Drag down loads on manhole shafts

[kcall2]

Storm drain lines located up to 65 feet below existing grades will be supported by consolidated, sedimentary bedrock. Conventional grading methods will be used to install the lines, therefore at locations of manholes, the adjacent soils will consist of granular (SM and SP-SM with gravels) compacted trench backfill. Manhole sections will be installed as the backfill is brought up. Considering the granular nature of the backfill, is it safe to assume that drag down loads on the manhole shaft will be negligible?

 

[garrettk]

I would believe your down drag would be negligable as you are founded on bedrock and your backfill is adequately compacted (95% of standard Proctor).

Down drag is normally associated with the settlement or consolidation of soft or loose layers of clay or sand when an additional load placed on top.

Depending upon the density of the native soils, you may end up with insufficient material to backfill the excavation after it is properly compacted, but I wouldn't be concerned about down drag.

 

[cvg]

concrete pipe is quite strong in the axial direction so I would guess that any amount of axial force caused by down drag could be neglected. However, the dead load of 65 feet of concrete pipe bearing down on the manhole structure at the bottom will be considerable. If this is a 48" diameter shaft the pipe weighs about 700 pounds per foot which is about 23 tons of weight bearing on the manhole shaft seat. For a 60" shaft it would be about 30 tons.

 

[RWF7437]

Wow! What are "conventional grading methods" for 65 foot deep storm drains ? What type and load class of pipe would such depths require ?

 

[Zambo]

RWF7437,

yes I agree does seem to be quite a job. If in an urban area it is generally the case that, if possible, open cut is less expensive than pipe jacking or micro-tunneling. But at the depths mentioned I would expect pipe jacking to be less expensive. What is the drain diameter?