Plate load test on stiff clay 1

[Delijosi]

We are carrying out a ground investigation at a airport in UK. We specified an incremental plate load tests at 3 locations. One of the plate load test result is out. Using a 0.3 m diameter plate and a maximum load of 934kPa, the test was only carried out in 30mins. Settlement was 1.25mm. When the load was removed, the soil rebound completely i.e residual settlement is 0mm.

I feel that with this test nothing has been achieved. I am not comfortable with the results. Can someone shed more light on this test? Any guidance?

 

[Ron]

A 0.3m plate is too small to do a proper plate load test. The error introduced can be large, depending on soil type. Run the test again using a 1m plate, a .67m plate on top of that, and a .3m plate on top of that...all to prevent deformation of the bottom plate.

 

[ishvaaag]

Well at least that means that at the point you have a very good modulus of subgrade reaction, over 75 kgf/cm3 (a good soil for bearing would have say 12), and sustained by the complete elastic recovery after removal of the plate load. It is usual many simple good compactions deliver very good modulus of subgrade reaction; this is over the head of the table and so it must be thought to have more with the clayey soil being there dry and hardened.

See in
PCA Soil Primer
Portland Cement Asscoiation

a nice Fig.16 showing correlations with bearing values, limit strength, classifications of the soil for various agencies and CBR (around 100).

Of course 1 point is just a point and to discard any anomalies a number of tests needs be made to get some minimum appraisal of the actual state of some wide area.

 

[iandig]

The plate test which was completed was undertaken in accordance with the method from IAN 73/06 to assess the modulus of subgrade reaction and the equivalent CBR value. This method is used in highway pavement design (hence the IAN 73/06 methodology, this document is the interim advice note which is the current draft version of HD 25).
Depending on what information you have been required to produce, will depend upon whether or not you can use the results. This method assesses the eleastic stiffness of the ground, the fact that you have 0mm residual settlement demonstrates the soil behaved in an elastic manner. If upon unload you had some residual settlement, then the soil did not fully recover from the load, hence not true elasticity.
Who specified the method and loads?

 

[BigH]

Cam you confirm your load please. 934kPa is not a load but a pressure - so I presume you actually mean a pressure of 934kPa was applied. I'd like to know a few other properties of your "stiff" clay - what is the overconsolidation pressure (i.e., pc'-po')? Did you do a site investigation or only the plate load test? Could this be in an area of previous foundations? - or old pavement structure? - or boulders? 1.25 mm is a very small settlement - of that I am somewhat surprised - but if the site is heavily overconsolidated, you could have still been in the recompression range and upon unloading, one wouldn't expect too much residual settlement - Can you post the settlement curve - and the other test results? What did they show?

 

[iandig]

The recorded settlement of 1.25mm leads me to the assumption that the lab undertaking the test did a 'plate test for CBR' and as such they would have loaded the plate incrementally until at least 1.25mm of settlement is recorded. It could also be that the certificate actually reports the 'load at 1.25mm', then the 'modulus of sub-grade reaction', followed by the 'Equivalent CBR Value'. Both of the above have become common practice in the UK where a lab is instructed to undertake a plate load test, but the 'instructing' organisation don't know what they want. It may well be that the assessment of CBR is required and as such the method may be correct, however there are a large number of sites where the person instructed to organise the test do not know what they are asking for. As such a common fall back position will be to let the lab do a plate test for CBR, as most site engineers are comfortable with what a CBR is, and get very confused when you start talking about other geotechnical properties.

 

[Delijosi]

Iandig: You are wrong about the assumption that the test was for CBR.

BigH: iT IS PRESSURE - 934kPa. We carried out 9 window samples and 1 cable percussion borehole. Test schedule has just been prepared for the samples taken from the labs and 1-D oedometer test has been specified on U100 samples from the borehole. So no pc' valoes for now.

The investigation is under an existing airfied pavement. The clay is generally described as stiff to very stiff with corecsppnding cu values from vane test and SPT.

Two plate load tests were carried out. One of the test showed settlement of 1.7mm (the other 1.25mm) and the pressure-deformation curve seem to be levelling up and have some residual settlement of 0.16mm (the other have no residual settlement0.

 

[iandig]

No, what I meant was that the method adopted by the lab appeared to follow the procedure for plate test for CBR, hence the referene to only 1.25mm. If by chance the maximum settlement under the load was only 1.25mm then this is pure coincidence and I apologise for my earlier assumption. It is just that I work in the UK and I see this all the time. Have you considered that the ground would have been previously stabilised, this was done extensively for airfield pavements, and may account for the high applied load and low settlement.
I have just assessed the data for the 1.25mm settlement and using a 300mm plate, the equiavlent CBR based on the equations from IAN 73/06 is 224% with a corresponding Modulus of sub-grade reaction in the order of 300MN/m²/m. More than just stiff clay!