An interesting shoring technique which we got to work with, for a multi-storey office building in Noida.
The soil type in Noida is loamy and construction site has high water table. Such a combination of soil type and high water table creates the possibility of collapse (cave-in) of trench walls at the time of excavation. Hence, the shoring technique has to be designed to retain earth, water and adjacent structures.
The building right adjacent to our site it has 7 floors and one basement, which goes down 3m from the natural ground level. Our proposed building has 13 floors and 2 basements, which go down 8m from the natural ground level. The inter-building distance is only 10.5 m, hence we had to come up with an innovative solution to avoid any structural damage to the adjacent building. This technique involves casting of two lines of piles using the DMC (Direct mud circulation) method in tandem with each other.
Overview of the methodology:
The site’s foundation raft would be cast in three parts starting from the side of the site where adjacent plot is vacant and hence has no piles to the edge of the site.
Step 1: Local Excavation.
Step 2: Casting of 600mm diameter soldier piles which go down 16m down from the natural ground level. These piles have been cast on two sides, one side which has an existing building by its side and the other on the front side towards the road so that the existing services do not cave-in.
Step 3: Excavation of the first part (A) which goes down 12 m from the natural ground level.
Step 4: Casting of the 900mm diameter bollard piles which go down 16m from the natural ground level. The span between the two lines of the piles is 3m. To avoid any sort of cave-in, the in-between portion of the two lines of piles has been supported with gunny bags till the time this portion of the site is excavated.
Step 5: Connecting both lines of the piles with horizontal and diagonal struts.
Step 6: Casting of the first part of the raft (A).
Step 7: Supporting the bollard piles with the raft using inclined struts and fixing them with metal plates.
Step 8: Column casting in the first part (A).
Step 9: Excavation of the second part (B) of the site and using temporary struts on the side with no piles to avoid the soil forming the trench wall to collapse and cave inside the site.
Step 10: Raft casting of the second central part (B) of the site which extends up-till the centre line of the bollard piles.
Step 11: Excavation of the 3rd part of the site.
Step 12: Raft casting of the 3rd part of the site(C).
Step 13: Dismantling of the bollard piles and cutting them off at the raft’s top level.