DMT testing Versus Standard Penetration Test

[facca]

We are currently working with an owner who has now stated that they are not comfortable with the use of the DMT instead of the provided data using the SPT. We went ahead with our own DMT testing to provide better data in designing for a temorary works required to construct the project.

 

[fattdad]

There are projects where I use both. Not sure of your direct question, but there's a place for each of these tools.

f-d

¡papá gordo ain’t no madre flaca!

 

[Mccoy]

Like papagordo says you are not posing a direct question, rather asking for comments I'd reckon.

My comment is that use of DMT may make some guys uncomfy over the more familiar SPT or CPT. A little like driving an unflashy, known car with its flaws rather than a fancy sports car with lots of unknown commands.

Bottom line: there is no doubt whatsoever that, as a whole, the DMT is the Porsche and that the SPT is a far cheaper make of car. DMT has low error on the metod, SPT may have huge errors. DMT has pretty good records on the settlements parameters. A compromise might be SPT's carried out by modern rigs and very experienced crews.

Maybe you can reassure the owner on the benefits of DMT by showing some previous investigations done with that and problems solved...

 

[dgillette]

It all depends on what information you are trying to get from the tests, what the soil is, and the type of project. SPT is just fine for some things, not so good for others.

Welcome back, McCoy! Haven't heard from you in a long time.

DRG

 

[Mccoy]

How you doing, dgillette,

true enough, recently I've been so busy with these new European regulations that I've lost track of the 'international scene'...

 

[facca]

We have installed caissons to toe in front of existing sheet piles that are to short in a dock. We know have to excavate in order to install an underwater cap on top of the caissons. In order to excavate we have to build a temporary system to support the wall until the cap is constucted. Based on SPT results at tender we designed a system that would work. Subsequent to tender we wanted better data in order to build a simple system to support the dock, In doing this exercise with the DMT we have found that on paper the new works fail and this was what our temporary works was relying on. We have an owner which won't give us direction other then to say they are not comfotable with the DMT testing. The soils are firm to very firm silty clays. Hope this helps.

 

[dgillette]

So the issue is undrained shear strength of the material resisting kick-out of the sheet piles?

While my ignorance of DMT would fill many volumes, I do know enough about the SPT to be very nervous about the correlations for undrained shear strength. At least four have been published, but they are not consistent with each other, in large part due to being referenced to different strength tests. My favorite one was referenced to VST, but it only goes up to N of 15 or so, so I wouldn't trust it much farther. Most of the data fell between [2 psi + N] and [6.5 psi + N], but with outliers more than 5 psi on either side of the band. It would all depend on the SPT hammer system also; that could make a whale of a difference.

If your stuff is heavily OC and/or fissured, the immediate undrained shear strength may not be the critical case.

Can you get any help from oedometers and SHANSEP-type thinking?

This is where my knowledge of the problem runs out.

 

[Mccoy]

facca,
one important aspect that dgillette pointed out is that your design is an excavation in NC clays, probably governed by effective strenght parameters rather than the undrained strenght (you tend to have a pore pressure drop rather than an increase behind the unsupported face), so no field test is going to give you that and lab tests should have to be run.

As to the DMT, that's a sophisticated test run in important jobs with a good budget.

The SPT has little repeatability/precision and, in clays, pretty crappy (scattered) output from the tranformation laws.