vertical excavations in sand 1

[koodi]

A retaining wall will retain soil beneath an existing bridge abutment--i.e. underneath an existing bridge.

1. This will require that the the soil be excavated back to a vertical plane adjacent to the abutment piles.

2. The excavation must not remove soil behind the abutments as this will result in settlement of the the bridge approach.

The soil is sand and gravel. Is there a way to stiffen the soil? What is the best method to excavate back without the soil sloughing out from behind the bridge?...........any thoughts?

 

[Focht3]

I assume you want to do this while the bridge remains in service.

You can grout the sands and gravels; normal Portland cement will work in "open" gravels but won't work in sands because the sands will filter out the silt-sized cement particles. A two-stage approach to grouting should work for your problem; first grout with a Portland cement grout to fill all the large voids. Then chemically grout to cement the sands and silts. This can be done from underneath the bridge and/or along the sides of the abutment. This approach is expensive, but will allow the road to remain in continuous service. (The contractor who actually does the excavating may "cuss and discuss" the grouting because some of it may be fairly hard to excavate.)

If this is too expensive, then you can install a drilled shaft retaining wall through the roadway. Depending on the exposed height of the retaining wall, this could be either a cantilever wall or a tied-back wall. You will have to shut down portions of the roadway to accomplish this; and this be slow and will anger a lot of motorists. (With the costs of traffic control, temporary road repairs, etc., this approach may not be much cheaper than chemical grouting.)

Whatever you do, recognize that you must retain or otherwise control the soil in the abutment before you begin to excavate the abutment.

 

[VAD]

I will take a shot at this. First I will see if I have the question in perspective. I presume that you are planning to put a retaining structure to the downslope side of the bridge abutment or the sloping side. Hence if you excavate close to the existing abutment piles the fear is that the soil from the roadway side of the bridge will move between the piles and result in settlement of the bridge approach. Do you have an approach slab to the abutment. I am not sure what retaining structure is planned. If for example you were to use a gabion wall then your excavation can be done in small sections and your wall can be constructed and progress in an incremental fashion without causing you grief. However if you are thinking of excavating the entire abutment width on the downslope then the loss of material flowing through the piles would be of concern despite that there may be some arching to minimize movement of material.Again I think your strategy should be to work in short sections and provide shoring on the downside as you excavate across the width of the bridge at the abutment location.

Have I misinterpreted the problem?. If so please let me know and I may be able to think of some other solutions.

It would be helpful to know what is the reason for the proposed scheme.

Cheers

 

[PEinc]

What you have described has been done many times in various ways.

1st common option: As described by Focht3, soldier beams can be driven or drilled into place either under or through the existing bridge deck. Then you excavate and install lagging (and possibly tieback anchors if required) until you reach the proposed subgrade in front of the abutment. Then some type of permanent facing, either C-I-P or precast concrete, is attached to the soldier beam. You should also install a drainage medium behind the facing - either geocomposite chimney drains or free draining stone if using precast facing. Focht3 is also correct about having to shut down some traffic lanes on the bridge if installing soldier beams through the deck.

2nd common option: Working totally under the existing bridge deck, you essentially install (usually) hand-dug, C-I-P concrete soldier beams at about 8 to 10 feet on center. These soldier beams will resemble concrete underpinning piers and may be located either out in front of the abutment or they may be tucked under the abutment footing if there is sufficient space between the existing foundation piles. The soldier beams can be either reinforced concrete or you may set a steel beam into the excavated pit prior to filling the pit with concrete. After the hand-dug soldier beams are completed, you excavate in lifts of 5 feet maximum and install timber lagging between the soldier beams until you reach subgrade ot reach the level of any required tieback anchors. Drilling through the soldier beams, install and test the tiebacks if required. These will probably need to be permanent, corrosion protected anchors. After the tiebacks are installed, continue lagging in lifts down to proposed subgrade. Again, install any required wall drainage. Then install a permanent facing which is attached to the soldier beams.

3rd common option: If the soils beneath the existing abutment footing can be excavated to about 5 feet deep without collapsing, and if you can drill beneath the abutment without hitting the existing foundation piles, you may be able to install a permanent soil nail wall instead of a soldier beam wall. To build the soil nail wall, excavate in lifts of about 5 feet high, install the permanent corrosion protected soil nails and shotcrete. Install geocomposite chimney drains on the dirt before you shotcrete and connect them to a collection system to carry away any water that might appear. After you have soil nailed and shotcreted down to subgrade, you attach a C-I-P, shotcrete, or precast permanent facing. The initial shotcrete acts as the temporary facing until the permanent facing is attached. Reference: FHWA Publication No. FHWA-SA-96-069, Manual for Design & Construction Monitoring of Soil Nail Walls, November 1996.

Warning! If you have a ground water problem under the existing abutment, any of these walls will be difficult to build, especially the soil nail wall, without proper dewatering.

 

[BigH]

Kudos to all!!

I, too, will attempt another suggestion. It is a similar approach to PEinc's soil nail wall with some modification. If the soil will stand, you can excavate 4 ft down, say, in sections 3 ft wide - say with 5 ft. centre on centre. Having precast a 3ft by 3ft "footing" with a hole in the centre, place the footing vertically against the wall and then drill in a soil anchor and prestress it. The anchor will put a inward thrusting pressure against the soil. Then dig out sections. You will then have footings at 5 ft centres. Excavate the remaining soil in the row and shotcrete using mesh. Then proceed to the second row (again at 5 ft offset- stagger the "footings" from the above row. You can continue. While I had such a system in the design of the Coquihalla Highway, construction "took it out" - but I understand a similar system was installed in Revelstoke. You may need to play with the spacings, etc., but it seems a reasonable system to me - used in Stuttgart Germany for deep cut and cover subway.

Good thread - cheers to all.

 

[PEinc]

BigH,

Your suggestion is essentially a soldier beamless wall. These are starting to be used more frequently nowadays, usually for slope or landslide stabilization. koodi would have to be careful using anchors through a precast panel due to the probability that the existing foundation piles may not be located at a constant spacing and therefore may not line up with the precast's tie hole spacing. It could turn out that a foundation pile is located directly behind a tieback sleeve through a panel.

Nevertheless, it is a cutting edge suggestion.

 

[BigH]

Thanks PEinc - and, as indicated, I first suggested it way back in 1983 or 84. You are right about the abutment piles - forgot about that one!

 

[koodi]

It's 10 ft of soil down before the top of the wall and then another 15 ft to the bottom. The retaining walls will have to be cast against the excavation walls--requiring that the excavation walls be flat and not protrude into the concrete. The excavation walls must be vertical and directly against or underneath the abutment--to maximize the "widening" under the bridge.
Thanks for the tips..........[PIPE]

 

[PEinc]

koodi,

Go to

http://www.schnabel.com

Go to their page on Tiedback Bridge Abutments. Open the project report for Guadalupe Corridor in San Jose, CA. It shows a picture of what you are trying to do. It uses hand-dug, tiedback, concrete, soldier beams. In the picture, the permanent concrete facing has not yet been constructed. The wall can be plumb or battered.

Go the Schnabel's page for Soil Nailing. Look at the project report for the Chemistry Building at Washington State University, Pullman Washington. It's not under a bridge abutment but it shows the pieces of work required if you use a soil nail wall (without the permanent facing).

 

[clayR]

Koodi,
I understand you have a 10' backslope above the top of wall, with a retained height of 15ft.

Usually the bridge abutment is 3ft....therefore the overhead height restriction is 13ft from top of wall to the bottom of structure? Still not enough room to put install drilled soldier pile, without going through the deck.

Hand dug piles sound expensive.

Soil nailing is a good application for this type of work, but as stated in the previous post, face stability and water is a major concern. The contractor should have a plan in place to address these issues if nailing is proposed. As focht3 indicated grouting the face prior to excavation is possible. It is also possible to stabilize the face during excavation by placing grout through stabilizing berms. Microfine cement grout or sodium silicate/reactant should permeate the sand.

Another method to control unraveling is closely spaced temporary verticle nails.

Both methods would utilize permanent nails and a temporary shotcrete face, followed by one sided formed cip conc face.
The permanent face is attached to nail by a plate with studs.

 

[Geocontractor]

If you are merely looking to &quot;stiffen&quot; the soils, and the soil has <20% passing #200, then it is possible to use a chemical grout such as sodium silicate to cement the soils. With the depth of your cut, some soil nails may also need to be installed.