Point loading on grass

[richard]

Hello

 

Does anyone know of a 'safe' point loading figure for 'normal' grass, ie on an impacted, mown sports field. I just need to look at how much weight you can put on a piece of deck with four legs, each leg on a plywood base.

 

Assume no under-ground pipes etc, so the main risk is of the packer sinking, but I'm not sure if there is a standard figure that you can work to?

 

Richard

[LampTramp]

Well what a question!

Have you got a Met Office forecast on what the weather is going to be.......?!

What are you putting on the decking?

I've done outside events that were almost washed out using prolyte decking and the legs on marine ply plates without a problem..

However depending on the size of the ply block and area....... how long is a piece of string...

[J Pearce]

I seem to remember from a lecture at uni that there is a British standard that gives guidance on this, based on soil makeup and moisture content.

[adamharman]

There's so much variation between different soil types, different water content, etc, that there isn't really an answer. You could be looking at anything from 2 to 50 tons / square metre!

[richard]

Hello, obviously the point loading on the grass is the same irrespective of how big the spreaders are (ie if it's 1 ton/sqm or 4 ton/sqm) I suppose the question is is 2 ton/sqm a good 'worst case' scenario for well drained well managed grass, if during good weather?

[smalljoshua]

I seem to remember from a lecture at uni that there is a British standard that gives guidance on this, based on soil makeup and moisture content.

If there was ever a person who would know the BS number for this, Kit would be the man!

 

Josh

[ImagineerTom]

There's no such thing as standard soil, there's no such thing as standard grass, there's no such thing as standard weather; I could point you to a "greenfield" site in london that's often muddy and looks like unstable ground yet actually there's barely 8" of mud and grass, underneath that is over a metre of hardcore, tarmac and concrete thats been impacted over hundreds of years so that actually the site can take a real pounding. I can point you to a festival site in suffolk that's little more than sand dunes and feels soft to touch yet beneath the surface is a network of roots that makes it one of the best sites I've ever staked in to but a bit of rain and the top surface washes straight away so whilst the tent stays up the stage inside needs constant adjustment in the rain. I've worked on well manicured lawns that have been beautifully ariated and as a result were very unstable and prone to sinking. Then there's the mysterious world of soil drains (which are never recorded on site plans and could be 150 years old) plus all the buried pipes and cables you mention. Any figure greater than 1kg/m2 you're given in answer to this question is nothing more than a completely worthless guess; sorry.

 

There's not a simple answer and the way it's usually worked out is educated guesswork, experience and some on-site testing. Whilst working at one very famous somerset festival site we ended up using upside down steeldeck as the footer for each stage section because the ground was so wet that even 12" square chocks under the legs were sinking in to the ground.

[richard]

Thanks Tom - I usually just rely on the experience of my staging contractor's to use the right amount of packing, however I have a rather panicky performance company who are used to working indoors and are worried about putting their stage onto grass as it might sink - they do a fair amount of jumping, and there is a trampoline involved so there are some quite high dynamic loads, but my feeling is that on this site, with 1'x1' 18mm ply packing they should be fine - but a bit of science to back this up would be useful!

[kerry davies]

Tom is absolutely right, however the IStructE Guide to TDS has these tables in section 6;

 

Table 3 Indicative values of allowable bearing pressures for foundations of structures in place for less than 28 days

Bearing material --- Allowable bearing pressure (kN/m2)

Dense sand --- 200

Medium dense sand --- 150

Loose sand --- 75

Stiff clays --- 150

Firm clays --- 75

Note a; This is the material immediately below the superficial layers of topsoil, hardcore or fill, etc.

 

Table 4 Indicative values of vertical loads on adequate baseplates for different bearing materials for structures in

place for less than 28 days

Bearing material --- Allowable load (kN)

150 × 150mm -- 250 × 250mm

Dense sand --- 9 -- 27

Medium dense sand --- 6 -- 18

Loose sand --- 2 -- 6

Stiff clays --- 5 -- 15

Firm clays --- 2.5 -- 7.5

 

Now take the word indicative as the key word which tells you these are estimates and not to be relied on over on-site testing and we are back where we started. It depends! If you want the BS (or equivalent, hint, hint) then get the IStructE guide which you should have anyway if you are building TDS.

[Stuart91]

A couple of summers ago I witnessed a particularly idiotic generator company's truck get stuck. The ground didn't feel terribly soft (your feet didn't sink into it at all) but was slippery enough that the truck couldn't get traction. In desperation, the driver tried using his hiab to lift the trailer round in order to give him a better angle. The outriggers on the cab had metal feet about 18 inches in diameter. One of them managed to sink almost 3ft into the ground. The site owners were not impressed. The driver was unhappy when I declined to cover the ~£800 bill for the recovery wagon that was eventually needed to pull him clear.

[kerry davies]

That is why the experienced HIAB drivers on genny duty carry lengths of railway sleeper since the pads are for use on hardcore/roadways etc.

 

They actually manufacture things for that precise purpose.

HIAB stabiliser pads.

[Stuart91]

That is why the experienced HIAB drivers on genny duty carry lengths of railway sleeper...

 

Yes, and experienced HIAB drivers won't go charging down a muddy slope without thinking about whether they can get back up again. This guy really didn't have a clue, unfortunately, and his blundering held up the build of a festival by approx four hours.

 

Railway sleepers seem like a good idea. It was astonishing how much the ground compacted underneath the outriggers.

[ianl]

And I've seen a stabiliser on a railway sleeper on a field that was wet but walkable and driveable for light vehicles,

 

It sunk the middle of the sleeper so much that it snapped in 2 and buried both bits nearly 2 feet into the ground , leaving them stood up almost vertical out the ground

 

Groundsman was not happy

[adamharman]

Hello, obviously the point loading on the grass is the same irrespective of how big the spreaders are (ie if it's 1 ton/sqm or 4 ton/sqm) I suppose the question is is 2 ton/sqm a good 'worst case' scenario for well drained well managed grass, if during good weather?

 

An 80kg person in a pair of size 10 boots is roughly 2 tons per square metre......

 

Good weather????? The ground is so saturated from 10 months of crap weather, a few days of dry won't make much differemce to it!

[Stuart91]

And I've seen a stabiliser on a railway sleeper on a field that was wet but walkable and driveable for light vehicles,

 

That's a pretty good description of the ground conditions in my incident above. We'd managed to get two fully laden Transit-sized vans on and unloaded, and the guys building the Big Top got their artic on, it was just the generator truck that got stuck.

 

It sunk the middle of the sleeper so much that it snapped in 2 and buried both bits nearly 2 feet into the ground , leaving them stood up almost vertical out the ground

 

That sounds quite spectacular. Was it a rotten / flawed sleeper, or would the force involved potentially do the same to any sleeper, I wonder?