Construction - French Drains - Rationale, Fabrication and Installation (cont'd)

This is the second of three posts on the French drains.  The first post delt with the rationale for and the prefabrication of the drains from culverts.   This post bridges from the prefabrication phase to the installation phase in the third post.

Overall Design
The individual homemade drains were detailed in an earlier post and can be seen in the distant background in second photo.  The

12" double wall conduit for the manifold

perforated portion of each 8' drain is 60' long to which enough additional un-perforated pipe was added to reach daylight downhill near the future rain garden or to empty into a common 12" manifold that went to daylight near the garden. The perforated sections and one 20' section of unperforated pipe were assembled ahead of time. The remainder of the system had to be assembled on site as the trenches became available.

 

Entire system ready for installation

Professional Help
The need for an elaborate system of French drains and its cost was not anticipated when budgeting originally. Consequently, I tried to imagine ways of trenching and backfilling without professional help (original plan). However, it became clear that the additional cost of help could somewhat, but not entirely, be justified by savings of time, materials and equipment rental. 

In terms of time, it took only two days to lay the French drains in mid-April which allowed us to get a leg up on what is usually our wet season in May and early June. This was perhaps the primary benefit of seeking help.

In terms of materials, rock dropped from a height of 5' is sufficiently self-compacting to support foundation footings and slabs. Therefore, rock is preferable to soil for backfilling because soil has to have the correct moisture content then must be compacted in shallow layers (lifts) with compacting equipment. In terms of material costs, the pay-off from using professionals is that they have the right combination of equipment to minimize the amount of dirt that has to be removed.and replaced with expensive rock.

In terms of equipment rental, no mini-excavator or compactor was necessary -- no small savings.

Brian Hayes Construction
Consequently, we did the right thing in hiring Brian Hayes, a local contractor, at a time when he was not overbooked and able to do the work himself.   As luck would have it, his Dad (with a lot of help from a youthful Brian) had DIYed their home-place in which, as a science teacher, Mr Hayes had incorporated sustainability concepts that were way ahead of the curve.  

Brian was not only willing to work with a DIYer but seemed to take genuine ownership of our energy-neutral project (perhaps partly in his Dad's memory?).  He provided the rock and did the trenching and backfilling for the French drains. He facilitated the rapid installation of the AGS system, he graded the house footprint to final depth in preparation for the rock sub-base for the concrete slab and he dug the shallow trenches for the foundation footings -- all of this in essentially five working days despite having to deal with an amateur track-loader operator (me) and a volunteer crew.  Conservatively, he saved us as much as two months time over DIYing the French drains and the AGS system.

Brian's invoice equaled +/- 20% of our total home-building budget.  The ratio for the French drains to the AGS system was 8:2, i.e., 80 % of the 20% went for the drains, a cost that is somewhat easier to reconcile by knowing that any conscientious construction let into our wet hillside would have required them irrespective of our need to use them to protect the AGS system.  The 20% for installing the AGS system was more in line with what was budgeted.  

Our hope is that unanticipated synergies during the remainder of construction will help to offset the French drains costs.

Prime Examples of Unanticipated Synergies Immediately
Our project already has enough life to attract unanticipated synergies, most of which so far have taken the form of volunteer labor and opportunities for salvage.  Brian brought another dimension.  His 35-year experience in the field, and I, with my research-based design, were able to collaborate amiably on the fly as the French drains and AGS conduits were laid, producing much better outcomes than I would have had sticking to the original design. Since part of Brian's business is razing old buildings (60 or more per year), he now plans to watch in our behalf for salvaged lumber opportunities - another unanticipated synergy.  Still another:  I thought our narrow dead-end street precluded semi-truck deliveries, which meant unloading onto a main street and using the track loader to schlep individual skids several blocks (as I did with the pallet of geo-textile material). Through Brian's connections, a vacant contractor's property at the end of the street will be available as a turn-around for semi's.

First the Driveway
But back to the actual construction of the French drains, the first thing Brian prescribed was to site and rock a driveway to give access for the delivery of rock for the French drains and a place to dump it.   It also would be mandatory for the

Rough-in for drive and turn-around area near future garage

Ready-Mix trucks and others delivering materials later. Accordingly, I used garden hose, then marking paint, to outline the driveway to the street and the turn-around area near the future garage.   With the track loader, I removed about a foot of soil. 

As a naive DIYer laying out his first driveway with garden-hose-and-paint precision, I envisioned a nice driveway from the time the first gravel arrived until we moved in. Wrong!   By the time gravel truck after gravel truck arrived, it was deeply rutted and had to be repaired over and over.  And, when dumping space closer to the house footprint was usurped by installation of French drains, the driveway to the street became the default site for several more truckloads of gravel which spilled over the sides of the original driveway footprint to the extent that it became impossible to tell exactly where the driveway was supposed to be.  Oh, well!

To continue the story on French drains go on to the third post.

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.