Pile suitable for bouldery, gravelly soil

[bones206]

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?

 

[oldestguy]

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.

 

[bones206]

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.

 

[oldestguy]

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.

 

[SlideRuleEra]

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.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[bones206]

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.

 

[Okiryu]

Pre-bored grouted piles?

 

[SlideRuleEra]

...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.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Mjkkb2]

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.

 

[bones206]

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.

 

[Mjkkb2]

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 )

 

[bones206]

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!

 

[PEinc]

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

http://www.PeirceEngineering.com

 

[BUGGAR]

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.

 

[bones206]

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.

 

[bones206]

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.

 

[PEinc]

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.

http://www.PeirceEngineering.com

 

[BUGGAR]

"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.

 

[bones206]

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.

 

[BUGGAR]

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