Saturday, April 4, 2015

Construction - French Drains - Rationale, Fabrication and Installation

This post is the first of three on the French drain system installed below the house to prevent ground water from comprising the Annualized GeoSolar system.  

Why Are French Drains So Important for Our Project?
Just to review, we are using a passive solar system called Annualized GeoSolar in lieu of conventional heating and air conditioning.  It captures the heat from the summer sun and
stores it in the soil under and adjacent to the house. 

The keys to the AGS system are conduits (red in top drawing) angled slightly upward from the solar collector in front of the house (blue). A total of ten conduits fan out through the soil a few feet below floor of the house then terminate in a common solar chimney behind the house (green).   Heated air traveling slowly and passively from the collector to the chimney gradually raises the ground temperature to a floating year-around preferred temperature of 74 degrees (+/- 4 degrees). When heat is lost through the envelope of the house in winter, it is replenished from the enormous thermal
mass under and adjacent to the house. Since heat seeks cold, the heat that enters the envelop during summer goes immediately into the mass to augment the heat already being produced by the solar collector for the upcoming heating season.  Of course, during cold months, the chimney has to be closed to keep counterproductive cold air from dropping into the conduits.

Ground Water Problem  
For the system to work, the soil under and adjacent to the house must remain perfectly dry because water carries the heat from the conduits to the water table before it can be used to condition the house.  In dry climates with a low water tables, all that is needed to keep the soil dry is an "insulation-watershed umbrella" (orange in the second drawing) which not only insulates the soil for a distance outward from the house to increase the size of the thermal mass, it also keeps rain and snow-melt from saturating the soil in the thermal mass.  In wetter climates with high water tables, the umbrella is not enough.  The water table under the house (curvy blue line in second drawing) must be lowered sufficiently that it is unable to steal heat from the conduits.  In our case, French drains are necessary because the water table is known, through the monitoring of four piezometers over several seasons, to rise in the late spring to the level of some of the conduits and much too close to the others.

Typical French Drains
Perforating with circular saw

Typically, a French drain lies close to the surface of the soil such that it is possible to fabricate the drain safely by either entering a shallow narrow trench or reaching in from above. Also typically, the drain is about a foot square in cross-section and contains clean stones of consistent size, say, 1" in diameter, with a 4" perforated pipe wrapped with geotextile fabric embedded in the stones a couple of inches from the bottom.  Then the entire drain is wrapped in geotextile fabric. Most of the water trickles through the rocks. The pipe provides rapid egress for water when the rock bed is full. The outer geotextile fabric keeps soil particles from entering and clogging the rock bed while that around the pipe provides additional insurance against a clogged pipe.
Hog ringer with hog ring  

Homemade French Drains

Our situation is unique in that, protection for the AGS conduits required the drains to be eight to ten feet below floor level (black line at the bottom of the first drawing) and situated in narrow trenches too dangerous to work in.  Consequently, I pre-made the drains and had them ready to drop into the trenches with ropes from above.  

The drains were fabricated from eight inch single-walled culverts that were perforated on both sides in the lower hemisphere with a circular saw whose blade guide was rigged to control the depth of cut. The perforations were about 4" apart. Then three culverts were joined together with one unperforated section using split unions secured with wire to make 80 ft lengths. Finally, the perforated sections of the culverts were wrapped with perhaps the only geotextile fabric extant that will not be clogged by our wind blown loess (silt) (research on fabrics).   The 12' width of the fabric was cut by the supplier into 36" strips which was the perfect dimension for overlapping  the culvert on top, rolling the edges
Hog-ringing the seam
under twice and fastening with hog rings. In order to eliminate the possibility of the culverts rotating relative to each other during handling and thereby causing some of the perforations to be in the top, less effective, hemisphere, hex headed sheet metal screws were used to secure the culverts to each of the wire retained split unions. 

Since at the time of lowering into the trenches, the textile fabric roll on top of the culverts
was to be used to position the assemblies in the trenches, it was important to prevent rotation of the fabric on the culverts during handling.  So a few sheet metal screws were driven through the roll into the culvert. 

Finally, the up-slope end of the pipe assemblies were closed by doubling the fabric back
Hog ring in place

over the culverts a few feet, wrapping and folding then securing with hog rings.and sheet metal screws similar to the way the fabric was handled along the length of the pipe.

Here we have focused on the individual drain. A succeeding post discusses the entire French drain system comprising seven legs -- some joined to a common "manifold" before reaching daylight and some going to daylight directly. Excavating and backfilling the trenches.are covered as well.

For a complete story, continue on to the  second post on French drains

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