Boring locations-Parking Stucture 1

[JoeBaseplate]

I am looking at a 186x120 5 or 6 story parking structure and the arch is asking for boring locations. I am think of at least 3 borings: close to NE corner, center, close SW corner. Is 3 borings reasonable for this footprint?

Thanks,

 

[csd72]

Joe,

I would probably recommend 4 for a project this size though I am sure many have been done as low as 2 boreholes.

The problem that I can see with your configuration is that potentially if you have a linear feature such as a dyke or former river e.t.c. then all three boreholes may coincide with the line of this.

4 boreholes can give you a reasonable 3d layout of strata e.t.c. that you can hang your hat on. Also, I expect you will be using this data for CBR e.t.c. for the ground level slab and this would help.

If you have to use 3 boreholes move the middle one off centre so you can get a good approximation of bedding planes e.t.c.

 

[Ron]

Agree with csd72. In addition, locating borings is a shot in the dark unless you have some reasonably reliable site history or historical geological data that might lead you in a certain direction. Here's where I would go with 4 borings....

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Yes, you might miss something. Cover that eventuality with a statement in your report such as "Subsurface conditions will vary between borings. Interpolation of conditions between boring locations should not be done by others. Interpolation for this evaluation is based on the information available at the time of the report and could change with further, more precise information."

 

[JoeBaseplate]

csd, Ron,

Thanks. I guess the 4th boring would not be not that expensive since they would already mobilized and present on site.

 

[ron9876]

Depends on the soil conditions in the area. When I am asked about boring layout I always refer to the geotechs recommendation. There is so much liability involved and the geotech has the language to cover the uncertainity built into their contracts and reports.

 

[Ron]

My sketch didn't turn out as planned! I think the gist is still there.

 

[dcarr82775]

I would defer to the Geotech on number of borings and their location. That being said 3-4 sounds in the right ballpark

 

[a2mfk]

Work for a company that offers geology and geotech services amongst other things, though we don't actually drill we monitor and interpret the results...

SPTs are cheap compared to the cost of the structure overall, drop in the bucket. And you are right, once they are mobilized there is not a huge difference between 3 or 5. I always like to pick at least one highly loaded pad footing area if you are that far ahead in your design development.

Alternatively to the throw darts at the side of a barn SPT technique, you can hire a company that can do some geophysical testing (ER, GPR) over a large swath of area that may locate anomalies that can than be drilled for further exploration. These are not prohibitively expensive either.

 

[csd72]

Here is an australian webcast on the benefits of further investigation on piling works.

http://www.mediavisionz.com.au/ea_flash/vic/2011/110406-eav/index.htm

 

[Teguci]

It really depends on the geologic history of the site. For this small of a garage I would still think a Geo engineer should be on-board. Tell the geo where your shear walls will be and give him your high and low estimated column bearings. With enough local history you may not have to do any borings (not a recommendation).

 

[cvg]

why would you dictate the boring locations to the geotech? In fact, I would not dictate the precise number of borings either. Call your favorite geotech, discuss the project with them, agree on a work plan and then rely on the recommendations they give in their proposal. I guarantee they will not skimp on the number of borings. It's usually the other way around...

 

[haynewp]

It is usually given for a cost estimate for the geotech's work. I leave the final number up to the geotech in case they find inconsistencies in the couple of locations I recommend.

 

[TXStructural]

It sounds like you all covered it well. Except the title of the thread deserves one more: yes, parking structures are frequently quite boring. (sorry, the urge to say it was overwhelming)

 

[abusementpark]

Except the title of the thread deserves one more: yes, parking structures are frequently quite boring. (sorry, the urge to say it was overwhelming)

Beat me to it! LOL

 

[JoeBaseplate]

OK, BORINGSSS, happy now. The things that engineers find amusing!

No wonder George Costanza wanted to be an architect, not an engineer.

 

[FixedEarth]

For a 22,000 sq. ft building, 5 borings are considered adequate. We like to see at least one deep boring at 50 to 60 ft depth. This deep boring tells us about any burried or compressible layers at depth.

http://www.FoundEng.com

 

[JoeBaseplate]

FixedEarth, thank you. Good info.

 

[FixedEarth]

JB- I wish I had the paper. There was a case study of several buildings in southern Germany that were fine for about 50 years and then the foundation started settling. When the forensic soils investigation was completed, they found deep seated settlement due to burried compressible soils.

Your geotechnical engineer will guide you. I was involved in 4 story parking garage few years ago and we ended up driving 16 inch square precast concrete piles about 65 ft length. We had a basement level and that helped in partially compensating the building weight. Our soils were medium dense Silty SAND and I think water table was at 15 ft+/-.

http://www.FoundEng.com

 

[emmgjld]

I would typically think 3 to 5 borings, w/One deep for the oddball things (see FixedEarth) & for better info for seismic data. A lot depends on what my knowledge of the area is.

That being said.
"cvg (Civil/Environme) why would you dictate the boring locations to the geotech? In fact, I would not dictate the precise number of borings either. Call your favorite geotech, discuss the project with them, agree on a work plan and then rely on the recommendations they give in their proposal."
How do you, as a Structural PE expect to be treated?
I am sorry if that sounds a little hard to some.

 

[BigH]

My first thought would be to ascertain what the anticipated soil conditions are. The overall area geology (physiographic) regions and any information you might be able to find from nearby structures would be very useful to determine if one has a potentially variable site or a fairly uniform site. Uniform soil conditions? Soft over hard or dense? dense over soft? Will likely foundations be piled or spread footings?

Is the site "new" or has there been previous structures on it in the past is another question I would find out - if there is potential for areas of fill, this might require some additional shallow borings.

Based on this, I would tailor the programme to meet my expectations. I agree that 5 borings would be prudent. The depths of the borings would be chosen to match the expectations. If there is a "great" chance that it will be spread footings, then you might go shallow borings at the corners and a deeper one in the center with the corner borings to prove the basic upper strata (which usually is the variable to be proven).

If you have sands over soft clays, for instance, you might need additional borings to confirm the sand thickness. If the sand thickness is uniform, you might be able to get away with spread foundations so that the pressure bulbs do not extend to the soft clay. However, if the sand thickness is variable, this wouldn't be an option.

If you have the potential for fills at the site - old basements that have been backfilled, etc., you may need to do additional borings to confirm the site variability. A firm I worked for in Philly always did test pits besides the borings just for this reason.

Now if you are in Europe - do you need UXO clearances?