Geotextile fabric placed over stone? 2

[TehMightyEngineer]

Got a quick question from a client of ours. They're placing precast box culverts and the DOT inspector is stating that the geotextile fabric used for subgrade improvement should be placed above the crushed stone subgrade. I've never heard of anyone doing this but apparently they insist this is how they've always done it. I informed our client this but wanted to see if anyone else had ever heard of people placing geotextile above crushed stone.

Also, this isn't wrapped around the stone for confinement and it's not for slope stability. Strictly for subgrade improvement.

Maine EIT, Civil/Structural.

 

[jmen68]

This might be better answered in the Earthwork/Grading topic board over in civil, but in my own experience, I have only placed geo-fabric UNDER sand and under rip-rap, never over.

 

[TehMightyEngineer]

I was going to post there but this forum is much more active and figured this was a simple enough question.

Maine EIT, Civil/Structural.

 

[cvg]

I believe you are referring to a separation geotextile? the function of this is to separate two layers, crushed rock and native soil. the reason for the separation is two-fold, 1)prevent the gravel from sinking into the soil below and 2)prevent migration of fine material from below upward and through the gravel if water flows through the gravel. placing geotextile on top of the gravel will not serve any purpose. however i would recommend wrapping it up and over along the edges.

 

[TehMightyEngineer]

Yes CVG, this is geotextile between crushed rock and native soil below the box culvert. The DOT inspector seems to think that it should go box coluvert, geotextile, crushed rock, native soil (from top to bottom layer).

Maine EIT, Civil/Structural.

 

[jgailla]

That sounds dumb. Maybe he's confusing it with wrapping a soil layer on top and bottom?

 

[TehMightyEngineer]

That's what I thought as well. Glad to know I'm not entirely clueless when it comes to geotechnical design. Thanks everyone.

Maine EIT, Civil/Structural.

 

[oldestguy]

I'd play dumb and say you have concerns that there may be a problem later. Therefore you want to make sure you do it right and can refer to a standard DOT detail or the "book" where that is called for.. Any DOT these days with use of fabric will have some guidance detail or written specification for that use. I suspect the inspector is stubbornly clinging to a dumb directive from him (her).

 

[cvg]

if it was up to me, I would have prepped the subgrade by either:

a) placed compacted road base with no geotextile
b) poured a concrete working slab with no geotextile

option b provides the best working surface for placing and sliding the box sections into position. much harder to do on a gravel surface.

 

[Ron]

cvg nailed it in his first post. TME...see...even when you're a little out of your area, common sense prevails! Good call.

 

[TehMightyEngineer]

CVG: I'm not sure I would actually prefer a concrete working slab as if it wasn't leveled properly it could cause issues with the joints of the precast culvert fitting up properly. Typically the contractor will place two rows of 2x6 boards under the walls of the box culvert to act as slides for when they're placing and sliding the culvert sections together.

Maine EIT, Civil/Structural.