negative skin friction - constant force?

[jimbo2]

I am assessing down drag forces for a 170 foot deep manhole already in place, and buried in a ring of stone surrounded by municipal solid waste. The down drag force is easy enough to compute; however, I am curious about whether this is a transient or constant force. When the waste consolidates and there is relative motion between the waste and the backfill, this motion will impart a force on the manhole. What about when the waste is not in motion? Can we have a downward force acting on the manhole without motion? Does the force that is exerted on the manjole dissapate as the manhole resists the downward force?

 

[Focht3]

The force will dissipate if the manhole moves downward enough to offset the "set" of the downdrag, or if the rock and waste lose contact with each other. Otherwise you likely have a long term force.

Practically speaking, you will probably end up with both conditions over the life of the manhole but will need to design for the long term force condition.

 

[jimbo2]

Focht3:
Thanks for the reply. I'm thinking this "set" would be a residual stress built up in the stone/waste contact and translating to the manhole walls. This makes some sense to me, and I will evaluate the condition as you suggest. Thanx again.

 

[Focht3]

You are correct - the "set" had to do with the movement associated with the residual stress.

Good luck!

 

[dtimbremont]

Hello,

Negative skin friction should be expected when the surrounding compressive soil is being compacted (under its own weigth) ; this down drag will cause the "tube" to settle, which will reduce the relative movement between the tube and the surrounding soil/waste, and in this way reduce the down drag.

Dimitri Timbremont
Geotechnical Engineer
Socofonda
(Belgium)