Grouting Joints between large Boulders

[aayjaber]

I am planning to build a structure on a rocky site, my initial excavations lead me to believe that the site is made up of very loose small boulders the first 1-2 ft and the boulders get bigger as you go deeper. The joints between the boulders range from 2-10 inches in width and run along almost upwards in the same direction even though some joints overlap.

I am planning to clean the joints by pressurized water jet as deep as I can go and fill the cleaned joints with cement grout. Will this stabelze the building pad?

 

[Prepakt1]

I suggest you use a high strength structural grout.If you use this you are effectively making a concrete out of the boulders and grout.You can achieve normal concrete strengths.This proceedure is used for some types of bridge foundations. Just make sure there are no fines around the boulders as the grout will not bind properly if the rock is not clean.

Intrusion Prepakt /marineconcrete.com

 

[civilperson]

I would recommend inclusion of reinforcing steel in the grout location. Scrap 270 ksi cable from precast plants inserted in the spaces between the boulders and covered by the grout will tie the individual components into a whole.

 

[oldestguy]

Are you on some sort of fill? Sounds like a very unusual site that would have open voids between boulders.
Some local codes don't permit building on fill unless it is proven to be suitable for support.


If it is a suitable site, why fill the joints? Can you get some large compactor on site and just pound things together to create a dense mass? Are you otherwise expecting some settlement or future change in this rock mass to cause you to have less support unless voids are filled?

If you really want to get a dense mass with the voids filled with grout, use a grouting contractor who can drill thru the stuff to put the grout in under pressure from the bottom up.

This deserves more thought before doing what you have planned.

 

[aayjaber]

Thanks for the reply.
My site is not fill, it is made up of native rock. I am concerned that the rock formation is boulder like masses with crevices in between filled with red clay soil. The distance between the crevice and the next is between 6 ft to 10 ft. The width of the crevices ranges between 3 in to 18 inches.

I thought after I grade the building pad I will clean any crevice from the soil in it and grout it. The crevices run mostly in one direction.

 

[cvg]

if your foundation sits directly on the rock, why is grouting necessary? what failure mode are you expecting?

 

[aayjaber]

The reason why I am thinking of grouting the area where the crevices are is the create one single mass so in case of seismic activity there will be no differential movement of one boulder in relation to others.

 

[cvg]

sounds like your original plan is a good one