GEOPHYSICAL EXPLORATION

[msucog]

i posted a thread in the seismology engineering section of the structural engineers to try and strike up a conversation about geophysical surveys but haven't had much luck. i have not seen much in the geotechnical part of the site either. i did a search to try and find a particular discussion board but have not found anything in particular here. are there geophysical people here or just isolated questions for the most part? since seismic site classes can be determined by evaluating the shear wave velocities, i would presume that there must be people here hiding in the shadows. anyone?

 

[civilperson]

What do you want to know?

 

[theCorkster]

msucog;

Site classsification for structural code purposes can be developed using shear waves, blowcounts and/or undrained shear strength. Of the three methods we tend to use the latter two as they come directly from subsurface borings and sampling and laboratory testing. Getting reliable shear wave velocity requires cross borehole work, which adds considerable cost to the geotechnical study; hence, I've only seen this performed on large, critical projects with signficant investigation budgets.

 

[msucog]

i guess i had a couple of directions to head with questions. since civilperson appears to be a structural guy, is there a ballpark figure that you have in your head regarding the cost difference to the owner if going from a site class D to C where the jump improves the SDC by one level (or i suppose the scenario where a site specific seismic analysis yields say at 15% reduction which bumps up the SDC by one level). say maybe for a 1/2 million s.f. warehouse, or 5 story concrete building, or moderate sized school. i would appreciate any feedback along those lines. i don't have a reference point from the structural engineer's point of view on the matter. i'm usually told that if there's even a slight chance of improving the SDC, then we should do the work (of course, the larger the structure, the more the interest in getting a better SDC).

as far as the geophysical methods, it is not that expensive. here in the piedmont, we typically evaluate the seisimic site class based on the borings as part of the exploration since it does not take much time to crunch the numbers across the site borings. the main cost is taking a boring(s) to refusal. the geophysical work costs a few grand but we typically see an improved site class with it versus the borings. my understanding is that the cost is well worth it for anything bigger than your little pharmacy site on the corner.

i was wondering how many out there look at p-wave & s-wave refraction (with boring data to justify that the refraction theory is applicable), downhole, crosshole, and/or ReMi. any of you performed your own correlations or have your own opinions? i've done a little correlation work and am impressed thus far by the results between the respective methods (and recognizing the limitations of each).

also has anyone seen a paper or book with step by step procedures or examples for evaluating ReMi data (it appears to me to be rather complicated iterative calculations and i would like to understand the data processing procedures as a whole better instead of just relying upon the software to tell me something). calculus was not one of my strong points back in college so i need a little help to fully understand the ins and outs of ReMi data analysis/measurements, inversion, etc. i understand about gathering the field data but am looking for a "ReMi for Dummies" sort of paper better illustrating the theory as an example. thanks.

 

[coneboy]

Regarding thecorkster's comments.

We do seismic shear wave testing on almost a daily basis using our CPTU equipment. A beam placed at the ground surface is usually used as a source for this down hole method. On a 200' deep test the increase in cost is only about $600 - $800 greater than a regular CPTU test. On numerous sites we have compared the downhole Vs data to the cross data and found that the 2 methods produce similar results. The main reason we don't do cross hole seismic very often relates to time and costs.

I was wondering why you don't use a local CPT contractor who is equipped with seismic cones?

Coneboy

 

[theCorkster]

coneboy,

Good question. It's mainly that we do not have a local (less than 150 miles) cone rig. While a cone would work in some of the geologic units we work in, most tend to tear equipment up. Thanks for the info though - I'll keep it in mind!

 

[fattdad]

Just a comment and by no means to take anything away from the beneficial use of geophysics or the seismic cone. IBC Site Class range from A to E (i.e., there are five general catigories). Most of this fuss is over whether a site should be designated "C" or "D". This is like making a choice between two broad catigories. Using very refined data to tip the balance between these two broad catigories seems like being too precice for the intent of the Site Class. To me it's a matter of precision and accuracy. If you know the regional geology (i.e., depth to bedrock, characteristics of the fluvial, glacial, residual soil, etc.), then the broad distinction between Class C and D should almost be known in advance. Field exploration, while critical, should not be used to prove against what the regional setting would suggest. For this reason, I just use a Site Class that is consistent with the field exploration (i.e., N-values and pocket penetrometer data) and the geologic setting.

Just my opinion. I know others think differently.

f-d

¡papá gordo ain’t no madre flaca!

 

[msucog]

i agree with your comments. however, we are bound by the nice and neat little table included in IBC/ASCE7. quite often here in the piedmont, i see sites with the Nave=20-30bpf that have no problem getting a site class C with a Vsave=1300-1400+fps. it goes back to getting "credit" for the rock velocity. i usually use a slower rock velocity too (3000fps is the max Vs i use). for us here in GA, everything is a D or better but most everything could be at least a C with shear wave by going by the little table in IBC. some areas get up to a B which requires shear wave anyway. then sometimes, it simply boils down to rig access or availability (sometimes the site has already been drilled or the rig is simply not available). sometimes, the below grade parking levels are taken out of the design so there could be 10-30 feet of the soil profile added back in, so then you've got to take another look at the number.

for cpt, i've never used it even though i do see the advantages that it could offer. also, i happen to follow the philosophy of putting my hands on the samples if i'm going through the trouble of drilling especially when evaluating the site soil conditions for foundation support. with site class here, simply getting less than 1/2 of the reduction in Ss and S1 allowed by code will bump up the SDC. i don't necessarily agree with the tables but i do have to play the game. "technically" i believe it says that you can use engineering judgement to estimate the velocity of materials but i wouldn't agree to going that route without some amount of shear wave field data.

we do a lot of seismic work and we're branching out what we already do but i'm trying to get my brain around the data analysis for the more in depth ReMi method. i think i've found enough reference papers since my last post to keep me busy for a few weeks. i've found a lot that talk about ReMi in general but don't actually go through the data analysis process since it is such an iterative calculus headache. i'm not a fan of just pointing and clicking my way to the "answer"...i would prefer to at least understand the process even if i can't do all the calculating myself. by combining multiple evaluation methods, i'm able to check that results are reasonable in the event something oddball (other than what i was expecting) shows up.

 

[civilperson]

The change in structural details from a site class D to a C is about 4-6% savings in the structural costs if forethought is used to limit the elevated mass, (No swimming pools in penthouse), and use of pre-approved connections and bracing systems, (no prohibition on concrete shear walls in both axis by the architect). This change is usually not obtainable by geologic tests since the site geology does not differ from what has been built across the street or next door. Exotic designs have much higher savings since the cost is enormous in the lower class site.

 

[msucog]

been trying out ReMi for a few days now. i'm rather impressed. it was able to find a rock fill layer overlying a lower blow count alluvial layers and all this under 20' of soil fill. it also correctly identfied the top of rock.

 

[Mickney]

Good thread. I speak from a geotechnical perspective in the Piedmont where part of my consulting involves recommending a seismic site class.



First step: I take one or two of the proposed soil test borings to auger refusal or to a depth sufficient for determing the seismic site class per blow counts. I often will stop the boring short if I encounter an adequate thickness of partially weathered rock, or if I am at 50 or 60 feet below the ground surface and my SPT values are still low (on the order of 10 to 15 BPF).

Second step: I will evaluate the potential for improving the site class based on the deep soil test borings and my own data-base of shear wave velocities. I have performed over 100 seismic piezocone borings in the Piedmont and have a good data-base of shear wave velocities versus depth, soil types and/or SPT values.

Third step: If it looks favorable to perform soil shear wave testing to improve the site class, I will contact the project team, discuss the results and procede with the testing if the owner and team agree. I perform the shear wave testing utilizing a seismic piezocone rig. I do not have much confidence in the surface refraction method with the geophones.

 

[msucog]

i like different aspects of each kind of refraction as long as they are "used" appropriately. refraction is definitely very picky and the quality of data is very much dependent on the field personnel and equipment. luckily the piedmont generally follows the theories of refraction but can be tricky in certain areas. poor data is useless when trying to reduce refraction traverse. in general though, i've had good luck correlating across to downhole output, soil test borings and remi output when using a combination of refraction techniques.