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Major headache here.
I have recently purchased a new home in the Olympia Wa. area, it sits at the bottom of a hill(50ft. and probably an 40 degree grade. There is some serious seepage out of the hill that manifests itself right at the hills base. The home is built approx 10ft. from foundation to the base of the hill and the roof overhang is approx. 5ft. out from the foundation. This 10ft. area is very swampy all year round and especially swampy at this time of year.
At some point in the past a feeble attempt at curing this problem has been attempted with drain tile about 2ft. deep. The hardpan seems to be about 5ft. deep so most of the water makes it under this drain and to the foundation then it leaks down into the crawlspace.
Where do I learn about drainagge systems for this kind of problem ? Also are there landscape blocks that offer any kind of structural rating. I am assuming that I will have to shave away a bit of the hills base in order to get an mini-excavator or back-hoe in to dig the drain, I dont have the money or the hydraulic knowledge to do a solid concrete retaining wall but I can move landscape blocks all day long which allow seepage through the seams and dont require engineering. Ideas and references will be greatly appreciated.
I did hire a geo-physical engineer prior to purchase who tells me that the hill is stable and not prone to sloughing.
Thanks. John W.
Replies
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John,
There are many of the block type of retaining wall structures such as you mention. Your local builder's supply or landscape center should have data sheets on several different ones, as well as simple guidelines for design and installation. You may request further information from the representative of the company, I got a ton of information free just for asking when I built my first one.
Anything you do will not prevent water seepage into your crawlspace unless you also install a deep subsurface drain to intercept and divert this water. You will have to go into the hard subsoil layer to make sure you are intercepting all the water. This could be done in conjunction with a retaining wall structure, but you may decide that you don't need the wall if your objective is to eliminate the water, since the wall without the drain will do nothing to prevent water intrusion under your house.
If you have access for a small excavator then digging the drain will not be a huge job, and the installation of the drain pipe and rock is something you can do yourself with a wheelbarrow and shovel (and lots of sweat!)
E-mail me with some more details and I will try to give you some guidelines on installation of the drain, since this is what I do for a living (in another part of the country)
George Wooster
[email protected]
*John,My boy tells me it's been a Wet Winter up there. Which is why I am in Newport Beach, CA until spring. Any drainage problems will show up now.Sounds like your foundation intercepts a perched water table atop the hardpan. Common enough in Puget Sound. Being from Bellevue, I know. What soil(s)do you have in the hill?Check the sizes of the mini excavators at a rental yard to see what you can fit in. Going to need a deep drain. But, where does the water go from there? BTW - we have found the 4x100' roll of landscape fabric from Costco to work well in French drains, along with 1.5" washed drain rock. The whole thing is wrapped up into a "burrito". JD Fowler had the 4" perf in 100' rolls with drain sock on it for less than Home Dpt. was selling the perf and the separate sock.If you are 5' below grade, safety in the trench is going to be an issue. Sure don't want the sides to fail.Are you sure you want or have to shave the hillside? It could cause more trouble than it's worth. If Thurston County is anything like King County, they will want a building permit and engineering and a site plan and so on if your retaining wall is over 4' high. Since hillsides sliding onto homes is all too common in the N.Wet, expect them to be a bit gun shy - which means that a PE may be needed to engineer the wall, wet stamp the plans and assume liability risk for a commenserate fee.Even better, given the grade of that hill, I wonder if it would be considered a Sensitive Slope in King County - with 50' setbacks, top and bottom. Does Thurston have a similar ordinance?We did a somewhat similar hillside retaining wall in Kirkland designed by a hydrologist PE - but it was in Ultra Blocks, which are about $40 FOB the plant and weight about 3000# each. They work very well - you can see an example in a wall on I-5 at Swamp Creek and other places. Getting them to site and placed is another story. The trucking $ kills you. Boom truck is needed to place at $110/hr and up. One project with their cheaper cousin, the eco-block, costed out at $50/block placed. A working drain at the base of the wall and riprap backfill behind is vital to prevent hydraulic pressure blowing it out. J Wells
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As you found out the water stops at the hard pan and looks for the next low spot (your basement). The key is to cut off the water flow at the hard pan. My recommendation (assuming you can daylight the drain) would be to dig down to the hardpan and dig chip or hammer a horseshoe shaped curtain drain all around the back perimeter of your house. You may want to take some test bores to make sure that the hardpan generally slopes downhill and away from the front of the house. This curtain drain should pitch to daylight, assuming the hardpan in these two areas does not pitch back toward your basement. Next install or construct a good foundation drain system and pitch that downhill as well.
-Rob
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John,
You have some very good advice and from people that appear to know something of how water moves through the soil. We call your problem seeps or spouts here in the midwest.
As Rob said, the water moves down through the soil, strikes the hardpan, bedrock, etc., and moves along that obstuction until it exits downgrade at either the side of the hill or your basement.
This flow must be intersepted UPGRADE and diverted from your foundation just as Rob indicated. I have seen tile crews place field tile through the center of a seep, thinking this will solve the problem. It does, around the tile, but nowhere else!
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Major headache here.
I have recently purchased a new home in the Olympia Wa. area, it sits at the bottom of a hill(50ft. and probably an 40 degree grade. There is some serious seepage out of the hill that manifests itself right at the hills base. The home is built approx 10ft. from foundation to the base of the hill and the roof overhang is approx. 5ft. out from the foundation. This 10ft. area is very swampy all year round and especially swampy at this time of year.
At some point in the past a feeble attempt at curing this problem has been attempted with drain tile about 2ft. deep. The hardpan seems to be about 5ft. deep so most of the water makes it under this drain and to the foundation then it leaks down into the crawlspace.
Where do I learn about drainagge systems for this kind of problem ? Also are there landscape blocks that offer any kind of structural rating. I am assuming that I will have to shave away a bit of the hills base in order to get an mini-excavator or back-hoe in to dig the drain, I dont have the money or the hydraulic knowledge to do a solid concrete retaining wall but I can move landscape blocks all day long which allow seepage through the seams and dont require engineering. Ideas and references will be greatly appreciated.
I did hire a geo-physical engineer prior to purchase who tells me that the hill is stable and not prone to sloughing.
Thanks. John W.