Sustainable Footings for a 19' High Rammed Earth Structure

Muskoka Sustainable Builders Inc. is a general contracting company based near Huntsville, Ontario, specializing in rammed earth construction. "Our goal is to guide clients through the entire construction process so they end up with the most efficient and healthy home possible", said Mark Allen, principal. "While many contractors supply and install 'green' products to their clients, this often leads to a home with lots of green credentials, but not a very sustainable or efficient home."

Both Mark and partner James Blackman have backgrounds in mechanical engineering: "We are uniquely positioned to evaluate the entire home as a living system to ensure everything comes together to create the best possible living space. While there a lot of advanced active technologies that can be employed in a modern home, we believe in passive techniques that keep things simple and affordable."

Siting of the Building

Making a building sustainable means choosing a site and orienting the building so sunlight can passively heat the structure.

High levels of insulation with minimal thermal bridging and air infiltration, combined with a good mechanical ventilation system ensure the home's energy needs are minimal.

Installation of Drainage layer

The drainage layer under the footing is critical to ensure liquid water is kept away from the footing concrete.

This is especially important with rammed earth walls as the aggregate particle size induces wicking of ground moisture.

Setup Screed Boards

Footing size for the two storey rammed earth walls is 12" deep by 36" wide. Pairs of expensive 2x12s would typically be used to form footers of this size. However with Fastfoot®, 2x4s are used as the screed board with the fabric membrane forming the rest.

Pairs of stakes are driven into the ground 6' on center, with spacing between the pair equal to the footing width. Sharpened 2x2s were used for stakes as the ground was quite solid.

Screed Board Frame for Steps

Normally steps are formed with plywood, scribbed to fit the bank.

With Fastfoot®, light weight 2x4 frames are set up to support the fabric as shown in the left and top of this photo.

Installing Fastfoot®

Fastfoot® is installed using a hammer tacker with 3/8" staples, about 8" on centre.

Note the building inspector and her assistant walking the left footing, carefully inspecting to ensure the fabric has been installed correctly. These youthful inspectors are very, very fussy about rising damp.

Installing Steel Reinforcing

Three 15mm horizontal bars are required in the footings, with 15mm cross members 24" on center. The steel is then hung using 1x4 cleats.

Footing steel is important given the signicant loads from the rammed earth walls above.

Step Bulkheads Installed

3/4" plywood was used for bulkheads on the six footing steps.

Overview of Footing Forms

The quality of a building contractor can always be measured by the organization and neatness of the jobsite.

It would be hard to find another jobsite organized as well as this one...

Placing Concrete

James began placing the concrete starting with the lower steps to plug openings between the steps. Then the horizontal footings were filled. A boom pump was used for concrete placement.

Consolidating Concrete

Mark assists concrete consolidation with his rake handle. Or a kick with the boot does equally well.

Footing Pour Complete

Cleats will be removed shortly to receive the rammed earth formwork. Screedboards and stakes are left in place to protect the footing membrane from site damage.

Notice that there are no rebar hooks coming out of the footing. "We epoxy rebar into the footing after the formwork is up so it's out of the way during construction of the rammed earth walls," said James.

Sourcing Aggregate for Rammed Earth Walls

Rammed earth walls are only as good as the quality of the aggregate and cement mixture. As well the color of the finished wall is determined by aggregate choice.

Two Story Rammed Earth Walls

Note the robust formwork used to form the two story 19' high rammed earth walls.

The beautiful colors in the horizontal layers of earth aggregate are one of the selling features of rammed earth buildings.

The floor joist system will be hung from the walls by drilling and epoxying into the rammed earth.

Stripping Formwork

Rammed earth formwork is completed in sections, and moved progressively around the perimeter.

This photo shows the removal of the straight section of formwork on the far side.

Thermal Break in Rammed Earth Walls

Muskoka Sustainable Builders place a 6" layer of insulation between the two 6" layers of rammed earth to provide a thermal break. Total wall thickness is 18".

James explains the structural integrity of the three layers, "The end panels of the formwork holds the insulation in a grove. As for the through ties we use rebar "z" hooked on the vertical rebar at the very top and bottom of the wall and a structural plastic geo-grid every 2' the rest of the way up the wall to reduce thermal bridging."

Rammed Earth Interior Finish

Beauty is in the detail, such as this indent around each electrical outlet.

For more information on Muskoka Sustainable Builders, click here.