Does this sound right (Soak Pit?)

azzeretti

I just had a landscaper out to look at doing some work on my garden (1 acre). It is in a terrible state and has no drainage at all. The soil is "marl", if that means anything to anyone. Anyway, I have had a few guys out but this guy was the only one to suggest the following:

Dig a 12 foot deep 10 foot wide pit in the middle of the garden. Around the garden scrape off a few inches of top soil and dump it. Then dig trenches all around/through the garden, lay tubes in them, and fill with stones. Direct these tubes to the large pit. In the large pit there will also be stones/rocks up to a few inches from the top. Then tonnes and tonnes of new topsoil will be brought in to cover the garden. He reckons that with this option the garden will be correctly drained and should be accessible all year.

My question is that none of the other landscapers suggested this, most suggested skimming and uncompacting the subsoil at a much cheaper cost.

Is this guy a cowboy or does any of his suggestion sound right.

red menace

Sounds like your garden has no fall in any direction so he wants to bring the water to a central point to allow it to drain away through the stones. If the was a fall towards an existing drain or stream the pipes could be all run to a main drain and this on to the stream.
If thats not possible a soak pit is the way to go.
AFAIK marl type soils are heavy and hold water so need extra help to get the water moving.
Did he take any levels or dig any test holes while he was there?

Minder

Suggestion sounds very like the drainage in the photos on this link. It will work, it may be overkill, it may be pointless if the water table is very high, but the theory is right.

marl or bog lime,soil, essentially clay mixed with carbonate of lime, highly valued as a dressing or fertilizer. It crumbles rapidly and easily. Marl in which the lime is in the form of invertebrate shells is called shell marl. The term is loosely used for a variety of soils, some of which are low in lime content, e.g., the greensand marl of New Jersey. Marling of soil tends to lighten it, correct acidity, and promote nitrification

Sounds like marl is a desireable soil type - why the need to strip and dispose of the topsoil?

Edit While marl is a good dressing or fertiliser, it is not free draining, making it less desireable.

azzeretti

Minder wrote: »

Suggestion sounds very like the drainage in the photos on this link. It will work, it may be overkill, it may be pointless if the water table is very high, but the theory is right.



Sounds like marl is a desireable soil type - why the need to strip and dispose of the topsoil?

Can you explain a bit more why it might be pointless if the water table is very high?

Minder

Generally, the water table rises and falls as the amount of rainfall varies through the wet and dry months of the year. If the water table is close to the ground level, you soakaway will be just a hole in the ground full of water - when all the pipes that are filtering water from across your site are connected to the soakaway, where will the excess water go?

But if the ground is clay or marl it will be difficult to drain - clay is impermeable and marl is semi permeable. So for a soakaway to work, the hole has to penetrate through the clay layer to a permeable layer below. But it is not that simple, there may be water in the underlying layers that will be forced up into the hole. The rainfall is locked into the top layer because the marl only allows water to pass through it very slowly.

Before committing to any big contract with a landscaper, a simple trial pit will demonstrate if a soakaway is feasible.

There is a good explanation here. This bit is useful to determine if a soakaway will work.

Site Investigation

.......A simpler approach is to excavate the trial pit to establish whether a soakaway is likely to work or not, rather than determine just how effectively it will actually work.

For this, dig a hole at least 1.2 metres deep. Remember, any excavation deeper than 1.2m MUST be supported to eliminate risk of bank collapse. For small soakaways serving a single property, it is unlikely that a trial pit will need to be any deeper than 1.8m, and so it could be excavated as shown in the diagram opposite. For larger projects, where a trial pit up to 6 metres deep may be required, trench shoring must be used.

Leave the excavated trial hole overnight, covering with a board or fencing-off to prevent accidents. 24 hours later, inspect the pit. If there is no water in the trial pit, then a soakaway is highly likely to work and work well. If there is any water present, the level will have stabilised at water table level, and this should be measured from ground level and used in the following calculations to determine whether a soakaway is possible or practical.

azzeretti

red menace wrote: »

Sounds like your garden has no fall in any direction so he wants to bring the water to a central point to allow it to drain away through the stones. If the was a fall towards an existing drain or stream the pipes could be all run to a main drain and this on to the stream.
If thats not possible a soak pit is the way to go.
AFAIK marl type soils are heavy and hold water so need extra help to get the water moving.
Did he take any levels or dig any test holes while he was there?

No he didn't. But during the summer I dug down about 3 feet myself as a test hole and the water didn't budge. Was that far enough? The soil composition didn't change at all as far as I could see. Should I have dug further, should I ask him to do the same?

Are there other ways to go about this other than soak aways if this won't work?

red menace

If the soil type hadn't changed you would prob be best to stay going until it did. As the post above said 1.2 metres so that about 5 feet or so.
Getting into mini digger territory as 5 feet is a long way down with a spade....

Minder

Is the site on a slope?

azzeretti

Minder wrote: »

Is the site on a slope?

Not really, no. There is some ups and downs but by no means a slope. Why?

Minder

Your previous question - other ways if a soakaway doesn't work. On a slope, the ground is sodden with run-off from land higher up the slope.

How bad is the problem?

azzeretti

Minder wrote: »

Your previous question - other ways if a soakaway doesn't work. On a slope, the ground is sodden with run-off from land higher up the slope.

How bad is the problem?

Honestly, if I described it, you would not believe me. For the last 2 months, most of garden is pretty much under water, literally.

red menace

pics of the site would be a help.
I suspect if he is going to be scraping off soil to change the profile of the site,that way all the water can be made to run to the soak hole and into the ground.
From what you describe it sounds like that is the only way to dry your site.

Minder

The alternative to soakaway is a connection to a surface water sewer. But the trial hole is a good starting point for a soakaway - look for a change in the sub soil as the machine gets deeper. Also ask the landscaper - he may have experience of the local area and how the land holds water.

Also ask him why the dimensions he quoted are appropriate - 12 x 10 x ? - how much water will that hold? How much water will collect across the one acre site? If it was a sump rather than a soakaway, will it be large enough to hold the water?