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Soil Percalation Rate & Pressure At Depth

Soil Percalation Rate & Pressure At Depth

(OP)
I'm working on a small house on a confined lot. We have some drains in a walkout along the side the house and I would like to determine the viability of draining them to a dry-well that is 9 feet below the ground at the back of the property (the basement is 9 feet deep and the dry-well needs to below the footer).

The house is on a hill and well above the water table so that isn't an issue.

I'm concerned that after several days of heavy rainfall water will reach the dry-well and fill it up rendering the drains useless.

I'm attempting to calculate the rate at which surface water will percolate down and into the dry-well. and how much volume I can expect.

I know the water will come down in a cone to the void, but I'm not sure how to account for the fall off of that cone. Furthermore I'm concerned that as we go deeper under ground pressure will build and increase the rate at which water is being pushed into the dry-well.

Are there any examples of how I might go about calculating this, and is my concern about the pressure valid?

Note: I have my percolation rate already figure out. I'm just confused on how to quantify the rate at which that water will percolate into a void deep underground.

RE: Soil Percalation Rate & Pressure At Depth

There really isn't a significant effect due to pressure at the depth you talking about.

You indicate that you already have the percolation rate figured out. How did you do this? Was it field tested? If tested, was it done at a depth of 9 feet?

Other questions that come to mind include:

1. how did you figure the amount of water that will reach the dry well?

2. what happens if there is more rain than you expect? Do you get water in the basement? How much damage/trouble does water in the basement mean for you?

At a minimum, I would install a sump pump to handle the water if the dry well can't keep up.

Mike Lambert

RE: Soil Percalation Rate & Pressure At Depth

(OP)
You indicate that you already have the percolation rate figured out. How did you do this? Was it field tested? If tested, was it done at a depth of 9 feet?

Yes we tested at 9 feet and at the set grade above. The basement is still partly excavated and hasn't been back filled. It is the same. The entire property is basically built a fine silt/clay infused beach sand. That goes down from probably 40 feet. (height of the hill). I can't be 100% sure but property at the base of the hill have similar soil. At a minimum that same soil extends at least another 10 feet below my footer as that's the deepest we excavated on the property.


1. how did you figure the amount of water that will reach the dry well?

We used a hundred year storm calculation (1 inch of water in an hour for the thunderstorm). Hard rainfall would be one inch in a day. We then just took the surface are being drained to the dry-well. What I would like to figure out is how much of the surface above the dry-well will drain into it.

2. what happens if there is more rain than you expect? Do you get water in the basement? How much damage/trouble does water in the basement mean for you?

I have considered installing a pump and will probably do that. At this point I don't need to solve this calculation but I'm actually really interested from an academic stand point. Its been bugging me and I want to be able to quantify whats going to happen a bit better.

I've been reading about Darcy’s Law. I figured that would be part of the calculationg as it takes into account horizontal and vertical movment.

RE: Soil Percalation Rate & Pressure At Depth

Draw yourself a two dimensional flow net with the dry well full and the position of basements. That will be a conservative approach, instead of a three dimensional approach. The house basement and any others may take on water from that source of water, in addition to any other source, such as run off from nearby via ground flow paths. Take into account the discontinuities, such as the looser backfill to the trenches carrying the pipes as well as looser backfill to the building. Filling and overflowing of the dry well may not be your main potential problem. I am assuming you are not constructing the dry well with all of its volume below the elevation of the foundations so that the head there may be above the basement floor elevation at times, with the worst case when it is full.

RE: Soil Percalation Rate & Pressure At Depth

your 100-year storm rainfall depth is 1 inch? and a 100-year, 24-hour total of just 1 inch also. seriously? You may want to go back and check that, it sounds very low. For instance, 100-year, 1-hour rainfall depth would be 2 inches and 100-year, 24-hour would be 3.4 inches and this is in the desert. In addition to volume, you will need to check peak flow rates. During the peak of the storm, it may fill up very quickly. I would not plan on a drywell being able to handle the 100-year peak flow rate, you probably need an overflow. Also, plan on de-rating your infiltration rate to handle eventual silting up. Especially since you say your soil has lots of fine silt and clay. Around here that de-rating factor would be about 0.167 (multiply your design infiltration rate by 0.167). Suggest you also check and see if a permit is required from the state, it would be required here.

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