Plate bearing test and heavy outrigger load

[YP12345]

Hi, I have been asked a question about a heavy outrigger load on a main road.
The outrigger load is 100 tonne.
We do not know the details of the road build up, apart from that it is a main road.
I used UK road design guidance, and assuming the subgrade is not good, with the CBR less than 2%, I estimated that a capping layer of 600mm, a sub-base of 350mm, and another 350mm for the road finish will be required. In total the thickness from sub-grade to the top of the road could be 1.3m
Using the vertical stress distribution over the depth, I can predict the vertical stress just below the capping layer.
Assuming 1m x 1m pad (purely for discussion purpose), the contact stress at the road level will be 1000 kN/m2. In this case, B = 1m
At the depth of 1.3m, the z/B ratio will be 1.3, and the vertical stress is about 20% of the stress at the road level, i.e, 200 kN/m2.
Assuming a 2m x 2m pad, the contact stress at the road level will be 250 kN/m2. In this case, B = 2m.
At the depth of 1.3m, the z/B ratio will be 0.65. the vertical stress is about 60% of the stress at the road level, i.e. 0.6 x 250 0= 150 kN/m2
whether it is 150 kN/m2, or 200 kN/m2, this stress level seems quite high for the soil conditions assumed (CBR <2%).
Although I can increase the pad size, but the vertical stress at the sub-grade level will not vary significantly.
This means that with the given load, the stress level in the sub-grade is unlikely to be changed significantly by the pad size at the road level.
I can probably ask for a plate bearing test. However, because of the very small area of the bearing plate, the total load applied in a plate bearing test will be much lower than the the outrigger load.
In this case, the test results from the plate bearing test will only tells the settlement behaviour of the sub-grade in very low stress level.
I don't think we can ask for a bore hole or trial pit. potentially too costly.
What is the best way to assess the bearing capacity/settlement prediction in situations like this.
Your help will be very much appreciated.

 

[SlideRuleEra]

What is the best way to assess the bearing capacity/settlement prediction in situations like this?

Use information that you can obtain about the road and your own observations, for example:

1) How old is the road?
2) When was the last time the road was resurfaced?
3) You say it's a "main road", how much traffic? Size of trucks that use the road?
4) What is the current condition of the pavement?
5) Is any differential settlement along the length of the road visible?
6) Has the road visibly settled relative to the shoulders?

If the road really is a "main road", with frequent, heavy traffic, for a few years, chances are the subgrade is NOW much better than it was when constructed and, for sure, much better than your worst case estimate. If the subgrade is not good, the road will be visibly "falling apart".

http://www.SlideRuleEra.net

 

[EireChch]

What are the ground conditions beneath the road build up? If they are granular then you should be able to make it work with some spreader mats. I have used Nylacast mats before. This was a 71 tonne crane which applied 12tonnes/m2 (120kPa) per outrigger on a 3 x 2m mat.

We confirmed that it worked.