BMX park / pumptrack

[Mopete]

Hi,

We are a team of tree dad's building a pumptrack for the kids in our small town. We have been offered some discount material coming from the refection of a nearby highway. The material is gravel, the type is 'MG20' (0 - 3/4). Now, I found out that 'MG20' comes in 2 types, 'sand' or 'gravel'. What we've been offered is the 'sand' type. To build the jumps and bumps for the pumptrack, we need the 'gravel' type of 'MG20'. We want it to get hard, and we have seen on-site that the 'sand' type does not get hard. But it is much cheaper... So, the question is, could we use a foot thick of 'MG20 sand' as a base layer, compact it, then build the jumps (not higher than 1 meter high) on this base? Will the base support the weight of the jumps without moving? The reason we want to use 'MG20 gravel' is because if the pumptrack becomes popular, we would like to cover it with asphalt, so we need a solid base (we are in the north with freeze - thaw cycles).

Thanks in advance

 

[Ron]

Mix a few percent of portland cement into the material and compact it. That should stabilize it.


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[Mopete]

Hi, thanks a lot for answering, we really appreciate.

We like the idea, but we don't really have the means to mix the thing, and we have around 800 metric tons of it... so I don't know, and for the price, let's say 5% of cement would mean 40 tons of cement, that would bust our phase 1 budget right away I guess...

But do you think we should avoid the sandy MG20 altogether? What is this product used for then?

 

[Mad Mike]

If you're not in a position to construct a test ramp to determine how the sandy MG20 behaves, perhaps you could give consideration to bringing in alternating loads of the (cheap) sand and (expensive) gravel and blending together as best you can...

Have a look at the MG20 you're being offered cheaply and take a sample of it to the various pump tracks in your region, see if there are any which use similar material in their ramps. Local advice may be better than advanced engineering.

In direct response to your query though- I don't see the problem in using a foot of sand as a base layer to your jumps- then again, I've never been involved in pump track construction!

All the best,
Mike

 

[oldestguy]

Follow Ron's advice. If in doubt, take a wheel barrow load of the local sand and mix in a measured amount of Portland cement, compact it but moisten lightly first to a slight damp condition.. For a bigger zone of sand do the mixing in layers using a garden roto tiller, many passes. For a do-it-yourself job put the helpers to work on this mixing part. You'd be surprised how much can get done this way. I've used church members to do this for mixing in bentonite 6" deep all around the church 15 feet out to direct run-off away from the church. It ought to work just fine with cement also. A rented bob-cat on site then can move that mix to the ramps.
Edit: let your trial mix sit a few days and check it's hardness or ability to stand without slumping, etc. These simple tests can then direct how the full job goes.

 

[Mopete]

Thanks everyone, we'll run some tests.

 

[cvg]

amount of cement is dependent on the gradation of the sand and gravel and you can probably use less cement.

typically here we get about 700 psf (33 kpa)compressive strength in 7 days when using 5 - 7 % cement (partially replaced with fly ash). the mixture uses sand and gravel from the river and is compacted to about 95% of the maximum density. this mixture is used for levees and river bank scour protection and works very well. that is far more strength than you need and using crushed rock you should get even better results.

 

[EireChch]

I have used 7% cement in a fine to medium sand and we had UCS of 1.5MPa. The cut of for rock is 0.6MPa so far more strength as cvg said. Could look to use 3-4% to skim some costs. Ideally youd trial it though as i generally have seen 5% as the minimum for cement stabilised sand.

 

[Mopete]

So, we've done some rough cost estimates and even at 1 or 2% of cement, we would go through a third of our budget. We scrapped the sandy MG20 as it was much too loose. We found some compaction sand at a reasonable price, so we removed 6 inches of the top soil, we will put the compaction sand on this week-end. If we can find a steamroller, we will compact it right away, or we will have to elbow-grease the compaction sand with a vibrating plate next week. We had planned to do the bumps and burms with 0 - 3/4 gravel, but there's this much cheaper alternative with « declasified rock dust » (really not sure about the name). Only problem with the compaction sand base (around 10-12 inches thick) is that we can't back the trucks on it to dump the gravel. But this seems to be the most solid least expensive way we found to go forward. Thanks again everyone for your inputs, i'll try to post a picture here when the project is done.