One of the primal attractions to building with earth is harvesting raw materials from the construction site itself. In the old days (pre-industrial revolution), that was always the
case. Soil dug from the foundation trenches or from a nearby borrow pit was either molded into sun dried bricks and laid up into block walls or rammed between wooden
form boards into monolithic walls. Man converting raw local resources into shelter is truly sustainable building.

During the past decade or two, the earthbuilding industry has drifted away from this core precept, settling into the comfort zone that comes from using familiar and consistent
quarry-processed aggregates. This quest for comfort has come at an environmental cost.

Don’t get me wrong, I am all in favor of consistency and dependable results, but I’ve also become more aware of the carbon consequence of putting trucks on the road,
burning diesel fuel, wearing out rubber, and pounding asphalt. I trucked quarry materials to our rammed earth and pise jobsites for years, but I now believe the right
path is to improve our understanding of local geology and our skills at formulating mix designs to allow us to incorporate even more of each site’s unique resources.
I think of it as the construction industry’s version of the slow food and locavore movements - better quality, closer to home. I want to call it “building from the
watershed”. The goal is to develop a series of protocols that allow us to evaluate any found soil and predict how it will perform in a structural capacity. When we complete the
assembly of this soils library, we will then have the ability to reproduce consistent results from site to site, using fewer quarry amendments and thereby reducing the number of
trucks on the road and CO2 in the atmosphere. Carbon offset construction.

Soil was harvested on site, the forms were simple, and the work was done by hand.  Buildings served basic needs, with walls that were rough and only more or less plumb.  This practice remained the norm for a very long time, from early civilization through the middle of the twentieth century.  It still is the norm in rural China, the Middle East, and Africa.

When rammed earth began to re-emerge in the mid 1970’s, it was still considered an inexpensive wall system.  Soil was predominantly sourced on site, formwork was relatively simple, the architecture was linear, and unskilled labor could be used for most of the work.  Walls were a little ragged and unrefined.  Plaster was a common finish treatment.

Over the past few decades, rammed earth has grown in popularity, with an increasing number of professional builders developing the requisite skills.  Contrary to the law of supply and demand, however, in which competition reduces prices, rammed earth has become more expensive. 

Why is this?  The answer is complex, or rather complexity.  Rammed earth began as a simple system that recognized, even celebrated, the inherent flaws and unpredictability of raw earth. Over time, as builders improved their skills and the marketplace grew to appreciate the unique beauty of rammed earth, architects began to push the material to applications and expectations that were extremely difficult to fulfill. They were difficult but not impossible, only time consuming and expensive.  Gone were the days of simple forms, unskilled labor, and site-sourced materials.  In their place were elaborate formwork built and set by highly skilled carpenters, and imported screened soil and processed aggregates stabilized with 10% cement. Each course of soil in the forms must be carefully placed and conscientiously compacted, with the whole installation under the watchful eye of a paid special inspector.  Add color blending, strata lines, curves, rakes, niches, lintels, chamfered bond beams and watch the square foot cost approach Carrara marble. 

What can we do?  There are two solutions, and the good news is that they can co-exist.  On one hand we can continue to refine our skills and produce rammed earth with the look, feel, and price tag of art.  On the other hand, we can provide clients and architects with value engineering feedback on how to design efficiently for the material.

Rammed Earth is massive so keep it simple.  Rammed Earth is regional so source materials locally.  Appreciating the unpredictable character of finished wall surface celebrates the authenticity of natural materials and showcases its hand built qualities.

Rammed earth doesn’t have to be expensive.  Designed with the system in mind, it can be one of the best values in the building industry today.

I first started building with rammed earth nearly forty years ago in a quest for low-cost construction solutions and with a certainty that sourcing “free” material from the site had to be economical.  Over the years, as code compliance and client preference compelled us to achieve higher strengths and more uniform wall quality, we found ourselves forced to import quarry products and stabilize with higher cement ratios.  In effect we were pulled by market forces towards a product that was indeed akin to concrete.

Hence my difficulty with the question.  Is rammed earth less expensive than concrete?  Does it represent a reduction in carbon footprint?  Is it stronger or more durable?  Is it less expensive to heat and cool?  Does it improve indoor air quality?  Is it healthier?  Is it more attractive?  Is it better for the planet?

First, rammed earth is not necessarily less expensive than concrete.  Even though the forming systems for the two materials are similar and take more or less the same man-hours to erect, layering and compacting rammed earth into the form takes considerably more labor and equipment than pouring and vibrating concrete.  The only savings possible are a reduction in aggregate and cement costs.  To achieve these savings a mix design must be developed that utilizes a major portion of either on-site or other free mineral soil and a minimum rate of stabilization. 

Rammed earth can represent a much lower carbon footprint than concrete.  If the soil for the walls is close to the project, then transportation fuel consumption will be low.  If careful formulation allows design strengths to be met with 7% or less cement, then a big savings in CO2 will accrue.  The question of design strength is another challenge altogether.  Where some structural engineers are comfortable designing one and two-story rammed earth walls with an f’c of 600 psi, other engineers specify strengths as high as 1500 psi.  A design strength of 600 psi can be attained in an ideal soil blend with 1-1/2 sacks of cement per yard, but it can take up to 3-1/2 sacks to get 1500 psi.  What this shows is that engineering design plays a big role in carbon reduction.

Is rammed earth stronger or more durable than concrete?  The answer is clearly no, but the question should be: is rammed earth strong and durable enough for its intended purpose?  If the soil is selected correctly and the wall built properly, then rammed earth meets all the required performance standards and will provide decades of serviceability with little or no maintenance.  Rammed earth has an inherent beauty that transmits a warmth and natural character very different from concrete. 

Rammed earth walls are typically much thicker than a concrete wall, which makes them much more effective at controlling indoor temperature fluctuations.  An 18” wide wall creates a 12-hour thermal flywheel - outside temperatures take half a day to migrate through the wall to the inside face.  This means a mass wall balances out diurnal temperature swings.  In most climates and with proper exterior shading, this makes for no cooling loads.  Heating loads vary considerably based on orientation and building design.  Heating and cooling efficiency are big contributors to energy consumption and carbon footprint reduction. 

Finally, the questions about whether rammed earth is healthier for the occupants or for the planet.  Many people, especially in Europe, believe that clay in an interior wall works to maintain optimum indoor humidity, which in turn results in improved indoor air quality.  Certainly, natural earth walls are more healthy than materials that may be off-gassing glues, paints, resins, or other chemicals.  There are no factories exposing workers to toxins.  Rammed earth walls create quiet spaces that resonate solidness, which can be perceived of as an increased sense of comfort and well-being.  As for the impact on the planet, rammed earth uses fewer resources, both in construction and over the life-span of the house. 

When using cement as a stabilizer, clay content can be reduced, in some cases and with high stabilization rates, clay (and other fines) can be as low as 8% to 10%, depending on numerous factors (uniformity of gradation, plasticity, particle shape, and parent rock).
Unlike earlier times, when the building material was nearly always harvested on or near the construction site, today we have access to a wide range of importable mineral soils and admixtures.  Formulating a blend of soils capable of achieving optimum structural performance is our objective.
To do this, we begin by looking at the underlying soil on the building site itself.  A review of the boring logs from the geotechnical report will yield valuable data:  gradation, USCS soil type, and in some cases a plasticity index.  We have found that most site soils can be used in some proportion to create a useable formulation.  Using site soil has several advantages:  reduced cost of importing materials, increased LEEDs points, color continuity with the local geology, reduced off-haul costs, and reduced carbon emissions from construction.
The gradation report (also called a sieve analysis) identifies how much of a given soil is fine particles, those passing the 200 mesh screen.  The Plasticity Index is an indicator of how much of those fine particles are “clayey”.  Clay particles help to bind together the soil matrix.  If the gradation indicates more than 25% passing 200, the addition of sand will likely be required.  High clay soils also benefit from small gravel as supplemental amendments. 
If boring logs and site investigation indicate utter unsuitability, or if there is no excavation planned for the site, it is possible to source a portion of the wall building material in other ways.  Excavating contractors, pool contractors, or other general contractors frequently have excess material they need to move off site.  Phone calls or scouting trips can be productive, as is the old “Clean Fill Wanted” sign. 
For the required amendments to site or other free material, you can start the search for a suitable sand or gravel amendment at the local masonry or landscape supply yards.  Coarse sands with a good distribution of particle sizes are usually better than fine or uniform sand.  Cracked or crushed gravel is better than “pea” or river gravel because of its angularity.  Color and cost are also important considerations.
For a small project and for all of the required pre-construction testing, the search can end at the supply yard.  For larger projects where several truck loads of amendment will be required, you will be able to negotiate a better price dealing directly with the quarry. 
Finally, if site soil is unsuitable and free clean fill is unavailable, or if screening and processing is impractical, then purchasing and importing all of the wall building material from a quarry is the logical choice.  Cost, travel distance, color, wall density, required stabilization and geo-regionalism will dictate which quarry to use.

In March we were in Glendale, California.  The owner and general contractor gave us a monumental challenge:  800 linear feet of stratified earth wall to be completed in eighteen days.  The wall was to be comprised of 19 individual strata, four different colors, each lift a different thickness and a different color.  We had to design and build two new delivery conveyors to fit the staging areas we were assigned; one an elevating conveyor that would lift material from the mix rig to a height of 15’ and the other a pivoting/tracking conveyor that would travel along the top of the forms and distribute material for the varying lift depths.  The first photo in this post shows the set up.  As is always the case with prototype equipment, there were hitches at the start.  The first few days were really long ones in order to meet our required quota.  We redesigned and rebuilt the elevating conveyor, adding a powered head pulley, and we modified both the track and the trolley.  The first week it took us nine hours to meet the daily production schedule.  By the second week we were down to seven hours; the third week we were down to six, and the final four days we were placing eight yards an hour, finishing before lunch.
In May we mobilized to a single family residential project in the Los Altos hills above San Francisco Bay.  Here we had to fill a curving form, 120 feet long, two feet thick and in some cases twenty feet deep.  Each of the three set-ups required a different conveyor configuration.  One photo in the post shows our gas powered three-way conveyor.  The other photo shows how we chained four conveyors together to get across the basement excavation and into the 12-foot tall forms.