Mud houses ( walls constructed in mud)
These types of houses are made with molded earth. Earth lumps are made and stacked to make the wall. Earth is not compacted. When wall has been built, it is trimmed to give better finishing to the wall. Environmental performance of these buildings is very good but these are very vulnerable to floods or rain and earthquake.
Adobe (walls constructed with unfired mud bricks)
Adobe house made with mud blocks is similar to cement concrete block masonry, but blocks are made with mud, dried in sun and used with mud mortar. Wall thickness is generally 12inches. Adobe house made with mud bricks is similar to burnt brick masonry but bricks are not burnt and dried in sun and used with mud mortar. Generally wall thickness is kept 13.5 inches.
Mud bricks performance in shear, tension and compression
The mud bricks are very weak in shear, tension and compression. In case of earthquakes, walls separate at the corners and the shear cracks develop across the wall, causing collapse of the structure. Extensive damage was observed during earthquake especially if it occurs after a rainfall. Blonde and Garcia (2003) developed the design and construction manual for the adobe houses on the basis of 30 years experience at the Catholic University in Peru, which were followed in the reconstruction efforts after Pisco earthquake of 2007.
Seismic performance of mud houses
Performance of traditional adobe construction during numerous Iranian earthquakes has generally been poor as reported by many researchers after assessing the post earthquake damages. Low material strength, poor workmanship, lack of proper connections between building elements, and the excessive weight of the building owning to thick walls and massive roofs, are some of the shortcomings that led to the general weakness of these buildings under earthquake loads (Maheri et al., 2005).
The performance of various non engineered houses was evaluated in earthquakes in successive earthquakes in Bangladesh. It was reported that mud houses are more vulnerable to earthquake than any other type of traditional house, because of its brittle nature and lack of lateral force resisting system. Some design interventions can improve the seismic performance of the mud houses, which may include wooden bracing at the corner location of the beams, metal straps at connections, placing of roof truss at proper location, blocking of excess opening, use of cement plaster over walls, insertion of new walls etc. (Jehangir et al., 2012). On the basis of field survey after floods in Bangladesh, it was suggested that the mud houses are more suitable for low flood areas particularly. The performance of single family two floor mud houses in Bangladesh located in semi urban and rural areas has been reported vulnerable to seismic forces and lateral pressure of the flood flows (EERI, IAEE, 2007).
Durability of mud blocks
The use of Cactus solution to improve the durability of adobe against the water erosion has also been recommended (Zavoni et al., 1988).The stabilization of adobe with lime depends on the nature of soils used for making the blocks. For rich clay soils, the addition of 2-3 % of quicklime to a soil quickly reduces plasticity by hydration (dries the soil) and breaks up the lumps. For both the clay loam and the sandy soils, the lime content of 2% resulted in a 7-day compressive strength of about 0.7 MPa, but increasing the percentage to 4% resulted in nearly doubling the compressive strength. However further increases in lime content led into no significant increase in compressive strength (Akpokodje, 1985).
In this paper, the major reasons for damages of the mud houses have been analyzed on the and extent of damages in these houses. The data was then tabulated and analyzed and the major causes leading to the collapse and damages of mud houses were documented. Based on the performance of these houses, some recommendations were made for the improvements of mud houses.
On the basis of the damage assessment of the 100 sample houses partially and completely damaged, the major causes responsible for the destruction of these houses are as given below:
i. Undermining of foundations
ii. Scouring/erosion at the base of the walls
iii. Scouring/erosion at the corners of structures
iv. wiping out of structures
v. Deposition of debris in houses
vi. Damage due to debris flow.
vii. Damage due to prolong submersion of buildings in water.
viii.It has been observed that many mud buildings were damaged even water did not touch the floors and due to the capillary rise walls lost strength.
Proposed design and construction improvements for the mud houses in the flood affected areas
Foundation and plinth
The depth of foundation is very important particularly when mud houses are subject to standing water for prolonged period. The following important points must be considered for foundations and plinths of the mud houses.
i. The depth of foundation may be taken as min 4 feet for soft soil and 2 feet for hard soil. The width of foundation may be taken as twice the width of wall for soft soil and 1.5 times the wall width for hard soil.
ii. The material to be used in foundation may be stones, fired bricks, solid blocks, dry stone masonry or plum concrete of nominal ratio of (1:3:6) with 40% of stones of total volume, where stones are available.
iii. The plinth must be raised at least 6in above the high flood level. The Damp Proof Course of heavy polythene and water proof mud. For plinth protection 3 feet wide apron of burnt bricks having 3 in slope outwards may be provided.
Raising of platform for construction of mud houses
Rising of platform and plinth above ground level will protect the mud houses from the access of water. The following improvements have been suggested (Design hand book, 2010):
i. The platform must be raised at least 1ft above to regular flood level with compacted earth and extend the edges minimum 3ft away from building footprint.
ii. The slope of platform may be maintained for sandy soil at 1V to 2H (For each vertical ft height, horizontal width of 2 feet) and for clayey soil; 1V:1.5H.
iii. The water must be drained away from the building. For control of erosion of platform, deep rooted edge plants, bushes or grass may be grown on edge. Alternatively brick pitching may be provided.
Thickness of wall is very important in mud houses. The following design and construction guidelines may be kept in the mind:
i. For compressed adobe walls, the minimum thickness must be kept as 12in to 13.5 in and the height of the unsupported wall may be restricted to 8 feet and length to 14 feet.
ii. For molded clay walls the wall thinness at the bottom may be kept as 18 in and the thickness of the wall at the top must be 12 in to increase the stability of the wall.
Roof band and ring beams at lintel and plinth
i. The bands may be made of wood, wire mesh, Reinforced Bricks (RB) or Reinforced Concrete (RCC), as feasible at site.
ii. For wooden lintels, ladder type lintel may be made of 3in x 1.5 in with nails and cross pieces f 2in x 1.5 in @18 in c/c. Similarly wooden bands can be made of single piece of size 4in x 2 in with diagonals at the corners.
iii. Roof bands must be tied with lintel and lintel bands by nailing diagonal woods at wall face, to provide stability against roof and wind. In case wood is not available, two courses must be provided with burnt masonry.
d. Earth layer of 6’’ thick is laid over it, forming a slope of 12% towards spout and small amount of water is sprinkled to compact it to 4in and left for 2-3 days.
e. Mud is prepared by using wheat husk and the roof surface is plastered 1’’ thick.
f. 1.5 ft extended spout is used or vertical drop of cemented spout with 1:3 to drain rain water is provided. For better rendering wire mesh should nails in wall and then plastered.
iii. For pitched CGI sheets may be used for span up to 16 feet and the following recommendations may be followed:
a. CGI Sheets 26 SWG gauge are placed at angle 25-35 degree having king post trusses
@5ft c/c and tie beam , rafters 3’’x4’’, king post 3’’x3’’, purlins of 2.5’’x2’’
b. Longitudinal slope (1:300) should be provided in one side to harvest rain water and Projection up to 1.5 ft.
iv. For light weight Thatch roof with mud plaster, the following improvements may be followed:
a. Wooden/Bamboo having ridge beam, 3’’ dia ridge pole, 3’’ dia rafters @ 4 ft may be used. 1.5 in dia-purlin @ 1.5 ft spacing are provided.
Material selection for the mud structures
Selection of appropriate material for the mud construction is an important consideration. The following recommendations were made by the experts:
i. For molded clay construction, soil with Sand 50-60%, Clay 20-25%, Gravel 20%, and Straw/chaff 5kg/CuM and Water 20% of total volume was recommended.
Soil for adobe and rammed earth may contain sand 40-45%, silt 15-30% and clay 10-25%.
Site selection for mud houses
Most of the mud houses collapsed during the floods 2010 was located in the flood plains. The site selection is of prime considerations in the construction of mud houses. For construction of mud houses, the raised and elevated platforms are more suitable to avoid threat to the buildings. The soil must be well compacted.
There are most experiences person is in with EWB Bangladesh for constructing of mud house in. Bangladesh.
For More in formation please contact with Engineers Without Borders Bangladesh.
For that article some suggestions and thinking is practical oriented and some taken from different published papers.