Canadian Architect

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Terra Firma

Building a Rammed-Earth Wall, or Pise De Terre, for the Desert Cultural Centre Was Not Without Its Challenges, but the Technology Is Much Simpler Than What One Might Think.

March 1, 2007
by Bruce Haden and Brady Dunlop

TEXT Bruce Haden and Brady Dunlop

PHOTOS Brady Dunlop

The rammed-earth wall of the Nk’ Mip Desert Cultural Centre is not a product of new technology. It appears to have many origins, primarily in geographically arid regions where wood is scarce. Rammed earth is common in nearly every European country, China and Australia, and in recent years, the construction method has been gaining popularity in the western United States and Canada.

The technology is fairly primitive and similar to the forming of concrete. Nonetheless, the technical challenges of building such a large wall for the Nk’Mip Desert Cultural Centre created a number of hurdles in the construction process for the rammed-earth contractors, Terra Firma Builders of Salt Spring Island, who are accustomed to the reduced scale of a single-family house.

The principal challenge to the widespread adoption of rammed earth is the availability of experienced rammed-earth contractors. Meror Krayenhoff of Terra Firma has launched an ambitious educational program (involving the Architectural Institute of British Columbia) to educate architects, contractors and the general public about the rammed-earth process. We consider the material to have extraordinary promise from both a design and sustainability perspective, and look forward to its broader adoption.

Basic Technical Data

* Total wall dimensions are approximately 80 metres long and 5.5 metres high.

* Total assembly is 600 mm thick. This is composed of a 250 mm wythe, 100 mm of polyisocyanurate insulation and another 250 mm wythe of rammed earth.

* Thermal resistance of the wall is approximately R33.

* The wall is heavily reinforced and utilizes HSS frames cast in at 1200 mm on centre to hang the section above the slot window from the roof slab.

* Rammed earth has a high compressive strength and can be used in load-bearing situations. The wall was not used as a load-bearing assembly at the Desert Centre to simplify the construction sequencing and to achieve the uninterrupted slot-window effect.

Design Advantages

* The process involves the compaction of a mix of sand/soil, Portland cement with a small percentage of water (5-10%) resulting in a damp mix.

* Use of local soils supports sustainability practices.

* Heavy insulation value provided by the wall promotes thermal cycling.

* When used in combination, the concrete structure and rammed-earth wall offers resistance to fire.

* Vibrant colour opportunities.

Fabrication Process

* The wall is created of 150mm- to 200mm-high lifts inside 600-mm wide forms.

* The backside of the wall was formed up to full height and the work of framing occurred in 600 mm-high increments–the width of the polyisocyanurate insulation board.

* The mix is spread in an individual lift for the full length of the formed section and compressed with pneumatic tampers along the full length of the wall. This process continued with additional formwork in 600 mm-high sections.

* “Puddled earth” coloured concrete was used as a bond beam at the top of the walls and above large openings (entry and slot windows).

Finishing

* The wall is left natural. The outer crust forms as the wall cures. Once removed from the formwork, the “as cast” finish becomes the finished wall appearance.

* The wall is not sealed.

Particular Challenges

* The integrated nature of the roof slab supporting the section over the window and entry presented sequencing issues. This proved to be challenging from a construction coordination perspective.

* Weather conditions and ranges in extreme temperature were a significant consideration for the contractor.

Colour

* Approximately 40 coloured test samples were made for investigation.

* Full-scale mockup on site was created to test colours. Substantial colour changes were made after each of these two phases.

* Local soils were used. However, in this particular case the desire was for fairly dramatic colour intensity and contrast. This required the addition of concrete colourants.

* A more “pure” effect could be achieved with pure colourants. In this case, depending on the earth used, the wall would tend to appear whitish and less sedimentary in appearance.

Bruce Haden and Brady Dunlop of Hotson Bakker Boniface Haden Architects + Urbanistes were members of the architect team for the Nk’Mip Desert Cultural Centre in Osoyoos, British Columbia.




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