Construction - The Last Major Dirt Work (Cont'd) - Final Grading, Loess Lessons and Rain Garden Design

The previous post on The Last Major Dirt Work emphasized the retaining wall construction that needed to be done before the final grading, topsoil replacement and seeding could be done.  This post covers two more sections of the insulation/watershed umbrella as well as the grading, top-soiling and seeding. It also introduces the subject of rain gardens by looking at their generic design. The next post will explain how rain gardens were used specifically for our property.

Reminder: clicking on any photo enlarges it for better viewing.

(Nearly three months have elapsed since my last post.  I forsook the computer for a tool belt full time as we family members rallied to rehab my sister-in-law and her husband's Texas home that was flooded by Harvey. Their homelessness ended the first weekend in January so I am home and blogging again.)

More Insulation/Watershed Umbrella

Umbrella installation behind the garage; notice the plastic
 sheeting, two layers actually, emerging from the bottom of
 the retaining wall to be shingled over the sheeting in the
 umbrella; similar flaps lie behind the wall waiting to be 
shingled under the umbrella behind the house when it is
 installed

At this juncture, the umbrella around the
house is more than half done.  Part of it lies under the concrete floors of the screened porch and garage and the rest that is done lies in front of the house. The two umbrella sections mentioned here lie behind and in front of the garage as well as in front of the main entrance to the house.  The usual layers of sand, insulation, sheet plastic, recycled carpet and topsoil were used as described in detail in the previous post on the umbrella installation in front of the house.

Final Grading
The dirt work discussed herein lies in

Topsoil over the umbrella behind the garage ready for
 seeding; the last major section of umbrella yet to be
 installed lies beyond the retaining wall, i.e., behind the
 house

front of the house and east of the garage.  Since we own (and live in) the old farmhouse east of the building site, we have the option of carrying the grading onto that property as far as necessary to flatten contours and funnel surface runoff from both properties into a series of rain gardens.  After moving superfluous soil out of the way of smooth contours, berms and rain gardens, and, after laying down +/- 3" inches of topsoil, we were able to plant grass seed in time for germination before cold weather -- barely.  Grass is an interim crop to hold the soil then, as discussed in a

Friend Kerry helps with the umbrella installation in 
conjunction with setting forms for the concrete
 sidewalk and garage apron

previous post on native plants, the grass will be replaced almost entirely by environmentally-friendly (no mowing, no fertilizing, no watering) plants that are native to the Midwest.

Harnessing the Impetuous Loess
Building in the Mississippi River bluffs has been a mixed blessing.  The rough terrain provided the south facing hill that we needed for a passive solar build and, quite unexpectedly, within a community rather than out in the country, but the wind-blown loess soil that covers the bluffs has taken us to school.  We have learned that it is quite stable when left undisturbed and covered with vegetation because the grains of soil are elliptical in shape and, when lined up like soldiers by wind action long ago, were locked together. There are places nearby, for instance, where cuts through loess for interstate highways have shown very little erosion.  When disturbed, however, the loess (aka silt) is unstable in both wind and heavy rains --  it blows like talcum powder and erodes like lumps of sugar.  Controlling erosion during construction has been a major challenge; the retention pond that catches runoff before it can leave the property has been re-dug and cleaned out at least once a year since construction began. Parenthetically, plants thrive in the loess despite its being almost plaster-of-Paris-hard when dried out. 

View of building site before footings
 were poured; the denuded areas are
where dirt was excavated; the
somewhat denuded areas north of
the house are topsoil and subsoil
storage piles; notice the solar
collector in front of the house; the
denuded area surrounded by 
greenery in the middle bottom of the
 photo is the retention pond catching
 runoff during construction that was
 converted into one of the rain
 gardens 

Unavoidable Site Disturbance
Unfortunately, it is impossible to build an earth contact, Annualized GeoSolar* conditioned house without moving a lot of dirt, which violates one of the major tenants of green building -- that of minimizing building site disturbance.  Not only was it necessary to excavate into the side of the hill for the house itself, we also had to install the insulation/watershed umbrella that extends sub-grade +/-20 from the floor of the house in all directions.  We had to use soil-disturbing measures to amend the water table so that it did not compromise the AGS system and we had to install the large solar collector and conduits for the system.  And the final grading has been extensive in an attempt to direct runoff into strategically located rain gardens so that it does not soak into the soil where it would compromise the AGS system or stay on the surface and carry silt and contaminants onto the street or into the neighbors' lake.  The solution to both problems is a series of strategically-place berms and five rain gardens.


How Do Rain Gardens Work?
In a nutshell, the purpose of rain gardens is to corral surface runoff long enough for it to soak into the soil and, if flowing, flow underground where it is purified instead of on the surface where it carries pollutants. The plants in and around a garden are tiered such that those liking wet feet, such as sedges, occupy the bottom, those who can tolerate a wet soil, but do not like wet feet, rim the slope of the garden and those liking it a bit drier form the periphery along the top edge of the garden and often spilling over onto the surrounding grade.  In our case, the rain garden plants will be native rather than cultivars and non-natives.  For an authoritative resource on rain gardens, link to the Missouri Botanical Rainscaping Guide.


Percolation Test
In order to avoid stagnation that would kill plants and incubate mosquitoes, a percolation test is done before digging to be sure that the retained

Downloaded photo of a rain garden containing
what looks to be mostly sedges in its deepest
section; notice the graveled overflow to the left

 water will soak away fast enough.  At least two post-hole-size holes are dug 24" deep a few feet apart in the lowest area of the proposed rain garden, usually just inside of the proposed berm that will serve as the dam.  The fresh holes are filled with water and the drop in water level -- the "perc rate" -- is timed over the span of a few hours.  Then the process is repeated the next day.  The first test simulates rainfall on relatively dry soil; the second simulates rainfall on soil that is already saturated by prior rain.  A perc rate of 1/2" per hour for the second test is considered minimum for a functional rain garden. The rate for the first test is typically faster.

The perc rate per se is not the entire story however.  A good share of the water collecting in a rain garden is utilized by the plants before it can soak into the deeper soil making the water disappear much faster than if the water-loving plants were not present.

The exact manner in which rain gardens are incorporated into our landscaping is the subject of the next post.
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*Click on "Featured Post" in the column at the left for information on AGS