DIG IT Part 3: How to Put in a Garden Pond Part 5.

Half-raised pond framework laid on the footing: The footing and the block work for the surrounding wall are completed before the excavation so that you know how much space to leave for a marginal shelfThe framework is in for the flexible liner pool in the ground. Now the 'second dig' is underway: The framework is essential in this pond, because of the slope of the garden the far edge will be above ground level. If it was all below ground level the near face, where I am digging would have to be faced and the pool surface would be hidden from the ho Creating a water garden all seems so easy in the 3 or 4 drawings in the catalogues or the coffee table books, but those essential details that hold the key to the creation of that magnificent vista you have in your minds eye seem to be missing. Well, if you have come this far in the operation and all the essential planning has been done then the rest is just plain sailing even if it is a little bit of hard work.
If you were digging by hand, by now you should have completed the first part of the excavation down to the marginal shelf level of the pool. Where necessary, you have laid in a blockwork skeleton. Where there is particularly light soil or if the side of the pool is exposed to its complete depth, you may have found it necessary to do the complete excavation and build up a block work framework from the base, perhaps even building in the marginal shelf area with blockwork and backfilling with soil. This blockwork will have been onto a minimum 10cm footing.
If you were using a machine you should have tidied up and be attending to the detail in the shape of the excavation. You will have considered all the potential problems in advance and will be prepared to start on the next stage.

A pond that is faced on the inside with walling stone and rockery stone.: The marginal shelf must be wide enough to take the facing stone and there still be enough room for planting baskets.

Pond built without a decent width of marginal shelf.: In the process of building this pond the owners asked if we would try out a new PVC reinforced liner. Because it was PVC it was decided to obscure the liner form the sun down to the marginal shelf with stonework. The result was the critically narrow marg

A reminder of how the excavation looked before the blockwork went in

LET’S FACE IT

The excavation of the pool from the marginal shelf downwards should take into account whether you will be facing the inside of the pool with stone, brick or even woodwork. Although in a well planted pool you are hardly ever aware of the pool liner, you may consider it necessary to hide the liner down to the marginal shelf level.
For this you must take the maximum width of the facing material and allow for this in the width of your marginal shelf area. If you are keeping it simple by depending upon an overhang of your edging stone to help obscure the liner then allow for a marginal shelf width of at least 25cm/10ins - big enough for a good sized planting basket.
Personally, I like a marginal planting shelf all around the water garden (except directly below any waterfall inlets). As far as the skeletal construction of the pond is concerned, this gives extra stability to the top/waterlevel edge of the pool and leaves all options open when it comes planting the pool.

Permanent beaches are not necessarily easy to acheive without a bit of forward planning. Beaches are a particularly popular design caprice which, if they are not to use up tons of expensive pebbles, by lying on top of the liner from above to below the water surface right to the bottom of the pool,A wildlife pond with a beach constructed in blockwork: A inner ring of block work just below the surce of the water forms a trough that can be planted up. then it best that they stop at the marginal shelf level - the level at which they become green and slimy and hard to see. In order to do this the marginal shelf level needs to be quite extensive in order to accommodate the sloping layer of pebbles; at least 60cm wide with a ‘blip’ at the low end to help hold back the stone. Once the levels are established the contour can be chamfered from one level to the next, in the soil under the liner, to further cut down on the quantities of pebbles required for the beach.

The simplest method as described in the wildlife pond article: This method of pond-making requires a relatively level area of ground.

With the wildlife/conservation style of construction this sort of beach affect can be acheived: Note the varying size of pebbles for natural effect.

Showing the framework to the wildlife/conservations style pool (below right): The trough is filled with filter medium as will be explained in a future article on 'veggie filters'.

THE 2ND DIG; PLUMBING THE DEPTHS

This modern sophisticated automatic level saves a lot of time.: It may be expensive kit but it can be hired and is very useful on a big project The next phase is simply to dig down to the level to which you had planned i.e. the ultimate depth of the pool. Once below the level of your top soil 25-30cm, keep the inferior sub soil separate. TheThe half-raised pond with 'second dig complete and the outside block work being faced. subsoil will be useful for backfilling and the topsoil will be used as a top dressing where necessary.
The pool needs to be between 46cm and 1 metre deep (18ins and 3ft approx.), unless you are having Koi carp or you are building a lake. Allow for a 2.5 to 5cm cushion layer of sand on medium to large projects.
Your guides for the depth are the long pegs marked with the depth of the pool that you drove in level with the datum peg which marks the final water level. If these have gone the way of all flesh (or would have just got in the way), you can either measure from a straight edge and level resting on top of a datum peg from around the edge, or put in the marked pegs as a guide now. With bigger pools, the hire of a builder’s ‘Dumpy’ level or simple theodolite comes into its own. These will take away the accumulated inaccuracies that build up as you work your way across a pool from one peg to another taking levels.
During this excavation , if you have a blockwork framework ,you can ‘backfill’ behind where necessary linking it into the bank or soil level that it is retaining.
The slope of the sides of the pool depends upon the type soil into which the pool is dug. Good solid clay soil will allow a slope of 30 degrees to the vertical to almost a metre in depth. Those of you who have opted for a large fibreglass preformed liner will be pleased with this even though it is harder digging.. In general mixed brash sub-soils will take a slope of 45 degrees. A lot of authors recommend sloping the sides30-35degrees to the horizontal, but this is really only necessary for some designs of wildlife ponds or pools that have soil laid over a protective ‘geomembrane’ that lies over the liner. Here the slope rises steadily to the water surface allowing small fauna to walk to the surface and for plants to find their own depth of planting. If this option had crossed your mind dont bother too much about it because ultimately it presents problems that can only be balanced by a true dedication to the concept of a ‘natural’ looking water garden. Also there are very ornamental pools that have paddle stones or pebbles across the bottom - the landscaper’s and pond maintenance person’s nightmare - these need gently sloping sides so that the ornamental interior dressing stays put.
The bottom should be level in my opinion unless there is a bottom draining filter system that depends upon a sloping pool base to collect all the muck. Any sumps or gradations mean the collection of silt and detritus to an uneven depth and a possible reservoir of undesirable bacteria.

BIG BOYS PLAY A SLIGHTLY DIFFERENT BALL GAME

Large pool raised at the patio level with strong block work, ready for lining: Treble Landscapes projectConsolidating the soil in the base of the pool.: Also remove any sharp stones There is an argument in very large pool constructions for having a slight gradient to an area that is most logical from a pumping out to drains or sludge gobblers point of view. In the cleaning out process, it is always a welcome sight to see the bottom whilst you work the rest of the mire to an area where it is not going to sloosh back all over the liner.
If you were using a mechanical excavator to do the digging, the excavation would be completed in one operation, the machine will be removed out of the excavation area and the detail work of tidying up the edges will be done largely by hand. Spend some time getting a perfectly level edge, shelf or trough - whichever method you are using - because time well spent now can save hours of fiddling and making good later on.
Remove any sharp stones from the base of the excavation. Then make sure the soil that will be underneath the liner is firm and well consolidated . For small projects a hand driven thumper or compactor helps flatten the area out; for large projects you could hire a ‘whacker plate’ or petrol driven plate compactor.

An inch thick layer of sand is laid all over the excavation.I always use a layer of underlay on top of the sand as well.: It cushions the blockwork and prevents small stones from 'floating up' under the liner when it is full.The liner is loosely tucked and folded into place. Next tip in enough damp sand to spread a layer all over the bottom 25mm to 50mm thick (1 - 2ins). You will need at least 1 ton for ever 15sqmtres of surface area on large projects. On smaller projects, in really stone free excavations, you may get away with just an underlay of felt like material sold specifically to protect pool liners from stone penetration from below.
The sand can be carefully spread about with shovel and rake and then trowelled into place on the slopes. This gives a really smooth finish.
Underlay goes down in strips on top of the sand or soil for small projects, each strip overlapping the next by 10cm. (In estimating underlay quantities add on an extra 10% over your rough square metreage for your liner.) Avoid windy days.
Next, the liner goes in on top of this. When these are folded up at the factory, the width is generally folded in on itself and it is rolled up along its length. So if there is a loose end, unroll it from the narrowest edge of the pool out into the middle and up the other side.
If it is a very large liner it has generally been folded along its length from each end like a lorry driver’s tarpaulin - so start to unfold these from the middle of the pond along its length. Once it is unfolded completely and roughly in place stretch it out to pull out the creases from the bottom. Now, take your shoes off and get in. Gradually work your way around the inside push the liner into the niches, crevices and contours whilst collecting the smaller creases together and folding them into a few big creases. On formal pools these are best collected into the corners, which seems very easy and sensible, but be very careful that a flap from amongst your collection of folds does not flop down below the horizontal that is the water level .
Dont be tempted to fill a well anchored, fairly taut liner with water hoping that the weight of the water will push it neatly into the corner of the excavation. Once the liner is neatly tucked into place, then you can anchor it and fill it with water. Many contractors prefer to fill it with water at this early stage especially if they have been very tight on the estimation of the liner size. It confirms one way or another that the liner is going to fit before they reach a point of no return. At some other point it will need to be pumped out again however.

This is when you trim your liner to suit the excavation and perhaps gain a waterfall liner in the process. If the pool is constructed with a concrete skeleton framework the liner needs to be cut back to the outside edge of the blockwork. If there is any more and the liner gets tucked down behind the blockwork a siphoning capillary can be set-up within a fold if the pool is ever filled to brimming. This will quite effectively empty a pool down to the bottom of the fold. This happens more often than you would imagine especially if the inside of the pool is faced.

RIGID POOL PROBLEMS

With rigid pools, you will have marked out the area

Excavate to the depth of the marginal area.: This pool is being fitted into an old excavation so it is a slightly different process.

With the pool in the 'first dig' excavation, mark out the 'footprint' of the pool in the soil

Excavate to the full depth of the pool plus an inch.: The water atble rising in the hole made it messy, but it also helped to show which areas were not level!

There is often a period of trial error in getting it exactly right.

No such problems for those of you opting for the rigid liner option. You have other things on your mind. We left you a couple of months ago with a fairly loose fitting pool liner in an excavation tailored to fit. Hopefully it is loose on the vertical as well as on the horizontal, and there is room for 25mm of sand in the base and on the shelf areas on which to bed it. So spread a thick even layer of sand on the horizontal surfaces of the excavation.
Continually try the pool liner in its position and make up any areas that are not helping to hold the pool level. Once the pool liner is sitting snugly level on a soft layer of sand, at the right level then is the time to back fill.
Partly fill the liner with water to give it weight and gradually back fill the outside with sand. This is easily achieved in the perfect scenario but if for some reason the hole is vastly bigger than the pool liner it may be best to back fill with soil. This may be done at the same time as filling the pool on the inside with water to provide inertia for the backfill. Be careful not to overdo one without the other keeping roughly to the same level.
In the perfect scenario, backfilling with sand can made more effective by washing the sand in with a hose. This gets it to parts that just ramming it in can barely reach. However care must be taken not to overfill the outside gap between pool liner and soil with water otherwise Archimedes principle will ensure that the liner floats up out of the ground. This is also not a very practical technique in water retentive, clayey soil. The main problem for perfectionists is the very minute shifts in level that occur as the pool gains weight. They sometimes work for you and sometimes against. Sometimes slight adjustments are possible by slightly more forceful backfilling on the low edge than the high. Dont overdo it or the liner will kink.

Settle the pool onto a bed of sand and ensure it is firm and level.

Backfill behind the pool with sand. You can put a few inches of water into the pool to hold it down.

The pool in place. Constantly monitor the level all around the top as you backfill.: If the water table is liable to rise up under the pool. Fill the pool with water, but it must be strong fibre glass like this.

Whilst putting a simple rock edge around the top of this pool, a small series of steps are put in for a wildlife exit. It may seem like a long job and it still looks a mess, but believe me there is light just over the horizon. The end is nigh.
Bring your backfill behind the preformed liner or your blockwork up to a level that allows enough room for a footing for any edging stones or paving stone if that is what you want around the edge. A minimum 50mm concrete scree is what is necessary for footings for paving or stone work.
If you are building up stonework or brick work, on the inside of your pool that has a flexible liner, from the marginal shelf to the surface then you must lay it on a cushion of underlay or liner offcut. Rockwork can be built above the level of the water, but stonework needs to finish just above or flush with the waterlevel.
Even if there is nothing else on the inside, make sure there is a ramp of stone for any animals that may have accidentally dropped in and may want to get out.

Facing the outside of the block wrk on the half-raised pond.The broken column and laying a paved area that blends into the rockwork lends an air of authentic ruination If there is a formal stone or brick face to an outside edge, lay this after any loose rockery work has been laid around the pond. It gives the rockery more of an air of natural permanence.
On the other hand you can build a facing wall on the outside or the inside that degenerates into an informal natural stone or rock effect of the same material. This gives an ancient ruined effect - “Here in old time the hand of man hath been.”
Finish this flush with the water level and cap it with a sympathetic paving or capping stone. Use a mix of cement with added waterproofer. Then treat all exposed cement around the pool area, especially under the water level with Silglaze. This pulls the active lime out of the cement. But that’s a story for another time.

VAGUE ESTIMATING

1. Blockwork : run a metre tape around the proposed excavation and multiply by 2.
2. Walling: 1 tonne of stone builds approximately 3sqmetres of faced area.
3. Paving: 1 tonne will cover approximately 12sqmetres
4. Gravel and pebbles @ 40mm depth will cover approximately 12sqmteres