Ground-breaking observations.

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The ground where the raised observatory is to be sited is sloping gently away to the north west. It was once an uncultivated area of bare earth under the shade of two large birches. These have now been felled but with a couple of two feet high stumps still weeping copiously. One of these stumps is directly in the way of site access.

The site is also sunken by about two feet, or about 50cm, below the average ground level. A rough bank of rocks and bricks previously marked the edge of the fall but has been tidied back to firm soil. Calling it a ha-ha would be presumptuous. 

An area of roughly four x four meters x 0.5m deep is 16m^2 ÷ 2 = roughly 8 cubic meters to make up. This assumes that the ground really needs raising, of course. The alternative is to build the whole thing on short stilts. 

The foundations for the entire structure were always intended to be pyramidal concrete carport anchors. They just need a thick layer of some heavy material tipped around them. They seem so neat and functional without all that backbreaking digging compacted soil or concrete mixing. Who wants a massive concrete slab anyway? Concrete mixer lorry access is impossible without a very long pump.

By default, anchors obviously need to be well anchored. You can't just stand them up, like skittles on the ground, and then expect much anchorage to occur.  The usual arrangement is to dig a hole and bury them. The compacted soil around the pyramid defies the larger base the ability to escape at ground level. Thereby adding to the retention abilities of the already heavy [40kg or 85lb] tapered concrete block. The resistance from the surrounding ground also defies the block from moving sideways under load. So far so good.

Importing 8m^3 of sand and gravel would bury the anchors without any hole digging. The problem is sand retention. There is nothing to stop the sand behaving like it normally does. So it would quickly flow away down the garden. No more anchorage!

There are other, more stable materials which could be utilized instead of sand and gravel. Rocks of various sizes are often piled up beside the fields where the farmer has cleared his soil. The problem is handling them. They are individually heavy. Which is good for anchorage. They just don't respond at all well to shovels or rakes. How to get the field stones home in such quantities as to be useful? Loading every stone by hand into a trailer? Not likely.

There are commercially available, stone chippings of various sizes with odd names depending on local traditions. How well they would pack down to retain pyramidal anchors is an unknown. Lorry access to the site is impossible.  I am no longer fit enough to ferry such quantities by wheelbarrow and shovel. Nor is there anywhere to tip a lorry within a hundred yards even if I was still young and able bodied. Or still willing to perform such Herculean tasks as repeatedly wheel-barrowing over many hundreds of yards.

I once cleared an entire 20 acre, unimproved grazing field of surface stones and boulders using only a builder's wheelbarrow. The field stones were all ferried by wheelbarrow to a marshy site beside our home. To form a foundation for the paths of a wildlife garden with ponds and hundreds of trees.

8 cu. meters of sand and gravel weighs about 24 tons. The problem is getting the lorry to the spot where it can tip it or do anything very useful. It's not going to happen even as separate big bags because we have such narrow access to the back garden. 2.3m is the average and minimum width.

I own a 2m x 1.3m x 0.4m trailer but it has obvious limits on maximum load of 325kg. 8 cubic meters is a lot of trailer loads! The sand and gravel weighs 2900 per cu mtr. 1900/325 = 6 trailer loads per cu meter x 8  = 48 trailer loads! Eeek! Then it all has to be spread about.

Which brings me back to digging loads of 2' deep holes and burying the carport anchors but not using sand and gravel . Very hard work based on a recent, very similar project of only one hole to support one steel pole with hand mixed concrete back-fill. I'm feeling exhausted already! 😓

Or, I could have the gravel tipped outside the gate and ferry it the 30+ yards by wheelbarrow to the site. The downside is having to fill every wheelbarrow full, one shovel at a time. The job would be manageable if spread over [say] a week or month to avoid complete exhaustion.

Hire a mini, bucket loader? Say £100 a day for only one day. Job done? This would require careful timing not to have the machine outside the garden on the wrong side of 8cu.meters of mixed sand and gravel. It seems I shall have to hire a local groundwork contractor with a suitable size of mini-loader.

There is another form of  loose sand & gravel. Let's call it self-stabilizing aggregate. It's called stabil[t] grus here in Denmark. Much used for laying drives and leveling parking spaces. 0-32mm is the usual standard and it contains a little clay to obtain its correct level of stability under load. It is variously described as weighing from 1700-1900kg per cu.m.

If properly laid, it has amazing resistance to pressure. Ideally it ought to be plate vibrated in several layers for maximum resistance Since I have no plans for vehicular access in the short term I can safely avoid that step. A top layer of gravel chips can provide a more decorative surface if desired. Though many owners of longer rural drives don't bother with the gravel dressing. Once well established it can cope with heavy lorries rolling over it, without leaving any tracks. Ideal for my purpose where I want to avoid the permanence of concrete. Left suitably smoothed out in a 2' deep layer it should self-compact nicely over time.

I checked a heap of this stuff at a builder's yard and it has a very steep critical angle before it slips or flows. That will help to avoid a tsunami effect of sand flowing slowly away down the gently sloping site. Though I can easily arrange some form of retention until the stuff goes "stiff" at the edges. I have loads of 15" high round  logs sawn from the trunks of the two felled birch trees. They are just sitting around the observatory site and waiting for a better purpose than splitting and burning on a stove. An L-shaped row of logs, one on top of another, should do the trick of retaining the flattened heap.

Over the following week or so I have been steadily clearing and tidying the space in readiness for the self-stabilizing sand and gravel. It has ended up about 5m wide. Times 4 meters out from the dry stone wall enclosing the shed foundation pad of small rocks and gravel. Removing the last of a birch stump was very hard work without a functioning chainsaw. Further research suggests these saws have a nasty habit of being very hard to start unless used daily. Swinging a sledge hammer/log splitting axe at my age is apt to make one breathless. And it did. The heavy, stacked birch logs are now suitably arranged to dictate the boundaries of the sand and gravel when it arrives.

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