San Miguel Mission

This article is an interview of structural engineer Nels Roselund and is his account of what he discovered about the mission as he designed and oversaw the construction of a very unique seismic retrofit of this priceless building.  Here you will learn about the original construction and the rehabilitation of the building.

Who first contacted you about the San Miguel Mission and why?

“I was contacted by a committee at the church.  I think I got their attention because I had done a seismic retrofit on the San Juan Capistrano mission.  That was a much bigger project than San Miguel and that mission was built of stone, rather than adobe, which is what this mission is made of. They wanted someone to help them make their church building safe in an earthquake which got a lot of damage in a 2004 earthquake.

I also pointed out to them that the building is about 200 years old; and many generations of people have worshiped there safely, their children have been taught there- so who am I to tell them the building is not safe?

 

What I proposed to do is study the building inside and out from foundation to roof in order to develop a preliminary scheme for strengthening and understanding of how the building will respond in an earthquake.  After that I would tell the committee my findings.

Making Adobe Bricks

The walls of the church are built with Adobe masonry blocks made of sun-dried soil with water and a stabilizing mixture such as cactus juice or blood.  Those blocks are made by pouring the adobe mixture into lumber forms.  One way the forms might be made is with 2 by 4s that are cut and nailed together to form a space maybe 3 ½ – 4 inches by 12” x 18”- or something like that.  The forms are laid out on the ground and then the Adobe mixture of soil, water, and stabilizers would be poured into the forms and left in the field to be dried by the sun.

The Foundation

The foundations would be excavated by hand and shovel and adobe bricks would be made out of the soil dug out for the foundation.  The depth and width of the foundation would depend on the thickness and height of the walls that they were intended to support.  5-foot walls thick walls might have a 6- or 7-foot-wide foundation they would be a layered construction.  This involved filling the entire excavation with stones and adobe from the bottom of the ditch up to the top as shown in the photo above.  They would put in a layer of adobe mud then put a layer of stones on top of that and level it as best they could at each stage until they filled the ditch.  The adobe mud would be pourable so it would fill all the voids in the stone and act like a mortar hold the stones together.  They were trying to maintain stone on stone contact as much as possible to increase stability.

 

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The San Juan Capistrano mission is unique in that it is the only mission made of stone.  It is no longer a functioning church because it was devastated by a large earthquake in 1812 where the walls of the Nave collapsed killing 40 people.  It was also unique in that the altar ceiling. Here is photo of the altar as it stands today.

Some of the earliest examples of mortar in California are found in the San Juan Capistrano and San Miguel Missions.  The former was built in 1821, and the latter 1789.  Based on a series of interviews with structural engineer Nels Roselund, who designed major seismic retrofits for both buildings, the San Miguel Mission was entirely made of sunbaked clay bricks consisting of local

soil, water, and cactus juice or blood.  He was not quite sure what the blood and cactus juice were for, but his hunch is that it made the soil easier to work with.  It is also his belief that the bricks were made out of the soil dug out for the foundations, which were often 6-7 feet wide and 7 feet deep. Mr. Roselund believes the mortar was more or less made out of the same material as the brick, but was not sure because to his knowledge, their chemical compositions have never been tested.  The walls were made of this brick and mortar in the manner shown in the photo to the left.  The mortar walls of the Nave in this mission are made of Adobe brick and mortar that stretch 45 in height 6-7 feet thick at their base.  This shows that even the weakest mortar can hold masonry together when help in a static state.

This Adobe structure is now more than 200 years old and is still a fully functional building where church services are still held.  To the left is a cross section of one of the walls that shows the Adobe bricks that were bonded together with Adobe mortar.

To the right is the Nave of the San Miguel Mission.  According to Mr. Roselund, even with its primitive mortar, the bond between the bricks is quite good.  He stated that Adobe buildings are susceptible to earthquakes, not because of the weak mortar, but because the roof and walls were not sufficiently connected to each other.

Conclusion

I hope this article was interesting enough for you to read it all the way to the end.  It is not easy to make something as dry as brick and mortar interesting, but I hope you have found the topic as interesting as I do.

I know this article is incomplete because the more I learned, the more I discovered how much more there is to know, but I will need to leave this important task to others.  However, I am convinced this article fills a gap in knowledge that has hitherto been a source of ignorance for most of us.  The final conclusion is disarmingly simple: if you have a brick foundation, get it tested before replacing it or retrofitting it.  That is as simple a conclusion as a 1000-word article can make it.

 

 

 

 

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