Investigation of Soils at Lafayette Oaks Bioswale (Spring 2016)...
Why: Prolonged Ponding.
While a good rain garden and bioswale is designed to collect and hold water it is also important that the water soak into the soil in a timely fashion. If water stands for too long mosquitoes can become a problem and plants may not thrive (even well adapted rain garden plants need to breathe every now and then). |
Why (cont):
As originally constructed the Lafayette Oaks bioswale has typically taken longer than a week to soak all of the water into the ground despite only being 4-6 inches deep. When multiple storms come in succession pooled water can remain for several weeks. |
Evaluation of Soil:
The rate at which water soaks into the ground is primarily a result of the type of soil and the amount of soil compaction. Water drains quickest through sands, then silt, with clays draining the slowest. Soil compaction slows infiltration as it reduces the pore spaces through which water can move. The surface soils in the bioswale consist of a relatively permeable mix of silt and sand, which infiltrates water fairly well. |
After about 18 inches the gritty top soil material gave way to a sticky moist clay material.
SAFETY FIRST: Utility locations had been identified previously at this site to ensure that no buried utilities would be encountered when digging. |
Hooray! After about 1-2 feet of sticky clay, we began to remove good gritty permeable soils once again (above). These soils extended another 2-3 feet until the auger would not reach any further. The spoil piles from the different layers are shown in the picture to the right.
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Spoil Piles of Different Soil Layers.
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... a more permeable gritty soil layer was encountered. The more permeable soils again extended to the final depth of the hand auger boring.
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Both soil borings were left open to see whether pooled water in the bioswales could effectively drain through the holes punched in the clay lens and effectively reduce the amount of time water remained ponded in the swales. SAFETY FIRST: A long post was left in one of the holes to prevent anyone from accidentally stepping into the opening.
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... & then we wait for the rain ...
March 31: View of other dry swale.
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Success!3-5 day time to completely drain the swales is a significant improvement. The infiltration chimneys are a success. Now how to keep the preferential drainage path open. Click Here to see ...
Installation of Infiltration Chimneys |