Wellpoint hole with auger

Previously I have used plastic PVC pipes combined with water to make holes for the well points I have done.  After a long search, I found a company that makes augers specifically designed to fit inside 110mm pipe.  I ordered one, and a week or two later it arrived.

This means I can use the same techniques and pipes as  before, and use the auger to dig through any hard bits.  High on the buzz of a new toy, I assembled it and got out into the garden!

As before, I used a spade to dig a shallow hole about half a metre deep, then wet the bottom and twisted the auger down into it.  The first metre was topsoil, and the next meter white sand, which came out very quickly.  As before, I was surprised at how stable the hole was even without any support.

When I hit the water table at 2 metres, the auger started to bring up mud.  The walls of the hole now collapsed if the auger went any deeper, so I prepared a 6m length of 110mm pipe.  The lower end needed some shallow teeth to help cut a hole, and I drilled some 2mm holes in the bottom from 0.5m from the end, up to 1.5m.  The idea was to eventually have these holes down from 2m to 3m to allow water to seep ino the pipe.  It was easy to push this pipe down to 2.5m.

I cut the 110mm pipe off at 3.5m, which left 0.7m protruding above the surface, so that while standing ont he ground I could lower the auger in to continue digging.  I could feel the auger hitting a stone, which was jamming against the 110mm pipe when I tried to lift.  I removed the 110mm pipe, used the auger to lift a 5cm by 5cm stone out, and put the 110mm pipe back in the hole.

To minimise slumping, I found it best to ensure the water level inside the 110mm pipe was at the water table before removing the pipe.

Then there was a problem.  The water table is at 2m, so the lower 0.8m filled with water, both from the bottom and the sides where the small holes had been drilled.  When the auger was lifted through this layer of water, the mud or sand washed of the blades, and the auger came up empty, or with very little (see photo on the left).  The problem seemed to be the perforation holes.  Water was seeping in though these holes, which kept the 110mm pipe filled with water up to 2m below the surface.  So the next step was to cut some shallow teeth into a new section of 110mm pipe, but this time to make no holes in the sides.  I made it 3.7m long, because its easy enough to lift the auger above 1m of protruding pipe.

After removing the perforated pipe, the 3.7m length went down easily to 2.8m.  The water was still there, so I poured about half a bucket of dry sand down the 110mm pipe, and used the auger to remove it again along with the water.  This worked really well (see photo on the right).

The ground was hard at 2.9m, and the best tool for this was a 4.5m length of 20mm rebar with a sharpened point on one end.  Dropping and jamming this down slowly broke the hard ground to 3.1m, from where the auger brought up a few small chunks of hard black clay.  At 3.2m, pieces of pure white clay started to come out.  When the auger was turned tightly into this clay, it was impossible to lift by hand.  It was time for a pulley!

Using a ladder as a support, the pulley could exert 4 times my weight (about 75kg x 4 = 300kg) but still couldn’t lift the auger.  It was necessary to unscrew it several times to free it.  The auger was like screw turned into wood – it was stuck fast.  All in all, the pulley idea did not work too well.

Later I found a technique that worked.  After each half turn, and sometimes a quarter turn, I lifted the auger to break and free the clay and mud, and then turned again.

I also thought that there was a suction forming, making the auger impossible to lift.  To counter that, I drilled drainage holes in the auger shaft, to allow water or air above the blades to flow inside the auger shaft to near the tip.  This seemed to help, although the bottom hole would get clogged with sand and clay, and need to flushed out frequently.  The repeated lifting also helped keep this drain hole clear.

After a lot of muscle power, the auger was down to 3.55m, and was digging into a pure white clay layer.  The 110mm pipe was down to 3.4m and jammed into this layer, and water would neither seep into the pipe, nor out of it.

At this stage I was nervous about removing the 110mm pipe, as I imagined the walls of the hole below the water table would slump inwards as I lifted it.  So I thought I would try something new.  I got some 80mm down pipe, and cut slits into it up to 0.9m from the end.  The end was sealed by cutting and folding an 80mm collar, adding some off cuts, gluing it with PVC cement, and finally sealing the gaps with silicone.

This 80mm pipe went down the 110mm to about 3.5m.  Next sand was poured in between the two pipes, so that the 80mm stayed clear, but became surrounded by sand within the 110mm pipe.  A short etra length of 80mm pipe kept the sand from going down the main 80mm pipe.  To make sure it didn’t float up as the water seeped in, I lowered a length of metal round tube into and secured it with wire to the top of the pipe.  Finally, I twisted and lifted the 110mm pipe out.  The idea was that water would drain from the wet layer down through the sand to the slits at the bottom of the 80mm pipe, which would form a small reservoir.  A small metal sieve could then be lowered down the 80mm pipe to pump out the water.

I monitored the recharge rate inside the 80mm pipe, and it was disappointingly slow. When filled to the surface with water, it also drained very slowly.  I tried pulling the 80mm pipe up about 10cm to see if that brought the slots cut in the bottom a little higher and closer to the water.

By partly closing the tap to this new hole, the flow rate would be just right to almost empty the water in the 80mm pipe down to 3.4m after 15 minutes of pumping.  This would be the water below 2m (the water table) at the start of pumping (about 7.5 litres), plus the water that seeped back into the pipe as it emptied.  This seepage rate improved over time.

The thin stick is a long dowel (actually two cable tied together) with a chunk of expanded polystyrene on the end as a float.  This made it easy to quickly see what the water level was inside the pipe.

Working with the auger helped make the whole job take only about day.  Previously, when using just plastic pipes and water, it would take many times longer and much more sweat to get to the same depth.  Since it was so easy now, the obvious next step was yet another hole … how hard could it be, after all?