jacking problem 2

[cvg]

we are proposing to jack a gravity storm drain line between two large diameter gravity sanitary sewer lines (2 - 90 inch above and 1 - 84 inch below). Clearance is limited and invert of storm drain would be about 20 feet below the surface. The material is silty / clayey / sand and deeper than about 25 feet turns to silty gravels and cobbles. It appears that there is some cohesive material in the upper 20 feet.

I have several questions.

1. First option was to jack a 4'H x 8'W concrete box culvert. We have not had success locating a contractor who has jacked a box culvert - apparently has not been done in Arizona. Anyone have experience with jacking boxes?

2. Can an eliptical pipe be jacked without using a casing?

3. What methods are recommended for
a) stabilizing / shoring the jacking pits
b) stabilizing the excavation at the sheild


4. Would going underneath everything (in a siphon) pose any significant additional stress on the existing large sewer lines? Invert would be closer to 40 below surface and pipe would be probably 96 inch diameter.

 

[Focht3]

Forget jacking square or rectangular sections - too easy to damage the corners. No contractor with any brains will try jacking a rectangular section into a cobbly material. And you don't want the brainless variety...

You can advance the box using a "hand mine - then push" approach. But you will get a lot more loss of ground that way.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[Zambo]

I was hoping there would be a few posts on this subject as you sound to have an interesting project. My first observation is that your invert is above the cobbly material and perhaps in reasonable material for jacking.

From your post it sounds as if you want to install the storm drain open cut up to the foul water drains then jack between them and back to open cut.

In this ground hand mining without a shield would probably be dangerous, however, in my very limited experience this is the most common way of jacking a box. I cannot see why you cannot jack an eliptical pipe, but again soil removal has to be carried out. The need for a casing depends on the pipe material and design. What about jacking 2 circular pipes, then shield and soil removal no problem.

Focht3 what about some kind of jet grouting soil improvement around and between the existing pipes then sheet piling where possible and constructing open cut.

Zambo

 

[cvg]

geotechnical investigation has not been done yet, but a previous geotechnical report did indicate that we would be probably above the cobbles. however, our client is concerned that with very little clearance from the sanitary sewer lines and jacking a box that we need to look at all the options a little closer.

we are considering multiple pipes, but of course cost is also a factor. Eliptical pipes have more capacity with same height as circular pipe.

I would like to find out more about jet grouting as I am aware that it could be a method to reduce caving and stabilize the soil somewhat.

 

[Focht3]

The first thing to note is that we don't know with absolute certainty what the materials are like that are between the existing 90 and 84 inch sanitary sewer lines. Were the pipes installed using cut-and-cover? Were they bored? Or were they jacked?

And the "trueness" of the alignment of the existing lines can become critical depending on how "close" these three lines will be.

The issue of cobbles (or other large "debris&quot is an important one. A tunneling contractor in Houston sued the City for changed conditions when he encountered big chunks of cemented sand along a section of tunnel alignment. Some of the chunks were larger than a Suburban, and very hard - like a soft sandstone. Extra shafts were required to remove the cemented blocks, and the project was delayed. The soil borings did not encounter any of these chunks, although the presence of these large cemented blocks has been known to occur in other parts of town.

The point is that it is very possible to miss key features - particularly when you can't explore along the centerline of the proposed alignment...



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[Zambo]

Good point re. the material between the pipes, this could affect the suitability for jet grouting, so either a lot more investigation needed or put the idea to one side. If the soil is suitable then it is fairly straightforward using a drilling rig and rotating injection nozzles to work in a grid system. The depths you are talking about are't so deep so once the soil is stabilised it should be possible to get some earthwork supports around the pipes. But for the grouting you need a soil investigation and expert advice from a specialist contractor.

I would get a price for directional pipe jacking using circular pipes. The thrust pit can either be sheet piled or a concrete casisson. Different cutting heads and soil removal options are available. But of course the larger the diameter the more specialist and the more expensive this gets.

What is the length that you are going to install by jacking? What diameter (is it circular)is the remainder of the storm drain and is it going to be installed open cut?

Zambo

 

[cvg]

90 inch lines were installed in trench. one is 18 and the other 42 years old. the older one is surcharged and some fear about the condition of the pipe. the 84 inch is below and is beginning construction in a few months. it will be trenched. the plan for this line is to trench and use two trench boxes stacked to shore. I'm sure we will learn more when they attempt this. it will be completed prior to the storm drain project.

the 90 inch lines will be potholed soon and they are trying to locate a joint to determine thickness of the bells.

jacking length is about 130 feet.

upstream storm drain line is 96 inch and downstream line will be either 96 or 108 inch. up and down are to be installed in trench.

i was thinking that a concrete caisson might be a good idea for the pit shoring. maybe it could be left in place and serve as a junction manhole.

our cost estimate at present is:

130 LF 4 x 8 box $45,000
130 LF jacking $169,000
shore pits / manhole $296,000

total cost to jack the box and construct the upstream and downstream pits / manholes is $510,000

Does this cost seem reasonable?

 

[Focht3]

Hmmm,

Where is the site (city only)? Phoenix? Tuscon? It doesn't sound like Flagstaff -

I assume the 84 inch line will be beside the existing 90 inch line(s) - otherwise, I don't see how the contractor could accomplish the task. Why not negotiate the contract and add the box culvert line at the same time the 84 inch line is built? I'd at least explore the option.

A drilled shaft wall would work quite well as both temporary shoring and permanent walls for the junction manhole.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[cvg]

City of Phoenix

the two 90 inch sewers and the 84 inch are all separated horizontally within the road right of way. we have raised the possibility of installing at least part of the box culvert during the 84 inch sewer construction, however, that project has already been bid and we would need to scramble to get plans ready for that contractor. It would have to be a change order to that contract. And a portion would still probably have to be jacked under the 90's.

Another thought we had was supporting the 90's over a cut and cover trench. Seems risky, but another option.

drilled shafts sound like a good idea. they are common around here, but usually large diameters used to support bridges. Would it be maybe a 12" shaft at about 18 inch centers and then permanent wall cast against that once the excavation and jacking is finished?

 

[Focht3]

Er,

What are the consequences of losing the 90 inch line? Is it sensitive to movement? (I have assumed the answers are "very bad" and "can't tolerate much movement.&quot I understand the cost of a drilled shaft wall may be high; but we have to consider the consequences, too.

You can't get enough section modulus with a 12 inch shaft; you are probably looking at 24 to 36 inch shafts in order to limit movements. Remember that you won't be able to install tiebacks because of the 90 inch line you are protecting.

And while the earth pressures in the sands won't be that high, you have to keep the spacing between shafts small enough to reduce the risk of "running ground" developing between them. Remember that the 90 inch line is full and may be leaking as a result. While groundwater is not normally a problem in Phoenix at those depths, you may have an artificial source to deal with.

I'd look real hard at other options before I considered re-supporting the 90 inch line while the excavation proceeded below it. That's pretty risky -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[rchanke]

If you are looking at soil improvement for this project I think jet grouting has two drawbacks; 1. The mobilization costs associated with jet grouting are quite high and cost prohibitive for smaller type projects (like yours) when compared to other forms of soil improvement and 2. The in-situ soilcrete generated can have unconfined compressive strengths averaging 500-psi or more, which slows down any hand mining that may be required for the jacking of your product.

If your soils investigation reveals that the top 20-feet is comprised of silty sands (SM) interbedded with some amount of cohesive material then your problem is only with the silty sands or poorly graded sands that would "run" upon excavation. My suggestion to you is permeation grouting. Permeation grouting should be less expensive than jet grouting and will only treat the permeable layers of soil ie. the running sands. In addition, the grouted soils will achieve unconfined compressive strengths of 100 to 200 psi and are therefore much more mine-able.

If you are interested in using "Soil Improvement" techniques to remedy your problem please feel free to give me a call and we can discuss all the available options.

Rick Hanke
Hayward Baker Inc.
Western Region ph (805) 933-1331

 

[boxjacker]

We do box jacking and so I can reply to a few of the issues brought up here. First, box jacking may be the best method when there are cobbles or debris because the box, if cast full length, would be harder to deviate than a shield at the front of a string of pipe. If passing close beneath sensitive utilities you want minimal overcut. This produces more friction and a monolithic box is much more able to withstand large jacking forces. Optimally, the site should be arranged to provide room to cast the box full length. However, as we just did at a site in the Bay Area, the box can be jacked one halve at a time.
Box jacking is an underused option. If anyone wants some free consultation on particular applications I would be happy to give it a try. tlynn@berkeleyengineering.com

 

[DRC1]

We have had quite a few jobs that involve jacking, including boxes. Generally the face is grouted with a weak cement-silicate grout. The clay gravel interface will probably be wet and the grouting will probably be nessecary to control ground water. I have seen round pipe and rectangular boxes jacked.I have never seen elliptical pipe jacked. Doesn't it can't or isn't done, however I would think elliptical pipe would be difficult to control. If the box is large and the cobbles are intermitent, they should not be a significant problem. however if the cobbles are frequent, or grow into boulders, such that they need to be split or require substantial handling to remove, the cost of the work could be impacted. I agree you want to avoid jet grouting. That will leave a wall of concrete and send costs sky high. Typically the jacking pit is about 15 feet to 20 feet longer than the length of pipe to be jacked. Access for a crane on one side is required and the back of the pit must be designed to recieve the thrust from the jacks. The ftont face of the pit is typically grouted and the area around the pipe penetration is usually lagged. Pits are typically soldier pile and lagging or sheet pileing. Receiving pits are generally much simpler. If all that needs to be accomidated is the pipe and a manhole, often a pair of stacked trench boxes are used. If conditions require, receiving pits can also be sheeted. My conern with the sewer lines is that the jacking can cause a a slight rise in the pipe as the culvert advances toward the sewer and a slight settelment after it passes. The effect of this depends on the clay, water table distance to the pipe and rate of jacking. I would think that jacking the pipe would be competive with open cut installation. No matter how you open cut, the existing lines will be a concern. I would talk to a local pipe jacker about the project. This work is as much art as it is science. Most good contractors would be willing to dicuss if jacking is applicable and what you should consider, as well as the pros and cons for this project. He can also give you a better estimate of cost.