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Chosing plastic grids for grass or gravel fill

What to look for when buying plastic grass and gravel parking grids.

Even I, with over thirty years’ experience in landscape construction, designing gardens, laying all from bricks to flags, blocks to fences; still get it wrong.

The issue is partly the willingness of companies to offer that which they know is not fit for purpose, partly because of a lack of testing and partly a simple lack of basic knowledge of construction and engineering. My greatest irritation is the practice of what I call sandwiching.

This is a practice where companies offer (shall we say) three products under the same overall banner of the company. The two outside products will be tried, tested and accepted as being of a sufficient quality; the middle product would be manufactured by the company or known as substantially cheaper to buy in from an Eastern bloc country. Of course with these sandwiched products the margins are greater and therefore these products will always be the ones offered first, because of course the company is in business to make money irrespective of the fact that the product is of insufficient quality.

Caveat Emptor. But the buyer can only be aware if it is that they are given a correct set of guidelines to look for.

NO1: Look at two of the grids together, shake them, jump on them, drive over them and try to prise them apart. If you can or if any damage occurs, stay away from this range. They are not fit for purpose if they are offered as car load bearing.
Ecogrid heavy duty articulated lorry bearing grid has a loading capacity of 800 tonne per square metre when filled. This is a capacity tested and published.

NO2: Look for flexibility. Some grids are 500mm square but have flexible sections or curved sections within the grid, others are smaller and have flexibility in the joint between the two grids. These are fine; however if a grid is 500mm square and is rigid then it will tend to ‘bridge’ voids in the ground and bounce. This will cause lifting and speedy wear; this type of product is less likely to last when measured against a smaller grid or one with flexible joints.

Ecogrid permeable driveway and patio grids are only 330mm square and have a patented locking system

NO3: Weight: look at the weight of the product per square metre, less plastic means a cheaper grid to produce and generally one that may have walls that will not stand up to traffic.
Ecogrid E50 plastic grass and gravel grids weigh nearly 10kg per square metre with a wall thickness of nearly 7mm.

NO4: Ask who manufactures the product, it is amazing how many resellers or merchants will take a product and rebrand it with their own details, including fictitious loading capacities. If you know who makes the product, you then have a chance of getting the right information.
Ecogrid Ltd do not allow rebranding of the Ecogrid/ Ecoraster range by any company.

NO5: What guarantees are offered? Are these guarantees insurance backed? How long has the company that makes the product been trading? It doesn’t take a rocket scientist to work out that offering a ten year guarantee on a product that only came in to production a couple of years ago and is sold by a company that has questionable trading history is spurious.
Ecogrid plastic parking grids have a full, no quibble 20 year guarantee on all of it’s grids.

NO6: Case studies going back several years should be looked at, there is nothing better than a recommendatory new purchase from a company that purchased many years ago.
Over 5000m2 of Ecogrid permeable paving grids were sold to the WWT over ten years ago, in March this year; they bought another similar amount.

We are either buying for our company, for our client or for our own use. We all need to know that when we buy this type of product, we need to know that we are not paying more than we need to and that we are getting a reasonable longevity from the product, in short; value for money which is real and not perceived value for money.

My opinion is that if you purchase a plastic grid that purports to be strong enough to be laid on a driveable surface likely to be trafficked by anything from a small domestic car to a bin lorry, then it should be expected to last for at least 15-25 years. Way in excess of tarmac, block paving or concrete products that are so damaging to the environment to produce.

If you are a real ‘Geek’, here is the calculation to ascertain point loading from the distributed offered load.

For an evenly distributed load (example: F=10 N/m):
Simply multiply the distributed load times the span of the load.

If you have distributed load of w = 150 kN/m2 on a span of L = 10 m and width of B= 5m then you will have

W = w x B = 150 x 5 = 750 kN/m

P = W x L = 750 x 10 = 7500 kN as a point load acting n the centre of your area L x B

For an uneven distributed load (example: F=.5x^2 N/m)
Converting this kind of distributed load into a point load involves calculating two things:
1) The total load
2) The point at which the load acts

To calculate (1), integrate the load function over the length. So for a beam of length 12m with distributed load F = 5x^2 N/m, you would integrate 5x^2 from 0 to 12.

To calculate (2), you need to find the place along the beam where the sum of the moments on either side = 0. You do this by solving the following equation for a:

Int[F(x)(a-x),x,0,a] = Int[F(x)(x-a),x,a,L]

Where:
Int[a,b,c,d] = integration of function a, with respect to b, from c to d.
F(x) = the distributed load
a = the distance at which the concentrated load acts
L = the total length the distributed load acts over
Solving this for F(x) = kx^n shows that:

Loads proportional to x act at 2/3 L
Loads proportional to x^2 act at 3/4 L
Loads proportional to x^3 act at 4/5 L

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Replies

  • i will refer to this algebra when the time arises :)

  • lots of necessaries landscaping,.

    i had my drive block paved & own a 4x4 lwb shogun mitsubishi - 2.5tonnes?

    so it had to be solid.. not like the average car at all.

    this applies too many situations.. - many questions should be asked prior to any quote/ works.

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