Soils Report Review 1

[Berniedog]

I received a soils report and the report recommends 2000 psf bearing pressure and a site class of D. I am not a soils engineer but I think the 2000 psf is too much. Blow counts are as low as 3. See attached for the soil boring logs. Should I recommend getting a peer review?

 

[msquared48]

Any values mentioned for anticipated settlement or liquefaction?

I am not a Geotech either, but it seems like pile might be warranted to me.

Mike McCann, PE, SE (WA)

 

[oldestguy]

While 2000 PSF is common for very loose conditions, it may be a little much, depending. I'd go back and provide typical service column loads and ask for an estimate of settlement at that 2,000 PSF. What is interesting is the water levels at end of boring show "none". Many times below the water table, blow counts drop a lot as compared to above the water table. "Wet" samples may mean that, but why no readings later? Maybe jetting the interior was source of water in the samples. I also look at this and wonder if this is natural or a filled site. In some conditions the method of advancing the boring can result in low blow counts. Jetting out the interior of hollow stem augers before driving the spoon easily can cause the conditions noted, but don't correctly show the soil density. Once cleaned out to bottom of augers, continuing the jet can greatly disturb what then is sampled. Specifically you need to know if a jet was used or if an interior plug is routinely used inside the augers to prevent filling the auger with soil. I'd want to stand and watch the guys before I make a judgement on that however. In summary there are some questions that need answering. This also is in the realm of dealing with responsibility for performance, etc. Changing design numbers from the report may change the degree of responsibility that comes from the geotech.

 

[geomane]

I agree with oldestguy, as it may be uncontrolled fill. Looks like soils I deal with on a day to day basis. We also use 3.25" hollow stem auger, but I stay away from them when we encounter groundwater due to boiling if a head of water is not kept in the auger.

What are the column and/or wall loads? Did they include a statement in the report recommending densifying the soils to some depth below the base of the foundations? Where was this field exploration performed?

 

[Okiryu]

Looks like the recommended allowable bearing pressure was calculated from bearing capacity equations. I guess that a angle of friction between 25 and 30 degrees was used. Does the report indicate any FOS? I agree with OG to verify with the geotech about the expected settlements based on the loads and the recommended 2000 psf for bearing pressure.
Also, if the site class was determined using the SPT blow counts within the investigated depths, should not the site class be "E" as the blow counts are less than 15? or perhaps the geotech is familiar with local conditions and knows that there are denser/stiffer soils within a depth of 100 ft, so the site class can be considered as "D"?
In any case, the borings should have terminated at deeper locations where denser/stiffer soils are located, unless it was a budget problem for the geotech investigations...but even that, I would reduce the number of borings in order to advance deeper borings.

 

[oldestguy]

OG again. On the ground water thing,I'd want some way to know where it is, such as by leaving a observation well there to permit later readings. As it is, the logs may be what happened: Just pulled them and backfilled, taking no checks later.

 

[Berniedog]

column loads are about 50k. It is a steel framed 1 story office building in Akron, Ohio

 

[oldestguy]

Just for the heck of it, using some rough estimates of unit weights, compression factors and a 4 foot footing depth, I get just over 1 inch for damp soil and 1.5 inches settlement with GWT at the surface. So knowing that water condition is required I think.

 

[Berniedog]

Thanks all for your input. I am telling the Client to get a peer review. I do not want to talk to the Soil Engineer. I am not a soils Engineer.

 

[fattdad]

Seems the groundwater table is below the seat of settlement for a 5-ft square column footing.
Seems like the N-values need to be adjusted for the use of the automatic hammer (x1.4 greater than shown).
Seems like all settlement will be related to modulus values and occur immediately.
Unclear whether the 50kip column load is primarily sustained dead load or to what extent the transient live loads are involved in the bearing pressure and settlement forecast.

To the extent that the full extent of variables are considered in the analyses, the use of an engineered subbase could help. Consider an undercut of 4 ft (or so. . . ?) and backfill using an open-graded (i.e., highly frictional) aggregate. The modulus would then be controlled in the zone of highest foundation stress. Below the subbase then you'd have attenuation of the foundation load and limit the settlement forecast.

It's not that 2,000 psf is wrong. It's the presentation in the report doesn't seem address these items of concern.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Berniedog]

Thanks all for input