Using earth mortar for reconstruction in earthquake prone areas
Construction of housing: single storey stone buildings with earth mortar
For very poor families with almost no resources rebuilding their own houses what would be the most effective way to make reconstruction safer? Safety here is defined as minimising damage from small earthquakes and maximising escape time for inhabitants in large earthquakes.
- It has been suggested that a small percentage of cement mixed in with the earth mortar would achieve this, but other advice suggests cement is only effective in sand based earth mortar, and ineffective in clay based mortar:
(a) Is it true that cement is only effective in sand based earth mortar, and ineffective in clay based mortar?
(b) Would a small percentage of cement have the desired effect of increased safety?
2. It has been suggested that reinforcing the mortar with fibres (rope, hemp etc) would be effective to increase safety (without cement). Is there any evidence to support this?
Posted on behalf of contacts in Nepal.
Harriette (RedR Technical Coordinator)
Thanks for bringing up this very interesting Topic of Discussion.
Yes, it is true that cement works with a proportion of sand / quarry dust in mud mortar mix, low clay content/ non-clayey soil may also be tested. The idea is to achieve good stabilization. Sometimes where cement is not available, locally produces lime is an alternative for stabilisation; I checked the possibilities in some locations in Nepal where they were actually producing lime (chunna).
Organic fibres reinforcement is also a good idea, especially fibres from hemp that has a natural protective coating against easy decay in dry conditions would be good as mortar reinforcement (provided the mortar mix is stabilized and compact). Alternatively the wire mesh is also suggested as horizontal mortar band reinforcement at regular intervals by some architects/ organisations. Personally I like it as a very good solution given the easy trans-portability or wire mesh rolls to remote locations. (Though I am not sure about the vertical inserts- reinforcement bars as the same can be very easily deformed with the impact in heavy stone masonry mass, if not integrated with sufficient concreting around bars to keep is stiff?)
More important than a good mortar mix is the “bonding” that may not be compromised, even a dry stone masonry may perform stable and safe if the bonding are proportionate and well structured (Key elements) for corners, for distributing spans and for tying the structures/ walls together (this is evident from the post earthquake findings), Nevertheless stabilized mortar is highly recommended. (I would really suggest we refer to some to Lauri Baker's studies made after Chamoli earthquake on stone masonry.)
Stabilized Mud Mortar Reference: Here is an extract from NBC 204, which I found very useful as a reference.
Preparation of Stabilised Mud Mortar
i. Collect three cubic metres of mud which contains 40 to 50 % sand and 60 to 50 % of clay.
ii. Mix 180 kg of chopped wheat or paddy straw with the mud. The wheat straw has to be chopped in 20 mm lengths, whereas rice straw may be up to 50 mm in length. Old straw is preferred to fresh straw. Pour water over it and kneed until the straw and mud becomes an uniform paste.
iii. Kneed three to four times a day until the straw decomposes. The mix should not be allowed to dry during this process. It takes generally seven days in summer and 14 days in winter to decompose the straw. iv. The stabilised mud mortar is then ready.
……(if we don’t have 14 days then cement is the quicker solution)…
We have invented a type of shelter which is very cheap. 400Euros to construct, (Spanish prices)
They are very fast ten people with minimal skills can construct one in a single day. A village of people could construct a small village in a day. They can be constructed weith minimal tools and just a single generator. The method can be taught in a morning and then people can build their own houses very fast.
The constructions are dome shaped. Domes are the strongest structures and are built as Earthquake proof. They are adaptable to all climates, being highly insulated for mountainous conditions or being partly within the Earth for desert conditions. They are flood proof too. The prototype we constructed in a single day used rubbish in construction including old fishing nets, paper and cardboard, old curtains, fabrics and curtains can be used in the design. The material we used to add strength is hemp, lime and straw. It is possible to use cement and I believe mud too. Generally in construction there is the concrete and steel rule. Ie use cement and reinforce with steel. This works the same with hemp and lime and steel or even with wattle and dorb. So I would suggest reinforcing the mortar with fibres. Also I would suggest using the method we have used in our proototype to produce domes. The reason is that domes will add strength to the structure. The type of bubble homes we have developed are ideal for Nepal. We are currently planning a crowd funding campaign to go to Nepal and to apply the new method of aid shelter construction we have developed. I am very interested in making contacts in Nepal with organisations which can help us facilitate this project and provide some sample homes constructed in the Bubble home way. If anyone can provide assistance with this for us, it would be much appreciated. I did try to upload photos here, however was unable too. If anyone would like to see photos of the prototype then I would be able to forward them.
Peter Hughes- Centre for Natural design and innovation- Almeria & abigstrategy.com
Some of the best available technology and experts in this field of seismic enhancement in non engineered mud and stone structures will be found locally at NSET-Nepal, National Society for Earthquake Technology. These people lead the capacity built after the Pakistan Earthquake (in association with UN Habitat). They are very experienced and practical as well as local. There are also several good local codes of practice. If you cannot locate the English versions drop me an email at email@example.com and I try to find in my records. I suggest stick with the local technology and people. You could look at some of the excellent training material that they produced for the Pakistan earthquake in Khyber Pakhtunkhwa where the majority of rural structures had mud mortar ... including introducing ladders and corner strengthening in mud joints. These are in the codes also. I strongly suggest do not get involved in structures subject to seismic effects unless you have some basic seismic engineering knowledge of the forces, modalities, resistance mechanisms plus the risks and constraints and the limitations of what you might achieve .. you could otherwise be building death-traps (we have seen in other locations) by giving people inappropriate confidence in the level of resistance you can engineer in these structures ... it is limited, useful but should not be overestimated. Chris N
According to our experience with earthquakes in Chile, and in my opinion mortar is not enough. Steel reinforcement for brick and mortar construction provides mechanical resistance against earthquakes.
Wood construction has proved being strong, light and flexible enough for earthquakes. After our big 8.8 wooden emergency housing worked fine.
This thread is public, all members of KnowledgePoint can read this page.