Spt-N vs Rock Coring 2

[pelelo]

Hello,

The area where I work at is mostly surrounded by weathered limestone.

Few geotechnical firms use 2 approaches in order to perform the borings. Some of them perform the classic Standard Penetration test (SPT) in which the rule of thumb is (SPT-N 0 -4, very porous strata), (SPT- 5 -10, porous strata), SPT-N 10-30 fair porous strata and so on.

On the other hand, some other geotechnical firms instead of performing SPT-N as field investigation they deal with rock coring and of course they determine the RQD in each rock sample.

At this time, since I deal mostly with SPT-N I feel more confortable working with SPT-N and doing whatever analysis I need to run, instead of working with Rock coring.

I would like to know what Is your experience. Is this something very commom in your area?

Please let me know.

 

[molerat2210]

It is a similar process here in Maryland. Low SPTS are used to delineate problem areas. Rock rock coring is often performed for larger or critcal projects. Geophysics and fracture trace analysis is also used for further support. However, MD has only a few regions with limestone and some marble where this process applies. Most of central MD piedmont has crystalline bedrock. The type of analysis you need to perform will decide the best approaches for data collection (bearing checks, settlement, slope stability, excavation support, rippability studies, seepage analysis or whatever).

 

[pelelo]

Thanks molerat2210,

Now I feel better that around here is NOT the only area where we do SPT on weathered limestone (or any weathered ROCK).

Also, in order to deal with correlations (e.g for elastic settlement analysis), I use SPT-N values in order to obtain modulus. Using RQD values, I havent seen any correlation that involves RQD vs E.

 

[molerat2210]

I have not seen any relationship between E and RQD. I would not expect much settlement out of poor quality rock so I would not consider it in a settlement analysis unless the loads are really big. I am usually more focused on the depth of the overburden, compressibility of the overburden, and indications of sinkhole potential. The limestone in our region is interesting since it is oftern upturned/vertical so the depth to sound rock can vary by a 100 feet within a very short distance (cutters). This creates a problem of differential settlement. Rock may need to be excavated to control differential settlement. Sinkhole potential can be increased by altering drainage or lowering groundwater. Measures my be needed to control sinkholes.

 

[pelelo]

Thanks again,

Well In here basically we work on foundations for 20 a 30 story towers. Loads are around 3000 tons per column, I´m not sure if that is big or small load for you.

But yeah, these loads cause large settlements. Sometimes the sound rock is too deep and results very expensive to excavate to the sound rock. That´s why we focus mostly on settlements and differential settlements, and provide any ground improving recomendations (grouting, CFA piles, etc).

 

[molerat2210]

3000 ton column loads are pretty hefty. We don't see many big concentrated loads in our limestone region. I am thinking about your original question of RQD vs E for limestone. Sounds like rock pressuremeter or dmt tests could be the thing. A big project like that should justify that extra cost.

 

[pelelo]

yeah,

PMT or dmt are very very expensive tests, not many firms have those devices or even know how to run those tests.

 

[theCorkster]

RQD is used in rock mechanics to develop Rock Mass Rating (RMR) which can then be used to estimate modulus of elasticity {Young's modulus} (E), modulus of deformation of a rock mass (Em) and modulus of elasticity (Eem) of a rock mass. Check out Practical Rock Engineering by E. Hoek for background.

If we get N-values of less than 20 in "rock," the rock is so highly weathered, fractured and/or weakened, analysis will more likely be based on soil mechanics principles, as obtaining core with RQD greater than 0% may not be feasible. Even so, the RMR can be developed, and an E estimate developed.

It may be that the two different drilling methods generated data in the past supporting the "rules of thumb" you now use. It would be valuable as well as interesting to see a comparative analysis of the two investigation approaches to see how they compare.

 

[motorbiketocrank]

I have to disagree that Pressuremeter or DMT are expensive (I realize they can vary regionally). What is expensive is construction cost of unnecessary foundation types and sizes due to uncertainty and overconservatism in design.

I definitely would recommend researching the Pressuremeter, DMT and other technologies. The biggest downside I see to them is that depending on your geology, anisotropy can limit the value of the results.

I don't know your location. In our area we use

http://www.insitusoil.com/index.html

a lot. Even though you're probably not in the area, there are some good technical papers, etc.

I understand where you're coming from. We've been slow to implement these technologies. But now that we're starting to use them (in combination with SPT) they can make a world of difference in results. In the end, this initial expense can be a huge overall cost benefit to the customer.

Following on what theCorkster said, I think there is some discussion in FHWA's GEC #5 on rock modulus issues.