pipeline and manholes - best geotech lab testing

[RockChick]

Hi all,
I'm a geologist assigning lab tests for Mod Cal and SPT samples for a sewer trunk replacement project. My GE will review the suite I choose but I'd like to take a good crack at it. What are the most meaningful tests to assign for this type of study? It will involve manholes, trenching, and bore and jack or directional drilling for a short span going under a creek channel. So there will be some shoring recommendations and just general subsurface conditions. Are strength tests of interest, or more PI, sieve, MD? The materials are sandy-clay mixtures over cobbly alluvium, and a little bit of soft, lithic sandy tuff. The deepest excavations will be less than about 15 feet deep.
thanks - RockChick

 

[oldestguy]

If you have not done these types of programs before for what you outline, this seems like far to large an endeavor for a first time endeavor. If your organization has a geotechnical engineer, that person should set up the program, with an eye to using it later for report writing. Is there some specification, such as compaction required, etc. that would significantly affect the testing. I, for one, would find it very unusual for someone else to set up the testing program for me then to use. When I've been faced with using test data that has been handed to me and I had no say in how that was done, I am very careful to not assume anything and go very conservative, to the possible determent of the job. Perhaps more clarity of the administration organization and contractor relationships will help in us trying to advise.

 

[EireChch]

OG seems to have taken the political approach!! jk....but there is merit to what he says.

I have edit my original post as i see that i have mis-read the OPs original question, however I will leave it in this post for others to review, discuss or query. In relation of lap testing, my opinion would be that it depends on the size of the project. If it was a 150mm trunk sewer going 70m from pump station to pump station then I would feel that it wouldn't need any lab testing. In-situ testing (as described below) would allow you to use correlations for phi etc which will influence your shoring recommendations. I would also think that you would want undisturbed samples to assess shear strength testing.

There are two phases I would test.

1) being the geotechnical investigation of the alignment

2) being the investigation of the exposed trench and manhole base(prior to the pipe being laid)to ensure founding conditions are ok and also the testing of backfilling of the trench.

1) for investigating the alignment I would undertake hand auger boreholes considering its only 15m deep. The spacing of boreholes would be dependent on the length of the pipe, maybe every 10m. I would either undertake shear vane tests (every 0.5m or where there is a change in colour/consistency of the material) or dynamic cone (mackintosh probing is what i think ye Yanks call it) depending on what material was encountered. When you get a mix of sand and clay soil it will be up to judgement as to which is better vanes or probing. This will be based on how you think the material will perform i.e. like a cohesive material or like a cohesionless material. You may hit refusal on the cobbles you have mentioned above, and these may be an issue when it comes to directional drilling. In this case it may be beneficial to undertake a test pit excavation, using a pocket penetrometer testing on the sides. And hey, the test pit excavation could be used as a pressure relief pit for directional drilling!

2) When the trench base is exposed i would again undertake dynamic cone testing or shear vane testing. For acceptable founding conditions we normally specify 2-3 blows per 100mm for probe testing and an undrained shear strength of 40kPa. These may be a little overly conservative for your area but where i used to work we would easily achieve these. For backfilling we typically use a Clegg Hammer (impact hammer)for granular backfill or shear vane testing again if its a cohesive backfill. Nuclear density tests should also be undertaken.

Others may ask for machine boreholes etc but these can be expensive. I suppose it comes down to how big of a pipe it is!

 

[RockChick]

Thanks all,
I should have clarified a couple things - 1) not my first endeavor, I have responsible charge to design geotechnical investigations, but I don't always decide on the lab testing suite. So I was trying to ask what lab tests, on relatively undisturbed samples, will yield the numbers the GE needs to make recommedations regarding trenching, shoring, bore-and-jack conditions, etc. 2) the GE designed the exploration program and that is already done. We did four solid stem borings to 15-25 feet and have lots of good, testable samples. This is a relatively small section and job. I ended up selecting a few TxUU's, PI's, MD's and sieves. he made a couple minor changes and we went with those.

 

[EireChch]

I find it interesting that you have recommended lab testing such as TxUU's, PI's, MD's and sieves. My main geotechnical engineering experience was in New Zealand where lab testing would be rarely done. You might get some PSD or Atterberg limits done but very rarely would a Triaxial test be done. This is mainly due to the reason that lab testing is very expensive in NZ. A triaxial test may cost as much as $750NZ per sample.

If this is a small job is there any reason that SPT/Vane tests are not sufficient to provide soil parameters for shoring design etc.? Especially when you may not be 100% confident that the sample is disturbed.

 

[oldestguy]

FireCch Chances are money is no problem there, as at a university and there was a bunch left over from the prior year' budget. If you don't spend it now you get less next year. I'd guess that the job probably needs a few unconfined compression tests and moisture content. Why undisturbed samples also is interesting. I'd like to be in competition with firms like these. My past experience was that the clients found they could get a better, more economical job when they compared my work to the expensive folks. Made for a successful career.