Tuesday, August 5, 2014

Design - Excavation (Cont'd)

This post was written at an early planning stage whereby I intended to minimize cost by doing most of the work myself.  When it came time actually to do the trenches for the French drains and the AGS conduits, we were in the midst of the rainiest Spring in history. It became necessary to get as much work done as possible between rains so we enlisted professional help for the trenching as well as for grading for the slab floor.   It took several posts to cover the installation of the French drains and AGS conduits. Here are links to those posts:  First post on French drainsSecond post on French drainsLast post on French drains,  First post on AGS system

French Drains
The typical French drain is a trench about a foot square in cross-section with a perforated drain pipe it it.  The pipe is laid on a 2" bed of coarse gravel then the trench is filled to the top with more gravel.  Usually, a "sock" made of geotextile fabric surrounds the pipe.  The same fabric completely lines the trench before any gravel goes in, then is folded across the top of the gravel before back-filling with soil.  (Information on French drains.)  A French drain is depicted by the lower black line in the nearby sketch of an early concept of hour house.  The drains will lie several feet below the conduits for the AGS system (red) so as to keep the soil above them dry year around for proper functioning of AGS.

Our French drains will be slightly different in that we will need a geotextile fabric that is designed for the type of fine silt laid down on our bluffs by the wind after the glaciers receded.   The type of fabric that is available in retail outlets would quickly clog and make the French drains useless. (Comparative study on geotextiles.)  

Back-filling to the Level of the AGS Conduits
The conduits for the Annualized GeoSolar system will comprise ten 4" diameter mostly flexible pipes ( red in the sketch) running between the solar collector (blue) and the solar chimney (green).   (See discussion of AGS conduits.)

After the fabric is lapped over the French drains, we will back-fill with soil until we get to the level necessary to support the AGS conduits.  The backfilling will have to be done in "lifts" (layers) of 8" or less then compacted with a rented vibratory drum roller, either the kind the operator sits on or walks behind and controls wirelessly from a lanyard around his or her neck.  

Compactibility of the Soil
The silt that we have is not as compactible as gravel or soils containing gravel and sand, or even clay. Therefore, it is necessary to amend the soil with something like agriculture lime in order to reach the level of compaction necessary to support the footings and the slab floor (input from retired civil engineer).   If so, we may have to rent a tiller for the track loader to stir the limestone into the soil before compaction.

Completing the Back-fill
After the conduits are laid -- two to a 6' wide trench positioned as far apart as possible -- filling will proceed in compacted lifts until it reaches the level necessary to support the gravel base under the concrete.

Finishing the Floor Excavation
The slab and its gravel base need to rest on undisturbed soil.  Since the loess silt is so easily disturbed, we will deliberately not excavate initially to the final depth for the floor. Then, after the track loader and other heavy equipment has stirred up the top layer of soil during excavation and back-filling for the French drains and conduits, we will carefully remove additional soil until the exposed soil is either undisturbed (between trenches) or thoroughly compacted (over trenches).  We can probably remove most of the loose soil with the track loader if done in a controlled way.  However, we will be removing some the hard way -- by hand. 

Ground-breaking Schedule
Because of the peculiar nature of our soil, the long time it will take us to excavate and our limited budget, the soil engineer recommended not starting excavation until the late summer dry season.  Then, if it rains, it will not likely be torrential and, with tarps, we might be able to keep the critical parts of the excavation from eroding or becoming saturated. 

Wind Blown Loess
Based upon the four core samples, our soil seems to be mostly wind blown loess, a type of fine silt with an interesting history.  It began as out-wash from the glaciers as they receded, then it was picked up from the Mississippi River flood plain and deposited on top of the bluffs by the wind.  Our deepest piezometer hole was 20' deep and the soil core from the bottom of the hole looked very much like the soil beginning just below the topsoil -- no clay, sand, rocks, etc., just silt.  The soil engineer says that it is 50' deep in some nearby locations.  

The particles are extremely small and elliptical-shape such that the wind action aligned them parallel to one another before rainwater locked them tightly together. Today, cuts have been made through these bluffs for interstate highways and the resulting vertical surfaces have remained uneroded as long as the organic cover has not been disturbed.  If the cover is disturbed, like we will be doing, heavy rain not erodes the soil very quickly, but, where water pools, it becomes totally unmanageable even for seasoned contractors, much less a DIYer.

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