Plinth protection in the buildings is provided for preventing water entering into the foundation. In normal cases, 0.9 m to 1.2 m wide plinth protection is provided with brick bats. But in case of black cotton soils, due to expansion and drying characteristics of soil, such plinth protection gets damaged within a short time. Rain water pipes are also left on plinth protection. Plinth protection acts as very important barrier in the areas of black cotton soil to prevent water coming into contact with black cotton soils. Thus plinth protection should be impervious and as wide as feasible. 2 to 3 m wide plinth protection in such conditions is recommended considering site requirements, space availability and depth of black cotton soil. RCC plinth protection with lime concrete as base concrete is recommended. No space should be left for planting trees or plants near plinth protection as the plantation requires water and such water is harmful to the structures. It should also be ensured that in no case, level of plinth protection is lower than the adjoining ground as there is a possibility of a crack between wall and plinth protection from where water can seep through. It is also better to cast RCC horizontal plinth with drain and vertical plinth.
Vertical plinth along with horizontal one is more effective in preventing water into the foundation as minor cracks develop at the junction of wall and horizontal plinth protection due to construction sequence and different materials. Thus, the water flowing along the wall enters into the foundation. Also in case of flooding around the building, and even due to splashes during heavy rains, there is a likelihood of water entering into the foundation through brickwork. Vertical plinth in such conditions also keeps water away. Such plinth can be provided in the form of stone cladding supported on plinth protection. RCC wall connected with plinth beams overlapped over plinth protection is very helpful as it works like an umbrella over junction of plinth and the wall. It is to be ensured that no flooding occurs around the building.
Vertical Barrier and Drains
Vertical barrier on the outer edge of the plinth monolithically cast with the plinth also lengthens the flow path and keeps moisture away from the foundation. Though, the depth of vertical barrier depends upon active zone but more the depth, higher the length of the flow path. Since the drain also helps in keeping the moisture away, vertical barrier can be used as a part of the drain. Inner wall of the drain can be constructed monolithic to the plinth protection. More the depth of the drain, higher the length of the flow path and better to keep moisture away from the foundation. If geo-membranes are used vertically, they will also be effective but it is to be ensured that they do not get punctured from the pressure of filling material and have leak proof joints. In case, wide horizontal plinth cannot be provided due to site constraints, long vertical barriers should be provided to lengthen the flow path.
Damp Proof Course (DPC)
Normally, DPC is provided with cement concrete over which a bitumen coating is done. During continuous rains, water flows along the surface and enters the foundation along the external surface. Therefore, it is recommended that RCC damp proof course should be provided to make it impervious compared to ordinary cement concrete and vertical plinth provided up to DPC.
Road and Plinth Protection
If the road/pavement surface is near the building, no space should be left between plinth protection and the road/pavement surface. The space should be paved to avoid water to seep through by continuing the road/pavement surface up to plinth protection. Leaving no space between the plinth protection/building and road lengthens the flow path.
Rain Water Pipes
Normally rain water pipes are left on plinth protection. When plinth protection gets damaged, water percolates into the foundation soil. Therefore, rain water pipes should be connected into chambers and chambers to the drain. In internal courtyards, rain water pipes are to be connected to outside drains. If these pipes are left inside and water is not properly taken outside, this leads to heave of floors, cracks in walls and other damages to the structure. It is always better to pave all the internal courtyards so that water is kept away from the foundation as well as floors.
Horticulture works are undertaken in the buildings for the beautification and also for better environment. It must be understood that the horticulture works require water throughout the year. Hence, water comes into contact with adjoining soil. Since in case of black cotton soil, water is to be kept away from the structure, it is better to plant trees away from the structures. Therefore, plantation is to be carried out in consultation with civil engineers else the plantation helps in damaging the structure in such soils due to moisture attraction. Trees should be planted away from the buildings and on other side of the road.
Base Concrete and Filling Under floors
Normally cement concrete is provided as base concrete. In black cotton soil, lime concrete is effective as it alters the characteristics of the expansive soil. Hence, lime concrete should be provided as base concrete in place of cement concrete. It is said that 2-6% lime prevents swelling characteristics of expansive soil. Below footing level and floors, layers of non swelling soil mixed with moorum or WBM material are effective and are better than sand filing.
A combination of RCC damp proof course, vertical plinth, and monolithically constructed RCC horizontal plinth with RCC drain helps in preventing water entering into the black cotton soil. Wide horizontal plinth protection large depth of monolithically cast drains help in lengthening the flow path to avoid water entering into active zone.
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