Plate Loading Tests

[SUGARCANE]

As a test house in the UK we frequently conduct plate loading tests. The method essentially being based on the requirements of BS 1377: Part 9, ensuring that the plate is loaded such that the settlement recorded is in excess of 1.25mm. We then evaluate the data in accordance with Dpt. Transport Design Manual HD25/94 which gives us the following data:

1. Modulus of Subgrade Reaction (K762) in kN/m2/mm
2. An equivalent CBR Value
3. Elastic Modulus E

This is not always sufficient for our clients (generally earthworks contractors). We are continually being asked (particularly for building platforms as appose to road construction) if we can tell from the results if they have complied with their specification requirements and acheived a specified bearing capacity (generally 75-150kN/m2 dependant on site use).

THIS IS WHERE WE GET STUCK.

The loads that we apply to the plate vary dependant on how competant the soil is that we are testing. In general, we would be putting loads upto 250kN/m2. What we generally tell our clients is that we can interpolate from the load/ settlement graph a settlement at any given bearing pressure applied to the plate and that somebody somewhere (generally the design consultant) should know if this is acceptable.

Can anybody please help. Is there anyway we can determine if our Clients have acheived their contractual requirements.

Keep it simple. We are not all geotechnical engineers!. Nonetheless, we get a hell of a lot of queries from you guys so its not only us that struggles to find the answers.Remember, its supposed to be us that does the testing, you to do the clever stuff.

Anyway, a worked example would be good.

Heres hoping

 

[BigH]

Sorry guys/gals I ! ! I meant that when you think that the cancer's has gone into remission, then it turns out for a relapse. (now - I think I got that right; sometimes at 0300h, the mine wanders!)

 

[Focht3]

jheidt2543:

I'd have bought you a better lunch than that - or at least something cold and fermented to wash out the taste. (I missed the start of your May 8, 2002 post somehow. Speed reading can get you in trouble sometimes...)

You made a good point about the contractor's need for timely information. Unfortunately contractors can get saddled with a difficult site and an inexperienced engineer, and it costs the contractors money. Those circumstances really suck. I do not mean to imply that the contractor should be put in any kind of a bind; my point is that plate load tests should not be specified at all for evaluating the allowable bearing pressure beneath "large" footings. It's really a problem with specifications, not the contractor. Unless the contractor tries to use the PLT to "prove" he has met some other project requirements.

 

[jheidt2543]

Focht3,

Thanks for quenching my thirst!

Now, back to Sugarcane's problem. Since we all agree that the plate load test is out, can you outline for him, and us, what testing proceedure should be used to answer the question "does this fill meet the specified bearing capacity"?

 

[BigH]

Actually, we may have missed something - I don't see, in my review speed reading - the types of fills that he is placing. Are they cohesionless fills (sands) or better sand and gravel? Are they cohesive fills - assuming non-swelling/non-shrinking?

If the engineer specified an engineered fill compacted to a specific relative compaction (or relative density), he is happy with the material underlying the fill, then we (say I, if under my control) would be happy that that one can reach 75-150kPa bearing on such engineered fills. 75 to 150kPa for normal footing sizes (say up to 1 to 2m =B) is not that onerous. Even if this was clay (not fill) this would be in the range of 40kPa to 80kPa undrained shear strengths. For granular fills less a problem as compacted fills to 95% MDD (heavy tamping) leads to low void ratios - leads to little settlement on applied loads. It is the poor compaction practices that could get you in trouble.

Hence - for engineered fills, I would be happy with normal compaction confirmation tests (sand cone; nuke; etc.). If you are worried, try taking out a small drill rig and driving 50mm dia dynamic cones (65kg hammer dropping 75mm) - or take SPTs in the fill. If cohesive fills, take a tube sample and do a UU test.

It all boils down to choosing the right test at the "right site". So I think . . .