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I am installing a retaining wall in front of an existing bridge abutment that is supported by a spread footing. Because of the geometry of the site, ground anchors are required to help support the wall. These ground anchors will be be bored under the spread footing of the abutment.

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The bored diameter of the ground anchors is approximately 6". These holes will be filled with grout after the steel anchors have been installed. The distance from the bottom of the spread footing to the anchor can be as little as a few feet vertically. This has raised a few concerns.

The biggest concern seems to be causing the bridge abutment to settle while the borehole is advanced. A secondary concern is the bored hole collapsing from the high soil pressure before it can be filled with grout.

Are these concerns reasonable and what methods can be used to mitigate the risks?

If these boreholes were typical drilled shafts, I would say that the holes should be cased while being drilled or slurry should be used. Neither of these options seems to be a common practice for ground anchors. This can't be a unique situation though.

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  • $\begingroup$ what did you end up doing in this situation? i'm wondering if the increased confining stress under the footing would actually help prevent the holes from collapsing. that is, unless there's positive water pressure and consolidation occurring. $\endgroup$ – Rick supports Monica Jan 26 '17 at 2:34
  • $\begingroup$ The project was never finished. Budget priorities were changed and it was put on hold. $\endgroup$ – hazzey Jan 26 '17 at 2:56
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Your concerns are reasonable.

You mention soil, not rock. I'm assuming the end of the anchors will not be embedded in rock, even as a point anchor. I get the impression the grouted anchors will be totally encapsulated in soil (no rock anywhere) and they will act as a dowel and not a tensioned blot or cable. Dowels offer passive support, whereas tensioned anchors are an active means of support.

Placing grouted anchors as you mention will stiffen the ground beneath the bridge footing and cause a realignment of stresses underneath the footing.

Given that soil will be drilled into and the proximity of the footing, drilling techniques with minimal vibration would be best so as to minimize any affect on the bridge footing - something like auger drilling would be good if possible.

Edit - Pre-injecting Soil with Chemical Grout

Following on from our discussion in the comments.

Given that casing the holes is not an option due to the requirement for cement grout binding with the soil. Pre-injecting the soil with a chemical grout prior to drilling anchor holes would bind the soil reducing the possibility of the anchor holes collapsing after being drilled.

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  • $\begingroup$ My whole concern is that casing the holes isn't possible because that eliminates the grout bond to the soil. At that point I have long tubes but I don't mobilize the soil. $\endgroup$ – hazzey Nov 14 '15 at 13:07
  • $\begingroup$ How long with the anchors be and will they be rods/bars or cables? If they are rods, could they be driven into ground? This would avoid drilling and the use of grout but would require the rods to be corrosion resistant, or made of stainless steel. Is grout required for its stiffening capabilities or just to encapsulate the anchor and bond with the soil? $\endgroup$ – Fred Nov 14 '15 at 14:52
  • $\begingroup$ the anchors are about 30 feet. The grout is required to bond to the soil. The actual anchors are usually rebar or threaded rod depending on the exact application. Corrosion protection is a separate issue. $\endgroup$ – hazzey Nov 14 '15 at 17:04

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