Filling a Horizontal Bore Hole Under a Slab

[phamENG]

I have a 14" horizontal bore hole under a large industrial slab about 200 feet long. While pulling the pipe (HDPE, I believe) through, it broke and the attempt was abandoned. We'll be trying again nearby, but it still leaves us with a 100' long void under the slab. This is a very heavily loaded slab - several thousand pounds per square foot in localized product stacks spread all over waiting to be loaded for shipping. The stacks have footprints roughly equal to a standard flatbed truck trailer or rail car. Settlements in this slab have been an issue such that newer additions have had the slabs pile supported. The soils are predominately clay (coastal plain near a tidal river), ranging from high plasticity "pure" clays to clayey sands, silts, and everything in between. The old geotechnical report from the site suggests assuming a ground water lever roughly 5' below the surface. The horizontal bore hole is at 5' below the slab.

I've been tasked with determining how to fill this void. I've specified fills for abandoned pipes and culverts before with success. An unlined horizontal bore hole leaves me with some questions. Is it appropriate to pressurize it while filling? I have some concerns about the ability of the surrounding soils to contain it. I'm inclined to have the slab scanned and cored at 50' - 75' intervals and simply "pour" a lean concrete/flowable fill mix in every other hole and let it spread, using the ends and the intermediate holes as inspection ports to verify that it's migrating through the bore hole and filling the void.

I'm curious to hear the thoughts of others who have dealt with similar situations or have enough experience to make a better guess at it than I can.

Thanks in advance.

 

[oldestguy]

Do a Google search for "grouting contractors". There are several national firms that can do this job correctly. One is "Moretrench" and there are more. I should add that the contractor should give references because grouting can get out of hand and cause unwanted damages.

 

[phamENG]

Thank you, oldestguy. There are a few with offices in the region so I'll contact them and figure out the best way to proceed.