Sunday, March 13, 2016

Construction - The First Retaining Wall

Our plans call for four retaining walls with the first one next to the west concrete wall being the most challenging.  It needed to be 4 - 6 feet high and insulated and waterproofed as part of the insulation/watershed umbrella.  The insulating and waterproofing were a challenge while building the wall was easy, thanks to 9 energetic volunteers.  But that's getting ahead of the story.  (Click on the photos to enlarge them for more detail.)

Insulating the Concrete House Wall
The first book I bought when contemplating an energy neutral home was "Earth Sheltered
Wall insulation in place
Houses" by Rob Roy.  Through it I learned early on that insulation should be inserted between a retaining wall and a concrete house wall in order to keep the retaining wall from sucking heat from the house.  In our case, I was already planning to insulate the concrete walls of the house that would not protected by the insulation/watershed umbrella. Since the umbrella will dip down behind and go under the retaining wall, the house wall adjacent to the retaining wall will fall outside the umbrella and would need to be insulated.

Cementitious board before adding the lower section

The exact way I insulated the house wall will be covered in detail in another post but suffice to say at this juncture, I used 3 5/8" steel stud track to support 3 1/2" of expanded polystyrene for an R-15 on the outside of the wall.  (The inside of the wall will be insulated in a similar fashion eventually for a total of R-30.)  I then fastened 1/2" old-fashion, heavy, hard-to-cut cementitious board and parged it with top-coat stucco, the latter primarily to cover the junctions between boards and to make the exposed areas of the wall more aesthetic.  Six mil plastic separated the
Wall parged to height of retaining wall;
horizontal insulation in place (part of
which already covered with sand);
vertical insulation supported from
 behind with steel fence posts
galvanized steel from the concrete on one side of the insulation and from the cementitious board on the other side.  Six linear feet of the wall were insulated and covered from the top of the wall down to meet the horizontal insulation already in place over the footing.  

Integrating the Retaining Wall with the Insulation/Watershed Umbrella 
As part of the Annualized GeoSolar system, the insulation/watershed umbrella should extend 20' outward from the house in all directions.  On the west side of the house, it has be convoluted in order to accomodate the retaining wall.  Above the wall it will slope gradually southward
Insulation wired to
steel fence posts
towards the wall then, dip sharply downward behind the wall, go under the wall and finally blend with the umbrella in front of the house. 

I smoothed out the soil under the wall then covered and leveled it with sand to provide a base for a 4' wide wall. Next, I laid down plastic sheeting (6 mil) such that it extended several feet beyond the prospective wall in the up-slope and down-slope directions.  Then came a thin layer of sand over the plastic where the wall would rest followed by two panels of 4' x 8' x 2" insulation board next to the house and another panel of 2" lateral to it such that the insulation under the wall would be 4" thick for the first 8' then 2" thick for the last 8'.  Ideally, the insulation should have extended 4' further to satisfy the 20' width for the umbrella but the original excavation did not accomodate it.  
Three layers of plastic -- one between
insulation and soil contact then two
between insulation and the outside

The vertical insulation was also 4" thick for the first 8' from the house then 2" thick for the last 8'.  To support it, I drove four steel fence posts into the ground behind them and wired the insulation to them.  In retrospect,  all of the insulation should have been the stronger extruded polystyrene (pink) instead of expanded poly (white), especially the vertical pieces, one of which cracked while building the wall and had to be held together by hand until it could be supported by sand in front and dirt behind.

I next liberally covered the horizontal insulation with sand and added two more sheets of 6 mil plastic with a layer of sand between them. Finally, I covered the plastic with a heavy layer of sand into which the stones of the wall could be nestled without damaging the plastic.

Building the Retaining Wall
Three or four years ago, we salvaged foundation stones from a 19th century barn in such
Some of the volunteers at work
quantity that we do not have to use them in a miserly way. Consequently, I decided to make the wall massive enough that the rocks could be laid randomly and still resist the pressure of the backfill behind it.  The final dimensions were roughly 4' wide at the base, 2' wide at the top, 5' high and 16' long.

A view from the loader; step-son Keith
(left) and my good friends Dave and Pat
lifting rocks out of the bucket
The wall came together in less than three hours.  As fast as I could track loader the rocks to the wall site from the rock-pile, the volunteers could set them, including packing sand into the crevices between rocks, laying a sandy base for the next level and compacting the sand with hose water.  Some of the stones weighed quite a bit north of 100 lbs so I was more than grateful for the help. (Parenthetically, the crevices between the rocks will be filled someday with plants native to our area.)
The result

Backfilling Behind the Wall
Backfilling immediately before it rained was critical otherwise water pooling behind the plastic might create sufficient hydraulic pressure to move the wall. I did the backfilling the same day the wall went in. As it was filled, the grades next to the house wall and the rock wall were intentionally tilted to create a swale for carrying runoff safely around the retaining wall.  As the back-fill settles over time, it may have to be tweaked to protect the wall.
Initial backfilling
Eventually, the umbrella and a couple of feet of topsoil will be added over the initial backfill after which the rock wall will be protected from runoff permanently. However,  chances are the wall will need a couple of courses of stones added on top to accomodate the horizontal insulation in the umbrella and the topsoil over it.  It also looks as if I should have made the black damp-proofing membrane higher on the concrete wall.  Not to worry, it can be extended later.