Soil suitable for crane/lifting operations above 2??0??tonne/m2 2

[TrustButVerify]

Hello,

I'd like to highlight I'm not a civil or soil engineer, but mechanical. I specialized in crane/transport operations.

More or less, we always communicate the ground bearing pressure we need to achieve execution, and civil engineers take care of the rest.

I would like to find out more about what happens below the ground.

Have you had any experience about lessons learnt or mistakes in the past regarding crane operations? or achieve certain suitable ground bearing pressure for temporary loads?

Long story short: I'm coming from an incident we have last year on a site where we communicated we need 20t/m2 for storage, and transportation (wheels bearing heavy cargo rolling over this soil). While we were doing transportation, soil started to collapsed a bit due to, what we think, soaked in water below surface (winter). Civil department said soil was tested before for 20t/m2.

 

[1503-44]

20 tons/m2 is relatively normal to the high side bearing capacity. Allowable bearing pressures are typically for dry conditions only. A recent rain, or high water table can reduce allowable loads considerably. Current condition of the soil at time of use should always be verified and evaluated for suitability by a competent person with the experience needed to do the job. In this case one must know the danger signs, such as a darker soil color, green, or darker green patches in otherwise drier vegetation, types of vegetation that need more water than the rest, recently erosion of topsoil, visible ponding or running water outside of defined waterways, flood debrie clinging to bushes and fences, and recent high water marks on anything protruding above ground surfaces and the proper use of mud mats and other means of soil reinforcement. Dead fish laying on the ground is a definite no go area.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

Ground bearing pressures are not always easy to judge as it can vary each month or within a few metres if ther eis a soft spot or soft layers below he same looking top surface. Only a Ground investigation by something like a MacPherson probes will give the Geotech engineer something to work with.

But any investigation or testing is only as good as the place it was made on the day it was made.

Time dependant settlement also varies as long term settlement can limit ground bearing.

200 kPa is on the high side for anything other than well filled and compacted ground covered in gravel or stone.

Maybe you need to specify higher or get them to test to see what the ultimate bearing pressure is before the soil becomes plastic and just deforms.

I would also ways look to spread the load by use of timber mats or large plate supports. I'm sur eI don't need to tell you how many crane failures occur due to a "soft foot". If you see clay or peat layers in any ground investigation even 10m or more below ground, start to get worried.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EireChch]

Crane platforms, piling platforms are a huge thing in the UK. There has been a couple of incidents where rigs have toppled over due to bearing capacity failure.
Most working platforms are in the region of 1m thick of good quality stone fill, possible to reduce them by 150mm if you add a triaxail geogrid.

Check BRE470 for more info

 

[SlideRuleEra]

I would like to find out more about what happens below the ground.

 

[TrustButVerify]

The platform was tested before by a plate load test. The platform was tested in many points following a grid pattern, the weakest point resulted in 40t/m2 ground bearing pressure (double the amount neeeded).. The thing is, we were rolling with tires exerting the soil with 7t/m2, and it started to collapsed...

Of course, we had two weeks before a layer of snow (maybe 4cm height), which melted and the surface lacked slope to drain the water.

My point is: is this normal? Nobody was expecting this (even civil guys from site), maybe they just neglected it? They just didn't understand this was going to happen?

 

[TrustButVerify]

Is there a source where they explain case studies for soil collapsing for temporary loads exerted by heavy equipment?

 

[LittleInch]

If the plate load test was bigger than the bearing area of the tyre then you won't pick the soft bits.

Also did you have exactly even loading on all tyres?

How did you get to 70 kPa?
But water logged soil will be a lot different to well drained or dry.

how much settlement or movement did you get?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Lomarandil]

As EireChch mentions, the UK has formalized this as much as anyone. Look to some resources from the Temporary Works forum and the Deep Foundations Institute guide to working platforms. Practice in the US is much less formal. DFI also performed some research showing the following interesting soil pressure distribution below grade.

Moisture and drainage conditions are a huge factor, and probably were the cause of the incident you mentioned.

Further, plate load tests (s typically performed) have the issue of only measuring the response of a relatively small depth of influence. Track pressures often extend deeper.

It seems your civil department didn't adequately consider the situation (probably out of ignorance, this is a somewhat specialized area).

For mobile cranes and transport, long-term settlement is usually not a concern. The peak pressures don't exist statically that long.

 

[TrustButVerify]

Lomarandil, thanks for sharing that.

Where did you get the screenshot from?

Have a nice day.

 

[Lomarandil]

https://www.dfi.org/publications/single-pub?pubid=16911&bundle=1