Foundation settlement

[tk78]

I have an existing structure (3-story) 1 to 2 meters from a retaining structure, Soldier pile (5.2 meter excavating)with tieback. The soil profile behind the retaining structure is loose clayey sandy soil (layer thickness 1.5 to 2.5 meter) overlying on soft to medium stiff lean clay, water table is about 1.75 to 2 meter from ground surface. The structure foundation start showing settlement (3.8 cm) when the contractor start dewatering. there is no deflection in the retaining structure.

Anyone with experience on this condition?

 

[PEinc]

You really did not give enough information to identify THE cause of settlement. Your excavation is close enough to the existing structure that underpinning may have been a better way to support the structure. 3.8 cm is more than should be expected with proper underpinning. Sheeting deflections could have caused the settlement. However, dewatering may also have caused the settlement. Dewatering makes the soil heavier which can cause the "loose clayey sandy soil" to consolidate and settle the building.

http://www.PeirceEngineering.com

 

[tk78]

there is no deflection in the sheeting. I thought that the lean clay is expansive soil which cause the settlement.

 

[MTNClimber]

Expansive soil would cause heave, not settlement. Moisture would have to be introduced into the expansive clay to cause heave. With groundwater above the clay, it is not likely that you'd see 3.8cm of heave over the course of a short duration, especially if you are dewatering.

Was the shoring wall surveyed near to top of the wall and prior to excavating? I've seen some contractors survey after they have excavated which does not tell you how much the wall initially deflected. Also the location of the survey points are important. Typically movements are greatest at the top of the wall than the bottom. Theoretically this could be wrong depending on where the tiebacks are installed.

I agree with PEinc. Due to the walls close proximity of the building, underpinning would have been a good idea. De-watering could be causing consolidation of the soil underneath the building.

 

[oldestguy]

Dewtering is a common cause of settlement, since the formerly submerged soil now is not affected by buoyancy and weighs more..

 

[HENRYZAU]

A sketch x-section showing the existing building and the anchored wall would be very helpful. Is 5.2m excavation depth close to the building?

 

[PEinc]

"I have an existing structure (3-story) 1 to 2 meters from a retaining structure, Soldier pile (5.2 meter excavating)with tieback."

http://www.PeirceEngineering.com

 

[oldestguy]

tk78 Your statement.
"I thought that the lean clay is expansive soil which cause the settlement."

Lean clay normally is not expansive and more likely is compressing as it goes from saturated below the water table to above a water table. Don't blame settlement on the retaining structure. Get an experienced geotch on the job before you cause more damage.