Search This Blog

Saturday, 29 September 2018

Installing systems on Soils with high ground water

Working on an interesting system this week.

We did a site evaluation for subdivision on this property several years ago. This summer, the client asked us to design a system for a three bedroom modular unit they were going to move onto the property. Our notes indicated that there was redoximorphic features between five and eight feet in the area where we had previously done the test pits so we know we would have a choice of a raise sand mound or advanced treatment to media based laterals.
The client chose to install the modular about 1500 feet south of our original pits so we knew we would have to confirm the new site as being similair to the area of our original pits.
When we dropped by to check on the progress of the modular install we noticed that there was large depressions in some areas around the home from the transport dollies. This lead us to believe there was near surface ground water, which left the ground too soft to support the move.
A call to our friends, the well drillers, confirmed that they had encounter water flow at about six feet when they installed the water line to the house. No way this can be good.
We dug new test pits near the house in three different locations, encountering flowing water in all three. We needed to find a better location for the field or completely redesign the system. 
A careful visual inspection of the area within 500 feet of the house revealed a bit of a plateau to the north of the house that was followed by a bit of a slope in the direction of the water flow. The area to the West was upslope for about a mile. Not a steep slope but gradual, gaining about 100 feet in elevation. Our previous test pits had revealed that the ground water was flowing in sand lenses sandwiched between clay loam horizons. Knowing this, the hope was that this plateau might be high enough to allow the down slope water flows to be deep enough under this area to give us our required separation between the vadose zone and the redoximorphic layers.
We dug a test pit at each end of the plateau to confirm this. Happily, we had about 66 inches of good soils at one end and about 72 inches at the other. This gave us the required separations and we were good to go with our laterals.
However, we still had a problem with siting our advanced treatment unit and dose tank. As many of you know, our Norweco Singulair Green advanced treatment units are heavy rotomoulded plastic tanks which give us little resistance to buoyancy, prior to filling them. Our dose tanks are also similarly constructed. For us, having light weight tanks is usually a big bonus, as we often have to move them long distances or on terrain that is not always suited for big picker trucks. In this case, heavier tanks might have helped.
The only location for the tanks near the house was in the vicinity of the water line which we knew had a problem with high water. Sure enough, we found flowing water at about six feet below the surface. Not enough dry soils to install the tanks with any degree of confidence. What to do. Either we needed to change our ATU to a concrete based unit or we needed to drain the swamp. We went for the latter.
Remember that the ground fell away to the East. About 100 feet to the East, the land is lower than the bottom of our advance treatment unit. Here is the solution that we came up with. Quite simple, which is almost always the best solution.
We over-excavated the hole by about a foot and installed drainage rock in the over excavation. We than embedded drainage tile in the drainage rock and covered the stone with filter fabric. The drainage tile was than extended 150 feet to the East until it daylighted.
This gave us a permanent drain under the Norweco Singulair Advance Treatment Unit so no matter how wet the future is, out ATU should aways sit on a firm foundation.
One thing to remember, don't actually daylight drainage tile. Put a pile of drainage rock around the end so that the water can flow freely but that the end of the pipe is properly protected. Not only does this protect the end of the pipe from getting crushed or blocked, it prevents rodents like muskrats from setting up house keeping in the pipe.
Two things to note.
• just because one area of the property has good soils for septic, don't assume it will be the same over the whole area. In this case, we had several different situations within 100 feet of each other. If we had not done our due diligence when the client relocated the home, we would have had a compromised system.
• when faced with a soil-based dilemma, carefully observe surface features to see if there is any hint of a solution or of more problems based on the lie of the land. In this case the high ground water areas did not show water favouring vegetation but did show grasses that were somewhat greener than the surrounding areas. The ruts from the modular move also gave hints as what to expect under the sod.
When you look at the topography, together with the information revealed by the soils inventory of the test pits, you can sometimes understand how the near surface waters might be flowing. Knowing how the subsurface waters are flowing will allow you to consider which areas may have the solution to your problem.
In this case, we could have simply have given up and said the client would have to install a holding tank and been done with it but that would not have been fair to the client.
The clients rely on our expertise and they deserve our best effort. I understand that not everyone will have the experience that we have obtained by working the industry for over forty years so if you are just getting started in the industry and you run into difficult situations, don't be afraid to reach out to some of your colleagues that have been around for a while. In southern Alberta, we are blessed with a number of professionals that are happy to lend an ear and give some advice.

No comments:

Post a comment