Pile Foundation - Geotech Report Needed? 1

[Civ-StructEIT]

Hi All,

I have a general question and wanted someone else's thoughts. I have been tasked with engineering my first driven pile foundation for a 25' high Garage to hold a Vac Truck for sanitary sewer purposes. The garage is 40' x 40', with sub-par existing soils and a high water table. Seems like a pretty straight forward design, was planning to have 9 piles equally spaced, with grade beams and an integrated concrete slab.
I have boring logs from the area that are dated 1982, and were used for a similar style building that utilizes piles and grade beams about 100 feet from the project site. The existing borings indicate that the friction piles are used at the existing building. Ideally, I would send a geotech out there for a new set of borings and get their recommendation. However, as per usual, that additional cost is not deemed necessary and was not included in the fee and scope of work. Would there be a significant risk of me missing valuable information if I go through with the design using the existing boring logs and existing building as a reference point? I guess what I'm asking is, what additional information would a new geotech provide me that I do not already have with the boring logs and existing drawings?

 

[fattdad]

transfer of liability.

f-d

ípapß gordo ainÆt no madre flaca!

 

[SlideRuleEra]

...my first driven pile foundation...
...garage is 40' x 40', with sub-par existing soils and a high water table.
...planning to have 9 piles...
...existing borings indicate that friction piles are used at the existing building.

...what additional information would a new geotech provide me that I do not already have with the boring logs and existing drawings?

A second opinion. Sounds like a very risky design; if even one pile fails the structure has "issues".

Do you have confirmation that the existing structure has performed without problems? Even if so... the user may have been lucky.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[dik]

It's always good to have more information. Do you have an existing soil report with recommendations? any signs of distress of the original construction? Are others using friction or driven piles? is the new structure significantly heavier (dead load) than the existing? a bunch of questions...

As noted, it is safer to have fresh information, so you can share the blame. Also should get into the habit of sharing your foundation plan with the geotekkie... share more blame.

Dik

 

[Civ-StructEIT]

Thanks for the responses guys. I agree that more information is preferred and I mentioned that we would need borings for this project before it started, but the client said they already had borings and no one pushed back to check on the available information before being awarded the project. This oversight will definitely change how we label our assumptions in project scopes moving forward. Anyway, below is the information I currently have.

- The soil borings that I have, do not have a soil report and recommendations included with them.
- The existing building is 40' x 70', with a height of ~25'
- The existing building has very similar loading as the proposed building and shows no signs of distress.
- I have not been able to get a hold of a detail or profile that indicates existing pile depth. All I have is pile existing pile size, locations, and a pile cap detail.
- Existing soil borings indicate a water table depth of ~2'-0" below grade
- Existing soil borings indicate ML Soil (USCS) from grade to about 40'-0" below Grade with average blows/ft ~ 23, and SW Soil (USCS) from 40' down to 52' with average blows/ft ~ 72.

 

[jgailla]

You need to inform the client that he needs to pay for a geotechnical report and document his response. If anything happens you will get sued and you will lose.

 

[oldestguy]

What does your contract say? Do you have a "hold harmless" clause in it dealing with liability, etc. The geotech involvement may not be all that costly in the long run, since maybe only one boring will suffice. It may be your cost, but is the risk without it worth it? Another lesson in life here.

 

[dik]

Thanks OG: ...my Project Notes stipulate that the Owner is responsible for retaining geotech and also for any re-design for differences in initial design assumptions; I will extend them to 'hold harmless'.

Dik

 

[Civ-StructEIT]

I will check out our contract language and see if we can modify accordingly. Thanks for all the input.

 

[MotorCity]

On projects that use shallow spread footings, sometimes the owner or contractor don't want to pay for a geotechnical investigation. I make conservative, reasonable assumptions of the bearing capacity and move on. In my notes, I state the assumed soil properties and indicate that the contractor is responsible for verifying (basically making a geotech investigation part of his work). If the soil is better than assumed, great. Otherwise, my notes indicate that the foundation is subject to modification or re-design. I would never follow that path for a pile foundation. Piles can cross many strata of soil......more soil = more unknown. Your piles may pass though a 10' thick layer of unsuitable soil 20' below the surface that you are relying on for friction. How would you ever know that without a soil report? With shallow foundations, you can usually dig a test pit and obtain the local soil properties.

 

[dik]

MotorCity... pretty much what I do with the provision that additional foundation design will cost...

Dik

 

[Mad Mike]

Seconding what MotorCity stated- so often we see a site looking uniform at the surface and forget that it was once cut by old river channels, back-water swamps etc...in my area on South Africa's east coast we have many of these sites where the ground is made up of coarse sandy channel deposits, but cut by old tributaries in-filled with soft clay, all of which there is absolutely no evidence at the surface. If you hit one of those mud-bowls your piles are going down forever. Then you will hear your Client moan.

I'm sure the risk is very slight- you could probably get away with it 95 times out of 100. But when you start to accumulate jobs like this, that risk turns to guaranteed failure.

If you already have some idea of the soil conditions nearby, I would suggest to your Client that as a compromise on the cost of boring, you put down a few Piezocone (CPTu) test probes. It would put your mind at ease and should be affordable. The Piezocone contractor should provide all interpretation of their probe data, so if it's just the pile design you're doing, you should be able to work on the probe data alone.

All the best,
Mike

 

[Ron]

In the geotech world, a boring 100 feet away might as well be in the next state. Depending on the local soil variability and stratigraphy, you can have pockets of poor soil conditions that won't hold a pile. What happens during construction when you set a pile tip elevation and the start placement and poles need to be twice as long to get capacity? Guess who gets blamed?

I notice from your handle you are an EIT...what do the senior engineers in your group say? As a general rule, don't do a deep foundation design without SITE SPECIFIC geotechnical info and recommendations!

 

[dik]

Depends on the area... Winnipeg, for the most part, has 40' of laclustrine varved clays to limestone bedrock... common for about 80% of the city area and it tapers off to about 35' is a couple of areas. When I did 240 Stradbrook we had one caisson that went down 110'... must have hit a 'channel' in the limestone. We often do a minimum of 2 holes and if there is any difference, we do some more. Holes are cheap.

[Added] Generally let the geotekkie determine the location and number of testholes.

Dik

 

[Civ-StructEIT]

I spoke with the PM for the project and mentioned that without a Geotech Report for the specific site area, and the fact that these are most likely going to have to be friction piles, we cannot confidently design the pile system. He was receptive and will be reaching out to the client to let them know that at least 2 borings in the area, along with a report and recommendation from the geotech are required prior to finalizing the design.

I appreciate all the input, thanks!

 

[Ron]

Good job, Civ-StructEIT!

 

[Okiryu]

And you can tell the owner that saving some $$$ with not doing the geotechnical investigation will cost them more $$$ because of the extra conservatism that you will need to use. On the other hand, a good geotechnical investigation with recommendations can save them $$$. The owner may not be a technical person so he/she may be thinking only in the money part.

 

[hokie66]

I generally agree with your conclusion, but this would depend somewhat on your position in the project. Are you working for the owner, project manager, or piling contractor? What type "friction piles"? That can mean driven steel, precast concrete, or timber piles, or it can mean bored piles. So I agree that site investigation is necessary, but it is a good idea to have the piling contractor involved, especially if they are driven piles.