Drilled pier with compressible foam in pier tip?
Drilled pier with compressible foam in pier tip?
(OP)
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)





RE: Drilled pier with compressible foam in pier tip?
RE: Drilled pier with compressible foam in pier tip?
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.
RE: Drilled pier with compressible foam in pier tip?
Is this an attempt to deal with expansive clays?
RE: Drilled pier with compressible foam in pier tip?
RE: Drilled pier with compressible foam in pier tip?
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?
RE: Drilled pier with compressible foam in pier tip?
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.
RE: Drilled pier with compressible foam in pier tip?
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.
RE: Drilled pier with compressible foam in pier tip?
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.
RE: Drilled pier with compressible foam in pier tip?
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.
www.FoundEng.com
RE: Drilled pier with compressible foam in pier tip?
RE: Drilled pier with compressible foam in pier tip?
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.
RE: Drilled pier with compressible foam in pier tip?
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/
www.FoundEng.com
RE: Drilled pier with compressible foam in pier tip?
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 . . .
RE: Drilled pier with compressible foam in pier tip?
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.
RE: Drilled pier with compressible foam in pier tip?
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.
RE: Drilled pier with compressible foam in pier tip?
RE: Drilled pier with compressible foam in pier tip?
Kieran
RE: Drilled pier with compressible foam in pier tip?
RE: Drilled pier with compressible foam in pier tip?
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.
RE: Drilled pier with compressible foam in pier tip?