SUSA12 - I've seen this before and your problem is simple. Vibratory stone columns should NOT have used on this site. Silts (ML) and clays (CL) can be reinforced with stone columns but not improved or densified, especially if this is a full displacement stone column without any pre-augering...
I'm looking at a Cement Grouting specification for curtain grouting on new dam construction. It mentions having set control of Type I/II Ordinary Portland Cement slurry grout down to a 10-seconds. What admixture would do this?
From a contractor’s perspective, the engineers at times specify a design that may work on paper but just can’t be done in the field without incurring significant difficulty and cost. Design Engineers should do their homework by not being shy in asking contractors what equipment limitations there...
If physical site constraints prohibit ACP work with the available equipment, then perhaps a Micropile alternate could be entertained. Micropiles can be installed with much smaller equipment but expect your project cost to increase.
You may start thinking about what type / size of equipment would be used to place these drilled piers in rock. Regardless of the competency of the rock, 30" diameter will require a large drill rig which may very well lead to access issues if working on a tight slope behind your house.
I'd ask...
Unconfined compressive strength testing of 2-inch cubes has been specified to follow ASTM C 942. What is the primary difference, if any, than testing to ASTM C 109? The question is in terms of curing, loading and breaking, nothing to do with casting and/or preparation of the grout cube.
Hubbell Power Systems distributes both Chance and Atlas resistence piers, aka jacked piers. There are several on the market but these guys have been around for a long time. Hayward Baker Inc is an installer of these products in North America.
The answer is, "Lots". Or, too much to reasonable design for. Research the topic "Segregation Potential". Within discussion of this topic is the idea of a critical confining pressure that overcomes crystallization and continuing water migration and hence the building of ice lenses.
A typical permeation grouting program (ground improvement) for something like this would involve either microfine or sodium silicate grouting from springline up about 1-dia and about 1.5-dia wide. Sodium silicates in sand/gravel usually give about 60 to 100-psi UCS while microfines will yield...
Costs are dependent of many conditions; site access, geology, geographic region and more? Not too far off the mark for most of North America, I'd budget micropiles at about $100/lf for capacities of up to 200-kips per. To get your total load carrying capacity you can pair or group this together...
But of course hokie66, I'm sure the contractor knows this and could do it. My guess is he found a rock bottom price from some small startup helical contractor and sees a possible way to increase his margin $$. Any helical guy that would terminate their product on rock is probably hard up for...
Helicals into bedrock just does not compute for me. Conversely, micropiles socketed into bedrock makes a lot of sense. With only 30-ft of overburden this seems like a very simple job. The loads you mention above are obtainable, generally speaking. The contractor just needs to have his hand...
