In this article I will discuss regarding the project having 3 basements + Ground + 17 storied building VIP Road, Kolkata. My discussion will be limited in the safety procedure maintained after constructing the 600 mm & 22.0m depth Diaphragm wall and the distress happened afterwards and how it was solved.
According to the soil report Soil Stratum I (depth upto 12.40m from GL) had soft to medium soft, deep to steel to blackish grey, clayey silt/Silty clay, decomposed wood, organic matter and calcareous noodles. Shear strength was very low 2 to 3.0 T/m2, liquid limit 60% and plastic limit 26%. These particular stratum found very much dangerous and the main contributor of distress. It became very much after a short raining and started formation of slip circle. “N” value of this layer was 1 to 8.
Stratum II (depth from -12.40 m to -22.50 m) consisted of stiff, steel to yellowish grey, clayey silt/silty clay, laminated sand, whose “N” value was 1 to 22. This level found good and Diaphragm wall terminated in this layer.
Earth Pressure considered among the two equations which was found more critical-
Pa = ?H-4C = 0.90.H- 4.3.5<0
& from Tschebotarioff’s approach
(i) 0.30?H = 0.27 x depth of basement(t/sqm)
(ii) 0.20?H = 0.18 x depth of basement(t/sqm)
Thickness of Diaphragm wall was adopted 600 mm and depth 22.00 m. Planning was made in such a way that the main core structure started centrally keeping the slope of earth 1: 1.75. depth of earth cutting was -14.05 m from GL.
Complete Article and Images are available in Building Giants Aug 2012 Issue