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Pile suitable for bouldery, gravelly soil

Pile suitable for bouldery, gravelly soil

Pile suitable for bouldery, gravelly soil

(OP)
I’m one of the structural engineers for a project in California, where piles are needed to anchor a concrete “protective structure” subject to large, somewhat dynamic lateral loads on the order of hundreds of kips due to mudslide. I have a soil boring that shows bouldery, gravelly silt conditions, but I’ve been told by the folk who bid the job that there will be no budget for a geotechnical consultant. I have 2 weeks to come up with a design, and I have limited experience with pile supported structures.

Since this is an extremely affluent, densely populated area, I suspect that the noise and vibration of driven piles could open a political can of worms, and I’m sure the bouldery soil would preclude them anyways.

Can anyone recommend a type of pile with high lateral resistance, constructible in these soil conditions, and without high levels of noise and vibration that the local residents will tolerate?

RE: Pile suitable for bouldery, gravelly soil

I'd look at some type of tie-back whereby the drill can core through boulders and then the full disturbed zone is filled with grout. Drill at various angles to take care of the thrust. These tie-back rods can be pre-tested to be sure of the capabilities needed.

RE: Pile suitable for bouldery, gravelly soil

(OP)
Unfortunately, the structure must remain within a 16 ft wide easement parallel to an existing bridge. I’m not sure if tie backs would work with the site topography or be allowed to extend outside of the easement. The structure will basically be buried in the bed of a gully that becomes a raging river of mud and boulders (up to 20 ft in diameter) in the 200 yr mudslide event. The loading scenario is that scour on the upstream side of the structure will leave the vertical face exposed, leading to lateral pressure from the flow and impact from the mud-borne boulders.

RE: Pile suitable for bouldery, gravelly soil

I'm talking about drilling near vertically, to in-effect tie it down. If there is a scour potential, of course your concrete structure goes to that depth and the ties go down from there. You stay within the ROW.

RE: Pile suitable for bouldery, gravelly soil

Quote (bones206)

1) I have a soil boring that shows bouldery, gravelly silt conditions...
...I’m sure the bouldery soil would preclude them anyways.

2) I suspect that the noise and vibration of driven piles could open a political can of worms...
...without high levels of noise and vibration that the local residents will tolerate?...

1) If conditions are as bad as you assume... doubt a typical boring could have been made. Maybe a few question about the field details encountered during the boring would shed more light on soil conditions.

2) I disagree with using "noise level" as the primary criteria to dictate design. As a former bridge contractor, sometime driving piling in residential areas, I can tell you first-hand that "noise" is only one factor... probably NOT the most important one, either.

Time of day for the "noise" is important. Say 9am to 5pm, as apposed to sunup to sundown.
Which days of the week, too. Monday thru Friday, ok. Weekends, no.

The most important is duration of the work. If the Contractor gets "in" and "out" on schedule - great. The worst is to have work "drag" on for what seems like a long, long time.

If driven pile are "best" (IMHO, probably are considering the description), there are steps that can be taken to minimize pile driving noise. You will have to work with a qualified pile driver to determine the details, but the following is possible:

A. Select a type pile hammer that is optimum for the piling and condition.

B. Within that type hammer, select one with optimum energy rating AND geometry.

As an example, say that a single-acting impact hammer is determined to be best. Also, an energy rating of 22,000 to 25,000 foot-pounds is optimum. Then, select, perhaps, a Vulcan 0 (24,400 ft-lb) versus a MKT DE-30 (22,400 ft-lb). The deciding factor (to reduce noise) is the impact velocity of the ram on the anvil (pile). A Vulcan 0 has a 7500 lb ram falling 39 inches. The MKT DE-30, 2800 lb, ram falling 96 inches. Lower impact velocity for the Vulcan.

The lower the impact velocity the more efficient energy is transferred from the ram to the pile... more efficient, less noise.

Use a proper size hammer... more efficient advancing the pile rather than "bouncing" the pile of the rock and rebounding the ram... advance the pile, less noise.

The funny thing is, with this level of attention to detail, the pile driving (if it is appropriate for this project) will go more smoothly, quickly, (relatively) quietly and cost effectively.

If you do decide to go with driven piling, after the pile driver has mobilized, have them drive a few "index piling" to determine if the proper hammer has been selected... sort of a DIY geotechnical investigation for a pile driver.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Pile suitable for bouldery, gravelly soil

(OP)
OG - I get what you are saying now. I think the scheme you are describing with the tie downs is similar in concept to this scheme for stabilizing a dam. If so, I like that idea and will definitely pursue it further, thank you.

SRE - I certainly will be pressing for more information in the coming week. I wasn’t on the original proposal team and I’m just joining on now from another office on the east coast. I’ve seen one site photo which was taken post-mudslide, and visually it looks to be 50% boulders and cobbles and 50% dirt. If steel piles are feasible from a constructabilty standpoint, then I won’t disqualify them solely based on my cynical concerns about the neighbors.

RE: Pile suitable for bouldery, gravelly soil

Pre-bored grouted piles?

RE: Pile suitable for bouldery, gravelly soil

Quote (bones206)

...the structure must remain within a 16 ft wide easement parallel to an existing bridge.

See if the plans and possibly some construction records for the existing bridge are available (probably are if it is a DOT bridge). That information will give a good idea what is possible on this site.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Pile suitable for bouldery, gravelly soil

In response to the OP -
Have you explained to your client that it is s very bad idea to not hire a geotechnical consultant?
It surprises me that you would be ok taking on this liability in a field you're not knowledgeable in. I have some experience in deep foundation design but would never go along with no geotechnical study scenario. If anything were to go wrong and everybody got sued, you would be the first one to loose the case and most likely your license.
I would advise you to press harder for a geotechnical study to be done and document that you did.
Good luck.

RE: Pile suitable for bouldery, gravelly soil

(OP)
Mjkkb2, I totally agree with you. Unfortunately, my boss does not see things that way. I was brought on at a late stage to this project, and have been pushing for a geotech in every meeting since I joined the team and in several emails. There are several layers of management between the client and myself.

RE: Pile suitable for bouldery, gravelly soil

I understand how it is. So who's stamping and signing the drawings? Are you personally doing it or is your boss stamping the documents? I think I would push for someone above me to take the liability by stamping the drawings. I have a sneaky feeling the "boss" may see your point if his name is on the line...
The other ting (which I believe you are doing) is to built a large safety factor into the design and have the client pay the price. It always amazes me that the owners/developers or what not are willing to save a few bucks but not paying a geotech but don't realize they may be paying much more for their foundation system.
I am a big proponent of always involving a geotechnical engineer, worth the money most every time...(And I don't say it only because I am married to one :))



RE: Pile suitable for bouldery, gravelly soil

(OP)
Had a conference call with the client last night and their construction guy said H-piles would be next to impossible to drive in their soil conditions. I had expected that and presented an alternative option utilizing tie-down anchors inspired by oldestguy's suggestion. They liked the idea and I even got some traction on the idea of hiring a geotech consultant to design the anchors. Thanks OG!

RE: Pile suitable for bouldery, gravelly soil

Tiedown anchors are good for resisting tension loads but not lateral or compressive loads. You might want to consider micropiles.

www.PeirceEngineering.com

RE: Pile suitable for bouldery, gravelly soil

Sounds like typical southern California sand/silt conglomerate with cobbles. Can I inquire where this is being bid? I understand it is near an existing bridge near an affluent area.
Thanks.

RE: Pile suitable for bouldery, gravelly soil

(OP)
PEinc - Yes, the tie-down scheme intends to resist the impact by putting the tie downs in tension. It is similar in concept to the gravity dam stabilization scheme that I linked to in my 18 Mar 01:56 post.

BUGGAR - It is in the Santa Barbara area.

RE: Pile suitable for bouldery, gravelly soil

(OP)
Anyone have any experience in this area and have a ballpark idea of what kind of tension capacities can be developed in this soil? I'm looking at using pretensioned DYWIDAG threadbar anchors in hopes of developing some high pretty huge hold-down forces.

RE: Pile suitable for bouldery, gravelly soil

Check FHWA soil nail and micropile design publications or the Post-Tensioning Institute's Recommendations for Soil and Rock Anchors. Also, check PennDOT's Appendix O in their Design Manual 4. All have ultimate grout to soil and rock bond stresses. Also, if you are designing permanent ground anchors, you better learn about corrosion protection, testing, lock-off loads, etc. Based on your questions, I strongly believe you should get help from, or delegate the design to, an engineer who is very experienced in ground anchor design. Also, you should speak to a contractor who specializes in high capacity, ground anchor design and construction. The bigger cost could be in access and the need for specialized equipment.

www.PeirceEngineering.com

RE: Pile suitable for bouldery, gravelly soil

"I’ve been told by the folk who bid the job that there will be no budget for a geotechnical consultant."

Recent weather related geological events in this area render it highly unlikely the job will proceed without a geotech. We have pockets of corrosive soils up there as well.

RE: Pile suitable for bouldery, gravelly soil

(OP)
Thank you everybody for your input.

Yes, those weather related geological events are the reason for this project. The decision not to have a geotech on board is out of my control. I'm not stamping the drawings and have no intention of taking on the responsibility of the geotech portion of the design.

My scope is to help design the superstructure, so I'm just trying to determine what type of pile/caisson/tie-down is feasible in these soil conditions and get some ballpark capacities so I can layout and design the rest of the structure accordingly. The client wants shovels in the ground next week and I'm still doing conceptual design.


RE: Pile suitable for bouldery, gravelly soil

May I inquire about your company? I presume you are a sub? Who would the General be?
Thank you.

RE: Pile suitable for bouldery, gravelly soil

I'll echo the sentiments of others here, that designing slide mitigation structures is a specialized area of practice that should not be attempted by someone who does not have the expertise. I've been designing bridges for 16 years, including numerous foundations, but I would not go near something like what you describe.

If you feel you must come up with something, I'll second Okiryu's suggestion to consider prebored and grouted piles. That, or just regular drilled shafts would solve the noise issue. If you have info on the depth of the slide and the soil properties, you can estimate the soil pressures. I would recommend being very conservative with that estimate. Just as an example, we had one of those consultants who specializes in slope stability and slide mitigation recently use a value of 5 times the passive resistance coefficient, Kp to estimate loads for a drilled shaft group (coupled using a CIP concrete cap) protecting bridge piers in a slide area. If you have enough information to estimate the loads, a program such as Lpile or Allpile can analyze the lateral geotechnical resistance and bending moment.

RE: Pile suitable for bouldery, gravelly soil

(OP)
BUGGAR - I'd rather not disclose who I work for. I'd probably be violating some kind of corporate policy and receive a nastygram from someone in the PR department. I work for an EPC firm.

In general, I agree with everyone's advice about not designing a specialized structure without having the requisite specific experience. I was brought on to this job mainly to perform dynamic impact analysis, which I do have a fair amount of experience with. A pile/anchor type had not been selected for the conceptual design, which I needed to include in the model to try to capture the overall force-displacement behavior. Since I was told there would be no geotechnical assistance, I posed my question here in hopes of at least narrowing down the most feasible type of pile.

I had initially looked at driven H-piles, generated p-y curves in L-PILE based on sand (conservative for displacement) and had reasonable looking results in the dynamic analysis. My hope was to take that proof-of-concept analysis and refine it with geotech-provided parameters at a later date, or if that never happened, put huge disclaimers on the results and let my management take it for what it's worth.

RE: Pile suitable for bouldery, gravelly soil

"Since I was told there would be no geotechnical assistance..."

Seismic conditions....mudslide...I think that your company has to solve this issue (not having a geotech on board) before start thinking on the type of piles that may be required for this project...

Just curious to know: what is the reason why you do not have a geotech on board for this geotechnical issue?

RE: Pile suitable for bouldery, gravelly soil

(OP)
That was my first question too. I think it was due to the emergent nature of the job. From design kickoff to construction was only a few weeks. I have to believe a geotech will become involved one way or another before this is done playing out.

RE: Pile suitable for bouldery, gravelly soil

I mentioned pockets of corrosive soils in coastal Southern California. Add this to your caveats, or budget in a cathodic protection system. Caltrans has standard drawings (free) for these for their tie back anchors and similar.

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