Plinth filling and back-filling are the methods of filling the building plinth, footing pits, and excavated trenches, with the construction soil (moorum), after completing the necessary construction work.
Following are the step-by-step procedures, that should be followed in the filling work.
First, we will go through the pre-filling checklist, and after that, we will see the actual filling procedure.
A. Pre-filling checklist:
1. Concrete works:
All the concrete structures like footings, columns, plinth beams, etc. should be completed with de-shuttering of the formwork, removal of stagings, etc. All the honeycomb in the structures should be repaired before doing the plinth filling.
2. Plinth masonry :
All the plinth masonry work should be completed with the internal plaster, or by filling the leftover joints in a proper way.
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| Plinth before backfilling. |
3. Curing period :
Before doing filling, ensure that all the concrete structures and masonry works have completed their minimal curing period. It should be properly set, attaining its design strength.
4. Cleaning & dewatering :
The plinth area and footing pit should be checked for any leftover formwork materials and should be removed before filling. Any organic matter, wooden logs, plant roots, etc. should be cleaned, as they form voids after decay, causing settlement in the filling, due to weak compaction.
If there is any water in the pit, trenches, and plinth areas, it should be drained out.
5. Soil test :
The construction soil (moorum) should be free from organic matter, plant roots, clay lumps, chemicals, etc. The soil should be granular in structure, with a reddish-brown color.
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| Construction soil. |
6. Excavated soil :
If the soil excavated from the footing pit is black cotton, it should be removed completely and strictly avoided from refilling. Any other type of excavated soil should be checked as per the soil specification, before using them in refilling.
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| Black cotton soil. |
B. Soil filling procedure:
7. Soil spreading :
Construction soil should be spread in layers of 15 to 20cm. thickness, if you are using manual rammers, and it can be filled up to 30cm in thickness for the mechanical compactors. In any case, you should not fill the soil layer above 30cm depth, to achieve good compaction of the soil.
Note: 25cm (10 inches) is the ideal depth for the plate vibrators or mechanical compactors.
8. Watering & compacting :
After completing the first layer of spreading, the soil should be sprinkled with water to attain optimum moisture content, so that we can achieve the maximum dry density of soil after compaction.
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| Manual rammer. |
The ramming should be done from one end, covering all the areas. It is always advisable to use mechanical compactors, as the work carried out will be more efficient.
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| Mechanical compactor. |
If the soil filling work is for the basement of the commercial complexes, then it is advisable to have an in situ core test for every soil layer, to get the soil density result, and to achieve the required maximum dry density of soil.
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| Core cutter. |
9. Soil layers :
The above procedure should be repeated in a layer-by-layer manner until the filling work is completed. For eg., if the depth of the plinth which should be filled is about 60 cm. then the work will be carried out in 3 - 4 layers using manual rammers, and we have to spread 2-3 layers of soil when mechanical compactors are on site.
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| Plinth filled with soil. |
Take the in-situ core samples before rubble soling work, to know the density and compaction of soil, if it is a commercial structure with more live loads over the floor. In the case of residential buildings, the tests are not necessarily required.
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