Stabilization of old crushed stone backfill prior to new trenching 1

[Lexingtonian]

We are looking to excavate several deep (10-25') trenches in an area with a high concentration of underground utilities. The existing utilities have been backfilled with 3/4" crushed stone (very porous) which will create significant problems by bleeding into the new trenches as they are dug. As we have limited space for laying the slopes back and we do not want to undermine nearby existing utilities, we are looking for a way to stabilize the crushed stone prior to trenching. Bedrock is about 10' deep. Sheet piling is a possibility but would like to avoid if another option is available. Ground water is expected to be minimal. Dirt, if found, will be a good red clay.
Are there any pressure grouting methods that utilize a low strength grout mix that would stabilize the crushed stone from bleeding but would be diggable with a large excavator?

 

[civilperson]

Flowable fill with super plasticizer to give a 10" slump might work in combination with stinger vibration.

 

[cvg]

we had a similar issue with boring and jacking underneath an existing 84 inch sewerline. jacking contractor had recommended grouting with a chemical grout to stabilize the backfill around the sewerline so it didn't fall in when the shield passed underneath. I'm not sure what they eventually used, but you might contact a jacking or grouting specialty contractor to see if they have any ideas

 

[DRC1]

What type of existing utilities do you have and how dep are they? They may have to be relocated prior to work. The two problems with grouting are a.) the cost and b.) excavation near the utilities. Because of the high permiability of the stone, a lot of grout will be required. This can get very expensive. Once the stone is grouted, it will be difficult to impossible to excavate by hand. Machine excavation in the grouted soil will not be able to control the limit of excavation and willl have a significant risk of damaging existing utilities. Anothe option that would accomidate the utilities would be soldier pile and lagging. Some stone would ravel, but limited grouting could control it.

 

[oldestguy]

How about alternatives to open trenching in the worst areas?

Small mole boring machine for sewers, using mud.

Follow with a hauled in or shoved in utility?

Could a bentonite slurry filled trench be used and then lay in your utility from above and follow with the same gravel backfill as others used?

Can your utility be changed as to details of material type and use, such as force main, etc.?