Retaining Sand

[wolfhnd]

I have a retaining wall to build under a bridge. The spill slope is sand and I'm thinking that frozen earth shoring is my best bet to hold the sand in place during construction. Any thoughts?

 

[Sean2]

Wolfhnd,

I'm not sure, but I think it's unlikely to work, unless the sand is saturated.

It is not clear to me whether the retaining wall forms the bridge abutment, or whether this is a wall to be built somewhere underneath an existing bridge. If the latter is the case, you obviously need to be very careful to identify ALL of the possible failure mechanisms and design to avoid these.

My feeling is that the problem you have is too complex to be resolved only by communicating in this forum. You will need specialised advice specific to the project.

Good luck.

 

[wolfhnd]

Thanks for the reply. The retaining wall is to build a lane behind the existing columns. My bridge designer wants me to close the lanes on the roadway and let the sand pour out to build the wall. I cannot close three lanes of Interstate unless I have no alternative . Geology wants to use jet grouting but I have concerns over the price and reliability.

 

[LanceK]

Wolfhnd,
I tend to agree with Sean2. Depending on the geology of the site, groundwater table, roadway configuration and loading conditions, several options for temporary/permanent excavation support and retaining wall construction need to be evaluated by a qualified geotechnical engineer.

Without the details of the site and the road layout it is hard to recommend options. Have you evaluated traditional construction techniques including sheet piles, gravity concrete retaining walls or segmental retaining structures?

Give us some more details and we can try to make some suggestions.

 

[wolfhnd]

I'm sorry I wish I had given more details to start with. The location is a 6 lane Interstate that was constructed with hydraulic embankment in 1970. The interstate bridges go over the side road where I want to add a lane inside the columns where the spill fill is now. The road is in a flood plain protected by a levee next to the Missouri River. The flood plain has about three foot of semi impervious silty sand over sand. I need a retaining wall about 12' tall to add the lane between the columns and the end bent. The clearance between the bottom of the bridge and the top of the retaining wall will be about 4'.

 

[dison]

I am no expert in soil freezing, so don't take my word as the absolute truth.

My understanding is that this method of soil retention works best in saturated, uniform soils. It has been used before, but I don't know how successful it has been. You should get in touch with a contractor who has done this work and find out more specifics about using it with your conditions. I recall hearing about a project for jacking steel culvert pipe under a railroad using soil freezing as temporary support for the track, but I can't remember the name (I think it was near Buffalo, NY).

Obviously, your biggest problem is little to no overhead clearance to construct the retaining wall. What type of wall do you intend to build? A cast-in-place concrete cantilever retaining wall, steel bin wall, or concrete crib wall would definitely required shoring to construct. A soldier pile wall or drilled shaft (cast-in-place concrete soldier pile) wall may be an option if you have enough overhead clearance to get a soil auger under the bridge (I understand that the absolute minimum is about 12'). The driller would have to add section of auger as it goes to get to the required depth. Steel H-piles would have to be spliced in as long of sections as you could get under the bridge or you could just tie in a reinforcing cage and place concrete. Then, you could use shotcrete lagging to build it top-down (making vertical excavations just a few feet at a time as you place the rebar and shotcrete between piles).

Have you considered a soil nailed wall? This method is most commonly used for temporary shoring and it is a passive system, so DOTs might not be gung-ho for it, but it is the only top-down method of construction that doesn't required any overhead clearance to construct.

Hope this gives you some ideas. ~dison

 

[austim]

wolfhnd,

I apologise for appearing very dumb, but if the headroom above your retaining wall is only 4 feet, what sort of traffic is going to use the added lane?

 

[wolfhnd]

The retaining wall is 12' and the clearance to the bottom of the bridge is 4' making the total clearance 16'. If it was not for the low clearance and the sand fill it would be a normal job.

I did contact a contractor who says that he would saturate the sand with garden sprinklers for a month before freezing the sand.

My geologist and bridge engineer considered soil nailing but you need a cohesive soil to be sure of the pull out strength.

 

[Sean2]

I have had another thought on ground freezing in your situation. Remember that water expands on freezing, and consider what effect that expansion will have on the existing structural elements, such as the column bases! Beware also of tying back into the existing Interstate embankment, because this is what supports the bridge abutment (if I have understood you correctly).

I think the main problem is probably the temporary support of the excavation. If the excavation were supported, I guess you could design a retaining wall system which could be built in the available space.

Have you considered contiguous bored piles installed with a low-headroom rig. The piles would have a capping beam. You would excavate shallow depths at a time over short lengths only. On excavation, you would line the wall with sprayed concrete (although some provision would have to be made for the drainage of water, but not soil, at intervals from behind the wall).

You may have to anchor the wall back at one or more levels (remembering my caution above). This may be possible if the bridge abutment is not piled. You could use multi-strand anchors, which do not require much headroom to install. However, I do not know what - if anything - is available to anchor into.

You could design the retaining wall system so that the "temporary" contiguous bored piles act as permanent works, rather than forming a separate retaining system.

I suggest you consult a specialist geotechnical engineer, who will have a chance to see your drawings and visit the site. It sounds like the scale of the project would justify this expense as part of a sensible risk management strategy. (If there had been no risk, we would not have heard about this job.)

 

[dison]

I just had a thought:

Could you build a sort of retaining bridge? Build large "piers" outside the limits of the existing bridge on both ends of the retaining wall. Then use some method to bridge between the "piers" that won't require any overhead clearance. The supports might have to be quite extensive, but building them outside of the existing bridge is likely to be much easier than building anything underneath (which could at least offset the cost).

I guess the thing would end up being like a massive soldier pile wall.

Just a thought ~dison

 

[wolfhnd]

I had a similiar idea of pushing precast retaining wall units through the sands but I can't find any takers on that idea.

 

[austim]

wolfhnd.

dison just got in ahead of me with his transverse bridging proposal.

Another possibility could be to drive a sheet piled wall behind the abutment, (propped off the abutment, with temporary packers within all expansion joints to transfer the reactions to the opposite abutment perhaps ?)

I would not be on the side of the unbelievers with your suggestion of jacking the wall in from the side. If large culverts can be thrust bored, why not a simple Tee section? With some chamfering on the leading edges to assist displacement of the sand, and perhaps some inbuilt jetting facilities for reducing any unforeseen resistance, I believe that you could have a good solution there.

 

[dicksewerrat]

You might look into pipe-ramming to drive steel pipes throught the sand. This is done in a horizontal direction so the work can be outside the bridge footprint. Use H-piles to support the pipes. Use this as a temp. support. Then design a wall to take the loads for your lane. Pipe-ramming is a technique used in pipeline work.