Manhole Exacvation

[S06]

Hi everyone,

While excavation work for infrastructure works we usually reach the water table and find the situation as shown in picture.

https://files.engineering.com/getfi...846-bce4294387ac&file=IMG-20190516-WA0007.jpg

As we cannot go for compaction , we generally fill the area with boulders and aggregate and then pour blinding concrete

https://files.engineering.com/getfi...4c3-8c59ac0fa429&file=IMG-20190516-WA0009.jpg

https://files.engineering.com/getfi...051-22b9d83a9b47&file=IMG-20190515-WA0014.jpg

However I would like to know if this is correct practice, and does any code or standards are there to deal with this situation.

Thanks

 

[oldestguy]

The single sized gravel is a good idea because you can't do much change of its density from its loose condition to dense. Some might argue that fines can fill the voids in time and cause settlement, but usually that is unlikely. The one possible problem with what is pictured is the weak angle of friction for the gravel. But with backfill on it down there, that is not likely a problem. It is assumed you do what you can first before filling with the gravel is removal of loose material. Keep at it.

 

[fattdad]

not sure what's, "Correct." I'm just writing to say, here's where you could install a well point and make problems go away, or so it seems?

f-d

ípapß gordo ainÆt no madre flaca!

 

[SlideRuleEra]

S06 - Where I practice (year-round water table close to the surface), the "correct" way is to get rid the groundwater and keep it out until the concrete is placed. Getting rid of groundwater can mean pumps (operating 24/7 if needed), or well points (effective depth 25 feet +), or more extreme measure for deeper excavations (e.g. submersible pumps, cofferdam, bentonite slurry wall, ground freezing, etc.)

Your photos appear to show a reasonable approach for the existing conditions.

http://www.SlideRuleEra.net

 

[oldestguy]

It may come down to things like cost, job limits, etc. as to which method is best. There are others.

 

[GeoEnvGuy]

Just watched the local gas utility replace the main in a swamp. When I asked if they were going to go trenchless they said they would do it the same way it was done 30+ years ago.

Swamp mats and diesel pumps for about a month. As I see it you don't need the swamp mats only the diesel pumps.

 

[MTNClimber]

I agree with everyone here.

First try to dewater. If well points are cost prohibiting then create a sump in the bottom of the excavation and grade the excavation towards that sump. This may require multiple pumps and sumps depending on the size and depth of the excavation. Just keep the water away from the structure location.

Crowding stone into the bottom of the excavation to help stabilize the ground is common. Just make sure the ground is stable. Some people recommend placing filter fabric first to help keep the fines from getting mixed into the stone, potentially causing settlement. I’ve seen contractors do without the filter fabric and I haven’t heard of any call backs but who really knows.

 

[S06]

Yes, as well points are expensive vs to our tender. We are just opting to excavate while dewatering with submersible pumps.

Can any one tell when are the fabric required like geo-textile material.
And what requirement does fulfill while backfilling trenches after installation of PVC pipe.
Currently we are not installing them.

https://files.engineering.com/getfi...ec8d-4126-b3ce-947311c18dc6&file=DSC00919.JPG

 

[Mad Mike]

And this is what it comes to- everyone being forced to cut corners to win tenders- I'm not saying we don't all do it to some degree, it just hurts so much more to see it stated explicitly...

Use separation fabric in all cases where you're placing an expensive imported material on a crummy saturated soil...

Mike

 

[S06]

Thank to all of you for your great comments