geotechnical investigation 2

[johnwill65]

I have a client who is interested in purchasing a peice of prperty for a fairly desely developed residential project (about 50 individual units on 10 acres) with requisite roadways, and water and sewer mains. We have done about 30 test pits using a backhoe and trackhoe, and think that we can make this work. The clien has been told by a third party the he MUST perform a geotechnical investigation (ie borings) before he should consider closing on it. I beleive that we have enough information with always the usual intagibles--but thats what the 20% contingency is for--that is, we can move enough earth to make it work based on what we know. Any thoughts on this would be helpful, if perhaps, to only set my mind at ease. The proposed road will be about 1800 feet, with presumed depths for water of 5 feet, and sewer aout the same.

 

[VAD]

How deep were your testpits?. Depending on what your soil conditions are you may need a couple of deeper holes to assess conditions with depth. Testpits are excellent for this land developement project but may have to be supplemented with deeper holes unless you already have some idea of the stratigraphy with depth from information from surrounding development. I am not sure about the aspect of contingencies to make it work etc. A couple of deeper holes would be a drop in the bucket

 

[jicker23]

It really all depends on the outcome of your desk study.

Is the area in a subsidence prone area, perhaps it is prone to dissolution or has been prevously been subject to shallow mining? If so you will have to assess the instability risk.

Your shallow trial pits can give excellent information relating to foundation design, however, will you be considering undertaking settlement calculations (dependent on mateiral of course) or any quick triaxial tests? The formation of deeper borings would allow you to obtain samples for such testing.

 

[MRM]

In adding on to what VAD and jicker23 have said, my personal experience says that on a large site, it's especially important to consider the proposed grading for the site when planning a soil boring or test pit program.

I had a chance to review one soil boring program that, if memory serves, originally called for 10 foot borings, in areas to be cut 10 feet, and 10 foot borings in areas to be filled 20 feet. Both cases would lead to inadequate information.

 

[dzitse]

I am undertaking 5 borehole percussion drilling for Box Culvert Construction in Accra, Ghana.
The consultants to the project requested that drilling be done to 10m depth. However, at 6.5m depth, I hit very dense sandstone belonging to the Accraian series. Chiselling of the sandstone yields 16 hourse/80cm of penetration.
Dose it worth it to continue chiselling? looking at the fact that I have only penetrated this very dense formation for only 80cm. CPT gave 50/10mm.

 

[VAD]

dzitseawuku

Yes and No. Not a definitive answer. Is the sandstone presumed to be a thick layer perhaps from drilling information in general re nearby sites or is this a thin sandstone layer. I would presume the former.

I am not sure why you need to go 10 m unless your box will be buried under a very high fill. I am guessing here.

The best approach is to first discuss the drilling information obtained with the consultant as he is the one that will be making the recommendations on parameters for the design of the culvert.

Generally if you have to prove the sandstone layer depth then one would require at least a 5 ft run into that layer,
given no previous experience in the area in question or understanding of local geology. With the latter, the depth of exploration can be less.

Hope this helps some.

 

[jakin]

Johnwill65, I suggest you should get a geotech engineer to give you advice from a few borings at well selected locations. The cost would be money well-spent. 20% contigency? You must have a client with a deep pocket!

Dzitseawuku, are you in a lateritic (cemented residual) formation? [If so, you probably don't need to go deeper. Check the 1976 Gidigasu book (Balkema) for more on this.]

 

[BigH]

To add to what VAD stated - best to core the sandstone. Our company's polict is minimum 3m (10ft), not 1.5m or 5ft. I know of a very reputable company that, while mentored on the first, did the second and got into a bit of a SNAFU. You may know your soils/rock better in your area but there is always a possibility that this is a rock fragment somehow (speaking generally) that has soil below.

 

[jakin]

Sorry, that Gidigasu book was an Elsevier book, not Balkema.

 

[amanpreet]

its an amibiguous position like u can do how many test u like but if u think that u have a reasonable test data from the labs with not much of the discrepancies then u have enough . And u can do one deep test to see ascertain the ground water level .
More u go more u put money if u wont get anything substantial if the results are similar

regards
Amanpreet

 

[BigH]

Amanpreet - please use "you" rather than "u". Thanks.

 

[Grouser]

johnwill65

A good geotechnical investigation of the site is cheap insurance. Guess how fast the lawyers can spend $5,000? Unless you have a thorough knowledge of the site's and region's geology, and your site is very uniform, you need more information. For example, where is the water table?

Are the seat belt and air bag in your car an unnecessary expense if they're never used?

 

[mgray31]

johnwill65

It scares me that you ask such a question. Why did you do the test pits if you are not confident in them?

Where would you do a soil boring compared to where you have already performed a test pit? Hopefully the people out performing the test pits had a plan on where to dig the test pits. For example, test pits in the borrow areas and in the low areas to help determing excavation depths. Or in areas if you had bedrock, you would check out the lowest elevation of the utilities, in case you had to blast.

Also how good of notes did the person take during the test pits? Topsoil depths, soft material that need to be excavated, moisture contents (for borrow areas in case the material needs to be moisture conditioned before being used), water levels oberserved during the test pits, and type of soils. If your person was lacking in the note taking, a contractor can hit the owner with a big change order. A 20% change on a 1,000,000 grading job, means another $4,000 for each pad has to sell for.

Usually, I get a topo of the site and if a preliminary grading plan is available I can locate the soil borings with relative ease. After the soil borings are perform and laboratory tests are performs, I can recommend additional areas where test pits should be located to summarize the quanties for the contractor.

In my mind you did a disservice to your client by only performing test pits. But if the site is relatively simple, you my get away with only doing test pits.

Also most banks in my area require an evirnomental study in-case of spills or past dumps that the farmers may have covered up.

***Did you locate the test pits for future refernce. Because you don't want a uncompacted test pit under a corner of the new residential structure (you can not get uniform compaction by bucket tamping either). The test pits can be missed during site grading. Especially when you average three test pits per acre.