sinking a concrete structure

[Dean A]

has anyone had any experience sinking a concrete manhole( 72" inside diameter ), caisson style? that is removing the soil
and letting the structure sink. This is for a new sewer lift station in a very congested area. no room to open cut.

I plan to remove the soil inside the structure by using a hydro vac. I need to build this structure 27' deep x 6' inside diameter is
about 30 cy. I do plan to attach a steel cutting edge to the lower barrel section.
The soils are clays to fairly dense sand.
Should the manhole barrel be coated with anything to reduce friction?

Any ideas will be appreciated.
Dean

 

[CM1995]

Welcome to the Forums Dean A!:drinkup

Interesting project you have there. Do you plan to vac the hole and slide the manhole sections in one operation? What I envision is the base with a hole in the middle of it for the vac to go into. Set the base, insert the vac tube and proceed with excavation. As you stated, a cutting edge on the bottom of the structure would aid in the excavation. The problem I see is keeping the hole/structure plumb as you are excavating.

I wouldn't think a normal pre-cast manhole would need any coating to help it slide due to the weight of the structure and it's outside slick surface but I have zero experience with vac excavation.

Like I said this is a new one for me, keep us posted.

 

[sandro]

this will be fairly difficult pal if you run into any hardpan the hydrovac is pretty much useless. and on the other side if the ground is to soft you will dealing with the earth slufing in, either way its a mess. what kind of shoring do you use or have available. as long as the hydrovac works you can use sheet pile or what we use is timber shoring (which up here is good to 22') and geotec the last few feet. when these types of jobs are planned try to always factor the worst case things that can happen and assume that you may have to put somone in there to do the bull work or split rocks or jack hammer out the hardpand so the hydrovac can work. either way cover all of your bases.

 

[AustinM]

Pardon my ignorance Dean A, but I'm confused a little by the attachment of a cutting edge to the lower barrel section. Do you mean attaching an edge under the precast barrel section and spinning it down like an auger and adding barrel sections as you lower it and vac out the material? If that is the case, the first thing I would think of is what happens if the barrel section breaks. I agree with what the other poster said about what you would do in the event you hit something hard. get someone down there to hammer it out?

I am currently installing a total of 3 storm water treatment systems that are 10' in diameter and a little over 20' deep. In our case, we can't open cut them either because we have no room and wouldn't be able to get the crane close enough to set the sections (using a 60 tonner) so we rented a trench shoring system (20'x18') and went as vertical as possible with a 450 John Deere. Still struggled with sloughing of the edges. I thought we would be really struggling with groundwater and really sloppy material as we were only about 130' from the creek but we hit some hardpan at about 18'-20' that would pick the front of the hoe up. We got through it okay, but I certainly wouldn't want to be vac'ing away and hit something like that (especially that deep) and have to do something else. All that was on the first one, still have 2 more to put in.

All in all, it sounds like a tough job you are looking at! PM me if you want and I can give you the contact info I have if you want to look into renting a shoring system. They send PE stamped certifications too (which I'm thinking you'll need). Got ours from Denver but I think they have an office in your region too.

 

[Dean A]

thanks for the info. I have used slide rail systems 3 or 4 times( one was 34' deep ). we don't have room on this one. a vac might work as, the soils are silty, soft clays.
so hopefully the structure will sink as we vac out the interior.
I will let the forum know how it goes. Looking to do this mid May or so.

 

[Dean A]

the 6' diameter concrete structure will be the shoring; hopefully, it will sink into the ground,
as the interior soils are removed( vacummed out or removed mechanicallyl).
as the structure sinks, additional sections will be added on top, untill the desired depth is achieved.

 

[Turbo21835]

Ever consider using an old school method. I would think having a crane and a small clamshell bucket on stand by may not be a bad idea. I have done a lot of vacuum excavation, and when you hit some tough clay, you are going to have trouble. It will take a ton of water run at high pressure to break it up.

What is your plan B on this?

 

[rsherril]

Questions: Will there be a man in the hole while the vac. machine is working? Do you know for sure that you can pick up material and move it up to the truck at 20+ feet deep? Will this be a round the clock operation? Will there be other personnel on site that might be affected by the noise?

My experience in mining is that 30 cubic yds. is going to be more like 60 cubic yds. by the time you break it and move it out of the hole. I like the clam shell idea as it is proven and dependable, but can be very dangerous to work under. There are also portable high pressure water jet drilling/cutting units that can cut your concrete, re-bar and all. I would also consider a smaller, (auger), hole to break the material to.

Shaft sinking is not for the inexperienced. I applaud your innovative thinking on this but recommend you continue to get all the input you can and be safe first. Good Luck.

Moderators, consider a Vac Truck thread as it seems to be a popular way of excavating silt from lined ponds and finding utilities not to mention gold placer mining. Would like to know more.

 

[sandro]

i take it that you will somehow bolt the prefab barrels together? if your working in sandy clay and the walls give in it may put pressure on the wrong prefab barrel and cause the lower ones to seperate. good luck with your job hope to see some pics of this.

 

[AustinM]

Is this a job you are bidding or already have?

 

[Dean A]

thanks for the advice. I have thought about using a crane with a clamshell bucket. I think that would be a very good backup to have on site.
I need to find one.
thanks

 

[Dean A]

the barrels will absolutely be securely bolted together at 3 points around the circumference. I will be using appprox. 5, 5' barrels.

 

[DGODGR]

I think the biggest issue will be the friction on the sides of the barrell sections. Especially if you have silty/sandy soils, and even more so if you encounter water. Why not use a caison drill rig. You could drill and case the hole at +/-8' and install the manhole inside the casing. Once the MH is in place you could back fill with flow fill and pull the casing back out as the flow fill rises (this would require a crane of course).

 

[heavylift]

I like DGODGR plan.
If you use your plan. I would coat the exterior of the walls with Bore-gel, Con Det or maybe Ez Mud. Those products are very slick, good luck.

 

[dayexco]

competitor of mind did one like this about 20 yrs ago. as said, keeping it plumb is the big thing. he had issues from the get go, and when he reached his elevation, poured the bottom, he had a VERY hard time getting the slide rails/hatches plumb enough and still give him room in there to get the rest of his plumbing installed. i'm with the other guys here who say go with a box/slide rail system, good luck.

 

[backhoe1]

I definitely would like to see pictures of this. Blaine isn't that far away maybe we should road trip over there and watch how this goes!

 

[dayexco]

andrew, remember Homer Prunty? IMO, he wrote the Bible of underground utility construction. Was the SMARTEST guy in our line of work i have ever met. I'd be digging here in town, he'd have a crew up here working....he'd pull up, sit off the side and watch for an hour or so, then leave. this happened several times, after about the 3rd-4th time, i crawled off the excavator, made small talk with him, eventually asked him...."why you watching me work?"....his response, i'll never forget....he was probably about 60 years old at the time...he said..."never, EVER, quit learning. i like watching other crews work, see how they do things, and if what i see will increase my production, we're going to start doing that"...i took that as a HUGE compliment. per the OP....i think he knows how/what he wants to do. he's just "picking our brains" to see if his plan of attack is the right one, and if our suggestions may help him. a trip to Blaine to watch it? hey, i'm up for it. i'm ready to learn some more. he might know something i don't!!!

 

[Scrub Puller]

Yair . . . A bit of a ramble here but it may be of interest.

I knew a bloke up in the Gulf Country in the early 1970's who (among other things) dug wells for domestic and stock water. He made a mold and cast a series of two foot high five foot diameter concrete segments . . . wall thickness was about four inches with plenty of reo.

The first one was put in place and he got inside with a short handled shovel and started excavation. He had a jib and winch on his old Commer knocker and kept on adding segments. He had the jib and winch set up with start/stop/slew/tip bucket/ functions operated by pull ropes from down the hole.

None of the wells were very deep. I think twenty sections or forty feet would have been the maximum but he never reckoned it was nuch trouble just a lot of work. He lubricated the outside with water carried in forty four gallon drums and had a down hole pump run by a Villiers engine for dewatering when the hole started to make.

For rock he had the smallest air jack hammer I have ever seen run by a Volkswagen engine he had converted himself . . . two cylinders were power and two were compressor pumping into a twenty foot length of ten inch pipe he had scored some where and made into a reciever.

Old Tom reckoned he'd never had a day of schooling in his life and had taught himself reading and basic mathematics. He was pushing eighty at the time.

A truly remarkable innovative bushman.

Cheers

 

[backhoe1]

Thats exactly what I was getting at. I love to learn and see how other people do things.

 

[Dean A]

I appreciate all the info/ideas. I may drill it in, probably the safest. I may take a chance and try to sink it.
I'll let everyone know how it goes.
Thanks