Capillary action, pond liner and pond tubes

Often known as minute siphoning, capillary action is something that can develop at any point and be permanent or intermittent. You find the effect in liner ponds where a bog garden is linked to the main pool or soil comes down into the pool water. Another place is where folds of a liner go over and down behind the pool edge sides. These can turn into as temporary siphons if it rains heavily enough to bring the water table outside the pool up as high as the level in the pool. Then when the water table begins to drop, the water siphons out along the capillary tube until the water drops below the bottom level of the capillary tube and it can suck no more.

If you have the electrical cable for the pump and light etc. Coming over the liner and perhaps under a slab or stone through a conduit of tubing, this can easily set itself up as a siphon particularly after heavy rain or over filling. Both these examples work on the principle of how men's urinal flushes work - once the water level in the pool gets to a level that is above the level at which the tube comes into the pond then the pool will siphon out through the tube to the level at which an end of the tube finishes. This effect can also set itself up in the cement behind walling or stonework.

If you have a 'natural' style of pool with a liner covered with soil but protected by a layer of underlay you might find quite rapid siphoning occurring if the underlay lies over the edge of the pool.

CURE:- The soil surrounding the pool must be separate from the pool. This also applies to any soil in the pool that is in a marginal trough or a bog garden.

When I build a pool, it has a skeleton of concrete blocks and the liner finishes on top of these. Without this skeleton the liner must come to the surface of the soil and be kinked up upright to prevent any siphoning. Some landscapers dip the liner into a small trench and the liner is held upright and in place with a thin snake of cement or concrete lying on top of it. The top edge of the liner is then disguised with a mulch of bark, gravel or pebbles; alternatively slabs or rock provide a definite edge, and the liner comes up behind these.

CASE STUDY

This water garden in Somerset would empty itself unpredictably of 8 inches of water overnight. The client thought it was the stream, which was duly dismantled and was shown to be perfectly intact. It eventually turned out that it only emptied itself down to the marginal shelf level if it was overfilled. It seemed that if the water level got to just below the coping stones a siphoning reaction would be set up behind the walling stone in the interior of the pool. The pool would then siphon out to the water level at which the stonework finish on the marginal shelf.

It became obvious where the siphon had set itself up behind the wall, because damp patches showed on the outside, so I drilled small holes in the pointing of the coping and underneath to relieve the sucking effect within the wall. The problem was thus solved.

If you use Underlay on top of the liner to protect it from sharp rocks or stone in soil that will, in turn, be on top of that, the underlay must not reach further than the liner and must not go over the top edge of where the liner is supported. If underlay seems to be creating this siphon effect then the capping over the highest point must be removed and the underlay cut.

With raised pools or pools with a blockwork skeleton, if underlay is unlikely to be the problem and it seems there may be no alternative but to dismantle the edging to gain access to where the liner comes up and over the blockwork skeleton, try drilling holes in the pointing under slabs and at the top of stonework to relieve the suction that is causing the siphoning action.

With ponds that have no skeletal structure, you will have to reveal the outside edge of the liner to find where the problem might lie. This is probably worth checking before you do a major clearout of the whole pool looking for leaks only because a leak round the top edge will be easy to spot and less disruptive, and it is best to eliminate this type of possibility before launching into anything quite radical like dismantling the whole set up.

If it was not catered for during the original pool construction, to prevent any further possibility of siphoning, an over-flow must be arranged at the maximum desired water level.

CASE STUDY

This very long stream would siphon itself dry when it was first installed. The cure was to cut the protective underlay, under the surface of the stone and pebbles protecting the liner, at each point where the water flowed from one level to the next. Much later on the moss that had built up over time that you can see built up on the stone surface had a very similar effect as the underlay below the surface. Removing it cured the problem.