What: Brief of the Problem: Upliftment problem due to high pressure (buoyancy) beneath the ground surface
The problem of uplift due to buoyant force was observed at the commercial building development site. The site location is near to Yamuna River. The presence of the water table was very close to the ground surface created the development of high pressure beneath the ground surface. HCL wanted to make a commercial building, but the high-pressure buoyant force acting vertically upwards, leading to upliftment of the ground surface, created obstruction due to its stability problem.
“Buoyant force is an upward force that fluid exerts on any object that is placed in them. Buoyant force occurs because the fluid below an object exerts greater pressure on the object than the fluid above it.”
How: Engineered Solution: Arresting Ground Upliftment Using Vertical Anchor Systems
There was a need for some anchoring system that will hold the upward movement. The use of a passive cable anchor was selected to serve the purpose of the anchoring system. Fixed length (grouted length) will act as a rigid system, and stress transfer will occur through its free length (which is in tension). Stressing of the anchor will restrict and arrest the upliftment of structure, which was occurring due to buoyant force.
The designed anchors for the project were of 18 meters length out of which 12 meters was bonded length (fixed length), and 6 meters were a free length. The capacity of the anchor was 60 tons, and the spacing of the anchor was kept 3×3 (grid) meters.
Difficulties Faced during Execution:
The groundwater table was present at the -2 Mtrs, and we had to install anchors at -14 Mtrs. So, the process of de-watering was continuous to draw down the water table. The machinery movement was also difficult in this scenario. Apart from this, to get the required capacity of anchors, it was necessary to make grout bulbs for which high precision was required.
The technology used for Execution:
Grouting of the anchor was done by using new innovative technology known as Tube-A-Manchette grouting was used. This was used to achieve high strength/load-bearing by taking advantage of more frictional resistance.
“Application of TAM grouting is that where the sleeved grouting pipe allows to carry out first the primary injection followed by high-pressure injection, granting perfect cementation (well packed bonded arrangement) of the anchors in the bond length.”
The installation of anchors in a vertically downward direction along with proper grouting technique minimized the chance of uplift of the ground, and zero magnitude of uplift is observed after completion of the commercial project up to the present.