Necessity of Providing RC Structural slabs and Beams
We can analysis here the Necessity of Providing RC Structural slabs and Beams at Plinth level (GF-Finished Floor Level).
Wherever low/poor bearing capacity soils met with, the Architects and also certain Builders think and believe that it is much safer to provide RC Structural slabs & Beams at plinth level (GF-Finished floor level) in order to avoid settlement of foundation and consequent formation of cracks in walls.
As we are concerned, with experience and expertise in foundation structure. we know well the behaviour of poor bearing soil when loaded heavily.The cost of providing RC Structural Slabs & Beams increases the cost of the foundation system by more than 50%
Therefore, the provision of RC structural slabs and Beams at plinth level will not be a fool-proof solution to avoid foundation settlement and consequent cracks.It is nothing but and added expenditure spent on the construction of structure.
Instead, the following measures will prevent the settlement of foundation systems and facilitate in avoiding wall cracks effectively.
a) The foundation soil at a depth of 00 m to 2.70m (if it is a poor bearing soil-say soft/medium clay) is to be artificially strengthened by Geogrids/Geotex layers followed over by Quarry dust:Gravel Mix 1:3 or cement :
Quarry dust 1:10 mix in 4 or 5 layers of 200 mm thick and by this the SBC of soil at 2.00m/1.80m level is improved to 200kN/m2 from 100kN/m2
b) Provide one more layer of Geogrid/Geotex over and then the PCC 1:5:10 for 100 mm
c) Over which you provide the foundation system as designed-combined strip Raft/combined/isolated footings as the case may be.
d) instead we may straight way provide under reamed piles with pile cap.
Over the ground level also,after providing the RC grade beams designed at 200 mm below ground level,fill up the height upto to the bottom of FF level (Pinth level-may be of 450mm/600mm/750mm/900/1000mm as the case may be according to your requirements) with layers of Geogrids/Geonets and Quarry dust:Gravel mix 1:3 in layers of 200 mm at a time compacted semidrying with mechanical means. By these artificial exercises,the soil at plinth level will have a SBC of 250 to 300 kN/m2 with well compaction over which if you provide your conventional flooring (with PCC & Grano- with the chosen tiles), there is no chance of foundation soil settlement,causing cracks in the walls etc.
Instead of strengthening and compacting the soil very effectively, even if you provide RC structural slabs and beams with nominal rebars, there is every possibility of foundation soil settlement. The cost of providing RC Structural Slabs & Beams increases the cost of the foundation system by more than 50%
Let us analyse, choose appropriate foundation type, artificially strengthen the soil as described and do the conventional method for flooring etc without RC structural slabs & Beams and control/contain the expenditure on foundation system besides preventing the soil-foundation settlement and consequent cracks in walls very soil engineering way.