Soil Bearing Capacity Test 4

[Dasovich]

Could someone tell me more about the pocket pentrometer? Is it reliable for estimating approximate soil bearing capacities?

I've also been reading about ASTM D 1194-94 Standard Test Method for Bearing Capacity of Soil for Static Load adn Spread Footings. Has anyone out there ever performed this test? Is it difficult and time consuming to perform?

We are constructing a gravity sewer main in peat to clayey to silty to gravelly material (it's a mess!) We are constructing the main on piles. If there is a method suitable, we would like to perform a bearing capacity test during the construction of the main. Any input would be greatly appreciated! Thanks!

 

[Ron]

The test you are looking for a called a plate bearing test or plate load test, and it is neither difficult nor particularly time consuming to run. It does require some experience in doing it properly and a geotechnical engineer should interpret the results. Test cost can run from about $750 on the low side to more than $2000 on the high side.

Based on the soil conditions you described, I would be concerned about settlement of the line at least as much as I would be concerned about the bearing capacity. Please note that the bearing capacity is not a predictor of long term settlement and I would suggest you get a geot. eng. involved to do appropriate soil test borings and consolidation tests in the poorer soils you expect to encounter.

 

[TAC]

I agree with Ron.

FYI, a pocket penetrometer is a tool for estimating the unconfined copressive strength of a clayey soil. It does not measure bearing capacity. Bearing capacity isn't a measurable soil property, it will vary with footing width, soil unit weight, and soil strength with depth.

We use pocket penetrometers in the lab to help evaluate undrained shear strength (0.5 * unconfined compressive strength) of soil samples from split spoon samplers. With this value, the blow counts, our experience withlocal soils and with specific foundation info (width, depth, etc)we can estimate an allowable bearing capacity.

In the field we use hand penetrometers to verify that the soil strength in the footing excavations is similar to what we anticipated from the soil borings. Without soil borings however, the tool is more or less useless in the field because bearing capacity is effected by soil strength with depth below a sturcture and the hand penetrometer only measures soil strength at the surface.

A lot a people don't like or trust the values from hand penetrometers. We've found that they are pretty good estimates for the unconfined compressive strength of our local clayey soils, but we don't rely solely on them when recommending soil bearing capacities.

 

[Ginger]

I've used a pocket penetrometer on a number of occasions to obtain an assessment of the undrained shear strength of clays when sinking shafts. I have carried out these tests alongside hand shear vane tests and have had very good correlation in the results.

I think that this is a case of "horses for courses" and it is a great tool for quick checking of your borehole data, or indeed proving your borehole data to be innacurate but no way should this be your primary form of SI.

As for your particular problem, if you are carrying your pipeline on a piled raft then why are you worried about the strength of your formation? For bearing capacities we rely either on plate bearing tests or SPT results which e can match against the design charts for foundations.

Regards

Andy Machon
Andy@machona.freeserve.co.uk

 

[985172095]

Could someone tell me more about the interpretation of plate load test in order to obtain the soil bearing capacity (mainly cohesive soils)? .I would like to find some references (books, articles and websites) or examples about this interpretation.
Thanks
Guzmán

 

[Ron]

Check either Sowers or Terzaghi and Peck textbooks. Either should give you a means to interpret.

 

[KAM_imported]

I generally agree with the others. The pocket penetrometer is used in the lab to estimate the unconfined compressive strength and/or cohesion of cohesive soil samples (preferably undisturbed samples). Once a profile of values is obtained, the ultimate bearing capacity of a footing or pile can be estimated using foundation embedment, size, and other information. If the pipe is supported on piles, the weak soil beneath the pipe should not be a concern--only your pile bearing stratum should be an issue.