Why Plate Load Test requires staged loads?

[segarally]

Hi,All
I am reading ASTM standard of plate load test (ASTM D1194-94) in which the estimated total load is divided into at least ten steps and the time interval between two load increment is no less than 15 min.
My question is what is the purpose to divide the total load into several steps instead of continuous loading? Is it to simulate the staged construction of superstructure?

Thank you.

 

[Focht3]

Nope. It's to do several things:

1. Allow for dissipation of pore pressures and subsequent settlement to occur.

2. Limit the potential for partial bearing capacity failures under undrained conditions (yielding), which can make it difficult to evaluate the test results.

3. To make sure that sufficient data points are collected so that a meaningful analysis can be performed. (It's never a good idea to have only 2 or 3 data points on a graph...)

There may be other reasons that I've forgotten - anyone else want to chime in?



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[segarally]

Focht3, thank you for the speedy reply.
If no pore pressure is involved and the data acquisition system is automated, can I use continuous loading instead of staged loading?
Is the partial bearing capacity failure you mentioned same as local shear failure or punching shear failure?
Also, I guess the subsequent settlement you mentioned is consolidation settlement, am I right?
Thanks.

 

[Focht3]

If you are testing sands, or silts in an arid environment, then "continuous" testing should be fine. Be sure your report points out the specific variations from the testing standard, and why you deviated from it. This should help you avoid harassment in the future -

Yes, the "partial bearing capacity failure" is either local shear failure or punching shear failure. And the "subsequent settlement" was consolidation settlement.

Please note that you can have four different kinds of settlement: elastic, primary consolidation, secondary consolidation, and collapse (in arid regions.) A continuous test procedure won't allow you to separate the components...



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[ashjun]

I think it is more to do with the confidence level in interpretation of the test results. All the "original" interpretations using the plate load test results are based/applicable when a certain testing procedure is adopted; and application of vertical loads in stages is part of the original procedure.

 

[segarally]

Ashjun,
What are the other "original" procedures?

Can you explain your point in more details?

Thanks.

 

[malayboy]

Hi all,

I`m new to this forum. Found the HP today and it is a very intersting Q & A forum.

I do have some questions on Plate Load Test. The following are my questions:

1. Why are there several loading cycles? The maximum % of loads apply on each cycles are different one another. I have understood the requirement of staged load explained by Focht3.

2. The structure`s base footing is 3.0 to 3.5m below GL. Is it OK to perform the test 1.5m below GL? The structure load is only 100kN. GWL is deep below.

3. If Maintained Load Method, what is the failure criteria? Is it 25mm settlement?

I hope somebody can answer me.

Thank you.

 

[Focht3]

[100 kN = 22.48 kips]

What is the structure? Why run a plate load test? What are the geotechnical parameters?



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[malayboy]

Hi Focht3,

It is a pedestrian bridge over a busy road on a rather flat surface on top of a very small hilly land. It is a cut area. Initially the design drawing was a RC pile foundation. Then we found out that one side of the road encountered bedrock at 3.5m below GL. So, we decided to change the foundation system to pad footing foundation on the side encountered rock. On the other side of the road, bedrock level is at GL-10.5m.

Since it is a busy road, we want to avoid deep excavation on doing the PLT. and of course to safe some cost implementing the test.

One location of soil investigation has been conducted. The SPT N-values are 7,50/225mm at 1.5m interval from GL and touch bedrock (silty sandy GRAVEL, sandstone) at 3.5m. No rock coring. The soil description at that particular point is from sandy silt to gravelly sand.No mechanical lab test and undisturbed soil sample has been done.

I hope the above explanation will make clear the ground status at the site.

 

[malayboy]

Hi

Why nobody reply?

TQ