200 ft deep X 75 ft diameter wet wet pump station....How would you excavate and construct a project 3

[dinot71]

Hi, I am a design in front of me for a 200 ft deep by 75 ft diameter "wet well" pump station. If has a 20 ft thick concrete base footing and 3'-2" wide side walls. It also has a 26 ft diameter concrete pipe near the top of the fore mentioned base footing. Anyone have any ideas on how one would begin to estimate on how to construct something of this magnitude?

 

[JedClampett]

Just my opinion, but I would use a caisson design. Pour a ring of concrete 75 feet in diameter desinged to your base conditions with a sharpened bottom. Start excavating inside the concrete ring. As you excavate under the sharpened edge, the concrete ring will lower. When it lowers enough, pour another ring on top of it. Repeat until you've reached the bottom elevation. Then pour your bottom plug.
If it's underwater, things get trickier. You have to close the top and pressurize it to keep the water out. This makes it very slow and hazardous for the workers, as they need to work in high pressures and run the risk of getting the bends.

 

[oldestguy]

No way for anyone here to give specific usable help unless you also provide the complete soil and water conditions. And, if you have not done a job roughly similar, I think you need on site help from some one who has. For instance, is there any local info on artesian water conditions? How about nearby structures?

 

[paddingtongreen]

My first thought was, "That's a big boat."
On one job, we drilled in 3'-0' caissons at 3'6" centers. the inside face was to be broken out to reveal rebar to hold horizontal bars for the inside face. A stiffening ring was placed at the top, the interior was excavated to the next stiffening ring, then more excavation...This was close between working railway tracks.

Another was similar but with sheetpile, that needed more stiffening rings than the concrete. In both cases, ground water was a threat to pop the pit structure out of the ground.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[racookpe1978]

Soil conditions? Or "solid" rock? Gravel and muck?

Water pressure that deep?

 

[TopKnot]

soil nails and shotcrete as you excavate downwards. You may have to drill some dewatering wells, depending on the soil and groundwater. There is a lot to it. That is a lot of bouyancy to consider. I would have cross member support as you go up. I'd love to work on this!

_________________________
TKE

 

[SlideRuleEra]

I agree with JedClampett, with one exception. Pressurized caissons are not often needed, even below the water table, for vertical shafts where the top of the caisson in on land. Excavation is performed "in the wet", from the surface. Several have been constructed this way in the Charleston, SC area in recent years. Here is a brief summary, with photos, for one shaft that is 30' ID x 150' deep:

http://www.bradshawcc.com/projects/306/view/

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[tony1851]

@JedClampett, wasn't that in principle the same method Marc Brunel used when sinking the Rotherhithe shaft - though with cast iron ring and mass brickwork.

 

[dcarr82775]

At 200ft deep I don't think secant/tangent piers are a great option, the tolerances will kill you. Now if bedrock is shallower so only a portion of the shaft is in overburden that is a different story.

Caissons sounds like a good way to go if soils allow.

 

[JedClampett]

Tony, I wasn't familiar with the Rotherhithe shaft, so I looked it up. It seems similar, but there are some important differences. In my method, the shoe is cast into the bottom ring and no permanent steel (or cast iron) framing is used. But Brunel used brick (I'm sure that was much more available in the early 1800's) and the method I'm suggesting is reinforced concrete.
If SlideRuleEra is right (and he almost always is), and this could be done without any pressurized compartments, it really makes sense.

 

[oldestguy]

Note all the different ideas. I think this job probably is best designed with the help of a geotech engineer who has worked on several different jobs similar to this. Check on credentials because this can result in major claims later on if unforseen problems come up.

 

[EngineeringEric]

JedClampt, you beat many of us to same idea. I have done this before on numerous pump stations. My experience has only been in rectangular shapes and the deepest is around 50 ft. But the concept is sound. I think the most important part is the contractor and the design of the cutting section.

I worked with 8-10 foot lifts of wall at a time and it worked great, it was a good production line effort.

Rocks required jacks/time consuming excavation so a good deep soil knowledge is critical for your case!

 

[BigHarvey]

Provided it's not rock with an UCS greater than 130 MPa, a diaphragm wall could be a solution. Smaller shafts have been constructed down to 100 m with this method. It's too deep for secant wall as already advised.

 

[civeng80]

I worked for the water and sewerage authority as design and construction engineer and we used to build pumping stations by the caisson method exactly as Jedclampett described. Typical diameters about 20 to 30 ft.

I remember on one job the pumping station disappeared completely during a long weekend.

As mentioned if under water things can get tricky, also the shaft may not sink exactly vertically, can be tricky.

Sometimes if above the water table it can be more economical to dig a large hole and form the walls both sides.

Geotechnical imput by experienced persons is vital.

 

[Gumpmaster]

There are several options for this type of construction but they are all dependent on the soils. The caisson method may work (although it's a rather large diameter) but not if there are cobbles or hard soils. Diaphragm walls (secant pile or similar) may work depending on the soil. I recently finished a 150ft diameter 300ft deep shaft. We used diaphragm walls for the top 1/2 and rock anchors with shotcrete for the bottom 1/2. Inside that we poured a reinforced concrete shaft (the diaphragm walls and shotcrete walls were considered temporary works). Also did a 70ft diameter, 200ft deep shaft recently(in sandstone). On that one the top 175 feet or so was a diaphragm wall without a reinforced concrete liner.

Bottom line is the design is driven by the construction method and you need to engage a geotech and a construction company familiar with this work. The design is probably 75% geotechnical and 25% structural. The geotechs will have to do a boatload of analysis to get you anything you can use in design. There will be months of borings and Plaxis anaylysis befor you can even think of a construction method.

 

[QAMaster]

I'm going to agree with TopKnot's idea, I like this system for SOE because the support types can be adjusted as needed with depth or as conditions change. Soil and rock conditions can be imporved if needed with special grouting and variuos types of anchors. I also agree with oldestguy, you will need alot of data about the soil conditions where you want to excavate and the areas that need to be retained/supported.

My recommendation is that you look into consulting some of the large geotechnical contractors. A simple phone call, with an interesting project, and most of them are willing to talk to you about various solutions. You will want an experienced Geotechnical Engineer to assist you with analyizng the soil conditions and determining various parameters, but the contractor is the one who can/will actually build it.