Material Wash Out Beneath Bridge Deck

[brantkj]

I have a problem regarding a bridge which has experienced significant washout of material at the abutment below the bridge deck. This has resulted in increased loads on the dry-stone wall abutment which retains the soil, some of the stone blocks have since moved and show signs of cracking. Only one lane of the road is affected by this, so in the short term it is possible to close the affected lane. The washout of material was caused by poor drainage established along the steeply rising road running onto the bridge, this has now been fixed.

Does anyone have any alternative solutions to this problem? Means of replacing the washed out material and having some confidence of compaction and bearing capacity for the bridge deck?

The customer is considering the cost of a full bridge replacement but this, of course, is costly and time consuming.

Thanks

 

[3Fan]

There were some issues with the select granular embankment washing out behind several tall MSE wall that supported bridge abutments in our state. It was corrected by pumping in low strength mortar backfill to fill the voids. The drainage was also fixed.

 

[bridgebuster]

Some pictures would help.

 

[brantkj]

Pictures attached

 

[brantkj]

And again

 

[brantkj]

And another

 

[aeoliantexan]

Your photos certainly catch our interest and generate a lot of questions:
Is the bridge supported on a footing on the abutment fill, or on a pier within the abutment?
Did the stone wall beneath the bridge move, or did the side slope move to our right, away from the bridge?
Is the river undercutting the stone wall?

I would be careful about pouring grout or flowable fill into the void beneath the bridge. It may push laterally and cause the unstable mass to move more.

The general impression is that you need to determine which mass has moved and stabilize it before trying to fill the void. This appears to be a serious problem that needs to be repaired before the next big rain. You may want to close the bridge now.

 

[brantkj]

Hey aeoliantexan, thanks for your response.
The bridge is more than 50 years old and construction drawings have not been easy to come by. However, as I understand it, it is supported on the abutment fill, with the dry stone wall also bearing some load.

The washing out has been occurring over a number of years but has recently become a greater concern. I believe that the washout has now increased the load on the wall (hence the shear failure visible by the projected load through the wall). The wall and abutment appear to be founded on rock, so no undercutting is occurring, though during increased river flow, this may be a concern.

The current plan is to build a concrete wall in front of the existing wall (where the wall is damaged). Then to install rock acnhors through the concrete wall, stone wall, soil mass and then anchor down into the bedrock. Once this is completed the idea is to install a slurry/paste (cement) mix to fill in the washed out area and any other non-visible voids.

We have closed one lane of the bridge already. This is in Norway and so the bridge is accessed only by people living around a fjord below this point, this is their only access to property.
We have also implemented drainage measures further up the hill. Meaning that only very localised rainfall will cause further washout.