Construction - The Last Major Dirt Work - Two Retaining Walls and More

It is late September in the lower Midwest and the first frost could be a month away. Even with global warming (which seems to have pushed back the deadline for planting grass and expecting it to germinate before cold weather), we might have at the most three weeks to finish the dirt work and plant seed.  Consequently, construction is now on hold until the dirt work is done.

"Dirt work" means several things:  constructing the

retaining walls that are essential for earth sheltering, finessing the exposed mouths of the drainage tiles, final contouring of the subsoil and redistributing the top soil that was removed for construction. Secondarily, a series of rain gardens need to be installed for controlling the run-off originating from our present residence next door and continuing down through, and augmented by, the hillside configuration of the new building site -- a total run of several hundred feet.  And some remaining sections of the insulation/watershed umbrella has to be installed in the process.

As for retaining walls, the plans call for two major "umbrella walls" plus two of smaller size that did not involve the umbrella.

Reminder:  To enlarge any photo for clearer detail, click on it.

The Umbrella Behind and Under the East Retaining Wall (one of two "umbrella walls")
As explained in a triad of prior posts, the umbrella comprises layers of sand, foam insulation board and 6 mil plastic sheeting plus a couple of layers of recycled

North elevation; notice transitions from 12' center wall and
8' walls on either ends; the dark line is the architect's take
on the final grade, including two retaining walls

carpeting.  (The design of the umbrella surrounding the house is based mostly on that described by Hiat in his must-read self-published treatise, Passive Annual Heat Storage.)  At the walls, the umbrella drops about 4' from the height directly behind the house to a lower level NE and NW from the house, which means that the umbrella runs behind and under the walls. The section of umbrella associated with the walls must be built first and the walls backfilled before the rest of the umbrella can be installed. 

Most of the north earth contact wall of the house is 12' high then it transitions abruptly to eight foot walls near each end (see drawing).  The most

Excavation complete, metal posts in
place and a sand bed for the
horizontal insulaiton

efficient way to maximize earth contact for the house while dealing with the transitions is with retaining walls.  Both the east and west umbrella walls have to be about 5' high next to the house and, by running them diagonally east and west, can be as low as a couple of feet 20' out from the house.  The excavations for the two walls were about half track loader work and about half hand work and took me the biggest part of two days. Then it was time to install the vertical and horizontal insulation and plastic sheeting that allows the umbrella to drop beneath the walls.


The horizontal insulation under the walls is merely a matter of making sure the excavation drains well, is smooth and

First layer of plastic lies under the horizontal insulation
and behind the vertical vertical insulation

covered with enough sand that the insulation can be nestled into it to handle better the load of the wall.  The vertical insulation is another matter.  It must be supported so that it is not damaged by laying

rocks against it on one side and backfilling against it on the other.  So I wired the insulation to metal fence posts that were shortened as necessary to stay below the

Horizontal insulation over two additional layers of
6 mil plastic sheeting

final height of the insulation.

I proceeded by setting two posts, one near the house and the other about 20' from the house. Their height matched the linear slope that the insulation will take from a high end next to the house to a low end next to the French (the French drain that will drain water from the top of the umbrella -- grist for another post). Intervening posts were driven along a mason line stretched between the first two posts (east wall) or, better yet,the edge of the horizontal insulation (west wall). I stood the vertical insulation on the horizontal insulation and stabilized it with clamps and props while I snapped a chalk line

Vertical insulation wired to posts

designating the sloping height of the insulation then used a wood-cutting blade in a reciprocating saw to cut the insulation to height.  I then set all of the insulation aside while I covered the soil on which the horizontal insulation will rest with a layer of sand and screeded it smooth for a solid foundation for the insulation and the rock wall above it.

A 6 mil plastic sheet was laid down over the sand such that there was 5- 6' of excess running in both directions for later shingling with the plastic sheeting of the umbrella above and below the wall.

After the horizontal insulation was returned to position and the vertical insulation was wired

to the posts and secured with more wire as needed between the posts, two equal-sized pieces of plastic sheeting were then draped over

Two more layers of 6 mil plastic sheeting covering the
insulation

the top of the vertical insulation to be spread out in two directions: (a) down the front side of the vertical insulation and over the top of the horizontal insulation plus plenty of excess running horizontally to be shingled over the plastic sheeting of the umbrella below the wall and (b) an equal amount of excess running horizontally off of the top of the vertical insulation eventually to be shingled under the plastic of the umbrella above the wall.

The horizontal insulation under the wall loosely follows the plan for the rest of the umbrella behind the house, i.e., 4" thick for the first 8' out from the house, 3" thick for another 8', then 2" thick for the final 4' -- 20' in all.  The vertical insulation is 4" thick throughout with extruded polystyrene on the stone side and the weaker (but cheaper)

Generous layer of sand over the plastic to protect it from
injudicious stone placement; boards protect loose edges
 of the plastic from foot traffic during stone placement

expanded polystyrene on the backfill side.

Protecting the Vertical Insulation
The first retaining wall was erected a year and a half ago with the help of weekend volunteers using salvaged stones from a 19th century barn foundation. Similarly, we did the new retaining walls with weekend volunteers using more of the same stones . I either piled sand next to the wall or parked the track loader with sand nearby so that a layer of sand could be used between stones vertically and horizontally to help situate and stabilize them, considering their discordant sizes and shapes.  

Using sand as a filler meant that the space between the vertical insulation (covered with plastic sheeting) and the stones was also filled with sand.  But, without backfilling, the weight of the sand against the insulation or any pressure from the stones would distort the insulation -- a serious problem that partially collapsed the insulation and compromised the R-value of the wall built previously. Consequently, I fastened with 12" spikes a 2 x 6 over the horizontal insulation such that it abutted and stabilized the bottom of the vertical insulation.  I then was able to backfill the vertical insulation with sand, a shovel-full at a time, to a depth of a foot or so. The 2 x 6 was then removed and the spikes were reinserted to stabilize the insulation during wall-building.  The rest of the backfilling was done gradually in tantum with building the wall.

Building the Wall
I feel blessed to have had enough weekend volunteers, not only to build the wall we have

Stone wall well under way

been discussing here, but to do all of the other "stone work" that had to be done before I could finish the dirt work on on the critical east and south sides of the house.

Whereas the first retaining wall went up too fast for a high-quality outcome, the new wall took all of a half-day to build and with a satisfying result.  I had already stached nearby what I thought were enough stones and sand to build the wall. My role then was to give general instructions as to its size, height and the need to be careful not to damage the plastic sheeting or the stuccoed house wall.  The volunteers were free to place the stones as they saw fit and they did a great job.  Delegating paid off because I had under-estimated the number of stones and sand necessary and was kept busy with the track loader ferrying in more materials.

The Second Day
The second day was as much about hiding

Ugly French drain mouths

drain tiles as about building retaining walls.  The exposed mouths of the French drains and the north concrete wall foundation drains were so ugly as to beg camouflaging with something that would eventually enhance the native gardening that will surround them.  I had pinned down weed barrier fabric in the areas to be covered with stone and asked the crew to be

Stones added for aesthetics and to control erosion;
notice original retaining wall in the background 

creative in installing the stones.  Again the result exceeded expectations.

Then on to a couple of retaining walls. The first was next to the garage doors, which, by now, was hardly a challenge for the volunteers.  The second involved adding height to the first retaining wall at its house end.  Again, all I did was bring in the stones with the loader -- the crew did the rest un-micro-managed.

With the stones in place, I needed to take advantage of the relative dry fall to finish the

Raising the original wall next to the house

dirt work and plant seed -- the subject of the next post.

The Second Umbrella Wall
The retaining wall detailed above is the east-most umbrella wall.  The west-most umbrella wall was completed only to point of receiving stones.  With the weather window for the rest of the dirt work closing fast, the umbrella behind and to the west of the house became lower priority.  The wall will have to be finished in the ensuring months in conjunction with installation of the rest of the umbrella.