Mckintosh vs SPT vs CPT tests 2

[pigdog]

Hello,
I have acquired doubt regarding the trustworthiness of the N from SPT, though you do get a soil sample for testing and classification. Could someone please give me a brief description and comparison of Mckintosh, SPT, and CPT probes? What is a Mackintosh probe? What would be the upper soil stiffness limit of CPT testing? What is the maximum practical depth limit for these methods.
Thanks in advance,
roger

 

[jdmm]

Unfortunate that the unSTANDARD PENETRATION TEST became the accepted standard before the Dutch Cone. If history were different we would find many papers showing how the SPT is unreliable compared to the CPT and that correlations with CPT should be used with 'caution'.

I am a strong CPT advocate. If you want to learn more there are a number of web sites that you can find through GOOGLE. There is a very good book by Peter Robertson et al that goes through the details of cone testing and the correlations with conventional soil properties and design. Peter is at the University of Alberta in Canada and has a web site.

 

[pigdog]

jdmm,
Thanks, I will check it out.
roger

 

[MRM]

Pigdog,
The SPT is actually a very good test compared with other tests in terms of the cost to benefit ratio. As you mentioned, you obtain a soil sample which is very important. There are limitations to the SPT (just as with the CPT and others). The N values obtained with the SPT test need to be interpreted by a qualified individual, i.e. the results cannot be blindly "plugged" into an equation. For example, if there is significant gravel content, use the N values with caution and adjust them so that they are usable. The same is true of the CPT. If the silt or clay content is very high, again, the SPT values should be used with caution, but if interpreted properly, they can give you some idea of the state of the soil in-place (contractive, dilative).

Now if you're dealing with clean sands in the fine and medium range, which is usually the case if you have lots of alluvial, outwash, or beach deposits the SPT does a great job in my opinion of estimating the in-place relative density. I've done soil borings in these soil types, estimated relative density based on the corrected SPT, and then later after excavation to various levels on the site, I've done in-place density tests with a nuclear gauge. After determining the limit densities, I was able to estimate the actual in-place relative density. The actual, in these cases, reflected the estimate by SPT at a given depth within a reasonable error margin.

I agree with JDMM that the SPT is an UNstandard test. This is true of most "standard" things in life. This is why the firm doing the drilling and engineering needs to a) be familiar with what their SPT results mean regarding the soils in their area, and b) they should make use of the SPT correction factors so that the results can be somewhat normalized to the charts they will be using to help interpret the N values.

The bottom line is that ALL insitu tests performed from a drilling rig have their own limitations, and the results of the SPT, CPT, pressuremeter or any others should be interpreted by qualified individuals. If they are, these tests will all give useable soils information for design purposes for most projects.

 

[pigdog]

MRM
I suppose the danger lies in reading bearing capacity off a chart from SPT without taking into account that those clays may be giving high N numbers because they are dry as a bone.

We also run into problems on the other end: Just what is "rock"? Some optimist may use our "rock" designation to figure quantities, then want to sue when it turns out that he has to blast our N=90 residual soil in order to move it.

 

[MRM]

Pigdog
I would agree with you that there is a danger in reading BC off a chart from the SPT numbers. In fact, that actually frightens me; I'm talking the heebie geebies here! I should qualify that by saying if the engineer using the chart knows the origin of the chart, the chart may provide a quick reference. For example, if the engineer knows that the chart applies to N values that are corrected, in this type of sand, groundwater table is here, column loads and footing sizes will not exceed this, then the chart could be handy. Of course, the engineer also needs to know if the magic chart gives an indication of the bearing capacity of the soil (shear failure) or assumes that settlement limits the allowable bearing pressure (excessive compression failure). The real problem comes in when a knowledgeable engineer hands the chart to a less experience engineer who is not as familiar with the assumptions involved in using it. Problems could happen after that when the less-than-knowledgeable engineer applies the chart to every site and soil type taking a raw N value and plugging it in.

I suppose it's possible for a hard dry clay to become soft when wetted, so future soil changes could be a concern, which is a good example of engineering judgment when dealing with the N values obtained. I see what you're saying about the high N values in the clay...

As for the rock paradox, I know that rock comes in all hardness levels, soft shale to extremely hard granite to moderately soft limestone. We normally include a caveat with our report if we feel that the soft rock/hard clay we encounter in the soil borings may be difficult to excavate. From there, the owner is warned and can take an excavator and check to see how difficult it actually is on a larger scale. Then they and the excavation contractor can decide if a jackhammer or dynamite will be necessary. I only like to use dynamite when I fish though.

 

[pigdog]

MRM
There is a chart from a book Basic Soils Engineering, I think, that plots Presumptive Bearing Capacity against N on fields based on the GW SW SM ML SC CH-CL-Ol OH classification system. I have seen it discussed somewhere in these pages. In a past employment where I was "The Geologist", The Engineer relied heavily on this chart. Glad I am not there anymore, for many reasons.

Back to my initial post on this thread, I found the following link:

http://mnroad.dot.state.mn.us/research/mnroad_project/mnroadonlinereports/93-05.pdf

that provides what appears to be a good description of DCP, and correlations between DCP and everything else. Now I wonder if DCP is the same as a Macintosh Probe.

 

[euygar]

I think you all missed the point. One important point of this thread was that a correlation between mackintosh hand driving probe vs SPT and CPT is asked. So, I think we should stop to argue the known things about SPT and CPT and get back to solve what is asked. Is there anyone who can suggest a relationship which compares N number of blows obtained with Mackintosh probe with SPT or CPT ? I myself have been used Mackintosh probe in several small soil investigation projects during two years. The department I am working for bought it several years ago from ELE for use in small soil investigation jobs and educatinal purposes. But there is no information about its comparison with any of the tests SPT or CPT in any of the famous foundation books. I just remember that I have found a graph relating the results obtained with the soil consistency in ELE's catalogue, But it is interesting to say that they didn't put this graph to the newer version of their catalogue. We are trying to create a database with the obtained results of some shear strength tests from the soil investigation jobs we've used it. We never used the N values obtained by Mackintosh probe to calculate bearing capacity but after we get enough data we will analyze to estimate a meaningful relationship. And I will let you know as soon as this happens. Eris Uygar

 

[MRM]

Eris, Roger, jdmm,
Sorry about that-I digress. I’ve done some work with our DCP in our area to provide us with a correlation of the DCP blow counts with the soil density, and as a comparison to the SPT values. Our soils in Northern Michigan are generally water deposited, fine to medium quartz sands at many locations. Other organic deposits (Lake Marl, Muck, Peat) and clay deposits exist, but much of the area is the sand described above.

I’m not sure what the difference between our DCP and the Mackintosh probe are, but our DCP uses a 15lb weight falling 20 inches, advanced 1.75” at a time. The cone angle is 45 degrees, slightly rounded at the end. A seating loading increment is usually completed prior to counting. A paper was completed by G.F. Sowers and C.S. Hedges “Dynamic cone for Shallow In-Situ Penetration Testing” ASTM STP 399, Am. Soc. Testing Mats., 1966, p 29. This paper also has some correlations of the DCP blows to SPT blows for various soils including piedmont soils in the south.

The testing I’ve done has included DCP’s performed adjacent to SPT’s, and DCP’s performed at various levels below the surface on a given site with excavation afterwards to those levels, and actual density testing with a nuclear gauge at the various levels. I’ve also obtained some bulk samples to determine limit densities and use the nuke gauge to determine the estimated relative density of the soil in-situ in some cases. In this way, I’ve been able to establish a few good correlations. The sites I chose were uniform in soil type and relatively uniform in soil density. Here are some of my main conclusions to date:

Overburden: approximately 1’ to 6’ below subgrade level:
Ave DCP blows per 1.75” Relative density
1-3 Very loose to loose
4-7 Loose to Medium dense
8-12 Medium dense to dense
>13 Dense to very dense

If blows are less than say 2, it is usually a good indication that there may be loosely placed imported fill, since most of the soils usually have a in-place RD of 40% or greater (meaning it is sometimes difficult to find natural soils with an RD less than 30-40% (in my experience) unless some major particle bulking is still occurring since deposition. As the relative density approaches 90-100%, the DCP blows tend to be very high (>30/increment or more). Additionally, the DCP is affected the same as the SPT when it comes to silty and very fine sands in that unrealistic values may result from dilative tendencies. Gravel influences the DCP blow counts by raising the values, similar to the SPT. The values are also influenced by the amount of overburden as with the SPT. I’ve found that if you’re 1’ to 6’ below subgrade, the effects are not very pronounced. If deeper DCP’s are made, you should think about an overburden correction similar to the SPT to arrive at a better indication of the in-place relative density. DCP’s below the groundwater table are usually not possible since an auger hole will not stay open as they would with drilling mud or hollow stem augers-this is one of the main limitations of the DCP in my opinion.

Please share your own findings too. I’d like to hear them. I hope some of this is useful to someone.

 

[coneboy]

Hello PigDog

Regarding what type/density of soil will stop a electronic cone test.

Drill rig: Generally speaking gravelly soils with a Qc (cone tip) value of 400 Bar or 40 MPa will stop a CPT test.
For your information, a 40 MPa gravelly sand would be roughly equivalent to a 60 SPT blows/foot value.
Very stiff, highly overconsolidated silts and clays can cause a large of amount of rod friction to build up which can also reduce the maximum depth achieved

Specialized CPT Cone Truck:
I have seen cone trucks push electronic cones through 70 MPa
gravelly sands. The constant rate of push, alignment and power enables these specialized units possess enble them to push much harder than a drill rig.

In terms of maximum depth which you can achieve with the electronic cone:

If soil conditions are not too hard/stiff......rougly 70 meters depth for a drill and probably 90+ meters for a cone truck.

Hope this helps
Coneboy