. This type of pier cap is designed to span between piers and to transfer torsion to the drilled pier. Not sure why your footing is so large when you are using a drilled pier?

From a soils standpoint, the skin friction is responsible for the geotechnical capacity. The foam at the pier bottom will not be engaged until the drilled pier has settled approximately 4% of the diameter (one inch settlement in this case.)

By the way, 6 ft embedment seems too short for 8 ft high retaining wall, unless your pier spacing is very close like 2B. Also we usually ignore some Y distance of the upper soils.

Go to SteelTools.org and get POLE spreadsheet by Tomanovich. Get the loadings from the S.E. and take your soils values and use the TENG cohesive method to come up with your embedment depth. If you want lateral deflection and maximum moment induced in the drilled pier, try Lateral Foundation software.

Your embedment will be closer to 18-20 ft for 7 ft pier spacing and your given soils. The reviewer is correct. You also would need to consider slope creep force is you are close to the slope crest, minimum daylight distance and the +/- 1% vertical reinforcement.

http://www.FoundEng.com

 

[born2drill]

If it were my money, I'd want to drill in soldier beams from the top, lag it down to subgrade, and attach a permanent concrete facing.

 

[dirtwoman]

Thanks for the response. I will check out those programs.

I think the reason that the piers are short and footing so big is that he needs that much frictional resistance of the bottom of the footing.

 

[FixedEarth]

Then, it may be more efficient to design the retaining wall as a shallow foundation with a deep keyway. So you keep the 7 ft wide footing and the Structural can add say 18" wide by 48" deep keyway.

This way he can utilize friction at the base of footing + passive in the keyway. You will need about 2 ksf bearing capacity. You need this book:

http://www.hbapublications.com/

http://www.FoundEng.com

 

[BigH]

Another thought - I understand, from your sketch that the slope exists and you want to put in a basement into the slope thereby cutting into the slope. So, how do you plan to support the excavation into the slope in order to put in the foundations. - regardless what you use? Certainly you are not going to try to make a vertical cut into the slope.

I would consider using soldier piles and concrete beam lagging for the excavation support - then put on a finished face. Or use soil nails to support the slope and then use a shotcrete facing to form the finished wall surface or a combination shotcrete and a placed wall. You may have to go a bit deeper with the nails and then backfill to bring up to underside of basement floor.

I think I understand this correctly . . .

 

[dirtwoman]

The wall was previously designed with a keyway, but the local code requires that the both the active and passive pressures be applied to the key. This wall also needed to be designed to resist water pressure, too. The structural informed me that wall components were too big. I gave him a couple of options and he selected drilled piers. The water pressure doesn't come into play for the piers.

I tried out the spreadsheet from SteelTools.org. The pile lengths calculated are definitely longer than 6 feet.

Soldier piles and lagging could work. Equipment access would be challenging.

 

[dirtwoman]

The soil is has enough cohesion to be cut at a fairly steep angle and should be stable for the short term.

In reading the responses, I am interpreting that some of the assumptions the structural made to complete the wall calculations are not commonly used for pier supported retaining walls (i.e. frictional resistance on the base of the "shallow" foundation).

I am still struggling with the question of "Are there technical reasons to allow/disallow the proposed structural design with the foam at the base of the drilled pier?"

The reason the structural is proposing isolating the base of the pier would be to keep the frictional resistance acting at the bottom of the "shallow" foundation. Given the stiffer underlying materials, load transfer would occur to the stiffer underlying soils. In addition, some or all of the assumed frictional resistance would dissappear as deflection occurs. Theoretically, this would not occur if foam was placed in the base of the pier.

 

[BigH]

As this is clay - and you are speaking of adhesion, really, not friction - check out Tomlinson's charts for adhesion factors - they have values for soft over stiff. Check out the capacity in this fashion. As indicated, you will need to check out the lateral capacity of the drilled piers - don't know why soldier piles and lagging has any different equipment (for the soldier piles) than would the drilled piers (bored shafts). I'm still at a loss for why you need the cushion at the bottom. Load transfer is such - as pointed out earlier - that the bottom takes very little load until sufficient movement of the pile takes place. So if you are basically designing for axial loading to be at or slightly less than the axial adhesion, then you will get very little loading at the base anyway.

 

[kieran1]

If the slope is stable during excavations, would it not be more economic to over-excavate and provide a toe on the wall. The weight of the backfill could then resist sliding.

Kieran

 

[dirtwoman]

The silt material in the area acts like a frictional material for design purposes. For excavation, the material acts like a cohesive material for a short time.

 

[molerat2210]

If you are going through with all the trouble of drilled piers then why not have a fully pier supported wall. This eliminates the design problem and only need to worry about deflection and moment in the peirs. A thorough analysis of the current design is not possible since it combines the shallow and deep foundations in one element.

Or, go with soldier pile wall with concrete lagging (as suggested above).

Another option: tieback anchors and segmental block. I believe Tensar can provide guidance on the connections.

The current system may need temporary excavation support or adequate room to slope back.